[FFmpeg-devel] [FFmpeg-cvslog] lavc/aac_ac3_parser: improve the raw AAC file bit rate calculation

[mypopy at gmail.com]

On Sun, Jun 28, 2020 at 5:30 AM Alexander Strasser <eclipse7 at gmx.net> wrote:
>
> On 2020-06-26 01:56 +0000, Jun Zhao wrote:
> > ffmpeg | branch: master | Jun Zhao <barryjzhao at tencent.com> | Sun May 17 12:10:05 2020 +0800| [60d79b1df9d4c6030010ccb0c134ede9e33158c2] | committer: Jun Zhao
> >
> > lavc/aac_ac3_parser: improve the raw AAC file bit rate calculation
> >
> > Now we just use one ADTS raw frame to calculate the bit rate, it's
> > lead to a larger error when get the duration from bit rate, the
> > improvement cumulate Nth ADTS frames to get the average bit rate.
> >
> > e,g used the command get the duration like:
> > ffprobe -show_entries format=duration -i fate-suite/aac/foo.aac
> >
> > before this improvement dump the duration=2.173935
> > after this improvement  dump the duration=1.979267
> >
> > in fact, the real duration can be get by command like:
> > ffmpeg -i fate-suite/aac/foo.aac -f null /dev/null with time=00:00:01.97
> >
> > Also update the fate-adtstoasc_ticket3715.
> >
> > Signed-off-by: Jun Zhao <barryjzhao at tencent.com>
> >
> > > http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=60d79b1df9d4c6030010ccb0c134ede9e33158c2
> > ---
> >
> >  libavcodec/aac_ac3_parser.c         | 9 +++++++--
> >  libavcodec/aac_ac3_parser.h         | 2 ++
> >  tests/ref/fate/adtstoasc_ticket3715 | 2 +-
> >  3 files changed, 10 insertions(+), 3 deletions(-)
> >
> > diff --git a/libavcodec/aac_ac3_parser.c b/libavcodec/aac_ac3_parser.c
> > index 54e459844f..0746798dab 100644
> > --- a/libavcodec/aac_ac3_parser.c
> > +++ b/libavcodec/aac_ac3_parser.c
> > @@ -97,8 +97,13 @@ get_next:
> >              avctx->audio_service_type = s->service_type;
> >          }
> >
> > -        if (avctx->codec_id != AV_CODEC_ID_EAC3)
> > -            avctx->bit_rate = s->bit_rate;
> > +        /* Calculate the average bit rate */
> > +        s->frame_number++;
> > +        if (avctx->codec_id != AV_CODEC_ID_EAC3) {
> > +            avctx->bit_rate =
> > +                (s->last_bit_rate * (s->frame_number -1) + s->bit_rate)/s->frame_number;
> > +            s->last_bit_rate = avctx->bit_rate;
> > +        }
> >      }
>
> Wouldn't it be better to sum up the individual bit_rate values in
> a private context variable and divide by number of frames?
>
I can't found a way in private context, so I change the AAC/AC3 parser

> This way or the other, it might be useful to think about maximum
> values of the total bits? I suspect the current int calculation
> might overflow.
>
Will fix the potential overflow for int calculation
>
>   Alexander
>
> >      return i;
> > diff --git a/libavcodec/aac_ac3_parser.h b/libavcodec/aac_ac3_parser.h
> > index c2506a5bfd..b04041f69d 100644
> > --- a/libavcodec/aac_ac3_parser.h
> > +++ b/libavcodec/aac_ac3_parser.h
> > @@ -55,6 +55,8 @@ typedef struct AACAC3ParseContext {
> >      uint64_t state;
> >
> >      int need_next_header;
> > +    int frame_number;
> > +    int last_bit_rate;
> >      enum AVCodecID codec_id;
> >  } AACAC3ParseContext;