Youtube shifting audio of upload

[Bencuri]

I have already mentioned this observation in another thread, that when you feed a video converter with an mpg that has wav audio, and you export it as mp4 with aac audio, if you compare the two afterwards, the audio timing won't match. In the mp4 the audio will slightly be shifted. The amount and the direction of the shift cannot be pre-determined (or at least I could not), because depending on the software you open it with, it will be shifted to various directions and amount.

I also noticed this on other 'something' to mp4 conversions as well, and I suspected if you upload a video to Youtube, as it will convert to mp4, it will do the same.

User jagabo had an idea that this might be caused by the processes during the compression of the audio, compression results in such a thing itself. That sounded logical. However since then I did some tests, and by now it seems to me it may not be, or not only be related to that, or something is happening that I don't see:

I had an mp4 with aac audio. I uploaded it to Youtube. Youtube has a feature that you can download what you uploaded, or actually it is not even the uploaded file, but what Youtube converted it to. So it is a good way to observe what Youtube is doing. And interestingly when I compared the uploaded file and the downloaded file (both mp4 aac), I noticed the audios do not match again. The converted one was shifted, no matter both the source and the target audio was the compressed aac.

This shift is usually 1 frame in an 50p video, or 20 milliseconds. This does not seem to be much, but the amount is not exact, it is fluctuating conversion by conversion, and there are video standards where the tolerance for lip sync is around 22-25 milliseconds. Now imagine that the original video was near to the tolerance limit, Youtube shifts the audio even more, and it will be out of sync, even in case of less stricter standards.

Does anyone have an idea what causes this and how to avoid it?

[2nHxWW6GkN1l916N3ayz8HQoi]

Does shifting the audio before uploading it to youtube work? So when youtube messes with it, it's actually perfectly synced.

[Bencuri]

Does shifting the audio before uploading it to youtube work? So when youtube messes with it, it's actually perfectly synced.

Didn't think of that. But might be a good idea.

The only reason I did not try experimenting with such things, is that when you check shifts after conversions on your computer, you find that software by software, the amount of shift displayed is different. It is different when you open the video in Goldwave, different in Wondershare, different in Vegas. And we are talking about the same file. So who knows how that audio behaves when the browser opens it. I know of no way to check that. So I decided to leave things untouched so far.

However, something that I managed to investigate is that when you export AAC only from Vegas, and skip the video channel, and you mux that with the video afterwards in another software, then when you load the full video with the audio back to Vegas, it is in sync. Through all platform. But again when you upload that particular file to Youtube, and you download it, it will be out of sync.

A bit chaotic problem. Something is happening on those conversions with the audio. And the strange is once it is shifted, as I mentioned, it is chaotically determined software by software where the audio starts. Maybe it is a muxing problem.