[MPlayer-users] A/V sync problem when appending AVIs

[danny]

Alle 09:18, venerdì 6 febbraio 2004, Daniele Forghieri ha scritto:
> Matthias Czapla wrote:
> >Hello!
> >
> >I have created three AVI files with (for each one)
...
> mp3 uses packet of sound of, say, 250 millisec.
>
> In any file you have a certain number of complete packet.
>
> If the length of the video file is not exactly a multiple of the packet
> size (in millisec) you' ve got an A/V desync in the second (and
> successive) files.
>
> At the end of the mencoder encode you got something like
>     video: 120.040 sec,  2401 frames
>     audio: 120.250 sec
>
> and the sound 'drift' is 210 millisec.
>
> To avoid the problem I encode the audio in PCM and, after you cat the
> files, convert in mp3
>
>     Daniele Forghieri
>
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I take a deeper look at this.

The mp3 packed is composed of 1152 samples (the tmp[2304] in mencoder source 
should give me an hint ...).

At 44100 hz is 1152/44100 -> 26.122 millisec, at 48000 hz is 24 millisec, so 
it is irrilevant.

I make a test with an avi file of 5 minutes splitted in 5 parts of about 1 
minute each with pcm audio.

if I use mp3lame i obtain this result (the video are of differrent len because 
i split on a keyframe)

video	frames	audio
60.040	1501	60.552
60.320	1508	60.840
59.640	1491	60.144
60.000	1500	60.504
60.000	1500	60.504
--------	-------	--------
300.000	7500	302.544

If i cat this file i obtain an A/V desync, very notable.

I try also using the mp3 encoder of lavc (-oac lavc -lavcopts 
acodec=mp3:abitrate=128) and the result are:

video	frames	audio
60.040	1501	59.976	( - 0.064)
60.320	1508	60.264	( - 0.056)
59.640	1491	59.592	( - 0.048)
60.000	1500	59.952	( - 0.048)
60.000	1500	59.952	( - 0.048)
--------	-------	--------
300.000	7500	299.736

Here some frames (2) are missed. The desync is less notable.

If i convert every avi in pcm i obtain the same video and audio size so no a/v 
desync but ....

if i use pcm sound and i use -endpos to cut the end of the files i obtain that 
the audio is at least 0.5 second longer than the video so, if i join the avi, 
i obtain on the last piece of video a 2.5 second of delay (and not 2.0 as i 
expected!)

To summarize: mencoder puts on the avi file more audio than needed. There is 
no problem if you watch the avi (the audio in the end of the file and you 
don't have any desync) but is a problem if you simply cat the avi to merge 
them.

A solution is to modifiy mencoder to output exactly the audio needed to match 
the video (with an option?) or build a program that read the avi and, at the 
end,. ignores the extra audio data.

	Daniele Forghieri