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feat: add smart audio copy and audio mix presets for configurable 5.1 downmixing

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This commit is contained in:
Trevor Mears
2026-01-17 13:31:13 -08:00
parent 9919975663
commit c1f8077438
8 changed files with 80 additions and 13 deletions
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server/services/transcodeSession.js
+37 -9
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@@ -222,15 +222,43 @@ class TranscodeSession extends EventEmitter {
this.addVideoEncoderArgs(args, encoder);
}
// Audio: Transcode to AAC
args.push(
'-c:a', 'aac',
'-ar', '48000',
'-b:a', '192k',
// Smart Stereo Downmix (ITU-R BS.775 Standard + LFE):
// Balanced mix: FL/FR=100%, FC=70% (Unity), Surrounds=70%, LFE=50%
'-af', 'pan=stereo|FL=FL+0.707*FC+0.707*BL+0.5*LFE|FR=FR+0.707*FC+0.707*BR+0.5*LFE,aresample=async=1'
);
// Audio: Apply mix preset
const audioCodec = this.options.audioCodec?.toLowerCase() || 'unknown';
const audioChannels = this.options.audioChannels || 0;
const audioMixPreset = this.options.audioMixPreset || 'auto';
const isStereoAac = audioCodec.includes('aac') && audioChannels === 2;
// Define pan filter presets for 5.1 -> Stereo downmix
const AUDIO_MIX_FILTERS = {
// ITU-R BS.775 Standard: Mathematically balanced, transparent
itu: 'pan=stereo|FL=FL+0.707*FC+0.707*BL+0.5*LFE|FR=FR+0.707*FC+0.707*BR+0.5*LFE',
// Night Mode: Heavy dialogue boost, reduced bass/surrounds for quiet viewing
night: 'pan=stereo|FL=0.5*FL+1.2*FC+0.3*BL+0.1*LFE|FR=0.5*FR+1.2*FC+0.3*BR+0.1*LFE',
// Cinematic: Wide soundstage, immersive (original "dialogue boost" mix)
cinematic: 'pan=stereo|FL=FC+0.80*FL+0.60*BL+0.5*LFE|FR=FC+0.80*FR+0.60*BR+0.5*LFE'
};
if (audioMixPreset === 'passthrough') {
// Passthrough: Always copy audio, no processing
console.log(`[TranscodeSession ${this.id}] Audio: Passthrough (copy)`);
args.push('-c:a', 'copy');
} else if (audioMixPreset === 'auto' && isStereoAac) {
// Auto + Stereo AAC source: Smart copy
console.log(`[TranscodeSession ${this.id}] Audio: Auto (Smart Copy) - Source is Stereo AAC`);
args.push('-c:a', 'copy');
} else {
// Transcode to AAC with selected mix preset (default to ITU for 'auto')
const mixPreset = (audioMixPreset === 'auto') ? 'itu' : audioMixPreset;
const panFilter = AUDIO_MIX_FILTERS[mixPreset] || AUDIO_MIX_FILTERS.itu;
console.log(`[TranscodeSession ${this.id}] Audio: ${mixPreset.toUpperCase()} mix (${audioCodec} ${audioChannels}ch -> Stereo AAC)`);
args.push(
'-c:a', 'aac',
'-ar', '48000',
'-b:a', '192k',
'-af', `${panFilter},aresample=async=1`
);
}
// HLS output options
args.push(