[FFmpeg-devel] [PATCH] avformat/mp3dec: prefer "fast_seek" to TOC seek for CBR files.

[Chris Cunningham]

Hey, sorry for the slow reply.

Use this sample file. Its a CBR file with a corrupt TOC.
http://happyworm.com/jPlayerLab/halite/jumptest/testcount.mp3

Before applying my patch:

./ffplay -ss 00:33:20 testcount.mp3  -usetoc 0
plays out "1395", which is the correct audio for the given seek time

./ffplay -ss 00:33:20 testcount.mp3  -usetoc 1
plays out "...378, 1379", way off because it used the bad TOC


After applying my patch these commands both play out the correct time of
"1395" because usetoc=1 is ignored/overridden by virtue of this file being
CBR.

However, I've *just* thought of another (better) approach. My goal is
really to improve the behavior of AVFMT_FLAG_FAST_SEEK
<https://github.com/FFmpeg/FFmpeg/blob/b9cd3e1add3a3db86aaeba8680ad5a4ee57d519b/libavformat/mp3dec.c#L348>,
which defaults to using the TOC whenever its available. Standby for a
second patch that will only change TOC behavior when the
AVFMT_FLAG_FAST_SEEK is being used.

Chris


On Mon, Nov 9, 2015 at 3:34 PM, Michael Niedermayer <michael at niedermayer.cc>
wrote:

> On Wed, Nov 04, 2015 at 05:21:57PM -0800, Chris Cunningham wrote:
> > "Fast seek" uses linear interpolation to find the position of the
> > requested seek time. For CBR this is more direct than using the
> > mp3 TOC and bypassing the TOC avoids problems when the TOC is
> > corrupted (e.g. https://crbug.com/545914).
> >
> > For VBR, fast seek is not precise, so continue to prefer the TOC
> > when available.
> > ---
> >  libavformat/mp3dec.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
>
> how can this be reproduced with FFmpeg ?
> do you have a sample which you can share ?
>
> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> I have often repented speaking, but never of holding my tongue.
> -- Xenocrates
>