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

[Martin Storsjö]

On Tue, 7 Feb 2023, Jan Ekström wrote:

> ISOBMFF (14496-12) made this field ('channelcount') in the
> AudioSampleEntry structure non-template¹ somewhere before the
> release of the 2022 edition. As for ETSI TS 126 244 AKA 3GPP
> file format (V16.1.0, 2020-10), it does not seem contain any
> references limiting the channelcount entry in AudioSampleEntry
> or in its own definition of EVSSampleEntry.
>
> fate-mov-mp4-chapters test had to be adjusted as it output a
> mono vorbis stream, which would now be properly marked as such
> in the container.
>
> 1: As per 14496-12:
>   Fields shown as “template” in the box descriptions are fields
>   which are coded with a default value unless a derived
>   specification defines their use and permits writers to use
>   other values than the default.
> ---
> libavformat/movenc.c            | 8 +-------
> tests/ref/fate/mov-mp4-chapters | 2 +-
> 2 files changed, 2 insertions(+), 8 deletions(-)
>
> diff --git a/libavformat/movenc.c b/libavformat/movenc.c
> index 8d31317838..f0e218e7b7 100644
> --- a/libavformat/movenc.c
> +++ b/libavformat/movenc.c
> @@ -1241,13 +1241,7 @@ static int mov_write_audio_tag(AVFormatContext *s, AVIOContext *pb, MOVMuxContex
>                 avio_wb16(pb, 16);
>             avio_wb16(pb, track->audio_vbr ? -2 : 0); /* compression ID */
>         } else { /* reserved for mp4/3gp */
> -            if (track->par->codec_id == AV_CODEC_ID_FLAC ||
> -                track->par->codec_id == AV_CODEC_ID_ALAC ||
> -                track->par->codec_id == AV_CODEC_ID_OPUS) {
> -                avio_wb16(pb, track->par->ch_layout.nb_channels);
> -            } else {
> -                avio_wb16(pb, 2);
> -            }
> +            avio_wb16(pb, track->par->ch_layout.nb_channels);
>             if (track->par->codec_id == AV_CODEC_ID_FLAC ||
>                 track->par->codec_id == AV_CODEC_ID_ALAC) {
>                 avio_wb16(pb, track->par->bits_per_raw_sample);

This looks reasonable to me. I didn't follow the references in the commit 
message, but it sounds plausible and probably correct to me.

// Martin