deho said:
About time stretching: Can you elaborate on the downsides to that? Especially when its just used filling a 20 ms gap.
I was also thinking about the analog capture ilovewaterslides made. I have absolutely no experiences with Laserdiscs. If a Laserdisc features an analog and digital audio track, are they always from the same master? Could it be that the analog track is superior to the digital one, because less compression was applied (similar to vinyl captures that offer a higher dynamic range than Audio-CDs)?
Logically if you time stretch a piece of sound that has distinct effects in it, those effects won't be at their original timecode, so won't be in sync anymore. Stretching make no sense in this case, as the same time you solve a problem, you create another one. In a different case, when the piece you want to time stretch is only an indistinct hum with no recognizable sounds, you could say the stretching won't be noticable. But if the piece is so monotonous why would you need stretching? You could just copy-paste instead.
What you have to see is the whole time stretching method contradicts the idea of syncing. If one frame is missing in a material, you have to fill in the space somehow and not do any harm to the rest of the sound. Especially not something that moves them out of sync. See what I mean? Time stretching is of course the right thing to do if there are speed differences, like framerate conversions etc. So all the cases when the sound is "stretched" and you have "unstretch" it.
Stretching is also risky because by default it affects the pitch as well. When it is done a way no to affect the pitch it often sounds poor. There are sound editors out there that can do speed adjustment with no pitch change and in good quality but it depends on the nature of sound (dialogues, music etc.). It's really not the most straightforward thing to do well.