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Stretching audio non-linearly
zambaretzu
post Jun 5 2006, 00:08
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I want to sync audio from a show with video from a different source. It turns out that the two are out of sync, and no simple stretch can fix it. For instance, if I stretch it so that the beginning and end match up, it'll be 30 seconds ahead 1/3 of the way through, and 45 seconds behind 2/3 of the way through. Basically, it's a bit of a nightmare.

What I want to be able to do is set a bunch of cuepoints saying:
- 00:00 -> 00:00
- 00:20 -> 00:23
- 00:35 -> 00:34
- 00:50 -> 00:51...
The software would somehow stretch the audio to accomplish the above, and just have a simple gliding stretch in between each pair of cuepoints.

Is this possible?
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Societal Eclipse
post Jun 5 2006, 01:41
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If you find an easy way I'd like to know too because I've wanted to do that with samples of audio for making weird music effects.


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PiezoTransducer
post Jun 5 2006, 02:40
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any non linear editor NLE should be able to do this. Premiere or Final Cut come to mind. I forget which ones have a checkbox to preserve pitch. Either set markers at the points to sync or split clips up. If AfterFX will let you do audio, it has better rate control tools (bezier curves instead of piecewise linear).
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Kees de Visser
post Jun 5 2006, 10:52
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QUOTE (zambaretzu @ Jun 5 2006, 01:08) *
I want to sync audio from a show with video from a different source. It turns out that the two are out of sync, and no simple stretch can fix it. For instance, if I stretch it so that the beginning and end match up, it'll be 30 seconds ahead 1/3 of the way through, and 45 seconds behind 2/3 of the way through. Basically, it's a bit of a nightmare.

Audio and video will only be 100% sync if both were resolved to the same clock during recording. In a lot of situations this is not practicle or even impossible. Usually the speed difference is so small that the total drift is a few frames per hour. The differences you mention are huge compared to that. Assuming you're playing the audio and picture at the correct sample/frame rate and that both are recordings from the same performance, I get the impression that the picture has been edited. Time stretching won't help you there. You'll have to edit the audio according to the picture.
Without having seen/heard the material, this is my best guess. Hope it helps.
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gameplaya15143
post Jun 7 2006, 01:23
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Sounds to me like variable framerate video. Or at least just switching between 23.976 and 29.97 frames per second. Which means, you would have to:

A) telecine the 23.976fps parts up to 29.97fps
B) inverse telecine the 29.97fps parts down to 23.976 (only if the 29.97fps parts were telecined)

Or the video track and audio track are completely different. Different parts in the video that aren't in the audio, and parts in the audio that aren't in the video.

Either way, it's a nightmare tongue.gif and it would help to know more.

ps. avisynth is your friend


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