FLAC in MP4 ignores WAVEFORMATEXTENSIBLE_CHANNEL_MASK Vorbis Comment

[John Regan]

When an MP4 file contains FLAC audio with a channel layout that requires signalling via Vorbis Comment in the FLAC Specific Box - ffmpeg ignores the comment and uses the incorrect channel layout.

Using the attached sample file:

% ffprobe 2_point_1.mp4 
ffprobe version N-111429-g9a2335444b Copyright (c) 2007-2023 the FFmpeg developers
  built with gcc 13.1.1 (GCC) 20230429
  configuration: 
  libavutil      58. 14.100 / 58. 14.100
  libavcodec     60. 22.100 / 60. 22.100
  libavformat    60. 10.100 / 60. 10.100
  libavdevice    60.  2.101 / 60.  2.101
  libavfilter     9.  8.102 /  9.  8.102
  libswscale      7.  3.100 /  7.  3.100
  libswresample   4. 11.100 /  4. 11.100
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x55f7561ffd80] non-STREAMINFO FLACMetadataBlock(s) ignored
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '2_point_1.mp4':
  Metadata:
    major_brand     : iso6
    minor_version   : 0
    compatible_brands: iso6aid3
  Duration: 00:00:06.00, start: 0.000000, bitrate: 185 kb/s
  Stream #0:0[0x1](und): Audio: flac (fLaC / 0x43614C66), 48000 Hz, 3.0, s16, 182 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
      vendor_id       : [0][0][0][0]

This should list a channel layout of 2.1 and instead lists 3.0.

[John Regan]

An MP4 file with FLAC audio. The FLAC specific box contains two FLAC metadata blocks - a STREAMINFO block and a Vorbis Comment block. The Vorbis Comment has a tag listing the channel layout.

[Elon Musk]

This is not really bug, metadata like comment are ignored.
flac in ogg have own channel layout set at codec level.

[John Regan]

Replying to Elon Musk:

flac in ogg have own channel layout set at codec level.

Just want to clarify - this ticket is for FLAC in MP4, not FLAC in Ogg.

I'll ask the FLAC project what the intended way to signal custom channel layouts is for MP4.

[John Regan]

After discussing with the FLAC project I do think this is a bug.

The primary definition of FLAC is considered the IETF spec - ​https://datatracker.ietf.org/doc/draft-ietf-cellar-flac/ - and the spec is considered to apply to all encapsulations of FLAC, including MP4.

Since the FLAC spec states that the WAVEFORMATEXTENSIBLE_CHANNEL_MASK comment within a Vorbis Comment Block is used to signal a channel layout, and the ISO encapsulation spec allows for including Vorbis Comment Blocks in the FLAC Specific Box - the comment block should be parsed and the tag honored, similar to native FLAC files.