On Thu, 22 Jan 2004, George Kola wrote:
> find that the generated MPEG-1 and MPEG-2 have perfect A/V sync (Thanks to
> people here for helping me with it). I find that the MPEG-4 (two pass) has
> A/V sync issue. The audio slowly drifts behind the video. In the beginning,
The alternate way to think of that is the video being slightly too
fast and moving ahead of the audio.
> both are in sync. I am using two pass because I want to get the best quality
> possible at a given bitrate of 250 kbps. We want to stream it over broadband.
Does the A/V sync issue also happen with the 1 pass method?
> Noticable lag starts occuring in the last 15 minutes of the clip. The lag is
> 0 at the beginning, becomes 4 seconds at 45 minutes and becomes close to 10
> seconds at 60 minutes. I wanted to know how to correct it.
Hmmm, so between 45 minutes and 60 minutes is a huge ~6 seconds of
A/V slip? If it was gradual that would be a continuation of the
previous slippage but 6 seconds loss in 15 minutes would seem to
indicate the player has lost its sanity.
At first I thought the problem was the .1% difference between
30 and 30000/1001 (29.97002997...) frames/second. If the encoding
is done at 30 instead of 29.97 then the video will be too fast and
gradually move ahead of the audio. .1% works out to about 3.6
seconds per hour (~2.7 seconds for 45 minutes).
The quicktime movies I have downloaded are encoded at various frame
rates but usually NOT at 29.97 - they've been at 30. I'm wondering
if windows media player is paying attention to the frame rate in the
file or rounding it up to 30...
> > What program(s) were you using to play the .mp4 file? Some
>
> I am using MPEG-4 Video and AAC Audio in MP4 container and
> using quicktime to play it. I also tried with Windows media player.
Thanks for the information - quite helpful. Have you tried MPlayer?
MPlayer understands enough of the MP4 container format to play
the files (and AAC audio is supported). Do you have a Mac that
could be used - I wonder if the QuickTime player would have the
same problem.
What frame size are you scaling down to? 320x240?
> > I've created numerous MP4 files that play well (no A/V sync issues)
> >
> It would be great, if I am also able to create such MP4 files :).
I think the longest one I have created has been around 18 to 20
minutes (a transcoded short subject movie from DVD) - nowhere near
an hour.
> It is a miniDV camcorder.
Does mencoder issue any diagnostic messages about "skipping frame"
or "duplicating frame"?
If you are seeing such messages it may be necessary to tell mencoder
to do no frame dropping or A/V sync correction issues.
mplayer -vc null -vo null -ao pcm -noframedrop -aofile wavfile input.dv
to extract the audio and
the use "-noskip -mc 0" when doing the video encoding.
> > I do not believe it will help. A/V sync issues are not, as a
> > general rule, introduced by the system that does the DV capture.
>
> Thanks, then I do not have to bother about the DV capture.
If the MPEG-1 and -2 encoding works then the problem you're having is
definitely not DV capture related.
I have a 85 minute concert I captured - I'll encode that to MP4 and
see if any A/V sync issues arise.
Cheers,
Steven Schultz
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