MP4 editing results in asynch

[Ghitulescu]

Well, I have a bunch of shows downloaded legally from the mediathek of the TV station. They do not have any copyright protection.
These are 720p MP4 having H264 for video and AAC for audio.
As I tried to remove the commercials (using AviDemux, per one thread found here), the audio starts gaining speed after a while. If I pause the player (a BD player via USB) or I wind/rewind for a splitsecond, the audio recovers the synch but again starts drifting as before.

Any ideas why it behaves this way and corrective measures?

Is there any other tool to "smart cut" (id est not only at I or IDR frames) MP4 files? Of course, with better results?

[sneaker_ger]

SolveigMM Video Splitter

[Ghitulescu]

What I find it strange is that I get the same asynch issues even if I am using ffmpeg concat demuxer function.
On one BDplayer has the file this asynch, a Pioneer BDP-140. The original files, as well as the cut ones remain in synch, only the joined ones start loosing it.
On an LG 370 as well as on another Pio (440) and as well as on other players the A/V is in synch.

This is the actual question - why this happens?
My hunch is that there is something, a parameter, a header, something that is probably wrong and ignored or corrected by many players but not all of them.

PS: my intention is to archive them, so I am concerned that these files may not be correctly played back later, much later.

[sneaker_ger]

The original files, as well as the cut ones remain in synch, only the joined ones start loosing it.
Lose sync how? First part sync, then every time it switches to a new part there's an additional delay? Then maybe the file ended up VFR (strictly speaking) e.g. because there are gaps in video or audio (original tracks not exactly same durations) and the player cannot handle it.

My hunch is that there is something, a parameter, a header, something that is probably wrong and ignored or corrected by many players but not all of them.
Sounds more like other way around: players who play with good sync are working correctly, other players are broken.

[Ghitulescu]

Well, I am still testing the joined files using various software and options.

And the MP4 created with a TS temporary files via h264_mp4toannexb option appear to be OK.

So I am now sure it's a some header thing...

PS: I do not consider a player being broken because it does not keep synch with a particular, processed file, when the unprocessed file plays OK. This is to me a perfect sign that the processing does something wrong.