Recording with external audio interface - sync problems

A

alekstef

Well-known member
Hi!

I am the TOM on a TV show we record for a client. The client has quite limited resources, so the whole setup is abit.. unstable, if I may say so.

Anyway, I use the M-Audio ProFire 2626 with an iMac to record the audio on multiple tracks. As of now, there are only two microphones being used. I also send a feed to the DSR-20P tape deck for reference sound. The M-Audio software sample rate is set to 48 kHz, and so is the sample rate in the Soundtrack Pro project. Still, after I'm finished with the sound post production, after exporting it (in 48 kHz), and importing it to FCP, it slowly "becomes" out of sync. I have to make a cut in the sound files on the timeline, and drag it one frame back about every three minutes or so. It varies. I can't understand what I'm doing wrong, why does the sound go out of sync? It's not a big problem, but it is kind of annoying, especially since it takes some time to adjust it so that everything is back in sync again...

Any thoughts? Any help is appreciated. Thanks!
 
This sounds like the classic sync "drift" issue. For two files recorded separately to stay in sync over the duration of a shot, the sample clocks on the camera and the audio recording device must run at the absolutely identical speed. For this to be accomplished reliably, they have to share a common timebase. If they don't, they will drift apart at a rate dependent on the degree of difference between the two clocks. One or the other of your devices (or both) is not running at EXACTLY 48.00000000000 kHz. The fix is to use devices with more relaiable (read ... expensive) clocks. The way broadcast studios do this is to drie the cameras and the audio recorders off of a common "studio clock" signal. Location shoots use cameras with genlock inputs, recorder with wordclock inputs, and Lockit Boxes tuned to each other to provide each device with a stable, identical clock signal.
 
Thank you, Steve.

Well.. That sucks. The DVX100's we are using don't have genlock inputs. The tape deck has this possibility, though. So bottomline - there's nothing I can do to make it work with the equipment that we are currently using?
 
It could also be one of the FCP "features/ bugs.
Are you shooting in a X.0 frame rate? I'm guessing yes (PAL). If so you have to be very careful in FCP or it will reverse telecine your sound. The "capture" settings have to be exactly the same as the project/ sequence settings, even if your file based and never capture a frame.
If you have that problem then you will see the same drift you would get from non drop to drop frame (3 sec./hour).
 
This may seem like a silly question....unless you need more mics, why not just record to the DVX? The DVX records decent audio and will do quite well if you have a mixer in front of it.

Also, you can sync your DVXs together with internal TC using the wireless remote, so potentially you could have more than two channels. If you aren't familiar with this technique, see the DVX book by Barry Green.

Grant
 
Hi. It's abit more complex than that :) To simplify, DVXs are connected to an MX-70 vision mixer, which outputs to the tape deck. We don't record video or audio using the cameras, we use the tape deck.

About syncing the DVXs, I'll look into it. Thanks! :)
 
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