[FFmpeg-devel] [PATCH] avformat/movenc: allow writing out channel count in MP4 and 3GP

[Jan Ekström]

On Thu, Feb 9, 2023 at 12:08 PM Martin Storsjö <martin at martin.st> wrote:
>
> This looks reasonable to me. I didn't follow the references in the commit
> message, but it sounds plausible and probably correct to me.
>

For the record, due to dumb ISO/IEC rule changes regarding how
specifications need to be authorized to be free after 2015, it's
relatively hard to verify the 14496-12 part.

The last version of 14496-12 that was freely available (2015) defined
channelcount in AudioSampleEntry as follows:

template unsigned int(16) channelcount = 2;

Meanwhile the latest 2022 edition defines the related structure(s) as follows:

class AudioSampleEntry(codingname) extends SampleEntry (codingname) {
   const unsigned int(32)[2] reserved = 0;
   unsigned int(16) channelcount;
   template unsigned int(16) samplesize = 16;
   unsigned int(16) pre_defined = 0;
   const unsigned int(16) reserved = 0 ;
   template unsigned int(32) samplerate = { default samplerate of media} << 16;
   // optional boxes follow
   Box ();      // further boxes as needed
   ChannelLayout();
   DownMixInstructions() [];
   DRCCoefficientsBasic() [];
   DRCInstructionsBasic() [];
   DRCCoefficientsUniDRC() [];
   DRCInstructionsUniDRC() [];
   // we permit only one DRC Extension box:
   UniDrcConfigExtension();
   // optional boxes follow
   SamplingRateBox();
   ChannelLayout();
}

aligned(8) class SamplingRateBox extends FullBox('srat') {
   unsigned int(32) sampling_rate;
}

class AudioSampleEntryV1(codingname) extends SampleEntry (codingname) {
   unsigned int(16) entry_version;   // shall be 1,
                     // and shall be in an stsd with version ==1
   const unsigned int(16)[3] reserved = 0;
   template unsigned int(16) channelcount;   // shall be correct
   template unsigned int(16) samplesize = 16;
   unsigned int(16) pre_defined = 0;
   const unsigned int(16) reserved = 0 ;
   template unsigned int(32) samplerate = 1<<16;
   // optional boxes follow
   SamplingRateBox();
   Box ();      // further boxes as needed
   ChannelLayout();
   DownMixInstructions() [];
   DRCCoefficientsBasic() [];
   DRCInstructionsBasic() [];
   DRCCoefficientsUniDRC() [];
   DRCInstructionsUniDRC() [];
   // we permit only one DRC Extension box:
   UniDrcConfigExtension();
   // optional boxes follow
   ChannelLayout();
}