[FFmpeg-devel] [PATCH] lavc: export flag for MPEG audio dual channel

[Scott Theisen <scott.the.elm@gmail.com>]

From: ulmus-scott <scott.the.elm@gmail.com>

The flag identifies two independant mono channels recorded as stereo.

This change has been kicking around in the MythTV modifications since 2006.
See https://github.com/MythTV/mythtv/commit/435540c9e8ac245ceca968791c67431f37c8d617
I have changed the names and comment.  For the current MythTV modification see
https://github.com/ulmus-scott/FFmpeg/commit/645fa6f9a61d23bac0665851d211bbeb3686deb0
---
 libavcodec/audiotoolboxdec.c    |  2 +-
 libavcodec/avcodec.h            | 11 +++++++++++
 libavcodec/mpegaudio_parser.c   |  2 +-
 libavcodec/mpegaudiodecheader.c |  4 +++-
 libavcodec/mpegaudiodecheader.h |  2 +-
 5 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/libavcodec/audiotoolboxdec.c b/libavcodec/audiotoolboxdec.c
index 82babe3d31..9d11844142 100644
--- a/libavcodec/audiotoolboxdec.c
+++ b/libavcodec/audiotoolboxdec.c
@@ -346,7 +346,7 @@ static av_cold int ffat_create_decoder(AVCodecContext *avctx,
         int bit_rate;
         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, &avctx->mpeg_audio_mode_dual_channel) < 0)
             return AVERROR_INVALIDDATA;
         avctx->bit_rate = bit_rate;
         in_format.mSampleRate = avctx->sample_rate;
diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index 7db5d1b1c5..bcf3a845a8 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -2076,6 +2076,17 @@ typedef struct AVCodecContext {
      *             The decoder can then override during decoding as needed.
      */
     AVChannelLayout ch_layout;
+
+    /**
+     * Audio only.  This flag is set when MPEG audio mode dual channel has been detected.
+     * This signals that the audio is two independent mono channels.
+     *
+     * 0 normally, 1 if dual channel flag is set.
+     *
+     * - encoding: currently unused (functionally equivalent to stereo, patch welcome)
+     * - decoding: set by lavc
+     */
+    int mpeg_audio_mode_dual_channel;
 } AVCodecContext;
 
 /**
diff --git a/libavcodec/mpegaudio_parser.c b/libavcodec/mpegaudio_parser.c
index d54366f10a..d957cf467f 100644
--- a/libavcodec/mpegaudio_parser.c
+++ b/libavcodec/mpegaudio_parser.c
@@ -70,7 +70,7 @@ static int mpegaudio_parse(AVCodecParserContext *s1,
 
                 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, &avctx->mpeg_audio_mode_dual_channel);
                 if (ret < 4) {
                     if (i > 4)
                         s->header_count = -2;
diff --git a/libavcodec/mpegaudiodecheader.c b/libavcodec/mpegaudiodecheader.c
index ef63befbf4..6c9a641906 100644
--- a/libavcodec/mpegaudiodecheader.c
+++ b/libavcodec/mpegaudiodecheader.c
@@ -117,7 +117,7 @@ int avpriv_mpegaudio_decode_header(MPADecodeHeader *s, uint32_t header)
     return 0;
 }
 
-int ff_mpa_decode_header(uint32_t head, int *sample_rate, int *channels, int *frame_size, int *bit_rate, enum AVCodecID *codec_id)
+int ff_mpa_decode_header(uint32_t head, int *sample_rate, int *channels, int *frame_size, int *bit_rate, enum AVCodecID *codec_id, int *dual_mono)
 {
     MPADecodeHeader s1, *s = &s1;
 
@@ -148,5 +148,7 @@ int ff_mpa_decode_header(uint32_t head, int *sample_rate, int *channels, int *fr
     *sample_rate = s->sample_rate;
     *channels = s->nb_channels;
     *bit_rate = s->bit_rate;
+    *dual_mono = (s->mode == MPA_DUAL) ? 1 : 0;
+
     return s->frame_size;
 }
diff --git a/libavcodec/mpegaudiodecheader.h b/libavcodec/mpegaudiodecheader.h
index ed5d1f3b33..e599d287f7 100644
--- a/libavcodec/mpegaudiodecheader.h
+++ b/libavcodec/mpegaudiodecheader.h
@@ -56,7 +56,7 @@ int avpriv_mpegaudio_decode_header(MPADecodeHeader *s, uint32_t header);
 /* useful helper to get MPEG audio stream info. Return -1 if error in
    header, otherwise the coded frame size in bytes */
 int ff_mpa_decode_header(uint32_t head, int *sample_rate,
-                         int *channels, int *frame_size, int *bitrate, enum AVCodecID *codec_id);
+                         int *channels, int *frame_size, int *bitrate, enum AVCodecID *codec_id, int *dual_mono);
 
 /* fast header check for resync */
 static inline int ff_mpa_check_header(uint32_t header){
-- 
2.34.1

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