v2.8.1 - reporting a problem with desynched audio in the preview

[phaolo]

Hello, I'm reporting that a video (AV1 from Youtube) is behaving differently if opened in v2.8.1 vs v2.7.8 (my previous version).

In the new one the audio gets out of sync right from the start (it actually feels the video plays a tad slowder tho?), whilst in the old one all seems good.

Luckily at least, this seems to be only a preview issue, even if it's annoying for cutting.

Source video:
youtube.com/watch?v=Hxa7JarZAhg

[eumagga0x2a]

If the performance is not sufficient for decoding and display of video in real time, video will get behind in Avidemux. If you haven't disabled post-processing or the video is HDR (which 2.7.8 AFAIR didn't support yet) and you have not disabled HDR tone mapping (which is technically also post-processing because it happens between the decoder and the cache while video filters live between the editor and the encoder), you impose a high additional performance penalty. Please check both last points thoroughly.

On my PC currently used to work on Avidemux, I've provisionally implemented hardware accelerated decoding of AV1 in Avidemux on Linux via VA-API, which allows real-time playback of 4k source videos with negligible CPU load. No estimates are possible whether this will be ready for official nightly build in a short term and even much less whether this can be achieved for Windows any time soon.

[phaolo]

I indeed have an old PC, but I don't think that video is HDR.
(Info doesn't say it and "tone mapping" in Avidemux is greyed out)
Plus other AV1 ones by that creator work fine (and they're just 1080p).

EDIT: oh, I changed the Postprocessing filter strenght from 3 to 1 and the video plays normally.
Nvm then, you were right

[eumagga0x2a]

Don't reduce the strength, please turn it off completely! (Else you also ruin the quality of the output if you re-encode the video.)

[sark]

please turn it off completely! (Else you also ruin the quality of the output if you re-encode the video.)

An oft repeated statement... so I'm curious. What exactly is the the purpose of Post Processing. Why is it there? Why not implement the options as a filter?

[eumagga0x2a]

Post-processing in Avidemux has a very valid reason to exist as it operates on pictures before they are accepted into cache, i.e. you see already processed pictures in the video display without giving up a lot of functionality to navigate which you have to give up when checking "apply filters" checkbox. Other than filters, post-processing is available in copy mode, allowing e.g. viewing interlaced videos deinterlaced (not very high quality deinterlacing, but definitely better than comb artifacts).

Regarding HDR tone mapping, it makes little sense to try to do it after conversion of original 10- (or 12-) bit video to 8-bit. With the existing architecture of Avidemux, this is possible only before pictures have joined the editor cache, i.e. as post-processing.

[sark]

you see already processed pictures in the video display without giving up a lot of functionality to navigate which you have to give up when checking "apply filters" checkbox

So would you turn on for editing (images), but turn off before exporting ?

[eumagga0x2a]

Deinterlacing via post-processing / deringing / denoising (if you ever need the last two), if you re-encode: yes, definitely.

If you are in copy mode: no, why should you? Copy mode completely bypasses the decoder, thus post-processing becomes irrelevant.

HDR tone mapping: if you re-encode a HDR source, I would advise to apply tone mapping (fine-tuned to look best for the given video), turning the video fed to encoder into SDR with correct colors. Do not add color information copied from the source video, this information may be needed in copy mode – and also when re-encoding without tone mapping, but the latter would likely have poor results.

[phaolo]

Don't reduce the strength, please turn it off completely! (Else you also ruin the quality of the output if you re-encode the video.)

Ah good to know thanks, but it was just a temporary test, since I haven't had issues with other videos so far.
Also, there's no option to disable it, unless I'd have to untick all options in Preferences > Post Processing

[eumagga0x2a]

This is exactly what you should do.