[FFmpeg-devel] [PATCH] libavcodec/mpegaudio_parser.c: differentiate MPEG audio dual mono

[Anton Khirnov]

Quoting James Almer (2024-11-30 14:41:20)
> On 11/14/2024 1:37 AM, Scott Theisen wrote:
> > When attempting to upstream this MythTV change in September 2022, it was recommended to
> > use AV_CHANNEL_ORDER_CUSTOM with two AV_CHAN_FRONT_CENTER channels. See
> > https://patchwork.ffmpeg.org/project/ffmpeg/patch/20220921192611.3241-1-scott.the.elm@gmail.com/
> > ---
> >   libavcodec/audiotoolboxdec.c    |  4 ++--
> >   libavcodec/mpegaudio_parser.c   | 12 +++++++++---
> >   libavcodec/mpegaudiodecheader.c |  4 +++-
> >   libavcodec/mpegaudiodecheader.h |  2 +-
> >   tests/ref/fate/pva-demux        |  2 +-
> >   5 files changed, 16 insertions(+), 8 deletions(-)
> > 
> > diff --git a/libavcodec/audiotoolboxdec.c b/libavcodec/audiotoolboxdec.c
> > index 0f7ce8e4eb..d279d7bbc4 100644
> > --- a/libavcodec/audiotoolboxdec.c
> > +++ b/libavcodec/audiotoolboxdec.c
> > @@ -346,10 +346,10 @@ static av_cold int ffat_create_decoder(AVCodecContext *avctx,
> >                   avctx->codec_id == AV_CODEC_ID_MP2 ||
> >                   avctx->codec_id == AV_CODEC_ID_MP3)) {
> >           enum AVCodecID codec_id;
> > -        int bit_rate;
> > +        int bit_rate, dual_mono;
> >           if (ff_mpa_decode_header(AV_RB32(pkt->data), &avctx->sample_rate,
> >                                    &in_format.mChannelsPerFrame, &avctx->frame_size,
> > -                                 &bit_rate, &codec_id) < 0)
> > +                                 &bit_rate, &codec_id, &dual_mono) < 0)
> >               return AVERROR_INVALIDDATA;
> >           avctx->bit_rate = bit_rate;
> >           in_format.mSampleRate = avctx->sample_rate;
> > diff --git a/libavcodec/mpegaudio_parser.c b/libavcodec/mpegaudio_parser.c
> > index d54366f10a..d1a4ee6434 100644
> > --- a/libavcodec/mpegaudio_parser.c
> > +++ b/libavcodec/mpegaudio_parser.c
> > @@ -65,12 +65,12 @@ static int mpegaudio_parse(AVCodecParserContext *s1,
> >               }
> >           }else{
> >               while(i<buf_size){
> > -                int ret, sr, channels, bit_rate, frame_size;
> > +                int ret, sr, channels, bit_rate, frame_size, dual_mono;
> >                   enum AVCodecID codec_id = avctx->codec_id;
> >   
> >                   state= (state<<8) + buf[i++];
> >   
> > -                ret = ff_mpa_decode_header(state, &sr, &channels, &frame_size, &bit_rate, &codec_id);
> > +                ret = ff_mpa_decode_header(state, &sr, &channels, &frame_size, &bit_rate, &codec_id, &dual_mono);
> >                   if (ret < 4) {
> >                       if (i > 4)
> >                           s->header_count = -2;
> > @@ -85,7 +85,13 @@ static int mpegaudio_parse(AVCodecParserContext *s1,
> >                       if (s->header_count > header_threshold) {
> >                           avctx->sample_rate= sr;
> >                           av_channel_layout_uninit(&avctx->ch_layout);
> > -                        av_channel_layout_default(&avctx->ch_layout, channels);
> > +                        if (dual_mono) {
> > +                            av_channel_layout_custom_init(&avctx->ch_layout, 2);
> > +                            avctx->ch_layout.u.map[0].id = AV_CHAN_FRONT_CENTER;
> > +                            avctx->ch_layout.u.map[1].id = AV_CHAN_FRONT_CENTER;
> 
> Kind of sucks we have used FC to represent mono for so long that we 
> can't cleanly introduce a new channel that's strictly mono (with no 
> speaker location implied) and give it meaningful use.

AV_CHAN_UNKNOWN?

-- 
Anton Khirnov