audio drifting out of sync with video

[John Rofrano]

[Griff Hamlin] “Is there something I’m doing wrong or missing here?”

What you’re missing is that every recording device has an internal clock that is slightly off from every other internal clock in every other device. Unless you use GenLock with an external clock that all cameras and audio recorders are connected to, you will have drift. You’re not doing anything wrong. it’s just the nature of the crystals used in the clocks.

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Dave Haynie]

First thing… I would record the audio at 48kHz sampling, not 44.1kHz, for video (well, me personally, I’d probably record at 96kHz, just because I can). 48kHz is pretty much standard for any video format. You’ll probably find some claims online that sampling rate can be a sync issue, but that’s not true.

John’s right about the crystals… every device runs from an independent quartz crystal, unless you’ve time locked them in some way (genlock, external word clock, etc). Every digital device uses some kind of clock for timing. Each has a stated frequency, and a spec that guarantees that frequency, within a certain accuracy. More accuracy generally means more money.

Now, that’s not the main problem… if you’re using an accurate enough clock. The problem is, some devices don’t. Take the plain old everyday bog standard quartz crystal or oscillator you’ll find in most PCs and other digital gear that doesn’t care about super-accurate timing. This has a stated accuracy of +/-100ppm (parts per million), which sounds pretty decent. But at worst case, if that’s the clock in your audio recorder, it’ll walk away by one frame at 24fps every 6.9 minutes. Now imagine if your camera and your audio recorder used the same clock, but each was off in the opposite direction!

On my day job, I do electronics design, including radio. The typical radio clock I’m using these days is 1.5ppm… that would translate to walking off one perfect 24fps frame every 7.7 hours, worst-case. In practice, you’d claim the field recorder and the camera were perfectly synced. Of course, I have to pay $8.00 or so for such a clock, while a 100ppm version at the same frequency might run $0.50. So you can guess which one the PC Industry is likely to use for regular gear.

In following some of the HDSLR sites, this has been a big problem for HDSLR users. Seems lots of these guys never did separate audio/video before (I’ve been doing it for… long), and so they picked up whatever field recorder they could find. Some of the early Samson/Zoom recorders had pretty crappy clocks, so they could walk off your video in 10 minutes or so. I have a later model, the Zoom H4n, and it pretty much holds lock for an hour or more with any of my camcorders. I hear tell folks have seen it wander a frame or so after 100minutes, suggesting maybe a 10ppm clock. Of course, I really don’t know the accuracy of my cameras, either, but pro gear tends to be well made in these kinds of things.

So the first question: what are you using for recording? You did say “record audio on a DAW” … so I’m guessing a PC and some sound card. Is this the PC’s default audio card, or something that’s got a name hooked up externally? Assuming the 2-3 cameras retain mutual audio sync, you can certainly factor them out of the equation.

I bet it clean up really nicely with a different recording device.

-Dave