Help: Weird Audio Sync Issue

Sol M. wrote on 5/1/2004, 1:18 AM
I'm editing a video of an event that I shot with two miniDV cameras. For my master audio track, I'm using audio recorded to cassette (I know, I know) from the sound mixer.

The problem is while the audio from both of the cameras sync up without any problem, I'm getting severe drift between the camera audio and cassette audio. The drift is to the extent that if I sync the camera's audio with the master audio at one point, after ~4minutes, the audio is already 1 second out of sync (and it continues to drift farther out of sync).

I know an old analog cassettte tape isn't the best medium to be capturing to, but it was all the people putting on the event could offer me. I captured the audio via the line-in on my computer from the line out of my cassette player at 48kHz/16-bit.

As I don't know the intricacies of cassette recordings nor analog line-in recordings (previously only worked with digital media), I would like to know if I am fighting a losing battle here trying to get these audio tracks synced up? Does anyone have any insight as to why the audio drifts out of sync so much? Any ideas on how to fix it?

Thanks.

Comments

Chienworks wrote on 5/1/2004, 5:34 AM
Most likely the problem is that your cassette deck isn't running the same speed as the one that recorded the tape. This in fact is the norm in these situations as it is extremely rare to find two cassette decks that do run exactly the same speed.

Hopefully that is the only problem and both decks ran at a consistant speed (though different from each other). If it is this simple case then it is very easy to fix. Align the sync as best you can at the beginning of the tracks. Find a cue near the end where you can also match up the sync. Use the Ctrl-drag feature on the cassette audio track to stretch (or shrink, as the case may be) it to match the camera tracks. Right-mouse-button click on the cassette audio track, click Properties, under Time stretch / pitch shift select "Change length and pitch". Hopefully you won't have to correct more than a few seconds either way, but you should be fine up to 5 or 10%.

This should cure almost all of the sync problem. However, there is also the possibility that one or both of the decks changed speed during the recording or playback. In this case, you may have to spot check areas through the program for sync as well. If you find any places during the program where sync is badly off, you can split the cassette audio track somewhere near those spots and Ctrl-Drag the ends of the split left or right to adjust as you go. Just keep in mind that if you have to make a large adjustment this way then there will be a noticeable change in pitch at these points. It's better to make lots of little adjustments than fewer big ones.
Sol M. wrote on 5/1/2004, 3:23 PM
Yeah, I realized this truth about the cassette speed sometime after I posted my question. I was thinking it was going to be about some highly technical aspect of cassette recordings that I had no idea about, when it really was just a simple fact of movings parts in technology.

At any rate, the workflow you suggested sounds good. I had actually started doing this last night as a last effort to make something work. Many sections did sync up to each other, so that was nice. I guess more work along these lines is in order.

I do have a question tho. When I playback any audio that I ctrl+dragged, the audio sounds very jerky/stuttery. Is this just a realtime preview of the audio that will be cleaned up in the final render? Or is what I'm hearing indicative of what my final product will be?

Thanks again!
MJhig wrote on 5/1/2004, 3:49 PM
Do you have your drivers set to Classic or ASIO in Preferences and NOT Microsoft Mapper?

Vegas has to do the math on that time stretched audio on the fly in real time plus all the video etc. you have in the project. Mapper adds additional on the fly calculations.

You also might try increasing your playback buffer size if the above does not solve it.

MJ