The Computer Blog

Wednesday, November 19, 2003

Fixing Out of Synch Audio and Video using FCE

It’s only been recently I learned that most camcorders come set from the factory to record using 12 bit audio. That’s not a problem if you’re just dumping the tape to a VCR; but if you’re dumping it onto your computer (whether PC or Mac) especially to burn to DVD, you’ll usually find that somewhere in your video editor’s timeline or, worse, in the DVD itself, the audio and video falls out of synchronization. This is because DVD’s are encoded using a 16 bit audio format, as I’ve mentioned before. I’ve since reset my Sony TVR720 camcorder to 16 bit audio, and there it stays. But I still have left some irreplaceable footage (like mountain lion and wedding videos) recorded in 12 bit audio. How was I going to fix the synch problem if it came up?

To experiment with that, I recorded an hour’s worth of footage using 12 bit audio and dumped it onto my Quicksilver PowerMac and into Final Cut Express. I had often paused the camcorder during filming and found that the pauses were often where the out of synch problem would begin. So, using the Razor Blade tool, I cut both the video and twin audio tracks. To move them right, I simply clicked on the appropriate track and typed in a number representing the number of frames I wanted it to move. Most of the time, the number was “+3”. This moved the track 3 frames to the right. I then played the remaining footage on the clip to see if it was in synch. If it was, I selected the “Modify/Mark in Synch” menu item with all the applicable clips selected.

In two out of three clips, no further manipulation proved necessary. In the last clip which was longer than the others, I noticed that the audio and video was in synch at first but then fell out again much later. Theorizing that the effect of an “out of synch” condition was cumulative and that I was only slightly out, I told FCE to move the audio tracks “0.5” to the right. That proved to be the magic bullet.

When I run the video in FCE, it all looks good. I’ll burn this footage to DVD using iDVD in another night or two and see how it does. I have no reason to think that it all won’t work, but it’s best not to assume anything.

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