Avidemux MP4v2 Muxer outputting variable frame rate (audio sync issues)

[ClintonJMolina112]

Source is from a Digital8 tape that I captured in WinDV to a DV-AVI Type 1 file, then ran through VirtualDub with direct stream copy saved as AVI, this is to convert it to DV-AVI Type 2 so that I can open it in Avidemux (DV-AVI Type 1 won't open in Avidemux). Here I'm using a few filters to de-interlace, resize to 640x480 etc. and output to MP4 (h264 and AAC resampled to 48000khz), but the audio in the output file is gradually losing sync. I have had success with these settings before but this video gets really bad towards the end.

In my quest to figure out the cause of this, I just noticed that when using the MP4v2 Muxer in Avidemux, MediaInfo shows this:

Frame rate mode : Variable
Frame rate : 59.940 (59940/1000) FPS
Minimum frame rate : 19.982 FPS
Maximum frame rate : 59.960 FPS

However, the AVI files are constant frame rate, and when I set Avidemux Output Format to MP4 Muxer instead of MP4v2 Muxer, the output file is constant frame rate. I suspect this is why the audio is coming out of sync. Can MP4v2 Muxer not output constant frame rate?

[AQUAR]

Not an expert on this as I don't use this program.
Generically, these issues may be caused by anything in the processing chain, or even hardware limitations.

Maybe try different directshow filters to see if you can improve the situation.
I find shark's codec pack to be very useful for this kind of thing (or just change the merit value of these filters).

Constant rate de-syncing of audio is often caused by capturing audio and video with corresponding hardware that run with individual clocks.

Mediainfo doesn't always interpret/calculate these information stats correctly either and maybe step through some frames (with ADM) at different spots, to see if the frame duration actually changes (avoid the start!). 

Wish you success.