fix: re-encode MP3 and multichannel Opus audio when converting - #147
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The copyable audio table lists codecs AVFoundation is known to decode from an MP4, but two entries do not hold up against the real thing. ffmpeg can only mux MP3 under the `mp4a` tag, and AVFoundation drops that track on open: the converted file plays video and is silent. Opus decodes at stereo and below, but above two channels it carries a channel mapping family AVFoundation will not load at all, so the whole MP4 fails to open rather than merely losing its audio. Re-encode both to AAC instead. Stereo Opus still copies untouched, so the common case keeps its original audio. The README also still named FLAC and Opus as codecs that get re-encoded, which stopped being true when they were made copyable.
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The copyable audio table lists codecs AVFoundation is known to decode from an MP4, but two entries do not hold up against the real thing. Verified on macOS 15.7.9 / ffmpeg 9.0.1 by muxing each codec with the app's own arguments (
-c copy -tag:v avc1 -f mp4) and decoding the result throughAVAssetReader.MP3 — ffmpeg can only mux it under the
mp4atag, and AVFoundation drops that track on open, soloadTracks(withMediaType: .audio)comes back empty. The converted file plays video and is silent. There is no tag that rescues it: ffmpeg's MP4 muxer rejects-tag:a ".mp3"outright.Opus above 2 channels — 5.1 and 7.1 need channel mapping family 1 in the
dOpsbox, which AVFoundation won't load. This one is worse than silence:assetProperty_Tracksfails with-11800 / 'what'and the whole file refuses to open. Mono and stereo decode fine.Both are re-encoded to AAC now. Opus is capped by channel count rather than dropped from the table, because stereo Opus is the common case in Matroska and a blanket re-encode would cost it both a lossy generation and the fast copy path. The cap lives in a new
isCopyable(audio:), mirroring the existingisCopyable(_:canDecodeAV1:)that already gates video on more than the codec name.After the fix, both cases produce an MP4 with a decodable AAC track where a straight copy previously produced no track and an unopenable file.
Also checked and left alone: DTS and TrueHD genuinely fail, PCM decodes but stays re-encoded for size, and FLAC decodes at 2/5.1/7.1 channels and 24-bit/96k.
Docs
README line 53 still listed FLAC and Opus as codecs that get re-encoded — untrue since #122 made them copyable. A
ConversionUITestscomment carried the same stale fact. Both corrected, plus a release note.