[FFmpeg-devel] [PATCH 1/2] lavf/mp3dec: pass Xing gapless metadata to AVCodecParameters

[Jon Toohill]

Michael et al., is this good to merge as-is? I just tested and a round trip
with ffmpeg from wav -> mp3 -> wav retains the correct number of samples.


Jon Toohill |  Google Play Music |  jtoohill at google.com |  (650) 215-0770

On Wed, Jun 1, 2016 at 5:58 PM, Jon Toohill <jtoohill at google.com> wrote:

> Based on my understanding of [1], these values in the Info tag specify
> only the encoder delay/padding, which matches the documentation for these
> fields. It looks like other formats are using the fields that way as well.
>
> I think the extra 528 + 1 samples are the decoder delay [2]. It looks like
> libmp3lame adds the 528 + 1 only to have mp3dec subtract it, so I'm not
> sure why that's done. IIUC start_skip_samples is the mechanism that
> actually accounts for the extra delay when decoding.
>
> [1]: http://gabriel.mp3-tech.org/mp3infotag.html#delays
> [2]: http://lame.sourceforge.net/tech-FAQ.txt
>
>
>
> Jon Toohill |  Google Play Music |  jtoohill at google.com |  (650) 215-0770
>
> On Thu, May 26, 2016 at 7:51 PM, Michael Niedermayer <
> michael at niedermayer.cc> wrote:
>
>> On Wed, May 25, 2016 at 09:56:59AM -0700, Jon Toohill wrote:
>> > ---
>> >  libavformat/mp3dec.c | 2 ++
>> >  1 file changed, 2 insertions(+)
>> >
>> > diff --git a/libavformat/mp3dec.c b/libavformat/mp3dec.c
>> > index 3725d67..192f5ef 100644
>> > --- a/libavformat/mp3dec.c
>> > +++ b/libavformat/mp3dec.c
>> > @@ -234,6 +234,8 @@ static void mp3_parse_info_tag(AVFormatContext *s,
>> AVStream *st,
>> >
>> >          mp3->start_pad = v>>12;
>> >          mp3->  end_pad = v&4095;
>> > +        st->codecpar->initial_padding = mp3->start_pad;
>> > +        st->codecpar->trailing_padding = mp3->end_pad;
>> >          st->start_skip_samples = mp3->start_pad + 528 + 1;
>> >          if (mp3->frames) {
>> >              st->first_discard_sample = -mp3->end_pad + 528 + 1 +
>> mp3->frames * (int64_t)spf;
>>
>> is the 528 + 1 difference intended to
>> start_skip_samples/first_discard_sample
>> ?
>> mp3enc stores par->initial_padding - 528 - 1
>>
>> [...]
>>
>> --
>> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>>
>> If you drop bombs on a foreign country and kill a hundred thousand
>> innocent people, expect your government to call the consequence
>> "unprovoked inhuman terrorist attacks" and use it to justify dropping
>> more bombs and killing more people. The technology changed, the idea is
>> old.
>>
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>>
>