Audio offset issue

[radpopl]

PXL-copy.mp4 - original file (shortened to 2s)
PXL-BestSource - avs script for this file

After loading the AviSynth script into VirtualDub, if you replace the audio track with the original MP4 using Audio->Audio from another file, the audio is shifted by ~0.02 seconds (1 frame).

ffmpeg seems to correctly convert the MP4 to WAV (PXL-wav.wav file), as the WAV perfectly matches the original MP4 on the timeline.

(BestSource from 7.01.2026)

AVS+BSSource:

original MP4:

ffmpeg's WAV:

PXL-wav.wav
https://github.com/user-attachments/assets/71bc1a79-30b4-4460-b901-45cf884e5142

AVS script:
BSSource("PXL-copy.mp4",timecodes=".PXL_20241224_181006359.txt",atrack=-1).TimeCodeFPS(".PXL_20241224_181006359.txt",fpsnum=60000,fpsden=1001,report=false)

[myrsloik]

Is it correct with any adjustdelay value?

[radpopl]

I haven't tried it, I thought BSSource should add the appropriate delay itself.

[myrsloik]

It will help me to figure out where the problem is. Does adjustdelay=-2 do the right thing?

[radpopl]

Yes, adjustdelay works.

[radpopl]

I investigated the problem further:

a) VirtualDub, after loading an MP4 file, shows the audio peak at frame 105,
b) LWLibavAudioSource does the same (via the AViSynth script),
c) I loaded the MP4 file (VFR) into Resolve, exported the audio to wave, loaded it, and the audio peak shows at frame 106 (wave loaded by both LWLibav and BestSource),
d) BestSource also shows the audio peak at frame 105 with an adjustdelay of -2, and with an adjustdelay of -1, it shows at frame 106..

I then changed the audio offset from 0.195 to 5.190 using "ffmpeg -itsoffset," and only BSAudioSource correctly offset the beginning of the audio track by those 5 seconds (LWLibav didn't account for this at all). VDub also offset the audio track by 5 seconds. But there was still 1 frame difference between BestSource and VDub (at 60 fps).

So it seems that only VDub and BSSource handle the offset correctly, but one of them is still off by 1 frame.

[myrsloik]

I haven't had time to look at it yet but that's even weirder. How do I even know which one is right? Without a good reference I don't feel motivated to do anything.

[radpopl]

I don't know either. But for now, I'm assuming Resolve and BSSource are doing it correctly.