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

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

[Jan Ekström]

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);
diff --git a/tests/ref/fate/mov-mp4-chapters b/tests/ref/fate/mov-mp4-chapters
index 34f84de1b1..75cd3b3438 100644
--- a/tests/ref/fate/mov-mp4-chapters
+++ b/tests/ref/fate/mov-mp4-chapters
@@ -1,4 +1,4 @@
-1fd844c2f5bf77c3344e88e30ad994e1 *tests/data/fate/mov-mp4-chapters.mp4
+7b6aaa99c86fa1f5abfc9f242abcfffa *tests/data/fate/mov-mp4-chapters.mp4
 111248 tests/data/fate/mov-mp4-chapters.mp4
 #extradata 0:     3469, 0xc6769ddc
 #tb 0: 1/44100
-- 
2.39.1

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Re: [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
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Re: [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@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();
}
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Re: [FFmpeg-devel] [PATCH] avformat/movenc: allow writing out channel count in MP4 and 3GP

[Jan Ekström]

On Thu, Feb 9, 2023 at 9:24 PM Jan Ekström <jeebjp@gmail.com> wrote:
>
> On Thu, Feb 9, 2023 at 12:08 PM Martin Storsjö <martin@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();
> }

Applied to master as 02ddfeadbee52c1ad8c023a93594a8cb957e11db .

Jan
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