feat(audio): add dither and resampler options for lossless conversion

Let users choose FFmpeg dithering when reducing bit depth and SoXr or SWR resampling when changing sample rate across single-track and batch lossless conversion flows.
2026-08-18 00:47:16 +02:00 · 2026-07-02 01:24:20 +07:00
parent dcfd95f276
commit 5424648158
7 changed files with 289 additions and 44 deletions
@@ -393,12 +393,19 @@ class FFmpegService {
    required String codec,
    int? targetBitDepth,
    int? targetSampleRate,
+    LosslessConversionProcessing processing =
+        const LosslessConversionProcessing(),
  }) {
-    if (targetSampleRate != null && targetSampleRate > 0) {
-      arguments
-        ..add('-ar')
-        ..add(targetSampleRate.toString());
-    }
+    final sampleFmt = _losslessOutputSampleFormat(
+      codec: codec,
+      targetBitDepth: targetBitDepth,
+    );
+    _appendLosslessAresampleFilter(
+      arguments,
+      targetSampleRate: targetSampleRate,
+      outputSampleFormat: sampleFmt,
+      processing: processing,
+    );
    if (targetBitDepth == null || targetBitDepth <= 0) return;

    if (codec == 'flac') {
@@ -431,6 +438,48 @@ class FFmpegService {
    }
  }

+  static String? _losslessOutputSampleFormat({
+    required String codec,
+    int? targetBitDepth,
+  }) {
+    if (targetBitDepth == null || targetBitDepth <= 0) return null;
+
+    if (codec == 'flac') {
+      return targetBitDepth <= 16 ? 's16' : 's32';
+    }
+    if (codec == 'alac') {
+      return targetBitDepth <= 16 ? 's16p' : 's32p';
+    }
+    if (codec == 'pcm') {
+      return targetBitDepth <= 16 ? 's16' : 's32';
+    }
+    return null;
+  }
+
+  static void _appendLosslessAresampleFilter(
+    List<String> arguments, {
+    int? targetSampleRate,
+    String? outputSampleFormat,
+    LosslessConversionProcessing processing =
+        const LosslessConversionProcessing(),
+  }) {
+    final hasSampleRate = targetSampleRate != null && targetSampleRate > 0;
+    final hasSampleFormat =
+        outputSampleFormat != null && outputSampleFormat.trim().isNotEmpty;
+    if (!hasSampleRate && !hasSampleFormat && !processing.hasDither) return;
+
+    final options = <String>[
+      'resampler=${processing.normalizedResampler}',
+      if (hasSampleRate) 'osr=$targetSampleRate',
+      if (hasSampleFormat) 'osf=${outputSampleFormat.trim()}',
+      if (processing.hasDither) 'dither_method=${processing.normalizedDither}',
+    ];
+
+    arguments
+      ..add('-af')
+      ..add('aresample=${options.join(':')}');
+  }
+
  static Future<String?> convertM4aToFlac(String inputPath) async {
    final outputPath = _buildOutputPath(inputPath, '.flac');

@@ -2349,6 +2398,8 @@ class FFmpegService {
    int? sourceBitDepth,
    LosslessConversionQuality losslessQuality =
        const LosslessConversionQuality(),
+    LosslessConversionProcessing losslessProcessing =
+        const LosslessConversionProcessing(),
  }) async {
    final format = targetFormat.toLowerCase();
    if (!const {
@@ -2380,6 +2431,7 @@ class FFmpegService {
        coverPath: coverPath,
        targetBitDepth: resolvedLosslessQuality.targetBitDepth,
        targetSampleRate: resolvedLosslessQuality.targetSampleRate,
+        processing: losslessProcessing,
        deleteOriginal: deleteOriginal,
      );
    }
@@ -2391,6 +2443,7 @@ class FFmpegService {
        artistTagMode: artistTagMode,
        targetBitDepth: resolvedLosslessQuality.targetBitDepth,
        targetSampleRate: resolvedLosslessQuality.targetSampleRate,
+        processing: losslessProcessing,
        deleteOriginal: deleteOriginal,
      );
    }
@@ -2403,6 +2456,7 @@ class FFmpegService {
        sourceBitDepth: sourceBitDepth,
        targetBitDepth: resolvedLosslessQuality.targetBitDepth,
        targetSampleRate: resolvedLosslessQuality.targetSampleRate,
+        processing: losslessProcessing,
        deleteOriginal: deleteOriginal,
      );
    }
@@ -2500,6 +2554,8 @@ class FFmpegService {
    String? coverPath,
    int? targetBitDepth,
    int? targetSampleRate,
+    LosslessConversionProcessing processing =
+        const LosslessConversionProcessing(),
    bool deleteOriginal = true,
  }) async {
    final outputPath = _buildOutputPath(inputPath, '.m4a');
@@ -2539,6 +2595,7 @@ class FFmpegService {
      codec: 'alac',
      targetBitDepth: targetBitDepth,
      targetSampleRate: targetSampleRate,
+      processing: processing,
    );
    arguments
      ..add('-map_metadata')
@@ -2553,7 +2610,9 @@ class FFmpegService {
    _log.i(
      'Converting ${inputPath.split(Platform.pathSeparator).last} to ALAC'
      '${targetBitDepth != null ? ' $targetBitDepth-bit' : ''}'
-      '${targetSampleRate != null ? ' @ ${targetSampleRate}Hz' : ''}',
+      '${targetSampleRate != null ? ' @ ${targetSampleRate}Hz' : ''}'
+      '${processing.hasDither ? ' dither=${processing.normalizedDither}' : ''}'
+      '${processing.normalizedResampler != 'swr' ? ' resampler=${processing.normalizedResampler}' : ''}',
    );
    final result = await _executeWithArguments(arguments);

@@ -2583,6 +2642,8 @@ class FFmpegService {
    String artistTagMode = artistTagModeJoined,
    int? targetBitDepth,
    int? targetSampleRate,
+    LosslessConversionProcessing processing =
+        const LosslessConversionProcessing(),
    bool deleteOriginal = true,
  }) async {
    final outputPath = _buildOutputPath(inputPath, '.flac');
@@ -2624,6 +2685,7 @@ class FFmpegService {
      codec: 'flac',
      targetBitDepth: targetBitDepth,
      targetSampleRate: targetSampleRate,
+      processing: processing,
    );
    arguments
      ..add('-map_metadata')
@@ -2642,7 +2704,9 @@ class FFmpegService {
    _log.i(
      'Converting ${inputPath.split(Platform.pathSeparator).last} to FLAC'
      '${targetBitDepth != null ? ' $targetBitDepth-bit' : ''}'
-      '${targetSampleRate != null ? ' @ ${targetSampleRate}Hz' : ''}',
+      '${targetSampleRate != null ? ' @ ${targetSampleRate}Hz' : ''}'
+      '${processing.hasDither ? ' dither=${processing.normalizedDither}' : ''}'
+      '${processing.normalizedResampler != 'swr' ? ' resampler=${processing.normalizedResampler}' : ''}',
    );
    final result = await _executeWithArguments(arguments);

@@ -2676,6 +2740,8 @@ class FFmpegService {
    int? sourceBitDepth,
    int? targetBitDepth,
    int? targetSampleRate,
+    LosslessConversionProcessing processing =
+        const LosslessConversionProcessing(),
    bool deleteOriginal = true,
  }) async {
    final isAiff = container == 'aiff';
@@ -2697,22 +2763,24 @@ class FFmpegService {
      inputPath,
      '-map',
      '0:a',
-      '-c:a',
-      codec,
-      if (targetSampleRate != null && targetSampleRate > 0) ...[
-        '-ar',
-        targetSampleRate.toString(),
-      ],
-      '-map_metadata',
-      '-1',
-      outputPath,
-      '-y',
    ];
+    _appendLosslessAresampleFilter(
+      arguments,
+      targetSampleRate: targetSampleRate,
+      outputSampleFormat: _losslessOutputSampleFormat(
+        codec: 'pcm',
+        targetBitDepth: targetBitDepth,
+      ),
+      processing: processing,
+    );
+    arguments.addAll(['-c:a', codec, '-map_metadata', '-1', outputPath, '-y']);

    _log.i(
      'Converting ${inputPath.split(Platform.pathSeparator).last} to '
      '${container.toUpperCase()} (${use24 ? 24 : 16}-bit'
-      '${targetSampleRate != null ? ', ${targetSampleRate}Hz' : ''})',
+      '${targetSampleRate != null ? ', ${targetSampleRate}Hz' : ''}'
+      '${processing.hasDither ? ', dither=${processing.normalizedDither}' : ''}'
+      '${processing.normalizedResampler != 'swr' ? ', resampler=${processing.normalizedResampler}' : ''})',
    );
    final result = await _executeWithArguments(arguments);
    if (!result.success) {