[FFmpeg-user] FFmpeg accuracy issues.

[Michael Bradshaw]

On Thu, Aug 16, 2012 at 8:42 AM, Roger Pack <rogerdpack2 at gmail.com> wrote:
>> Actually, wait a bit on that. MP3s suffer from added encoder/decoder
>> delays. LAME FAQ talks about it here:
>> http://lame.sourceforge.net/tech-FAQ.txt
>>
>> So re-encoding an MP3 just keeps accumulating delays, so it's slowly
>> getting longer. But three seconds (the first transcode) is a very
>> large jump... the other ones are more consistent with LAME's delays.
>
> Oops the 3 seconds one was a typo :)
>
> Ok with .wav it seems to keep consistent timings.
>
> However, using lame "straight" (lame.exe) results in the following timings:
>
> lame1:  00:00:53.28
> lame2:  00:00:53.28
> lame3:  00:00:53.28
>
> (and the files stay the same size, so maybe lame has fixed their
> problem since then?)

No, it's still an issue in LAME.

> This is using the same version of lame that ffmpeg is compiled with.
> Any feedback?

LAME is kinda special. I just did some tests (and I plan on contacting
the LAME list about this), but here's what I've found: LAME adds the
delay, just like all MP3 encoders do. BUT, it also has the special
ability to skip this delay when it reads an MP3 file (or at least it
skips the delay it added; dunno if it works with MP3s encoded with
another encoder). So if you do lame in.wav out.mp3 and open it with
Audacity or something, you'll see the delay at the start (there's also
about ~277 samples missing at the end... I'm trying to get feedback on
this from the LAME devs). However, if you then do lame --decode
out.mp3 out.wav and open out.wav in Audacity, the added delay to the
beginning is gone and the missing ~277 samples are back at the end
again.

So it looks like LAME is the only tool that properly reads LAME
output. I suspect that if you select the libmp3lame codec for the
decoder (and don't use FFmpeg's MP3 decoder) it'll go away. But if you
use another MP3 decoder, it doesn't do whatever magic LAME does (so
the delay is still there and the missing samples are still gone), the
delay just gets accumulated.

Like I said, I'm trying to get feedback from the LAME devs on this (I
just came across it this week). If anyone else has any input on this,
I'd be happy to hear it.

--Michael