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

[James Almer]

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.

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