Opus in MPEG-TS: findOpusChannelCount does not handle custom channel-mapping-family (255) correctly

[ditogam]

Describe the feature

Context

When muxing Opus into MPEG-TS with GStreamer and using more than stereo channels, the PMT contains an extension descriptor (tag 0x80) carrying channel/mapping information.

Example pipeline (custom channel count):

audiotestsrc is-live=true wave=ticks \
  ! audio/x-raw,channels=CHANNEL_COUNT,rate=48000,format=S16LE,layout=interleaved \
  ! audiomixmatrix mode=first-channels out-channels=CHANNEL_COUNT \
  ! audio/x-raw,channels=CHANNEL_COUNT,rate=48000,format=S16LE,layout=interleaved \
  ! capssetter caps="audio/x-raw,channel-mask=(bitmask)0" \
  ! opusenc bitrate=510000 bitrate-type=cbr inband-fec=true frame-size=10 \
  ! capsfilter caps="audio/x-opus,channel-mapping-family=(int)255"

Stereo (standard mapping):

audiomixmatrix mode=first-channels out-channels=2 \
  ! audio/x-raw,channels=2,rate=48000,format=S16LE,layout=interleaved \
  ! capssetter caps="audio/x-raw,channel-mask=(bitmask)0" \
  ! opusenc bitrate=510000 bitrate-type=cbr inband-fec=true frame-size=10

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Observed Behavior

• For stereo or family 0, the descriptor is short, and astits.Descriptor.Extension.Unknown contains just the channel count.
  Example:

  [2]
  

• For custom mapping (family=255), the payload is longer:
  • 6 channels →

    [129, 6, 255, 160, 20, 229]
    

  • 8 channels →

    [129, 8, 255, 112, 1, 35, 69, 103]
    

Currently, in mediacommon/pkg/formats/mpegts/track.go, findOpusChannelCount only inspects the first element, so it fails to extract the correct channel count in the custom mapping case.

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Expected Behavior

findOpusChannelCount should:

• Skip a leading sub-tag byte (≥0x80, e.g. 129).
• Correctly read the channel count (the following byte).
• Optionally expose the channel-mapping-family (e.g. 255).
• Safely ignore or parse trailing bytes if present.

So in the above examples, the function should return:

• 6 channels, family=255 for [129, 6, 255, 160, 20, 229]
• 8 channels, family=255 for [129, 8, 255, 112, 1, 35, 69, 103]

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Request

Please update findOpusChannelCount to correctly handle custom mapping families (at least to extract the proper channel count when channel-mapping-family=255).

If you agree, I’m happy to contribute a PR with a parser that:

• Detects and skips a sub-tag byte,
• Reads channel count and family,
• Falls back gracefully when only a single byte is present (stereo/mono case).