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

[Scott Theisen]

On 12/9/24 02:30, Anton Khirnov wrote:
> 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?
>

Using AV_CHAN_UNKNOWN would be fine with me.

However, would using AV_CHANNEL_ORDER_UNSPEC instead of 
AV_CHANNEL_ORDER_CUSTOM be better?

Regards,

Scott Theisen