SPOT - Audio/Sync Question

MUTTLEY wrote on 8/21/2004, 12:23 AM

Eh buddy, you had said in one of your articles " Be certain that your audio file is 48K when working with synchronized video. Audio ripped from CD will be 44.1, and cannot be correctly synchronized. "

( http://www.digitalprosound.com/articles/viewarticle.jsp?id=16025-1 )

Could you expand on this just a bit ? I'm not understanding why this is. Unfortunatly I'm 90% on one that I'm working on right now and have had issue with the cd version not synching up with the lipped synch version, falls off after a min or so. And is there a reason Vegas cant/doesnt do this when you extract audio from a cd ( which is how I've always done it, figured since it was there they must know what their doing ). I'm also assuming that it wouldnt be worth changing it at this point and that if I tried it would throw everything else out of whack, am I right on this ?

Anyway, would appreciate any light you could shine on this for me. Can't tell ya how mad I am I didn't see this earlier !!!

Thanks.

- Ray

www.undergroundplanet.com

Comments

farss wrote on 8/21/2004, 4:49 AM

Mutley,
maybe I can help. Lets see if I understand what you're trying to achieve. Presumably you have a video of someone miming to a CD and you have the same CD.
Rip the CD and drop that onto the TL and the thing looses sync after a while. I cannot see how the difference in sample rate should make a difference, even if they were both 48K this could still happen. Unless everything is running at exactly 48K then you would get drift due to the difference in the true clock frequencies.
However the differences should be minute, enough to be noticable after a while but well within what you could fix with minor time stretching. The error should be pretty linear, so you could just get it in sync at the start and then time slip the vision to get a point near the end in sync. Alternatively if they're any cuts in the vision you maybe able to trim a frame or two here and there to get it back into sync.

I'd did this ages ago with a station promo I recorded off air in the middle of China. The vision was OK but the audio was shot and just by chance the same song was on a $2 CD we'd bought. Took a bit of work as the dancers were dancing to a different arrangement of the song.

Ah, just thinking about your problem again, what source did the video come from. Analogue sources can be much more marginal than digital.

Bob.

Spot|DSE wrote on 8/21/2004, 6:46 AM

Muttley, with Vegas this is less of an issue. Where this used to come into play was mostly with other NLE's where you'd have a CD that was ripped, to be matched to audio that was recorded on DV. The two sample rates wouldn't line up in other NLE's. Vegas resamples this automatically. However, as Farss mentions, sync in the sample rate is nearly impossible to achieve, due to clock differences. For years, Canon XL1 for instance, had a sample rate of 48.009 that made it swing out of sync with a Sony cam used at the same time, that used 48.001. Over time, these would be out of sync. This is one of the smaller reasons that Vegas' stretch capability is really useful, stretching audio a frame or partial sample. You'll never hear the shift.

MUTTLEY wrote on 8/21/2004, 3:45 PM

Thanks for clearing that up guys. There are a bunch of cuts so yea, I was able to just bump things around a bit and make it sync back up but having to resync every cut gets fairly tedious. Was basically wanting to know just how much my " err " of not doing what Spot had suggested had added to the tedium.

This was shot with two XL1's and an XL1s. Three cameras and many takes adds up to a whole lotta out of sync layers.

So with all that was said and based on what you mentioned, would having 48K audio do anything to help with the sync issue in the future or not so much ? And would I be better advised to import in another way than Vegas' " Extract Audio From CD " if the vid were to get any play on MTV/CMT/GAC or other ? And if so, is there a cheap/free program that you would advise for capturing CD audio ? Or, should I have the artist provide me with something else if available ?

Thanks again, you guys rock. Its insight like yours that perpetually keeps me humbled.

- Ray

www.undergroundplanet.com