feat(library): measure every track's loudness in the background (M464 #4995)

The first step of loudness normalization: the server measures each track with ffmpeg's EBU R128 filter (true peak, mono as dual mono) and stores the integrated loudness, true peak and loudness range in track_loudness (migration 0065). It also keeps a histogram of the 400 ms gating blocks at 0.1 LU, so album loudness can be computed exactly later with no second decode (#4996). The histogram reproduces ffmpeg's own figure (-10.68 against -10.7 on the captured fixture), and the analyzer logs a warning if the two ever drift. - A background worker, cloned from the fingerprint backfill, measures every track, new ones included. Measuring inline in the scan was dropped: the analysis decodes the whole file, and a large import could pass the scan's one-hour stuck threshold. The scan only deletes a changed file's measurement; the worker ticks every 10 minutes. - Timeouts, the cancel/missing-binary split and settled verdicts follow the fingerprint runner. Silence and undecodable files are stored as verdicts; stalls are retried. The deadline scales with track length. - loudness_settings (enabled, files at once) and an admin card with the coverage gauge, under GET/PUT /api/admin/library/loudness-settings and GET /api/admin/library/loudness. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
2026-10-06 13:01:01 -04:00
parent edd9a3a6db
commit f3196b3443
22 changed files with 2139 additions and 3 deletions
@@ -132,6 +132,13 @@ func run() error {
 	}
 	scanner := library.New(pool, logger, cfg.Library.ScanPaths, fpSettings)

+	// Loudness analysis settings (M464 #4995): shared by the loudness backfill
+	// and the admin API, and falls back to the defaults like the above.
+	loudSettings, loudErr := library.NewLoudnessSettingsService(ctx, pool)
+	if loudErr != nil {
+		logger.Warn("loudness settings: using defaults", "err", loudErr)
+	}
+
 	contact := cfg.Library.ContactEmail
 	if contact == "" {
 		contact = "https://git.fabledsword.com/bvandeusen/minstrel"
@@ -228,6 +235,11 @@ func run() error {
 	// internal/library/fingerprint_backfill.go for why.
 	go library.NewFingerprintBackfillWorker(pool, logger.With("component", "fingerprint_backfill"), fpSettings).Run(ctx)

+	// Loudness backfill (M464 #4995): measures every track's loudness for
+	// normalization, new tracks included; the scan only drops a changed file's
+	// measurement. See internal/library/loudness_backfill.go.
+	go library.NewLoudnessBackfillWorker(pool, logger.With("component", "loudness_backfill"), loudSettings).Run(ctx)
+
 	// Duplicate sweep (M400 #3910): proposes groups of tracks holding one
 	// recording, from the fingerprints above. Sweeps only when fingerprints have
 	// changed since the last sweep.
@@ -377,6 +389,7 @@ func run() error {
 	srv.RecSettings = recSettings
 	srv.TagSettings = tagSettings
 	srv.FingerprintSettings = fpSettings
+	srv.LoudnessSettings = loudSettings
 	// The sweeper above holds this same instance, so a save from the admin
 	// card changes what it does on its next tick (#3936).
 	srv.ReacqSettings = reacqSettings
@@ -0,0 +1,82 @@
+package api
+
+import (
+	"encoding/json"
+	"errors"
+	"net/http"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
+)
+
+// loudnessCoverageResp is the wire shape for GET /api/admin/library/loudness
+// (M464 #4995). measured + silent + unreadable + pending = total; missing
+// tracks are not counted. Enabled travels with the counts because with
+// analysis off, pending never shrinks, and a gauge implying progress would be
+// promising work nothing is doing.
+type loudnessCoverageResp struct {
+	Total      int64 `json:"total"`
+	Measured   int64 `json:"measured"`
+	Silent     int64 `json:"silent"`
+	Unreadable int64 `json:"unreadable"`
+	Pending    int64 `json:"pending"`
+	Enabled    bool  `json:"enabled"`
+}
+
+// handleGetLoudnessCoverage implements GET /api/admin/library/loudness: how
+// far the loudness backfill has got. Always 200; zeros on an empty library.
+func (h *handlers) handleGetLoudnessCoverage(w http.ResponseWriter, r *http.Request) {
+	row, err := library.LoudnessCoverage(r.Context(), h.pool)
+	if err != nil {
+		writeErrWithLog(w, h.logger, "admin: get loudness coverage", apierror.InternalMsg("lookup failed", err))
+		return
+	}
+	writeJSON(w, http.StatusOK, loudnessCoverageResp{
+		Total:      row.Total,
+		Measured:   row.Measured,
+		Silent:     row.Silent,
+		Unreadable: row.Unreadable,
+		Pending:    row.Pending,
+		Enabled:    h.loudnessSettings.Get().Enabled,
+	})
+}
+
+// loudnessSettingsBody is the wire shape for GET and PUT
+// /api/admin/library/loudness-settings.
+type loudnessSettingsBody struct {
+	Enabled             bool  `json:"enabled"`
+	BackfillConcurrency int32 `json:"backfill_concurrency"`
+}
+
+func loudnessSettingsBodyOf(s library.LoudnessSettings) loudnessSettingsBody {
+	return loudnessSettingsBody{Enabled: s.Enabled, BackfillConcurrency: s.BackfillConcurrency}
+}
+
+// handleGetLoudnessSettings implements GET /api/admin/library/loudness-settings.
+func (h *handlers) handleGetLoudnessSettings(w http.ResponseWriter, _ *http.Request) {
+	writeJSON(w, http.StatusOK, loudnessSettingsBodyOf(h.loudnessSettings.Get()))
+}
+
+// handleUpdateLoudnessSettings implements PUT /api/admin/library/loudness-settings.
+// A whole-row write: a body that leaves out the concurrency decodes it as zero,
+// which is refused rather than saved.
+func (h *handlers) handleUpdateLoudnessSettings(w http.ResponseWriter, r *http.Request) {
+	var req loudnessSettingsBody
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeErr(w, apierror.BadRequest("invalid_body", "malformed JSON"))
+		return
+	}
+	saved, err := h.loudnessSettings.Set(r.Context(), library.LoudnessSettings{
+		Enabled:             req.Enabled,
+		BackfillConcurrency: req.BackfillConcurrency,
+	})
+	if err != nil {
+		if errors.Is(err, library.ErrLoudnessSettingOutOfRange) {
+			writeErr(w, apierror.BadRequest("invalid_setting", err.Error()))
+			return
+		}
+		writeErrWithLog(w, h.logger, "admin loudness settings: update failed", apierror.Internal(err))
+		return
+	}
+	writeJSON(w, http.StatusOK, loudnessSettingsBodyOf(saved))
+}
@@ -34,7 +34,7 @@ import (
 // Mount attaches /api/* handlers to r. Public endpoints (login) are outside
 // RequireUser; everything else is gated by the middleware. The events writer
 // is shared with the Subsonic mount so /rest/scrobble feeds the same store.
-func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playevents.Writer, recCfg config.RecommendationConfig, recSettings *recsettings.Service, lidarrCfg *lidarrconfig.Service, lidarrReqs *lidarrrequests.Service, lidarrQuar *lidarrquarantine.Service, tracksSvc *tracks.Service, playlistsSvc *playlists.Service, coverEnricher *coverart.Enricher, coverSettings *coverart.SettingsService, tagSettings *tags.SettingsService, scanner *library.Scanner, scanCfg library.RunScanConfig, dataDir string, sender mailer.Sender, bus *eventbus.Bus, playlistScheduler *playlists.Scheduler, streamSecret []byte, netSettings *netsettings.Service, reacqSettings *reacquisition.SettingsService, fpSettings *library.FingerprintSettingsService) {
+func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playevents.Writer, recCfg config.RecommendationConfig, recSettings *recsettings.Service, lidarrCfg *lidarrconfig.Service, lidarrReqs *lidarrrequests.Service, lidarrQuar *lidarrquarantine.Service, tracksSvc *tracks.Service, playlistsSvc *playlists.Service, coverEnricher *coverart.Enricher, coverSettings *coverart.SettingsService, tagSettings *tags.SettingsService, scanner *library.Scanner, scanCfg library.RunScanConfig, dataDir string, sender mailer.Sender, bus *eventbus.Bus, playlistScheduler *playlists.Scheduler, streamSecret []byte, netSettings *netsettings.Service, reacqSettings *reacquisition.SettingsService, fpSettings *library.FingerprintSettingsService, loudSettings *library.LoudnessSettingsService) {
 	rng := rand.New(rand.NewSource(rand.Int63()))
 	setupToken, err := auth.NewSetupToken()
 	if err != nil {
@@ -65,6 +65,7 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 		netSettings:         netSettings,
 		reacqSettings:       reacqSettings,
 		fingerprintSettings: fpSettings,
+		loudnessSettings:    loudSettings,
 		librarySize:         recommendation.NewLibrarySize(nil),
 		loginGuard:          auth.NewLoginGuard(),
 		setupToken:          setupToken,
@@ -237,6 +238,9 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 				admin.Get("/library/fingerprints", h.handleGetFingerprintCoverage)
 				admin.Get("/library/fingerprint-settings", h.handleGetFingerprintSettings)
 				admin.Put("/library/fingerprint-settings", h.handleUpdateFingerprintSettings)
+				admin.Get("/library/loudness", h.handleGetLoudnessCoverage)
+				admin.Get("/library/loudness-settings", h.handleGetLoudnessSettings)
+				admin.Put("/library/loudness-settings", h.handleUpdateLoudnessSettings)
 				// Duplicates report (#3912): proposals from the duplicate sweep, a
 				// trigger to sweep now, dismissal, and the merge (#3911), which deletes
 				// the removed copies' files after moving their history onto the kept one.
@@ -331,6 +335,9 @@ type handlers struct {
 	// instance the scanner and the fingerprint workers read, so a save from the
 	// admin card reaches them without a restart. Nil serves the defaults.
 	fingerprintSettings *library.FingerprintSettingsService
+	// loudnessSettings is the loudness analysis policy (M464 #4995), the same
+	// instance the loudness backfill reads. Nil serves the defaults.
+	loudnessSettings *library.LoudnessSettingsService
 	// setupToken must accompany the first registration while no users exist
 	// (see auth.SetupToken). requireSetupToken is set by Mount, the only
 	// production constructor; tests that build handlers directly leave it
@@ -0,0 +1,208 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.31.1
+// source: loudness.sql
+
+package dbq
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgtype"
+)
+
+const deleteTrackLoudness = `-- name: DeleteTrackLoudness :exec
+DELETE FROM track_loudness WHERE track_id = $1
+`
+
+// The scan saw new bytes at this path. The stored measurement describes the old
+// ones, so it goes, and the backfill measures the file again.
+func (q *Queries) DeleteTrackLoudness(ctx context.Context, trackID pgtype.UUID) error {
+	_, err := q.db.Exec(ctx, deleteTrackLoudness, trackID)
+	return err
+}
+
+const getLoudnessCoverage = `-- name: GetLoudnessCoverage :one
+SELECT count(*)::bigint AS total,
+       count(*) FILTER (
+           WHERE l.analysis_version >= $1
+             AND l.integrated_lufs IS NOT NULL
+       )::bigint AS measured,
+       count(*) FILTER (
+           WHERE l.analysis_version >= $1
+             AND l.integrated_lufs IS NULL AND NOT l.unreadable
+       )::bigint AS silent,
+       count(*) FILTER (
+           WHERE l.analysis_version >= $1
+             AND l.unreadable
+       )::bigint AS unreadable,
+       count(*) FILTER (
+           WHERE l.track_id IS NULL OR l.analysis_version < $1
+       )::bigint AS pending
+  FROM tracks t
+  LEFT JOIN track_loudness l ON l.track_id = t.id
+ WHERE t.missing_since IS NULL
+`
+
+type GetLoudnessCoverageRow struct {
+	Total      int64
+	Measured   int64
+	Silent     int64
+	Unreadable int64
+	Pending    int64
+}
+
+// The admin gauge. measured + silent + unreadable + pending = total. Missing
+// tracks are excluded, or the gauge could never reach the end.
+func (q *Queries) GetLoudnessCoverage(ctx context.Context, currentVersion int16) (GetLoudnessCoverageRow, error) {
+	row := q.db.QueryRow(ctx, getLoudnessCoverage, currentVersion)
+	var i GetLoudnessCoverageRow
+	err := row.Scan(
+		&i.Total,
+		&i.Measured,
+		&i.Silent,
+		&i.Unreadable,
+		&i.Pending,
+	)
+	return i, err
+}
+
+const getLoudnessSettings = `-- name: GetLoudnessSettings :one
+SELECT id, enabled, backfill_concurrency, updated_at FROM loudness_settings WHERE id = true
+`
+
+func (q *Queries) GetLoudnessSettings(ctx context.Context) (LoudnessSetting, error) {
+	row := q.db.QueryRow(ctx, getLoudnessSettings)
+	var i LoudnessSetting
+	err := row.Scan(
+		&i.ID,
+		&i.Enabled,
+		&i.BackfillConcurrency,
+		&i.UpdatedAt,
+	)
+	return i, err
+}
+
+const listTracksNeedingLoudness = `-- name: ListTracksNeedingLoudness :many
+SELECT t.id, t.file_path, t.duration_ms
+  FROM tracks t
+  LEFT JOIN track_loudness l ON l.track_id = t.id
+ WHERE t.missing_since IS NULL
+   AND (l.track_id IS NULL OR l.analysis_version < $1)
+   AND t.id > $2
+ ORDER BY t.id
+ LIMIT $3
+`
+
+type ListTracksNeedingLoudnessParams struct {
+	CurrentVersion int16
+	AfterID        pgtype.UUID
+	BatchLimit     int32
+}
+
+type ListTracksNeedingLoudnessRow struct {
+	ID         pgtype.UUID
+	FilePath   string
+	DurationMs int32
+}
+
+// The backfill's work queue: tracks with no measurement, or one taken by an
+// older method. Keyset-paged on id so a pass visits each track at most once;
+// an inconclusive attempt writes no row, and without the cursor a file that
+// keeps timing out would be listed again straight away. Missing tracks are
+// skipped: there is no file to read.
+func (q *Queries) ListTracksNeedingLoudness(ctx context.Context, arg ListTracksNeedingLoudnessParams) ([]ListTracksNeedingLoudnessRow, error) {
+	rows, err := q.db.Query(ctx, listTracksNeedingLoudness, arg.CurrentVersion, arg.AfterID, arg.BatchLimit)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ListTracksNeedingLoudnessRow
+	for rows.Next() {
+		var i ListTracksNeedingLoudnessRow
+		if err := rows.Scan(&i.ID, &i.FilePath, &i.DurationMs); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
+const updateLoudnessSettings = `-- name: UpdateLoudnessSettings :one
+UPDATE loudness_settings
+   SET enabled              = $1,
+       backfill_concurrency = $2,
+       updated_at           = now()
+ WHERE id = true
+RETURNING id, enabled, backfill_concurrency, updated_at
+`
+
+type UpdateLoudnessSettingsParams struct {
+	Enabled             bool
+	BackfillConcurrency int32
+}
+
+// Whole-row write from the admin card; migration 0065's CHECK is the backstop
+// behind the service's own validation.
+func (q *Queries) UpdateLoudnessSettings(ctx context.Context, arg UpdateLoudnessSettingsParams) (LoudnessSetting, error) {
+	row := q.db.QueryRow(ctx, updateLoudnessSettings, arg.Enabled, arg.BackfillConcurrency)
+	var i LoudnessSetting
+	err := row.Scan(
+		&i.ID,
+		&i.Enabled,
+		&i.BackfillConcurrency,
+		&i.UpdatedAt,
+	)
+	return i, err
+}
+
+const upsertTrackLoudness = `-- name: UpsertTrackLoudness :exec
+INSERT INTO track_loudness (
+    track_id, integrated_lufs, true_peak_dbtp, loudness_range_lu,
+    block_hist_start, block_hist, unreadable, analysis_version
+) VALUES (
+    $1, $2, $3,
+    $4, $5, $6,
+    $7, $8
+)
+ON CONFLICT (track_id) DO UPDATE SET
+    integrated_lufs   = EXCLUDED.integrated_lufs,
+    true_peak_dbtp    = EXCLUDED.true_peak_dbtp,
+    loudness_range_lu = EXCLUDED.loudness_range_lu,
+    block_hist_start  = EXCLUDED.block_hist_start,
+    block_hist        = EXCLUDED.block_hist,
+    unreadable        = EXCLUDED.unreadable,
+    analysis_version  = EXCLUDED.analysis_version,
+    analyzed_at       = now()
+`
+
+type UpsertTrackLoudnessParams struct {
+	TrackID         pgtype.UUID
+	IntegratedLufs  *float32
+	TruePeakDbtp    *float32
+	LoudnessRangeLu *float32
+	BlockHistStart  *int16
+	BlockHist       []int32
+	Unreadable      bool
+	AnalysisVersion int16
+}
+
+// Written when the backfill measures a track (#4995). Replaces the row
+// wholesale: a measurement of the old bytes has no standing once the file has
+// changed.
+func (q *Queries) UpsertTrackLoudness(ctx context.Context, arg UpsertTrackLoudnessParams) error {
+	_, err := q.db.Exec(ctx, upsertTrackLoudness,
+		arg.TrackID,
+		arg.IntegratedLufs,
+		arg.TruePeakDbtp,
+		arg.LoudnessRangeLu,
+		arg.BlockHistStart,
+		arg.BlockHist,
+		arg.Unreadable,
+		arg.AnalysisVersion,
+	)
+	return err
+}
@@ -417,6 +417,13 @@ type LidarrRequest struct {
 	LidarrAddConfirmedAt pgtype.Timestamptz
 }

+type LoudnessSetting struct {
+	ID                  bool
+	Enabled             bool
+	BackfillConcurrency int32
+	UpdatedAt           pgtype.Timestamptz
+}
+
 type MissingReacquisition struct {
 	AlbumID       pgtype.UUID
 	Attempts      int32
@@ -713,6 +720,18 @@ type TrackFingerprint struct {
 	ChromaprintLengthSec int32
 }

+type TrackLoudness struct {
+	TrackID         pgtype.UUID
+	IntegratedLufs  *float32
+	TruePeakDbtp    *float32
+	LoudnessRangeLu *float32
+	BlockHistStart  *int16
+	BlockHist       []int32
+	Unreadable      bool
+	AnalysisVersion int16
+	AnalyzedAt      pgtype.Timestamptz
+}
+
 type TrackSimilarity struct {
 	TrackAID  pgtype.UUID
 	TrackBID  pgtype.UUID
@@ -0,0 +1,2 @@
+DROP TABLE IF EXISTS loudness_settings;
+DROP TABLE IF EXISTS track_loudness;
@@ -0,0 +1,65 @@
+-- 0065_track_loudness.up.sql — measured loudness per track, for loudness
+-- normalization (Scribe milestone #464, #4995).
+--
+-- Measured with ffmpeg's EBU R128 filter rather than read from ReplayGain tags:
+-- tags in the wild are written against four different reference levels, and
+-- most files have none. internal/library/loudness.go says how it is measured.
+--
+-- A table of its own rather than columns on tracks, for the reason
+-- track_fingerprints is (0058): tracks is read with SELECT * on the hot path,
+-- and the block histogram is a few hundred integers only album loudness reads.
+--
+-- What a row means, which the backfill depends on:
+--   no row                          never analyzed, or the file changed since
+--   analysis_version < current      measured by an older method; measure again
+--   analysis_version = current      settled until the file changes:
+--     integrated_lufs NOT NULL        measured
+--     integrated_lufs NULL, unreadable false
+--                                     read fine, but no 400 ms block was above
+--                                     the -70 LUFS gate: silence, or too short
+--     unreadable true                 ffmpeg could not decode the file
+-- A failure that says nothing about the file (a timeout, a cancelled pass, a
+-- missing ffmpeg) writes no row, so the backfill tries again.
+CREATE TABLE track_loudness (
+    track_id          uuid        PRIMARY KEY REFERENCES tracks (id) ON DELETE CASCADE,
+    -- Gated integrated loudness (ITU-R BS.1770), mono measured as dual mono.
+    integrated_lufs   real,
+    -- Highest inter-sample peak, from 4x oversampling, in dB relative to full
+    -- scale. NULL for digital silence, whose peak is -inf.
+    true_peak_dbtp    real,
+    -- Loudness range (EBU Tech 3342): how much the loudness moves within the
+    -- track. Not used for gain; kept because it costs nothing here.
+    loudness_range_lu real,
+    -- How many 400 ms gating blocks fell in each 0.1 LU bin. Bin i holds blocks
+    -- measuring -70.0 + (block_hist_start + i) / 10 LUFS; the array is trimmed
+    -- to the first and last non-empty bins. Album loudness is the gated loudness
+    -- of every block on the album, so it is computed from these exactly, with no
+    -- second decode (#4996).
+    block_hist_start  smallint,
+    block_hist        integer[],
+    unreadable        boolean     NOT NULL DEFAULT false,
+    analysis_version  smallint    NOT NULL,
+    analyzed_at       timestamptz NOT NULL DEFAULT now(),
+
+    CONSTRAINT track_loudness_hist_pair
+        CHECK ((block_hist IS NULL) = (block_hist_start IS NULL))
+);
+
+-- Loudness analysis's knobs, in admin Settings. Rule 25: an operator setting is
+-- a database row, changed without a restart. Singleton in the style of
+-- fingerprint_settings (0061).
+CREATE TABLE loudness_settings (
+    id                   boolean PRIMARY KEY DEFAULT true,
+    -- Off stops the background analysis. Tracks already measured keep their
+    -- values, so normalization keeps working for them.
+    enabled              boolean NOT NULL DEFAULT true,
+    -- Files analyzed at once. Each is a full decode, competing with playback
+    -- transcoding for CPU and with streaming for the mount.
+    backfill_concurrency integer NOT NULL DEFAULT 2,
+    updated_at           timestamptz NOT NULL DEFAULT now(),
+
+    CONSTRAINT loudness_settings_singleton CHECK (id = true),
+    CONSTRAINT loudness_settings_concurrency_range
+        CHECK (backfill_concurrency >= 1 AND backfill_concurrency <= 8)
+);
+INSERT INTO loudness_settings (id) VALUES (true) ON CONFLICT (id) DO NOTHING;
@@ -0,0 +1,77 @@
+-- name: UpsertTrackLoudness :exec
+-- Written when the backfill measures a track (#4995). Replaces the row
+-- wholesale: a measurement of the old bytes has no standing once the file has
+-- changed.
+INSERT INTO track_loudness (
+    track_id, integrated_lufs, true_peak_dbtp, loudness_range_lu,
+    block_hist_start, block_hist, unreadable, analysis_version
+) VALUES (
+    sqlc.arg(track_id), sqlc.narg(integrated_lufs), sqlc.narg(true_peak_dbtp),
+    sqlc.narg(loudness_range_lu), sqlc.narg(block_hist_start), sqlc.narg(block_hist),
+    sqlc.arg(unreadable), sqlc.arg(analysis_version)
+)
+ON CONFLICT (track_id) DO UPDATE SET
+    integrated_lufs   = EXCLUDED.integrated_lufs,
+    true_peak_dbtp    = EXCLUDED.true_peak_dbtp,
+    loudness_range_lu = EXCLUDED.loudness_range_lu,
+    block_hist_start  = EXCLUDED.block_hist_start,
+    block_hist        = EXCLUDED.block_hist,
+    unreadable        = EXCLUDED.unreadable,
+    analysis_version  = EXCLUDED.analysis_version,
+    analyzed_at       = now();
+
+-- name: DeleteTrackLoudness :exec
+-- The scan saw new bytes at this path. The stored measurement describes the old
+-- ones, so it goes, and the backfill measures the file again.
+DELETE FROM track_loudness WHERE track_id = $1;
+
+-- name: ListTracksNeedingLoudness :many
+-- The backfill's work queue: tracks with no measurement, or one taken by an
+-- older method. Keyset-paged on id so a pass visits each track at most once;
+-- an inconclusive attempt writes no row, and without the cursor a file that
+-- keeps timing out would be listed again straight away. Missing tracks are
+-- skipped: there is no file to read.
+SELECT t.id, t.file_path, t.duration_ms
+  FROM tracks t
+  LEFT JOIN track_loudness l ON l.track_id = t.id
+ WHERE t.missing_since IS NULL
+   AND (l.track_id IS NULL OR l.analysis_version < sqlc.arg(current_version))
+   AND t.id > sqlc.arg(after_id)
+ ORDER BY t.id
+ LIMIT sqlc.arg(batch_limit);
+
+-- name: GetLoudnessCoverage :one
+-- The admin gauge. measured + silent + unreadable + pending = total. Missing
+-- tracks are excluded, or the gauge could never reach the end.
+SELECT count(*)::bigint AS total,
+       count(*) FILTER (
+           WHERE l.analysis_version >= sqlc.arg(current_version)
+             AND l.integrated_lufs IS NOT NULL
+       )::bigint AS measured,
+       count(*) FILTER (
+           WHERE l.analysis_version >= sqlc.arg(current_version)
+             AND l.integrated_lufs IS NULL AND NOT l.unreadable
+       )::bigint AS silent,
+       count(*) FILTER (
+           WHERE l.analysis_version >= sqlc.arg(current_version)
+             AND l.unreadable
+       )::bigint AS unreadable,
+       count(*) FILTER (
+           WHERE l.track_id IS NULL OR l.analysis_version < sqlc.arg(current_version)
+       )::bigint AS pending
+  FROM tracks t
+  LEFT JOIN track_loudness l ON l.track_id = t.id
+ WHERE t.missing_since IS NULL;
+
+-- name: GetLoudnessSettings :one
+SELECT * FROM loudness_settings WHERE id = true;
+
+-- name: UpdateLoudnessSettings :one
+-- Whole-row write from the admin card; migration 0065's CHECK is the backstop
+-- behind the service's own validation.
+UPDATE loudness_settings
+   SET enabled              = sqlc.arg(enabled),
+       backfill_concurrency = sqlc.arg(backfill_concurrency),
+       updated_at           = now()
+ WHERE id = true
+RETURNING *;
@@ -91,6 +91,7 @@ var dataTables = []string{
 	"duplicate_groups",
 	"duplicate_sweeps",
 	"track_fingerprints", // M400
+	"track_loudness",     // M464
 	"tracks",
 	"albums",
 	"artists",
@@ -141,4 +142,11 @@ func ResetDB(t *testing.T, pool *pgxpool.Pool) {
 	); err != nil {
 		t.Fatalf("dbtest.ResetDB reset fingerprint settings: %v", err)
 	}
+	// Loudness analysis settings (M464 #4995), reset the same way.
+	if _, err := pool.Exec(ctx, `
+		UPDATE loudness_settings
+		   SET enabled = DEFAULT, backfill_concurrency = DEFAULT, updated_at = DEFAULT`,
+	); err != nil {
+		t.Fatalf("dbtest.ResetDB reset loudness settings: %v", err)
+	}
 }
@@ -0,0 +1,392 @@
+package library
+
+import (
+	"bufio"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"log/slog"
+	"math"
+	"os/exec"
+	"regexp"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// Loudness analysis (M464 #4995).
+//
+// Every track is measured with ffmpeg's EBU R128 filter, which reports the
+// ITU-R BS.1770 values loudness normalization works from:
+//
+//	integrated loudness  the gated, K-weighted average loudness of the whole
+//	                     track, in LUFS. A client levels a track by playing it
+//	                     at (target - integrated) dB.
+//	true peak            the highest inter-sample peak, in dBTP. It bounds how
+//	                     far a quiet track can be raised before it clips.
+//	loudness range       how much the loudness moves within the track.
+//
+// Tags are not trusted: ReplayGain values in the wild are written against four
+// different reference levels, and most files have none.
+//
+// The filter also logs the loudness of every 400 ms gating block (one every
+// 100 ms). Those are kept as a histogram, because an album's loudness is the
+// gated loudness of every block on the album, not an average of its tracks'
+// values. With the histograms stored, album loudness is exact and needs no
+// second decode (#4996).
+
+// loudnessVersion stamps how a track_loudness row was measured. Bump it when
+// the measurement changes (the filter's options, the histogram's bins) and the
+// backfill measures every row below it again.
+const loudnessVersion int16 = 1
+
+// Unlike a fingerprint, the analysis decodes the whole file, so a fixed
+// deadline would either cut off a long mix or be useless for a three-minute
+// song. The deadline is a base plus the track's length at loudnessMinSpeed:
+// a decode slower than that is a stall, not a big file.
+const (
+	loudnessBaseTimeout = 2 * time.Minute
+	loudnessMinSpeed    = 4
+)
+
+// errLoudnessTimeout marks an analysis that ran out of time: a fact about the
+// mount, not about the file.
+var errLoudnessTimeout = errors.New("loudness analysis timed out")
+
+// The block histogram's bins: 0.1 LU wide (the precision ffmpeg prints) from
+// the -70 LUFS absolute gate up to +10 LUFS. Blocks below the gate do not take
+// part in gating at all, so they are not counted; anything above the top bin,
+// which only clipped noise reaches, is counted in it.
+const (
+	loudnessHistFloor  = -70.0
+	loudnessHistPerLU  = 10
+	loudnessHistBins   = 800
+	loudnessStderrTail = 20 // lines kept for an error message
+)
+
+// histogramCrossCheckLU is how far the loudness recomputed from the histogram
+// may stray from ffmpeg's own figure before it is logged. They agree to a few
+// hundredths when the log means what the parser assumes, so a larger gap says
+// ffmpeg's output has changed underneath us.
+const histogramCrossCheckLU = 0.3
+
+// ebur128Args measures the file's first audio stream.
+//
+// peak=true asks for true peak (4x oversampled) rather than sample peak, which
+// misses the inter-sample overs a boosted track would clip on. dualmono=true
+// measures a mono file as if played on both speakers of a stereo pair, which is
+// how it is heard; measured as one channel it would read 3 LU quiet and be
+// boosted too far. framelog=info makes the per-block lines print at the log
+// level asked for here, independent of ffmpeg's default.
+func ebur128Args(path string) []string {
+	return []string{
+		"-hide_banner", "-nostdin", "-nostats",
+		"-loglevel", "info",
+		"-i", path,
+		"-map", "0:a:0",
+		"-af", "ebur128=peak=true:dualmono=true:framelog=info",
+		"-f", "null", "-",
+	}
+}
+
+// loudnessTimeout is the deadline for a track of durationMs. An unknown length
+// (0) gets the base alone, which covers an ordinary song.
+func loudnessTimeout(durationMs int32) time.Duration {
+	return loudnessBaseTimeout + time.Duration(max(durationMs, 0))*time.Millisecond/loudnessMinSpeed
+}
+
+// loudnessResult is one attempt at measuring a track.
+type loudnessResult struct {
+	// integratedLUFS is nil when no block passed the gates: silence, or a
+	// file shorter than one 400 ms block.
+	integratedLUFS *float32
+	truePeakDBTP   *float32
+	rangeLU        *float32
+	hist           blockHistogram
+	err            error
+}
+
+// inconclusive reports whether the attempt failed for a reason that says
+// nothing about the file. Such a result is never stored: stamped at the current
+// version it would read as "this file cannot be measured", and the backfill
+// would never try it again.
+func (r loudnessResult) inconclusive() bool {
+	return isInconclusive(r.err) || errors.Is(r.err, errLoudnessTimeout)
+}
+
+// blockHistogram counts gating blocks per 0.1 LU bin, trimmed to the occupied
+// range: counts[i] is the number of blocks measuring
+// loudnessHistFloor + (start+i)/loudnessHistPerLU LUFS.
+type blockHistogram struct {
+	start  int16
+	counts []int32
+}
+
+func (h blockHistogram) empty() bool { return len(h.counts) == 0 }
+
+// gatedLoudness applies BS.1770's two gates to the histogram and returns the
+// integrated loudness of what passes. Every counted block is already above the
+// absolute gate; the relative gate drops blocks more than 10 LU below the
+// loudness of the blocks that passed it. ok is false when nothing passes.
+//
+// The same function measures an album, over the sum of its tracks'
+// histograms (#4996).
+func (h blockHistogram) gatedLoudness() (lufs float64, ok bool) {
+	energy := func(i int) float64 {
+		l := loudnessHistFloor + float64(int(h.start)+i)/loudnessHistPerLU
+		return math.Pow(10, (l+0.691)/10)
+	}
+	mean := func(threshold float64) (float64, bool) {
+		var sum, n float64
+		for i, c := range h.counts {
+			if c == 0 {
+				continue
+			}
+			if loudnessHistFloor+float64(int(h.start)+i)/loudnessHistPerLU < threshold {
+				continue
+			}
+			sum += float64(c) * energy(i)
+			n += float64(c)
+		}
+		if n == 0 {
+			return 0, false
+		}
+		return sum / n, true
+	}
+	ungated, ok := mean(math.Inf(-1))
+	if !ok {
+		return 0, false
+	}
+	relative := -0.691 + 10*math.Log10(ungated) - 10
+	gated, ok := mean(relative)
+	if !ok {
+		return 0, false
+	}
+	return -0.691 + 10*math.Log10(gated), true
+}
+
+// computeLoudness measures the file at path. durationMs sets the deadline.
+func computeLoudness(ctx context.Context, path string, durationMs int32) loudnessResult {
+	timeout := loudnessTimeout(durationMs)
+	runCtx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+
+	cmd := exec.CommandContext(runCtx, "ffmpeg", ebur128Args(path)...)
+	cmd.WaitDelay = fingerprintWaitDelay
+	stderr, err := cmd.StderrPipe()
+	if err != nil {
+		return loudnessResult{err: fmt.Errorf("ffmpeg: %w", err)}
+	}
+	if err := cmd.Start(); err != nil {
+		return loudnessResult{err: fmt.Errorf("ffmpeg: %w", err)}
+	}
+	// The per-block log runs to ten lines a second of audio, about 12 MB for a
+	// two-hour mix, so it is parsed as it streams rather than buffered.
+	p := newEbur128Parser()
+	p.consume(stderr)
+	waitErr := cmd.Wait()
+
+	switch {
+	case ctx.Err() != nil:
+		// The caller gave up. Report that rather than the kill it caused, so it
+		// is never mistaken for a verdict on the file.
+		return loudnessResult{err: fmt.Errorf("ffmpeg: %w", ctx.Err())}
+	case errors.Is(runCtx.Err(), context.DeadlineExceeded):
+		return loudnessResult{err: fmt.Errorf("ffmpeg: no result within %s: %w", timeout, errLoudnessTimeout)}
+	case waitErr != nil:
+		var exitErr *exec.ExitError
+		if errors.As(waitErr, &exitErr) {
+			return loudnessResult{err: fmt.Errorf("ffmpeg exited %d: %s", exitErr.ExitCode(), p.tail())}
+		}
+		return loudnessResult{err: fmt.Errorf("ffmpeg: %w", waitErr)}
+	}
+	return p.result()
+}
+
+var (
+	// A per-block line: "[Parsed_ebur128_0 @ 0x…] t: 2.49998  TARGET:-23 LUFS
+	// M: -31.1 S:-120.7  I: -31.1 LUFS ...". M is the 400 ms block just ended.
+	ebur128BlockRe = regexp.MustCompile(`\bt:\s*\S+\s+TARGET:.*?\bM:\s*(-?(?:\d+(?:\.\d+)?|inf))`)
+	// The summary's values, each on a line of its own after "Summary:".
+	ebur128SummaryRe = regexp.MustCompile(`^(I|LRA|Peak):\s+(-?(?:\d+(?:\.\d+)?|inf))\s+(?:LUFS|LU|dBFS)$`)
+)
+
+// ebur128Parser reads the filter's log: the per-block lines into the
+// histogram, and the closing summary.
+type ebur128Parser struct {
+	bins      [loudnessHistBins]int32
+	blocks    int
+	inSummary bool
+	summary   map[string]string
+	lastLines []string
+}
+
+func newEbur128Parser() *ebur128Parser {
+	return &ebur128Parser{summary: map[string]string{}}
+}
+
+func (p *ebur128Parser) consume(r io.Reader) {
+	sc := bufio.NewScanner(r)
+	sc.Buffer(make([]byte, 0, 4096), 64*1024)
+	for sc.Scan() {
+		p.line(sc.Text())
+	}
+	// A line past the buffer (not something ffmpeg prints) stops the scanner;
+	// drain the rest so ffmpeg is never blocked writing to a full pipe.
+	_, _ = io.Copy(io.Discard, r)
+}
+
+func (p *ebur128Parser) line(raw string) {
+	line := strings.TrimSpace(strings.TrimRight(raw, "\r"))
+	if line == "" {
+		return
+	}
+	if len(p.lastLines) == loudnessStderrTail {
+		p.lastLines = p.lastLines[1:]
+	}
+	p.lastLines = append(p.lastLines, line)
+
+	if strings.HasSuffix(line, "Summary:") {
+		p.inSummary = true
+		return
+	}
+	if p.inSummary {
+		if m := ebur128SummaryRe.FindStringSubmatch(line); m != nil {
+			p.summary[m[1]] = m[2]
+		}
+		return
+	}
+	m := ebur128BlockRe.FindStringSubmatch(line)
+	if m == nil {
+		return
+	}
+	v, err := strconv.ParseFloat(m[1], 64)
+	if err != nil || v < loudnessHistFloor {
+		// -inf, or below the absolute gate: such a block takes no part in
+		// gating.
+		return
+	}
+	bin := int(math.Round((v - loudnessHistFloor) * loudnessHistPerLU))
+	p.bins[min(bin, loudnessHistBins-1)]++
+	p.blocks++
+}
+
+func (p *ebur128Parser) tail() string {
+	return strings.Join(p.lastLines, " | ")
+}
+
+func (p *ebur128Parser) histogram() blockHistogram {
+	if p.blocks == 0 {
+		return blockHistogram{}
+	}
+	first, last := 0, loudnessHistBins-1
+	for p.bins[first] == 0 {
+		first++
+	}
+	for p.bins[last] == 0 {
+		last--
+	}
+	counts := make([]int32, last-first+1)
+	copy(counts, p.bins[first:last+1])
+	return blockHistogram{start: int16(first), counts: counts}
+}
+
+// result turns a clean exit into a measurement. A run with no summary means
+// ffmpeg decoded nothing it could measure, which is a verdict on the file.
+func (p *ebur128Parser) result() loudnessResult {
+	integrated, ok := p.summary["I"]
+	if !ok {
+		return loudnessResult{err: fmt.Errorf("ffmpeg printed no loudness summary: %s", p.tail())}
+	}
+	r := loudnessResult{
+		hist:         p.histogram(),
+		truePeakDBTP: parseLoudnessValue(p.summary["Peak"]),
+		rangeLU:      parseLoudnessValue(p.summary["LRA"]),
+	}
+	// ffmpeg reports -70.0 when no block passed the gate. The empty histogram
+	// says the same thing directly.
+	if !r.hist.empty() {
+		r.integratedLUFS = parseLoudnessValue(integrated)
+	}
+	if r.integratedLUFS == nil {
+		r.rangeLU = nil
+	}
+	return r
+}
+
+// parseLoudnessValue reads one summary figure; -inf and anything unreadable
+// come back nil.
+func parseLoudnessValue(s string) *float32 {
+	v, err := strconv.ParseFloat(s, 32)
+	if err != nil || math.IsInf(v, 0) || math.IsNaN(v) {
+		return nil
+	}
+	f := float32(v)
+	return &f
+}
+
+// loudnessOutcome is what storeLoudness did with one attempt.
+type loudnessOutcome int
+
+const (
+	loudnessMeasured     loudnessOutcome = iota // integrated loudness stored
+	loudnessSilent                              // read fine; no block above the gate
+	loudnessUnreadable                          // ffmpeg could not decode the file
+	loudnessInconclusive                        // nothing stored; worth trying again
+	loudnessStoreFailed                         // the write itself failed
+)
+
+// storeLoudness records one attempt. It never fails its caller: an unmeasured
+// track simply plays without a gain adjustment.
+//
+// An inconclusive attempt leaves any existing row alone. The scan deletes a
+// row when its file changes, so a row still here describes these bytes, and a
+// value from an older method is a better gain than none until it is redone.
+func storeLoudness(
+	ctx context.Context, q *dbq.Queries, logger *slog.Logger,
+	trackID pgtype.UUID, path string, r loudnessResult,
+) loudnessOutcome {
+	if r.err != nil {
+		logger.Warn("loudness: analysis failed", "path", path, "err", r.err)
+		if r.inconclusive() {
+			return loudnessInconclusive
+		}
+	}
+	params := dbq.UpsertTrackLoudnessParams{
+		TrackID:         trackID,
+		Unreadable:      r.err != nil,
+		AnalysisVersion: loudnessVersion,
+	}
+	outcome := loudnessUnreadable
+	if r.err == nil {
+		outcome = loudnessSilent
+		params.IntegratedLufs = r.integratedLUFS
+		params.TruePeakDbtp = r.truePeakDBTP
+		params.LoudnessRangeLu = r.rangeLU
+		if !r.hist.empty() {
+			start := r.hist.start
+			params.BlockHistStart = &start
+			params.BlockHist = r.hist.counts
+		}
+		if r.integratedLUFS != nil {
+			outcome = loudnessMeasured
+			if got, ok := r.hist.gatedLoudness(); ok && math.Abs(got-float64(*r.integratedLUFS)) > histogramCrossCheckLU {
+				// Stored regardless: ffmpeg's own figure is the track's
+				// loudness. But album loudness is computed from the
+				// histogram, and this says it would be wrong.
+				logger.Warn("loudness: block histogram disagrees with ffmpeg's integrated loudness",
+					"path", path, "ffmpeg_lufs", *r.integratedLUFS, "histogram_lufs", got)
+			}
+		}
+	}
+	if err := q.UpsertTrackLoudness(ctx, params); err != nil {
+		logger.Warn("loudness: storing measurement failed", "path", path, "err", err)
+		return loudnessStoreFailed
+	}
+	return outcome
+}
@@ -0,0 +1,200 @@
+package library
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"sync"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgtype"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// Loudness backfill (M464 #4995).
+//
+// Every track is measured here, the new ones included: the scan only deletes a
+// changed file's measurement (see scanFile), and this worker measures it again.
+// Measuring inline in the scan was the first plan and was dropped, because the
+// analysis decodes the whole file. Added to the scan, a large import would run
+// several times longer and could pass StuckScanThreshold (1h), at which point
+// the run is reaped and a second scan started beside it. The fingerprint
+// backfill is a worker of its own for the same reason.
+//
+// Until a track is measured it plays with no gain adjustment, which is how
+// every track played before normalization existed.
+
+// loudnessBackfillTick is how often the worker looks for work. Shorter than
+// the fingerprint backfill's hour, because a new track plays unleveled until it
+// is measured; once the library has caught up, a tick is one indexed query.
+const loudnessBackfillTick = 10 * time.Minute
+
+// loudnessBackfillBatch is how many tracks one query hands the worker.
+const loudnessBackfillBatch = 50
+
+// loudnessBackfillConcurrency is the shipped value of the concurrency setting.
+// Low for the reason fingerprinting's is: each analysis is a full decode,
+// competing with playback transcoding and streaming.
+const loudnessBackfillConcurrency = 2
+
+// BackfillLoudnessResult tallies one pass.
+type BackfillLoudnessResult struct {
+	Processed    int
+	Measured     int
+	Silent       int // read fine, no block above the gate (settled)
+	Unreadable   int // ffmpeg could not decode the file (settled)
+	Inconclusive int // nothing stored; tried again on a later pass
+}
+
+func (r *BackfillLoudnessResult) add(o loudnessOutcome) {
+	r.Processed++
+	switch o {
+	case loudnessMeasured:
+		r.Measured++
+	case loudnessSilent:
+		r.Silent++
+	case loudnessUnreadable:
+		r.Unreadable++
+	default:
+		r.Inconclusive++
+	}
+}
+
+// LoudnessBackfillWorker measures every track that has no current measurement.
+type LoudnessBackfillWorker struct {
+	pool     *pgxpool.Pool
+	logger   *slog.Logger
+	settings *LoudnessSettingsService
+	tick     time.Duration
+	batch    int32
+	// analyze is a field so an integration test pins which tracks a pass
+	// touches, not what ffmpeg prints.
+	analyze func(ctx context.Context, path string, durationMs int32) loudnessResult
+}
+
+// NewLoudnessBackfillWorker builds a worker with the production cadence.
+// settings is shared with the admin API; nil runs on defaults.
+func NewLoudnessBackfillWorker(
+	pool *pgxpool.Pool, logger *slog.Logger, settings *LoudnessSettingsService,
+) *LoudnessBackfillWorker {
+	return &LoudnessBackfillWorker{
+		pool:     pool,
+		logger:   logger,
+		settings: settings,
+		tick:     loudnessBackfillTick,
+		batch:    loudnessBackfillBatch,
+		analyze:  computeLoudness,
+	}
+}
+
+// Run blocks until ctx is cancelled: one pass at start, then one per tick.
+func (w *LoudnessBackfillWorker) Run(ctx context.Context) {
+	w.runOnce(ctx)
+	t := time.NewTicker(w.tick)
+	defer t.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-t.C:
+			w.runOnce(ctx)
+		}
+	}
+}
+
+// runOnce contains a pass so that nothing it does (an error, a panic) can stop
+// the next tick from firing (rule 157).
+func (w *LoudnessBackfillWorker) runOnce(ctx context.Context) {
+	defer func() {
+		if r := recover(); r != nil {
+			w.logger.Error("loudness backfill: pass panicked", "panic", r)
+		}
+	}()
+	res, err := w.pass(ctx)
+	if err != nil && ctx.Err() == nil {
+		w.logger.Warn("loudness backfill: pass failed", "err", err, "processed", res.Processed)
+	}
+	if res.Processed > 0 {
+		w.logger.Info("loudness backfill: pass complete",
+			"processed", res.Processed, "measured", res.Measured, "silent", res.Silent,
+			"unreadable", res.Unreadable, "inconclusive", res.Inconclusive)
+	}
+}
+
+// pass walks every track needing a measurement once, keyset-paged on id. The
+// cursor is what lets a pass end: an inconclusive attempt writes no row, so a
+// file that keeps timing out would otherwise be listed again immediately.
+// Settings are read before every batch, so switching analysis off ends the
+// pass and a new concurrency applies to the next batch.
+func (w *LoudnessBackfillWorker) pass(ctx context.Context) (BackfillLoudnessResult, error) {
+	q := dbq.New(w.pool)
+	var (
+		res BackfillLoudnessResult
+		mu  sync.Mutex
+	)
+	// The all-zero uuid sorts before every real id. Valid must be true: a NULL
+	// cursor would make "id > NULL" match nothing and every pass a no-op.
+	after := pgtype.UUID{Valid: true}
+	for {
+		if err := ctx.Err(); err != nil {
+			return res, err
+		}
+		cfg := w.settings.Get()
+		if !cfg.Enabled {
+			return res, nil
+		}
+		rows, err := q.ListTracksNeedingLoudness(ctx, dbq.ListTracksNeedingLoudnessParams{
+			CurrentVersion: loudnessVersion,
+			AfterID:        after,
+			BatchLimit:     w.batch,
+		})
+		if err != nil {
+			return res, fmt.Errorf("list tracks needing loudness: %w", err)
+		}
+		if len(rows) == 0 {
+			return res, nil
+		}
+
+		sem := make(chan struct{}, max(1, int(cfg.BackfillConcurrency)))
+		var wg sync.WaitGroup
+		for _, row := range rows {
+			if ctx.Err() != nil {
+				break
+			}
+			sem <- struct{}{}
+			wg.Add(1)
+			go func(row dbq.ListTracksNeedingLoudnessRow) {
+				defer wg.Done()
+				defer func() { <-sem }()
+				defer func() {
+					if r := recover(); r != nil {
+						w.logger.Error("loudness backfill: track panicked", "path", row.FilePath, "panic", r)
+					}
+				}()
+				outcome := storeLoudness(ctx, q, w.logger, row.ID, row.FilePath,
+					w.analyzeFile(ctx, row.FilePath, row.DurationMs))
+				mu.Lock()
+				res.add(outcome)
+				mu.Unlock()
+			}(row)
+		}
+		wg.Wait()
+		after = rows[len(rows)-1].ID
+	}
+}
+
+func (w *LoudnessBackfillWorker) analyzeFile(ctx context.Context, path string, durationMs int32) loudnessResult {
+	if w.analyze == nil {
+		return computeLoudness(ctx, path, durationMs)
+	}
+	return w.analyze(ctx, path, durationMs)
+}
+
+// LoudnessCoverage reports how much of the library carries a current
+// measurement, for the admin gauge. It lives beside the backfill so the
+// version it counts against is the one the backfill writes.
+func LoudnessCoverage(ctx context.Context, pool *pgxpool.Pool) (dbq.GetLoudnessCoverageRow, error) {
+	return dbq.New(pool).GetLoudnessCoverage(ctx, loudnessVersion)
+}
@@ -0,0 +1,230 @@
+package library
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"log/slog"
+	"path/filepath"
+	"sync"
+	"testing"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// TestLoudnessBackfill_Integration pins which tracks a pass measures, what it
+// stores for each kind of result, that a pass ends, and that the gauge counts
+// what the passes wrote.
+func TestLoudnessBackfill_Integration(t *testing.T) {
+	pool := newPool(t)
+	ctx := context.Background()
+	q := dbq.New(pool)
+	dir := t.TempDir()
+
+	_, album, artist := seedTrack(t, pool, filepath.Join(dir, "unmeasured.mp3"))
+	addTrack := func(name string) dbq.Track {
+		t.Helper()
+		tr, err := q.UpsertTrack(ctx, dbq.UpsertTrackParams{
+			Title: name, AlbumID: album.ID, ArtistID: artist.ID,
+			DurationMs: 180000, FilePath: filepath.Join(dir, name+".mp3"), FileSize: 100, FileFormat: "mp3",
+		})
+		if err != nil {
+			t.Fatalf("track %s: %v", name, err)
+		}
+		return tr
+	}
+	lufs := func(v float32) *float32 { return &v }
+	current := addTrack("current")
+	stale := addTrack("stale")
+	missing := addTrack("missing")
+	for _, seed := range []struct {
+		track   dbq.Track
+		version int16
+	}{
+		{current, loudnessVersion},
+		{stale, loudnessVersion - 1},
+	} {
+		if err := q.UpsertTrackLoudness(ctx, dbq.UpsertTrackLoudnessParams{
+			TrackID: seed.track.ID, IntegratedLufs: lufs(-9), AnalysisVersion: seed.version,
+		}); err != nil {
+			t.Fatalf("seed loudness: %v", err)
+		}
+	}
+	if _, err := pool.Exec(ctx, "UPDATE tracks SET missing_since = now() WHERE id = $1", missing.ID); err != nil {
+		t.Fatalf("mark missing: %v", err)
+	}
+
+	settings, err := NewLoudnessSettingsService(ctx, pool)
+	if err != nil {
+		t.Fatalf("loudness settings: %v", err)
+	}
+	w := NewLoudnessBackfillWorker(pool, slog.New(slog.NewTextHandler(io.Discard, nil)), settings)
+	// A batch of one forces the keyset cursor across several queries in a pass.
+	w.batch = 1
+	var mu sync.Mutex
+	calls := map[string]int{}
+	durations := map[string]int32{}
+	start := int16(500)
+	w.analyze = func(_ context.Context, path string, durationMs int32) loudnessResult {
+		name := filepath.Base(path)
+		mu.Lock()
+		calls[name]++
+		durations[name] = durationMs
+		mu.Unlock()
+		switch name {
+		case "stall.mp3":
+			return loudnessResult{err: fmt.Errorf("ffmpeg: %w", errLoudnessTimeout)}
+		case "corrupt.mp3":
+			return loudnessResult{err: errors.New("ffmpeg exited 1: invalid data")}
+		case "silent.mp3":
+			return loudnessResult{}
+		default:
+			return loudnessResult{
+				integratedLUFS: lufs(-12.5), truePeakDBTP: lufs(-0.4), rangeLU: lufs(6),
+				hist: blockHistogram{start: start, counts: []int32{3, 0, 7}},
+			}
+		}
+	}
+	callCount := func(name string) int {
+		mu.Lock()
+		defer mu.Unlock()
+		return calls[name]
+	}
+
+	// 1. Only the unmeasured track and the stale one are measured, never the
+	// current one or the missing one, and the track's length reaches the
+	// analyzer (it sets the deadline).
+	res, err := w.pass(ctx)
+	if err != nil {
+		t.Fatalf("first pass: %v", err)
+	}
+	if res.Processed != 2 || res.Measured != 2 {
+		t.Fatalf("first pass = %+v, want 2 processed, 2 measured", res)
+	}
+	for name, want := range map[string]int{
+		"unmeasured.mp3": 1, "stale.mp3": 1, "current.mp3": 0, "missing.mp3": 0,
+	} {
+		if got := callCount(name); got != want {
+			t.Errorf("%s measured %d times, want %d", name, got, want)
+		}
+	}
+	if durations["stale.mp3"] != 180000 {
+		t.Errorf("analyzer got duration %d for stale.mp3, want 180000", durations["stale.mp3"])
+	}
+	var (
+		gotLUFS, gotPeak *float32
+		gotStart         *int16
+		gotHist          []int32
+		gotVersion       int16
+	)
+	if err := pool.QueryRow(ctx, `SELECT integrated_lufs, true_peak_dbtp, block_hist_start, block_hist, analysis_version
+	                                FROM track_loudness WHERE track_id = $1`, stale.ID).
+		Scan(&gotLUFS, &gotPeak, &gotStart, &gotHist, &gotVersion); err != nil {
+		t.Fatalf("read stale row back: %v", err)
+	}
+	if gotLUFS == nil || *gotLUFS != -12.5 || gotPeak == nil || *gotPeak != -0.4 ||
+		gotStart == nil || *gotStart != start || len(gotHist) != 3 || gotHist[2] != 7 ||
+		gotVersion != loudnessVersion {
+		t.Errorf("stale row after re-measuring = lufs %v peak %v hist %v@%v version %d",
+			gotLUFS, gotPeak, gotHist, gotStart, gotVersion)
+	}
+
+	// 2. A pass after a complete one is a no-op.
+	res, err = w.pass(ctx)
+	if err != nil {
+		t.Fatalf("second pass: %v", err)
+	}
+	if res.Processed != 0 {
+		t.Fatalf("second pass processed %d tracks, want 0", res.Processed)
+	}
+
+	// 3. A stall is tried once and the pass ends; silence and a corrupt file are
+	// verdicts, stored and not tried again.
+	addTrack("stall")
+	addTrack("corrupt")
+	silent := addTrack("silent")
+	res, err = w.pass(ctx)
+	if err != nil {
+		t.Fatalf("third pass: %v", err)
+	}
+	if res.Processed != 3 || res.Inconclusive != 1 || res.Unreadable != 1 || res.Silent != 1 {
+		t.Fatalf("third pass = %+v, want 3 processed: 1 inconclusive, 1 unreadable, 1 silent", res)
+	}
+	if got := callCount("stall.mp3"); got != 1 {
+		t.Fatalf("stalling file tried %d times in one pass, want exactly 1", got)
+	}
+	var silentHist []int32
+	var silentUnreadable bool
+	if err := pool.QueryRow(ctx, "SELECT block_hist, unreadable FROM track_loudness WHERE track_id = $1",
+		silent.ID).Scan(&silentHist, &silentUnreadable); err != nil {
+		t.Fatalf("read silent row: %v", err)
+	}
+	if silentHist != nil || silentUnreadable {
+		t.Errorf("silent row = hist %v unreadable %v, want no histogram and readable", silentHist, silentUnreadable)
+	}
+	res, err = w.pass(ctx)
+	if err != nil {
+		t.Fatalf("fourth pass: %v", err)
+	}
+	if res.Processed != 1 || callCount("stall.mp3") != 2 || callCount("corrupt.mp3") != 1 {
+		t.Fatalf("fourth pass = %+v; want only the stalled file retried", res)
+	}
+
+	// 4. The gauge counts what the passes wrote, and its buckets add up. Six
+	// present tracks: unmeasured, current, stale, stall, corrupt, silent.
+	cov, err := LoudnessCoverage(ctx, pool)
+	if err != nil {
+		t.Fatalf("coverage: %v", err)
+	}
+	if cov.Total != 6 || cov.Measured != 3 || cov.Silent != 1 || cov.Unreadable != 1 || cov.Pending != 1 {
+		t.Errorf("coverage = %+v, want total 6, measured 3, silent 1, unreadable 1, pending 1", cov)
+	}
+	if cov.Measured+cov.Silent+cov.Unreadable+cov.Pending != cov.Total {
+		t.Errorf("coverage buckets %+v do not sum to the total", cov)
+	}
+
+	// 5. A changed file loses its measurement, so it is measured again.
+	if err := q.DeleteTrackLoudness(ctx, current.ID); err != nil {
+		t.Fatalf("delete loudness: %v", err)
+	}
+	// 6. Switched off, the backfill does nothing, even with work waiting.
+	off := DefaultLoudnessSettings
+	off.Enabled = false
+	if _, err := settings.Set(ctx, off); err != nil {
+		t.Fatalf("switch analysis off: %v", err)
+	}
+	res, err = w.pass(ctx)
+	if err != nil {
+		t.Fatalf("pass with analysis off: %v", err)
+	}
+	if res.Processed != 0 || callCount("current.mp3") != 0 {
+		t.Fatalf("pass with analysis off = %+v, want nothing done", res)
+	}
+	if _, err := settings.Set(ctx, DefaultLoudnessSettings); err != nil {
+		t.Fatalf("switch analysis on: %v", err)
+	}
+	res, err = w.pass(ctx)
+	if err != nil {
+		t.Fatalf("pass after switching back on: %v", err)
+	}
+	if callCount("current.mp3") != 1 {
+		t.Fatalf("changed file measured %d times after switching back on (pass %+v), want 1",
+			callCount("current.mp3"), res)
+	}
+}
+
+// The Go defaults must match the migration's, or a database that cannot be
+// read would analyze differently from a fresh install.
+func TestLoudnessSettings_DefaultsMatchMigration(t *testing.T) {
+	pool := newPool(t)
+	s, err := NewLoudnessSettingsService(context.Background(), pool)
+	if err != nil {
+		t.Fatalf("load: %v", err)
+	}
+	got := s.Get()
+	got.UpdatedAt = DefaultLoudnessSettings.UpdatedAt
+	if got != DefaultLoudnessSettings {
+		t.Errorf("migration defaults = %+v, Go defaults = %+v", got, DefaultLoudnessSettings)
+	}
+}
@@ -0,0 +1,106 @@
+package library
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"sync"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// Loudness analysis settings (M464 #4995). Rule 25: an operator setting is a
+// database row, changed without a restart. One instance is shared by the
+// backfill and the admin API, so a save reaches the worker at once.
+
+// LoudnessSettings mirrors the loudness_settings row.
+type LoudnessSettings struct {
+	// Enabled off stops the background analysis. Measured tracks keep their
+	// values, so normalization keeps working for them.
+	Enabled             bool
+	BackfillConcurrency int32
+	// UpdatedAt is set by the database; ignored by Set.
+	UpdatedAt time.Time
+}
+
+// DefaultLoudnessSettings mirrors migration 0065's column defaults, so a
+// database that cannot be read still analyzes the way a fresh install does.
+var DefaultLoudnessSettings = LoudnessSettings{
+	Enabled:             true,
+	BackfillConcurrency: loudnessBackfillConcurrency,
+}
+
+// ErrLoudnessSettingOutOfRange is returned by Set for a value migration 0065's
+// CHECK would reject, so the API answers 400 naming the field.
+var ErrLoudnessSettingOutOfRange = errors.New("loudness setting out of range")
+
+// LoudnessSettingsService caches the settings and owns their persistence.
+type LoudnessSettingsService struct {
+	pool *pgxpool.Pool
+
+	mu  sync.RWMutex
+	cur LoudnessSettings
+}
+
+// NewLoudnessSettingsService loads once and caches. It always returns a usable
+// service, holding the defaults when the load fails; the error says so.
+func NewLoudnessSettingsService(ctx context.Context, pool *pgxpool.Pool) (*LoudnessSettingsService, error) {
+	s := &LoudnessSettingsService{pool: pool, cur: DefaultLoudnessSettings}
+	row, err := dbq.New(pool).GetLoudnessSettings(ctx)
+	if err != nil {
+		return s, fmt.Errorf("loudness settings: load: %w", err)
+	}
+	s.cur = loudnessSettingsFromRow(row)
+	return s, nil
+}
+
+// Get returns the cached settings. A nil service answers with the defaults.
+func (s *LoudnessSettingsService) Get() LoudnessSettings {
+	if s == nil {
+		return DefaultLoudnessSettings
+	}
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.cur
+}
+
+// Set validates, persists and re-caches.
+func (s *LoudnessSettingsService) Set(ctx context.Context, in LoudnessSettings) (LoudnessSettings, error) {
+	if err := validateLoudnessSettings(in); err != nil {
+		return LoudnessSettings{}, err
+	}
+	if s == nil {
+		return LoudnessSettings{}, errors.New("loudness settings: no settings service")
+	}
+	row, err := dbq.New(s.pool).UpdateLoudnessSettings(ctx, dbq.UpdateLoudnessSettingsParams{
+		Enabled:             in.Enabled,
+		BackfillConcurrency: in.BackfillConcurrency,
+	})
+	if err != nil {
+		return LoudnessSettings{}, fmt.Errorf("loudness settings: save: %w", err)
+	}
+	out := loudnessSettingsFromRow(row)
+	s.mu.Lock()
+	s.cur = out
+	s.mu.Unlock()
+	return out, nil
+}
+
+func validateLoudnessSettings(in LoudnessSettings) error {
+	if in.BackfillConcurrency < minBackfillConcurrency || in.BackfillConcurrency > maxBackfillConcurrency {
+		return fmt.Errorf("%w: backfill_concurrency must be %d-%d",
+			ErrLoudnessSettingOutOfRange, minBackfillConcurrency, maxBackfillConcurrency)
+	}
+	return nil
+}
+
+func loudnessSettingsFromRow(row dbq.LoudnessSetting) LoudnessSettings {
+	return LoudnessSettings{
+		Enabled:             row.Enabled,
+		BackfillConcurrency: row.BackfillConcurrency,
+		UpdatedAt:           row.UpdatedAt.Time,
+	}
+}
@@ -0,0 +1,250 @@
+package library
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"math"
+	"os"
+	"os/exec"
+	"slices"
+	"strings"
+	"testing"
+	"time"
+)
+
+// parseEbur128 runs the parser over a whole log, as computeLoudness does over
+// ffmpeg's stderr.
+func parseEbur128(log string) *ebur128Parser {
+	p := newEbur128Parser()
+	p.consume(strings.NewReader(log))
+	return p
+}
+
+// The fixture is real ffmpeg 6.1 output for 12 s of stereo tone whose first
+// second is digital silence (testdata/ebur128_fixture.txt). Pinning the parser
+// to captured output, not to a hand-written imitation of it, is the point: the
+// per-block lines are where a format drift would hide.
+func TestEbur128Parser_RealOutput(t *testing.T) {
+	raw, err := os.ReadFile("testdata/ebur128_fixture.txt")
+	if err != nil {
+		t.Fatalf("read fixture: %v", err)
+	}
+	r := parseEbur128(string(raw)).result()
+	if r.err != nil {
+		t.Fatalf("result err = %v", r.err)
+	}
+	for name, c := range map[string]struct {
+		got  *float32
+		want float32
+	}{
+		"integrated": {r.integratedLUFS, -10.7},
+		"true peak":  {r.truePeakDBTP, -5.6},
+		"range":      {r.rangeLU, 2.0},
+	} {
+		if c.got == nil || *c.got != c.want {
+			t.Errorf("%s = %v, want %v", name, c.got, c.want)
+		}
+	}
+
+	// 120 blocks, of which the 10 covering the silent second are below the
+	// absolute gate and not counted. The loudest is -8.1 LUFS, the quietest
+	// that passed -22.6.
+	var blocks int32
+	for _, c := range r.hist.counts {
+		blocks += c
+	}
+	if blocks != 110 {
+		t.Errorf("histogram holds %d blocks, want 110", blocks)
+	}
+	if r.hist.start != 474 || len(r.hist.counts) != 146 {
+		t.Errorf("histogram spans bins %d..%d, want 474..619 (-22.6..-8.1 LUFS)",
+			r.hist.start, int(r.hist.start)+len(r.hist.counts)-1)
+	}
+	if r.hist.counts[0] == 0 || r.hist.counts[len(r.hist.counts)-1] == 0 {
+		t.Errorf("histogram not trimmed to its occupied bins: %v", r.hist.counts)
+	}
+
+	// Album loudness is computed from these histograms (#4996), so the
+	// histogram has to reproduce ffmpeg's own figure.
+	got, ok := r.hist.gatedLoudness()
+	if !ok || math.Abs(got-(-10.7)) > 0.05 {
+		t.Errorf("loudness from the histogram = %.3f (ok=%v), want ffmpeg's -10.7 within 0.05", got, ok)
+	}
+}
+
+func TestEbur128Parser_SilenceIsAVerdictNotAMeasurement(t *testing.T) {
+	log := `[Parsed_ebur128_0 @ 0x1] t: 0.1  TARGET:-23 LUFS    M:-120.7 S:-120.7     I: -70.0 LUFS       LRA:   0.0 LU  FTPK:  -inf  -inf dBFS  TPK:  -inf  -inf dBFS
+[Parsed_ebur128_0 @ 0x1] t: 0.2  TARGET:-23 LUFS    M:-120.7 S:-120.7     I: -70.0 LUFS       LRA:   0.0 LU  FTPK:  -inf  -inf dBFS  TPK:  -inf  -inf dBFS
+[Parsed_ebur128_0 @ 0x1] Summary:
+
+  Integrated loudness:
+    I:         -70.0 LUFS
+    Threshold:   0.0 LUFS
+
+  Loudness range:
+    LRA:         0.0 LU
+    Threshold:   0.0 LUFS
+    LRA low:     0.0 LUFS
+    LRA high:    0.0 LUFS
+
+  True peak:
+    Peak:       -inf dBFS
+`
+	r := parseEbur128(log).result()
+	if r.err != nil {
+		t.Fatalf("err = %v, want a clean result", r.err)
+	}
+	if r.integratedLUFS != nil || r.truePeakDBTP != nil || r.rangeLU != nil {
+		t.Errorf("silence measured as integrated=%v peak=%v range=%v, want all nil",
+			r.integratedLUFS, r.truePeakDBTP, r.rangeLU)
+	}
+	if !r.hist.empty() {
+		t.Errorf("silence left blocks in the histogram: %+v", r.hist)
+	}
+	if r.inconclusive() {
+		t.Errorf("silence reported as inconclusive; it is settled until the file changes")
+	}
+}
+
+func TestEbur128Parser_NoSummaryIsUnreadable(t *testing.T) {
+	r := parseEbur128("[in#0 @ 0x1] Error opening input: Invalid data found when processing input\n").result()
+	if r.err == nil {
+		t.Fatal("a log with no summary produced a measurement")
+	}
+	if r.inconclusive() {
+		t.Errorf("err %v reads as inconclusive; ffmpeg ran and found nothing to measure", r.err)
+	}
+	if !strings.Contains(r.err.Error(), "Invalid data found") {
+		t.Errorf("err %q does not carry ffmpeg's reason", r.err)
+	}
+}
+
+func TestEbur128Parser_LoudBlocksLandInTheTopBin(t *testing.T) {
+	p := parseEbur128(`[Parsed_ebur128_0 @ 0x1] t: 0.4  TARGET:-23 LUFS    M:  12.4 S:  12.4     I:  12.4 LUFS
+[Parsed_ebur128_0 @ 0x1] t: 0.5  TARGET:-23 LUFS    M:  -5.0 S:  -5.0     I:  -5.0 LUFS
+`)
+	h := p.histogram()
+	if int(h.start)+len(h.counts) != loudnessHistBins || h.counts[len(h.counts)-1] != 1 {
+		t.Errorf("a +12.4 LUFS block was not counted in the top bin: %+v", h)
+	}
+	if h.start != 650 || h.counts[0] != 1 {
+		t.Errorf("a -5.0 LUFS block is not in bin 650: start %d, first %d", h.start, h.counts[0])
+	}
+}
+
+func TestBlockHistogram_GatedLoudness(t *testing.T) {
+	bin := func(lufs float64) int { return int(math.Round((lufs - loudnessHistFloor) * loudnessHistPerLU)) }
+	hist := func(blocks map[float64]int32) blockHistogram {
+		lo, hi := loudnessHistBins, 0
+		for l := range blocks {
+			lo, hi = min(lo, bin(l)), max(hi, bin(l))
+		}
+		h := blockHistogram{start: int16(lo), counts: make([]int32, hi-lo+1)}
+		for l, n := range blocks {
+			h.counts[bin(l)-lo] = n
+		}
+		return h
+	}
+
+	cases := []struct {
+		name   string
+		blocks map[float64]int32
+		want   float64
+	}{
+		// One level throughout is that level.
+		{"steady", map[float64]int32{-14: 50}, -14},
+		// The quiet half is 30 LU below the loud half, past the relative gate
+		// (10 LU below the ungated loudness, here about -13), so it is dropped
+		// and the result is the loud half alone.
+		{"quiet passage gated out", map[float64]int32{-10: 100, -40: 100}, -10},
+		// 6 LU apart is inside the gate, so both count, energy-weighted: the
+		// result sits nearer the louder level than the midpoint (-15) does.
+		{"both inside the gate", map[float64]int32{-12: 100, -18: 100}, -14.037},
+	}
+	for _, c := range cases {
+		got, ok := hist(c.blocks).gatedLoudness()
+		if !ok || math.Abs(got-c.want) > 0.01 {
+			t.Errorf("%s: gatedLoudness = %.3f (ok=%v), want %.3f", c.name, got, ok, c.want)
+		}
+	}
+	if _, ok := (blockHistogram{}).gatedLoudness(); ok {
+		t.Errorf("an empty histogram reported a loudness")
+	}
+}
+
+func TestEbur128Args(t *testing.T) {
+	args := ebur128Args("/music/a.flac")
+	joined := strings.Join(args, " ")
+	for _, want := range []string{"peak=true", "dualmono=true", "framelog=info", "-map 0:a:0", "-nostdin", "-f null -"} {
+		if !strings.Contains(joined, want) {
+			t.Errorf("args %q lack %q", joined, want)
+		}
+	}
+	// The path is its own argument, never spliced into the filter string.
+	if i := slices.Index(args, "-i"); i < 0 || args[i+1] != "/music/a.flac" {
+		t.Errorf("args %q do not pass the path after -i", args)
+	}
+}
+
+func TestLoudnessTimeout_ScalesWithLength(t *testing.T) {
+	if got := loudnessTimeout(0); got != loudnessBaseTimeout {
+		t.Errorf("unknown length: %s, want the base %s", got, loudnessBaseTimeout)
+	}
+	if got, want := loudnessTimeout(int32((2 * time.Hour).Milliseconds())), loudnessBaseTimeout+30*time.Minute; got != want {
+		t.Errorf("two-hour mix: %s, want %s", got, want)
+	}
+	if got := loudnessTimeout(-5); got != loudnessBaseTimeout {
+		t.Errorf("negative length: %s, want the base %s", got, loudnessBaseTimeout)
+	}
+}
+
+func TestLoudnessResult_Inconclusive(t *testing.T) {
+	for _, err := range []error{
+		fmt.Errorf("ffmpeg: %w", errLoudnessTimeout),
+		fmt.Errorf("ffmpeg: %w", context.Canceled),
+		fmt.Errorf("ffmpeg: %w", exec.ErrNotFound),
+	} {
+		if !(loudnessResult{err: err}).inconclusive() {
+			t.Errorf("%v: not inconclusive, so it would be stored as a verdict", err)
+		}
+	}
+	if (loudnessResult{err: errors.New("ffmpeg exited 1: moov atom not found")}).inconclusive() {
+		t.Errorf("a decode failure read as inconclusive; it would be retried every pass")
+	}
+	if (loudnessResult{}).inconclusive() {
+		t.Errorf("a clean result read as inconclusive")
+	}
+}
+
+func TestBackfillLoudnessResult_Add(t *testing.T) {
+	var r BackfillLoudnessResult
+	for _, o := range []loudnessOutcome{
+		loudnessMeasured, loudnessMeasured, loudnessSilent, loudnessUnreadable,
+		loudnessInconclusive, loudnessStoreFailed,
+	} {
+		r.add(o)
+	}
+	// A failed write stored nothing, so it is retried like an inconclusive one.
+	want := BackfillLoudnessResult{Processed: 6, Measured: 2, Silent: 1, Unreadable: 1, Inconclusive: 2}
+	if r != want {
+		t.Errorf("tally = %+v, want %+v", r, want)
+	}
+}
+
+func TestValidateLoudnessSettings(t *testing.T) {
+	for _, n := range []int32{minBackfillConcurrency, maxBackfillConcurrency} {
+		if err := validateLoudnessSettings(LoudnessSettings{BackfillConcurrency: n}); err != nil {
+			t.Errorf("concurrency %d rejected: %v", n, err)
+		}
+	}
+	for _, n := range []int32{0, maxBackfillConcurrency + 1} {
+		if err := validateLoudnessSettings(LoudnessSettings{BackfillConcurrency: n}); !errors.Is(err, ErrLoudnessSettingOutOfRange) {
+			t.Errorf("concurrency %d: err = %v, want ErrLoudnessSettingOutOfRange", n, err)
+		}
+	}
+	var nilSvc *LoudnessSettingsService
+	if got := nilSvc.Get(); got != DefaultLoudnessSettings {
+		t.Errorf("nil service Get = %+v, want the defaults", got)
+	}
+}
@@ -402,6 +402,13 @@ func (s *Scanner) scanFile(
 			// must not outlive them, or the sweep would compare audio that is gone.
 			s.logger.Warn("fingerprint: clearing stale fingerprint failed", "path", path, "err", err)
 		}
+		// Loudness is measured by its own worker, never inline: it decodes the
+		// whole file (see loudness_backfill.go). The scan's part is to drop a
+		// measurement of bytes that are gone, so clients stop leveling this
+		// track by the old file's loudness and the worker measures it again.
+		if err := q.DeleteTrackLoudness(ctx, track.ID); err != nil {
+			s.logger.Warn("loudness: clearing stale measurement failed", "path", path, "err", err)
+		}
 	}

 	if knownTrack {
@@ -0,0 +1,149 @@
+Input #0, flac, from 'fixture.flac':
+  Metadata:
+    encoder         : Lavf60.16.100
+  Duration: 00:00:12.00, start: 0.000000, bitrate: 789 kb/s
+  Stream #0:0: Audio: flac, 44100 Hz, stereo, s32 (24 bit)
+Stream mapping:
+  Stream #0:0 -> #0:0 (flac (native) -> pcm_s16le (native))
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 0.0999773  TARGET:-23 LUFS    M:-120.7 S:-120.7     I: -70.0 LUFS       LRA:   0.0 LU  FTPK:  -inf  -inf dBFS  TPK:  -inf  -inf dBFS
+Output #0, null, to 'pipe:':
+  Metadata:
+    encoder         : Lavf60.16.100
+  Stream #0:0: Audio: pcm_s16le, 44100 Hz, stereo, s16, 1411 kb/s
+    Metadata:
+      encoder         : Lavc60.31.102 pcm_s16le
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 0.199977   TARGET:-23 LUFS    M:-120.7 S:-120.7     I: -70.0 LUFS       LRA:   0.0 LU  FTPK:  -inf  -inf dBFS  TPK:  -inf  -inf dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 0.299977   TARGET:-23 LUFS    M:-120.7 S:-120.7     I: -70.0 LUFS       LRA:   0.0 LU  FTPK:  -inf  -inf dBFS  TPK:  -inf  -inf dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 0.399977   TARGET:-23 LUFS    M:-163.2 S:-120.7     I: -70.0 LUFS       LRA:   0.0 LU  FTPK:  -inf  -inf dBFS  TPK:  -inf  -inf dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 0.499977   TARGET:-23 LUFS    M:-163.2 S:-120.7     I: -70.0 LUFS       LRA:   0.0 LU  FTPK:  -inf  -inf dBFS  TPK:  -inf  -inf dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 0.599977   TARGET:-23 LUFS    M:-163.2 S:-120.7     I: -70.0 LUFS       LRA:   0.0 LU  FTPK:  -inf  -inf dBFS  TPK:  -inf  -inf dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 0.699977   TARGET:-23 LUFS    M:-163.2 S:-120.7     I: -70.0 LUFS       LRA:   0.0 LU  FTPK:  -inf  -inf dBFS  TPK:  -inf  -inf dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 0.799977   TARGET:-23 LUFS    M:-163.2 S:-120.7     I: -70.0 LUFS       LRA:   0.0 LU  FTPK:  -inf  -inf dBFS  TPK:  -inf  -inf dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 0.899977   TARGET:-23 LUFS    M:-163.2 S:-120.7     I: -70.0 LUFS       LRA:   0.0 LU  FTPK:  -inf  -inf dBFS  TPK:  -inf  -inf dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 0.999977   TARGET:-23 LUFS    M:-163.2 S:-120.7     I: -70.0 LUFS       LRA:   0.0 LU  FTPK:  -inf  -inf dBFS  TPK:  -inf  -inf dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 1.09998    TARGET:-23 LUFS    M: -15.4 S:-120.7     I: -15.4 LUFS       LRA:   0.0 LU  FTPK:  -7.1 -10.7 dBFS  TPK:  -7.1 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 1.19998    TARGET:-23 LUFS    M: -12.2 S:-120.7     I: -13.5 LUFS       LRA:   0.0 LU  FTPK:  -6.6 -10.9 dBFS  TPK:  -6.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 1.29998    TARGET:-23 LUFS    M: -10.3 S:-120.7     I: -12.2 LUFS       LRA:   0.0 LU  FTPK:  -6.3 -11.1 dBFS  TPK:  -6.3 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 1.39998    TARGET:-23 LUFS    M:  -9.0 S:-120.7     I: -11.1 LUFS       LRA:   0.0 LU  FTPK:  -6.0 -11.4 dBFS  TPK:  -6.0 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 1.49998    TARGET:-23 LUFS    M:  -8.8 S:-120.7     I: -10.6 LUFS       LRA:   0.0 LU  FTPK:  -5.8 -11.7 dBFS  TPK:  -5.8 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 1.59998    TARGET:-23 LUFS    M:  -8.7 S:-120.7     I: -10.2 LUFS       LRA:   0.0 LU  FTPK:  -5.7 -12.1 dBFS  TPK:  -5.7 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 1.69998    TARGET:-23 LUFS    M:  -8.6 S:-120.7     I:  -9.9 LUFS       LRA:   0.0 LU  FTPK:  -5.6 -12.4 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 1.79998    TARGET:-23 LUFS    M:  -8.5 S:-120.7     I:  -9.7 LUFS       LRA:   0.0 LU  FTPK:  -5.6 -12.8 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 1.89998    TARGET:-23 LUFS    M:  -8.6 S:-120.7     I:  -9.6 LUFS       LRA:   0.0 LU  FTPK:  -5.6 -13.3 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 1.99998    TARGET:-23 LUFS    M:  -8.6 S:-120.7     I:  -9.5 LUFS       LRA:   0.0 LU  FTPK:  -5.7 -13.8 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 2.09998    TARGET:-23 LUFS    M:  -8.8 S:-120.7     I:  -9.4 LUFS       LRA:   0.0 LU  FTPK:  -5.8 -14.3 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 2.19998    TARGET:-23 LUFS    M:  -8.9 S:-120.7     I:  -9.4 LUFS       LRA:   0.0 LU  FTPK:  -6.0 -14.9 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 2.29998    TARGET:-23 LUFS    M:  -9.2 S:-120.7     I:  -9.3 LUFS       LRA:   0.0 LU  FTPK:  -6.3 -15.6 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 2.39998    TARGET:-23 LUFS    M:  -9.5 S:-120.7     I:  -9.4 LUFS       LRA:   0.0 LU  FTPK:  -6.7 -16.3 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 2.49998    TARGET:-23 LUFS    M:  -9.9 S:-120.7     I:  -9.4 LUFS       LRA:   0.0 LU  FTPK:  -7.1 -17.2 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 2.59998    TARGET:-23 LUFS    M: -10.4 S:-120.7     I:  -9.5 LUFS       LRA:   0.0 LU  FTPK:  -7.7 -18.1 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 2.69998    TARGET:-23 LUFS    M: -11.0 S:-120.7     I:  -9.5 LUFS       LRA:   0.0 LU  FTPK:  -8.3 -19.2 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 2.79998    TARGET:-23 LUFS    M: -11.6 S:-120.7     I:  -9.6 LUFS       LRA:   0.0 LU  FTPK:  -9.1 -20.4 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 2.89998    TARGET:-23 LUFS    M: -12.4 S:-120.7     I:  -9.7 LUFS       LRA:   0.0 LU  FTPK: -10.0 -21.8 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 2.99998    TARGET:-23 LUFS    M: -13.4 S: -11.5     I:  -9.9 LUFS       LRA:  20.0 LU  FTPK: -11.1 -23.6 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 3.09998    TARGET:-23 LUFS    M: -14.5 S: -11.5     I: -10.0 LUFS       LRA:  20.0 LU  FTPK: -12.5 -25.8 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 3.19998    TARGET:-23 LUFS    M: -15.8 S: -11.4     I: -10.1 LUFS       LRA:  20.0 LU  FTPK: -14.1 -28.2 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 3.29998    TARGET:-23 LUFS    M: -17.3 S: -11.4     I: -10.3 LUFS       LRA:  20.0 LU  FTPK: -16.2 -25.4 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 3.39998    TARGET:-23 LUFS    M: -19.1 S: -11.4     I: -10.5 LUFS       LRA:   0.1 LU  FTPK: -19.2 -23.3 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 3.49998    TARGET:-23 LUFS    M: -21.1 S: -11.4     I: -10.5 LUFS       LRA:   0.1 LU  FTPK: -23.7 -21.6 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 3.59998    TARGET:-23 LUFS    M: -22.6 S: -11.4     I: -10.5 LUFS       LRA:   0.1 LU  FTPK: -22.6 -20.2 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 3.69998    TARGET:-23 LUFS    M: -22.5 S: -11.4     I: -10.5 LUFS       LRA:   0.1 LU  FTPK: -18.5 -19.0 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 3.79998    TARGET:-23 LUFS    M: -20.9 S: -11.3     I: -10.6 LUFS       LRA:   0.2 LU  FTPK: -15.8 -17.9 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 3.89998    TARGET:-23 LUFS    M: -18.9 S: -11.3     I: -10.9 LUFS       LRA:   0.2 LU  FTPK: -13.8 -17.0 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 3.99998    TARGET:-23 LUFS    M: -17.1 S: -11.2     I: -11.0 LUFS       LRA:   0.2 LU  FTPK: -12.2 -16.2 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 4.09998    TARGET:-23 LUFS    M: -15.6 S: -11.4     I: -11.1 LUFS       LRA:   0.2 LU  FTPK: -10.9 -15.5 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 4.19998    TARGET:-23 LUFS    M: -14.3 S: -11.5     I: -11.2 LUFS       LRA:   0.2 LU  FTPK:  -9.8 -14.8 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 4.29998    TARGET:-23 LUFS    M: -13.2 S: -11.7     I: -11.3 LUFS       LRA:   0.4 LU  FTPK:  -8.9 -14.2 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 4.39998    TARGET:-23 LUFS    M: -12.3 S: -11.8     I: -11.3 LUFS       LRA:   0.4 LU  FTPK:  -8.2 -13.7 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 4.49998    TARGET:-23 LUFS    M: -11.5 S: -11.9     I: -11.3 LUFS       LRA:   0.5 LU  FTPK:  -7.6 -13.2 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 4.59998    TARGET:-23 LUFS    M: -10.8 S: -12.0     I: -11.3 LUFS       LRA:   0.6 LU  FTPK:  -7.0 -12.8 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 4.69998    TARGET:-23 LUFS    M: -10.2 S: -12.0     I: -11.2 LUFS       LRA:   0.7 LU  FTPK:  -6.6 -12.4 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 4.79998    TARGET:-23 LUFS    M:  -9.7 S: -12.1     I: -11.2 LUFS       LRA:   0.7 LU  FTPK:  -6.3 -12.0 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 4.89998    TARGET:-23 LUFS    M:  -9.3 S: -12.1     I: -11.1 LUFS       LRA:   0.8 LU  FTPK:  -6.0 -11.7 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 4.99998    TARGET:-23 LUFS    M:  -8.9 S: -12.1     I: -11.0 LUFS       LRA:   0.8 LU  FTPK:  -5.8 -11.4 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 5.09998    TARGET:-23 LUFS    M:  -8.6 S: -12.0     I: -11.0 LUFS       LRA:   0.8 LU  FTPK:  -5.7 -11.1 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 5.19998    TARGET:-23 LUFS    M:  -8.4 S: -11.9     I: -10.8 LUFS       LRA:   0.8 LU  FTPK:  -5.6 -10.8 dBFS  TPK:  -5.6 -10.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 5.29998    TARGET:-23 LUFS    M:  -8.3 S: -11.8     I: -10.7 LUFS       LRA:   0.8 LU  FTPK:  -5.6 -10.6 dBFS  TPK:  -5.6 -10.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 5.39998    TARGET:-23 LUFS    M:  -8.2 S: -11.7     I: -10.5 LUFS       LRA:   0.8 LU  FTPK:  -5.6 -10.4 dBFS  TPK:  -5.6 -10.4 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 5.49998    TARGET:-23 LUFS    M:  -8.1 S: -11.5     I: -10.4 LUFS       LRA:   0.8 LU  FTPK:  -5.7 -10.2 dBFS  TPK:  -5.6 -10.2 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 5.59998    TARGET:-23 LUFS    M:  -8.1 S: -11.4     I: -10.4 LUFS       LRA:   0.8 LU  FTPK:  -5.8 -10.1 dBFS  TPK:  -5.6 -10.1 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 5.69998    TARGET:-23 LUFS    M:  -8.2 S: -11.2     I: -10.3 LUFS       LRA:   0.8 LU  FTPK:  -6.0 -10.0 dBFS  TPK:  -5.6 -10.0 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 5.79998    TARGET:-23 LUFS    M:  -8.3 S: -11.1     I: -10.2 LUFS       LRA:   0.8 LU  FTPK:  -6.3  -9.9 dBFS  TPK:  -5.6  -9.9 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 5.89998    TARGET:-23 LUFS    M:  -8.4 S: -10.9     I: -10.2 LUFS       LRA:   1.0 LU  FTPK:  -6.7  -9.8 dBFS  TPK:  -5.6  -9.8 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 5.99998    TARGET:-23 LUFS    M:  -8.7 S: -10.7     I: -10.2 LUFS       LRA:   1.0 LU  FTPK:  -7.2  -9.7 dBFS  TPK:  -5.6  -9.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 6.09998    TARGET:-23 LUFS    M:  -8.9 S: -10.6     I: -10.1 LUFS       LRA:   1.2 LU  FTPK:  -7.7  -9.7 dBFS  TPK:  -5.6  -9.7 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 6.19998    TARGET:-23 LUFS    M:  -9.2 S: -10.4     I: -10.1 LUFS       LRA:   1.3 LU  FTPK:  -8.4  -9.6 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 6.29998    TARGET:-23 LUFS    M:  -9.6 S: -10.3     I: -10.1 LUFS       LRA:   1.5 LU  FTPK:  -9.2  -9.6 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 6.39998    TARGET:-23 LUFS    M: -10.0 S: -10.2     I: -10.1 LUFS       LRA:   1.6 LU  FTPK: -10.1  -9.6 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 6.49998    TARGET:-23 LUFS    M: -10.4 S: -10.0     I: -10.1 LUFS       LRA:   1.8 LU  FTPK: -11.2  -9.6 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 6.59998    TARGET:-23 LUFS    M: -10.9 S: -10.0     I: -10.1 LUFS       LRA:   1.9 LU  FTPK: -12.6  -9.7 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 6.69998    TARGET:-23 LUFS    M: -11.4 S:  -9.9     I: -10.1 LUFS       LRA:   2.0 LU  FTPK: -14.3  -9.7 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 6.79998    TARGET:-23 LUFS    M: -11.9 S:  -9.8     I: -10.2 LUFS       LRA:   2.1 LU  FTPK: -16.5  -9.8 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 6.89998    TARGET:-23 LUFS    M: -12.5 S:  -9.8     I: -10.2 LUFS       LRA:   2.1 LU  FTPK: -19.5  -9.9 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 6.99998    TARGET:-23 LUFS    M: -12.9 S:  -9.8     I: -10.2 LUFS       LRA:   2.2 LU  FTPK: -24.3 -10.0 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 7.09998    TARGET:-23 LUFS    M: -13.3 S:  -9.8     I: -10.3 LUFS       LRA:   2.2 LU  FTPK: -22.2 -10.1 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 7.19998    TARGET:-23 LUFS    M: -13.5 S:  -9.8     I: -10.3 LUFS       LRA:   2.2 LU  FTPK: -18.2 -10.3 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 7.29998    TARGET:-23 LUFS    M: -13.5 S:  -9.8     I: -10.3 LUFS       LRA:   2.2 LU  FTPK: -15.6 -10.5 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 7.39998    TARGET:-23 LUFS    M: -13.3 S:  -9.9     I: -10.4 LUFS       LRA:   2.2 LU  FTPK: -13.6 -10.7 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 7.49998    TARGET:-23 LUFS    M: -13.0 S:  -9.9     I: -10.4 LUFS       LRA:   2.2 LU  FTPK: -12.0 -10.9 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 7.59998    TARGET:-23 LUFS    M: -12.6 S: -10.0     I: -10.4 LUFS       LRA:   2.2 LU  FTPK: -10.8 -11.2 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 7.69998    TARGET:-23 LUFS    M: -12.1 S: -10.0     I: -10.5 LUFS       LRA:   2.2 LU  FTPK:  -9.7 -11.5 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 7.79998    TARGET:-23 LUFS    M: -11.7 S: -10.1     I: -10.5 LUFS       LRA:   2.2 LU  FTPK:  -8.9 -11.8 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 7.89998    TARGET:-23 LUFS    M: -11.2 S: -10.1     I: -10.5 LUFS       LRA:   2.2 LU  FTPK:  -8.1 -12.1 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 7.99998    TARGET:-23 LUFS    M: -10.8 S: -10.2     I: -10.5 LUFS       LRA:   2.2 LU  FTPK:  -7.5 -12.5 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 8.09998    TARGET:-23 LUFS    M: -10.4 S: -10.3     I: -10.5 LUFS       LRA:   2.2 LU  FTPK:  -7.0 -12.9 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 8.19998    TARGET:-23 LUFS    M: -10.1 S: -10.4     I: -10.5 LUFS       LRA:   2.2 LU  FTPK:  -6.6 -13.4 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 8.29998    TARGET:-23 LUFS    M:  -9.8 S: -10.4     I: -10.5 LUFS       LRA:   2.2 LU  FTPK:  -6.2 -13.9 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 8.39998    TARGET:-23 LUFS    M:  -9.5 S: -10.5     I: -10.5 LUFS       LRA:   2.2 LU  FTPK:  -6.0 -14.4 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 8.49998    TARGET:-23 LUFS    M:  -9.3 S: -10.5     I: -10.4 LUFS       LRA:   2.2 LU  FTPK:  -5.8 -15.0 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 8.59998    TARGET:-23 LUFS    M:  -9.2 S: -10.6     I: -10.4 LUFS       LRA:   2.2 LU  FTPK:  -5.7 -15.7 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 8.69998    TARGET:-23 LUFS    M:  -9.1 S: -10.6     I: -10.4 LUFS       LRA:   2.2 LU  FTPK:  -5.6 -16.5 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 8.79998    TARGET:-23 LUFS    M:  -9.0 S: -10.6     I: -10.4 LUFS       LRA:   2.2 LU  FTPK:  -5.6 -17.3 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 8.89998    TARGET:-23 LUFS    M:  -9.0 S: -10.6     I: -10.4 LUFS       LRA:   2.2 LU  FTPK:  -5.6 -18.3 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 8.99998    TARGET:-23 LUFS    M:  -9.1 S: -10.7     I: -10.3 LUFS       LRA:   2.2 LU  FTPK:  -5.7 -19.4 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 9.09998    TARGET:-23 LUFS    M:  -9.2 S: -10.7     I: -10.3 LUFS       LRA:   2.2 LU  FTPK:  -5.9 -20.6 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 9.19998    TARGET:-23 LUFS    M:  -9.4 S: -10.7     I: -10.3 LUFS       LRA:   2.2 LU  FTPK:  -6.1 -22.1 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 9.29998    TARGET:-23 LUFS    M:  -9.7 S: -10.7     I: -10.3 LUFS       LRA:   2.2 LU  FTPK:  -6.4 -23.9 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 9.39998    TARGET:-23 LUFS    M: -10.0 S: -10.7     I: -10.3 LUFS       LRA:   2.1 LU  FTPK:  -6.7 -26.3 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 9.49998    TARGET:-23 LUFS    M: -10.4 S: -10.7     I: -10.3 LUFS       LRA:   2.1 LU  FTPK:  -7.2 -27.7 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 9.59998    TARGET:-23 LUFS    M: -10.9 S: -10.7     I: -10.3 LUFS       LRA:   2.1 LU  FTPK:  -7.8 -25.0 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 9.69998    TARGET:-23 LUFS    M: -11.4 S: -10.7     I: -10.3 LUFS       LRA:   2.1 LU  FTPK:  -8.4 -23.0 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 9.79998    TARGET:-23 LUFS    M: -12.0 S: -10.7     I: -10.3 LUFS       LRA:   2.1 LU  FTPK:  -9.2 -21.3 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 9.89998    TARGET:-23 LUFS    M: -12.7 S: -10.7     I: -10.4 LUFS       LRA:   2.1 LU  FTPK: -10.2 -20.0 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 9.99998    TARGET:-23 LUFS    M: -13.5 S: -10.7     I: -10.4 LUFS       LRA:   2.1 LU  FTPK: -11.3 -18.8 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 10.1       TARGET:-23 LUFS    M: -14.4 S: -10.7     I: -10.4 LUFS       LRA:   2.1 LU  FTPK: -12.7 -17.8 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 10.2       TARGET:-23 LUFS    M: -15.3 S: -10.8     I: -10.4 LUFS       LRA:   2.1 LU  FTPK: -14.5 -16.9 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 10.3       TARGET:-23 LUFS    M: -16.2 S: -10.8     I: -10.5 LUFS       LRA:   2.1 LU  FTPK: -16.7 -16.1 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 10.4       TARGET:-23 LUFS    M: -17.1 S: -10.9     I: -10.5 LUFS       LRA:   2.1 LU  FTPK: -19.8 -15.4 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 10.5       TARGET:-23 LUFS    M: -17.8 S: -11.0     I: -10.6 LUFS       LRA:   2.1 LU  FTPK: -24.9 -14.7 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 10.6       TARGET:-23 LUFS    M: -18.0 S: -11.1     I: -10.6 LUFS       LRA:   2.1 LU  FTPK: -21.7 -14.1 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 10.7       TARGET:-23 LUFS    M: -17.6 S: -11.1     I: -10.6 LUFS       LRA:   2.1 LU  FTPK: -17.9 -13.6 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 10.8       TARGET:-23 LUFS    M: -16.7 S: -11.2     I: -10.7 LUFS       LRA:   2.1 LU  FTPK: -15.4 -13.1 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 10.9       TARGET:-23 LUFS    M: -15.7 S: -11.3     I: -10.7 LUFS       LRA:   2.1 LU  FTPK: -13.4 -12.7 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 11         TARGET:-23 LUFS    M: -14.6 S: -11.4     I: -10.7 LUFS       LRA:   2.1 LU  FTPK: -11.9 -12.3 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 11.1       TARGET:-23 LUFS    M: -13.6 S: -11.5     I: -10.8 LUFS       LRA:   2.1 LU  FTPK: -10.7 -11.9 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 11.2       TARGET:-23 LUFS    M: -12.7 S: -11.6     I: -10.8 LUFS       LRA:   2.1 LU  FTPK:  -9.6 -11.6 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 11.3       TARGET:-23 LUFS    M: -11.8 S: -11.7     I: -10.8 LUFS       LRA:   2.1 LU  FTPK:  -8.8 -11.3 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 11.4       TARGET:-23 LUFS    M: -11.1 S: -11.7     I: -10.8 LUFS       LRA:   2.0 LU  FTPK:  -8.1 -11.0 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 11.5       TARGET:-23 LUFS    M: -10.5 S: -11.7     I: -10.8 LUFS       LRA:   2.0 LU  FTPK:  -7.4 -10.8 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 11.6       TARGET:-23 LUFS    M:  -9.9 S: -11.8     I: -10.8 LUFS       LRA:   2.0 LU  FTPK:  -6.9 -10.6 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 11.7       TARGET:-23 LUFS    M:  -9.5 S: -11.8     I: -10.8 LUFS       LRA:   2.0 LU  FTPK:  -6.5 -10.4 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 11.8       TARGET:-23 LUFS    M:  -9.0 S: -11.7     I: -10.7 LUFS       LRA:   2.0 LU  FTPK:  -6.2 -10.2 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 11.9       TARGET:-23 LUFS    M:  -8.7 S: -11.7     I: -10.7 LUFS       LRA:   2.0 LU  FTPK:  -5.9 -10.1 dBFS  TPK:  -5.6  -9.6 dBFS
+[Parsed_ebur128_0 @ 0x5ebc43357b00] t: 12         TARGET:-23 LUFS    M:  -8.4 S: -11.6     I: -10.7 LUFS       LRA:   2.0 LU  FTPK:  -5.8 -10.0 dBFS  TPK:  -5.6  -9.6 dBFS
+[out#0/null @ 0x5ebc43345c00] video:0kB audio:2067kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
+size=N/A time=00:00:11.90 bitrate=N/A speed= 161x    
+[Parsed_ebur128_0 @ 0x5ebc43357b00] Summary:
+
+  Integrated loudness:
+    I:         -10.7 LUFS
+    Threshold: -20.8 LUFS
+
+  Loudness range:
+    LRA:         2.0 LU
+    Threshold: -30.9 LUFS
+    LRA low:   -11.8 LUFS
+    LRA high:   -9.8 LUFS
+
+  True peak:
+    Peak:       -5.6 dBFS
@@ -105,6 +105,9 @@ type Server struct {
 	// fingerprint workers, so a save from the admin card reaches them without a
 	// restart. Router() constructs a fallback when nil (tests).
 	FingerprintSettings *library.FingerprintSettingsService
+	// LoudnessSettings is the DB-backed loudness analysis policy (M464 #4995),
+	// shared with the loudness backfill. Nil makes the router load its own.
+	LoudnessSettings *library.LoudnessSettingsService
 	// ReacqSettings is the DB-backed missing-file re-acquisition policy
 	// (milestone #290) — the same instance the sweeper in cmd/minstrel/main.go
 	// reads, so a save from the admin card reaches it without a restart
@@ -214,7 +217,15 @@ func (s *Server) Router() http.Handler {
 				s.Logger.Warn("fingerprint settings unavailable; serving defaults", "err", err)
 			}
 		}
-		api.Mount(r, s.Pool, s.Logger, writer, s.RecommendationCfg, recSettings, lidarrCfg, lidarrReqs, lidarrQuar, tracksSvc, playlistsSvc, s.CoverEnricher, s.CoverSettings, s.TagSettings, s.LibraryScanner, s.ScanCfg, s.DataDir, smtpSender, bus, s.PlaylistScheduler, s.StreamSecret, netSettings, reacqSettings, fpSettings)
+		loudSettings := s.LoudnessSettings
+		if loudSettings == nil {
+			var err error
+			loudSettings, err = library.NewLoudnessSettingsService(context.Background(), s.Pool)
+			if err != nil {
+				s.Logger.Warn("loudness settings unavailable; serving defaults", "err", err)
+			}
+		}
+		api.Mount(r, s.Pool, s.Logger, writer, s.RecommendationCfg, recSettings, lidarrCfg, lidarrReqs, lidarrQuar, tracksSvc, playlistsSvc, s.CoverEnricher, s.CoverSettings, s.TagSettings, s.LibraryScanner, s.ScanCfg, s.DataDir, smtpSender, bus, s.PlaylistScheduler, s.StreamSecret, netSettings, reacqSettings, fpSettings, loudSettings)
 		// /api/admin/scan is the only admin route owned by the server package
 		// (it needs the Scanner). Register it as a single inline-middleware
 		// route — using r.Route("/api/admin", ...) here would create a second
@@ -364,6 +364,38 @@ export async function updateFingerprintSettings(
  return api.put<FingerprintSettings>('/api/admin/library/fingerprint-settings', s);
 }

+// Loudness analysis (#4995) -------------------------------------------------
+
+export type LoudnessCoverage = {
+  total: number;
+  measured: number;
+  // Read fine, but no part of the track was loud enough to measure: silence,
+  // or a file shorter than half a second.
+  silent: number;
+  unreadable: number;
+  pending: number;
+  // False when the operator has switched analysis off: pending then never
+  // shrinks, and nothing should read as progress.
+  enabled: boolean;
+};
+
+export async function getLoudnessCoverage(): Promise<LoudnessCoverage> {
+  return api.get<LoudnessCoverage>('/api/admin/library/loudness');
+}
+
+export type LoudnessSettings = {
+  enabled: boolean;
+  backfill_concurrency: number;
+};
+
+export async function getLoudnessSettings(): Promise<LoudnessSettings> {
+  return api.get<LoudnessSettings>('/api/admin/library/loudness-settings');
+}
+
+export async function updateLoudnessSettings(s: LoudnessSettings): Promise<LoudnessSettings> {
+  return api.put<LoudnessSettings>('/api/admin/library/loudness-settings', s);
+}
+
 // Cover-art providers ------------------------------------------------------

 export type CoverProviderCapability = 'album_cover' | 'artist_thumb' | 'artist_fanart';
@@ -0,0 +1,173 @@
+<script lang="ts">
+  import { onMount } from 'svelte';
+  import {
+    getLoudnessCoverage,
+    getLoudnessSettings,
+    updateLoudnessSettings,
+    type LoudnessCoverage,
+    type LoudnessSettings
+  } from '$lib/api/admin';
+  import { errMessage } from '$lib/api/errors';
+  import { pushToast } from '$lib/stores/toast.svelte';
+
+  // Loudness analysis (#4995): the background measurement that loudness
+  // normalization levels playback from, its progress, and its two knobs.
+
+  let saved = $state<LoudnessSettings | null>(null);
+  let form = $state<LoudnessSettings | null>(null);
+  let coverage = $state<LoudnessCoverage | null>(null);
+  let saving = $state(false);
+  let loadError = $state(false);
+
+  const dirty = $derived(!!saved && !!form && JSON.stringify(saved) !== JSON.stringify(form));
+  const concurrencyOk = $derived(
+    !!form &&
+      Number.isInteger(form.backfill_concurrency) &&
+      form.backfill_concurrency >= 1 &&
+      form.backfill_concurrency <= 8
+  );
+
+  async function load() {
+    try {
+      saved = await getLoudnessSettings();
+      form = { ...saved };
+      loadError = false;
+    } catch {
+      loadError = true;
+    }
+    await refreshCoverage();
+  }
+
+  // The gauge moves a few tracks a minute while the backfill runs, so a slow
+  // poll is enough; a failed read keeps the last figures rather than blanking.
+  async function refreshCoverage() {
+    try {
+      coverage = await getLoudnessCoverage();
+    } catch {
+      // Keep what we had.
+    }
+  }
+
+  onMount(() => {
+    load();
+    const timer = setInterval(refreshCoverage, 30_000);
+    return () => clearInterval(timer);
+  });
+
+  async function save() {
+    if (!form || !concurrencyOk) return;
+    saving = true;
+    try {
+      saved = await updateLoudnessSettings(form);
+      form = { ...saved };
+      pushToast('Loudness analysis settings saved.');
+      await refreshCoverage();
+    } catch (e) {
+      pushToast(errMessage(e), 'error');
+    } finally {
+      saving = false;
+    }
+  }
+
+  const inputClass =
+    'mt-1 w-28 rounded border border-border bg-background px-2 py-1 text-sm text-text-primary ' +
+    'focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent';
+</script>
+
+<section class="space-y-4 rounded border border-border bg-surface p-4">
+  <div>
+    <h2 class="font-display text-lg font-medium">Loudness analysis</h2>
+    <p class="mt-1 text-sm text-text-secondary">
+      Measures how loud each track is, so playback can even out the volume between tracks. It runs
+      in the background; until a track is measured, it plays at its own volume.
+    </p>
+  </div>
+
+  {#if coverage && coverage.total > 0}
+    <div class="flex flex-wrap items-center gap-3 text-sm" data-testid="loudness-coverage">
+      <span>
+        {coverage.measured.toLocaleString()} of {coverage.total.toLocaleString()} tracks measured
+      </span>
+      {#if coverage.pending > 0}
+        <span class="text-text-muted">·</span>
+        <span>{coverage.pending.toLocaleString()} pending</span>
+      {/if}
+      {#if !coverage.enabled && coverage.pending > 0}
+        <span class="text-text-muted">·</span>
+        <span>paused while analysis is off</span>
+      {/if}
+      {#if coverage.silent > 0}
+        <span class="text-text-muted">·</span>
+        <span
+          class="cursor-help"
+          title="These tracks are silent or too short to measure. They play without adjustment."
+        >
+          {coverage.silent.toLocaleString()} silent
+        </span>
+      {/if}
+      {#if coverage.unreadable > 0}
+        <span class="text-text-muted">·</span>
+        <span
+          class="cursor-help"
+          title="ffmpeg couldn't decode these files. Each is tried again when its file changes."
+        >
+          {coverage.unreadable.toLocaleString()} unreadable
+        </span>
+      {/if}
+    </div>
+  {/if}
+
+  {#if loadError}
+    <p class="text-sm text-action-destructive">
+      Couldn't load loudness analysis settings.
+      <button type="button" class="underline hover:no-underline" onclick={load}>Try again</button>
+    </p>
+  {:else if form === null}
+    <p class="text-sm text-text-secondary">Loading…</p>
+  {:else}
+    <label class="flex items-start gap-3">
+      <input type="checkbox" bind:checked={form.enabled} class="mt-1" />
+      <span>
+        <span class="text-sm text-text-primary">Analyze tracks</span>
+        <span class="block text-xs text-text-secondary">
+          Off stops the background analysis, including for new tracks. Tracks already measured keep
+          their values, so they stay leveled.
+        </span>
+      </span>
+    </label>
+
+    <label class="block">
+      <span class="text-sm text-text-primary">Files analyzed at once</span>
+      <span class="block text-xs text-text-secondary">
+        Each analysis reads the whole file, competing with playback for CPU and with streaming for
+        the disk. Lower it if playback stutters while it runs.
+      </span>
+      <input
+        type="number"
+        min="1"
+        max="8"
+        bind:value={form.backfill_concurrency}
+        class={inputClass}
+      />
+    </label>
+
+    {#if !concurrencyOk}
+      <p class="text-xs text-action-destructive" data-testid="settings-problems">
+        Files analyzed at once must be a whole number from 1 to 8.
+      </p>
+    {/if}
+
+    <div class="flex justify-end">
+      <button
+        type="button"
+        class="rounded-md bg-action-secondary px-4 py-2 text-sm text-action-fg hover:opacity-90
+               focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent
+               disabled:cursor-not-allowed disabled:opacity-50"
+        disabled={!dirty || saving || !concurrencyOk}
+        onclick={save}
+      >
+        {saving ? 'Saving…' : 'Save'}
+      </button>
+    </div>
+  {/if}
+</section>
@@ -0,0 +1,94 @@
+import { afterEach, describe, expect, test, vi } from 'vitest';
+import { render, screen, fireEvent, waitFor } from '@testing-library/svelte';
+import type { LoudnessCoverage, LoudnessSettings } from '$lib/api/admin';
+
+vi.mock('$lib/api/admin', () => ({
+  getLoudnessSettings: vi.fn(),
+  updateLoudnessSettings: vi.fn(),
+  getLoudnessCoverage: vi.fn()
+}));
+
+vi.mock('$lib/stores/toast.svelte', () => ({ pushToast: vi.fn() }));
+
+import LoudnessSettingsCard from './LoudnessSettingsCard.svelte';
+import { getLoudnessCoverage, getLoudnessSettings, updateLoudnessSettings } from '$lib/api/admin';
+import { pushToast } from '$lib/stores/toast.svelte';
+
+const base: LoudnessSettings = { enabled: true, backfill_concurrency: 2 };
+const coverage: LoudnessCoverage = {
+  total: 1200,
+  measured: 900,
+  silent: 2,
+  unreadable: 3,
+  pending: 295,
+  enabled: true
+};
+
+afterEach(() => vi.clearAllMocks());
+
+async function renderCard(
+  over: Partial<LoudnessSettings> = {},
+  cov: Partial<LoudnessCoverage> = {}
+) {
+  vi.mocked(getLoudnessSettings).mockResolvedValue({ ...base, ...over });
+  vi.mocked(getLoudnessCoverage).mockResolvedValue({ ...coverage, ...cov });
+  const r = render(LoudnessSettingsCard);
+  await screen.findByRole('spinbutton', { name: /files analyzed at once/i });
+  return r;
+}
+
+const saveButton = () => screen.getByRole('button', { name: /save/i });
+const concurrency = () => screen.getByRole('spinbutton', { name: /files analyzed at once/i });
+
+describe('LoudnessSettingsCard', () => {
+  test('shows how far the analysis has got', async () => {
+    await renderCard();
+    const gauge = await screen.findByTestId('loudness-coverage');
+    expect(gauge.textContent).toMatch(/900 of 1,200 tracks measured/);
+    expect(gauge.textContent).toMatch(/295 pending/);
+    expect(gauge.textContent).toMatch(/2 silent/);
+    expect(gauge.textContent).toMatch(/3 unreadable/);
+    expect(gauge.textContent).not.toMatch(/paused/);
+  });
+
+  // With analysis off, pending never shrinks; the gauge must not read as progress.
+  test('says the work is paused when analysis is off', async () => {
+    await renderCard({ enabled: false }, { enabled: false });
+    const gauge = await screen.findByTestId('loudness-coverage');
+    expect(gauge.textContent).toMatch(/paused while analysis is off/);
+  });
+
+  test('save is disabled until something changes, then saves the form', async () => {
+    vi.mocked(updateLoudnessSettings).mockResolvedValue({ ...base, backfill_concurrency: 4 });
+    await renderCard();
+    expect(saveButton()).toHaveProperty('disabled', true);
+
+    await fireEvent.input(concurrency(), { target: { value: '4' } });
+    await waitFor(() => expect(saveButton()).toHaveProperty('disabled', false));
+    await fireEvent.click(saveButton());
+
+    await waitFor(() =>
+      expect(updateLoudnessSettings).toHaveBeenCalledWith({ enabled: true, backfill_concurrency: 4 })
+    );
+    await waitFor(() => expect(pushToast).toHaveBeenCalledWith('Loudness analysis settings saved.'));
+    // The gauge is read again after a save: switching analysis on or off changes it.
+    expect(getLoudnessCoverage).toHaveBeenCalledTimes(2);
+  });
+
+  test('an out-of-range value is named and cannot be saved', async () => {
+    await renderCard();
+    await fireEvent.input(concurrency(), { target: { value: '9' } });
+    await waitFor(() =>
+      expect(screen.getByTestId('settings-problems').textContent).toMatch(/1 to 8/)
+    );
+    expect(saveButton()).toHaveProperty('disabled', true);
+  });
+
+  test('a failed load offers a retry', async () => {
+    vi.mocked(getLoudnessSettings).mockRejectedValue(new Error('boom'));
+    vi.mocked(getLoudnessCoverage).mockResolvedValue(coverage);
+    render(LoudnessSettingsCard);
+    await screen.findByText(/couldn't load loudness analysis settings/i);
+    expect(screen.getByRole('button', { name: /try again/i })).toBeTruthy();
+  });
+});
@@ -2,6 +2,7 @@
  import { pageTitle } from '$lib/branding';
  import { Disc3, Album, Music2, Check, X, RotateCcw, Trash2, Cloud, ChevronRight } from 'lucide-svelte';
  import StageBadge from '$lib/components/StageBadge.svelte';
+  import LoudnessSettingsCard from '$lib/components/LoudnessSettingsCard.svelte';
  import { stageState } from './stage-state';
  import { useQueryClient } from '@tanstack/svelte-query';
  import {
@@ -463,6 +464,10 @@
    {/if}
  </section>

+  <!-- Loudness analysis (#4995): a background worker like the fingerprint
+       backfill, so its progress lives on its own card. -->
+  <LoudnessSettingsCard />
+
  <!-- Cover art bulk refetch -->
  <section class="rounded border border-border bg-surface p-4">
    <h2 class="font-display text-lg font-medium">Cover art</h2>
@@ -48,7 +48,13 @@ vi.mock('$lib/api/admin', async () => {
    deleteQuarantineViaLidarr: vi.fn().mockResolvedValue({}),
    triggerScan: vi.fn().mockResolvedValue({}),
    refetchMissingCovers: vi.fn().mockResolvedValue({ started: true }),
-    researchMissingArt: vi.fn().mockResolvedValue({ version: 1 })
+    researchMissingArt: vi.fn().mockResolvedValue({ version: 1 }),
+    // LoudnessSettingsCard, rendered by the page and tested on its own.
+    getLoudnessSettings: vi.fn().mockResolvedValue({ enabled: true, backfill_concurrency: 2 }),
+    updateLoudnessSettings: vi.fn(),
+    getLoudnessCoverage: vi.fn().mockResolvedValue({
+      total: 0, measured: 0, silent: 0, unreadable: 0, pending: 0, enabled: true
+    })
  };
 });