MainConcept / AVISynth sync issue

[julesh]

Hi, I'm currently trying to convert a video using a combination of AVISynth and MainConcept MPEG Encoder, but I'm getting a sync issue; audio is being delayed by about 500ms.

My AVISynth script is:

# 704x304, 23.976 fps => 704x442 (704x576) 16:9 25.000 fps
DirectShowSource ("<my video file>.avi")
BicubicResize (704,442)
AddBorders (0,66,0,68)
AssumeFPS (25, true)

MainConcept is set up to produce a 704x576 PAL DVD-compliant output stream with 48kHz MP2 audio, average bitrate is set to 2000 kbps (yes, I know this is low quality).

Encoding the video in MainConcept directly without using AVISynth doesn't produce the same issue; previewing the AVISynth script directly *also* doesn't produce the same problem.

Does anyone have any clue what's going on, suggestions how to fix it (preferably without having to go through a demux / mux step), etc.?

[guns1inger]

Is the audio actually being delayed, or drifting out ?

If it is out by a constant 500ms then the simplest solution to demux, delay cut, remux.

If it is drifting then it is your method for changing framerates. AssumeFPS adjusts sample rates to keep the audio in sync. I suspect that when MC encodes the audio and corrects the sample rates, it puts the audio out of sync.

[Alex_ander]

Assuming FPS 25 with original 23.976 changes video length (-4%). This method of framerate conversion demands processing audio (for changing its length correspondingly). If you began with a progressive source (looks like this), it's better to encode it as a 23.976 progressive elementary stream (if MC allows this non-standard template with PAL size; CCE does), then apply DGPulldown and you'll keep original video length at 25fps. Encode audio separately, mux or author for DVD. To get .wav audio from your script you can 'save wav' in VDub(Mod); 'source audio' mode. This also helps to decode audio properly when original .avi has VBR audio. It's also useful to check audio vs video length in the beginning by opening .avs in Vdub (file information).