problems joining 2avi with ac3

[Lupo Silenzioso]

I use VDMOD 1.5.10 to join 2 avi with ac3 audio (often 5.1 ch)
Every time (says the 90%) there is a problem with the audio exactly when the 2 avi joins together.
In example if the 1st avi is 1h 05' , then when i listen to the avi i have just created (appending the 2nd part to the 1st) i hear a terrible noise when i reach 1h 05' . After that the sound gets lower.
I have solved this problem by demuxing the ac3 and fixing it with besliced, and then muxing again over the other one.
But why does this problem happen?

Thanks

[Asmodian]

I think it is an incomplete ac3 'frame' at the end of the first file from an improperly split avi/ac3 file (maybe done with Nandub?).

There may be other ways this could happen.

[Matthew]

I've noticed this recently with 2 AVIs, one muxed in nandub, the other in virtualdubmod.

The fact the avi has been split part way through an audio frame is not a problem in itself...when the files are joined the audio frame spanning the split point should in theory be recreated no probs.

However looking at these 2 cases in a hex editor what has happened is that data has been duplicated at the end of audio1 and the start of audio2 (1 byte in one case, 24 in the other). Deleting this duplicate data in a hex editor results in a correct stream.

If you don't delete the duplicate data then a frame is skipped and the result is 32 ms desync from the split point onwards. This can also be corrected by joining fixedaudio1+1framedummy+fixedaudio2 using besplit (or a file joiner).