Combining Recordings From Two Devices

I’m experimenting with recording my jazz trio. I’m trying an interesting setup:

I record with a Zoom H2.
I also record by plugging my electric keyboard directly into my laptop, and recording with Audacity. So, the Zoom records the three of us, and the laptop records only the piano. I then combine the two recordings in audacity, lining up the two audio streams. This gives me the ability to adjust the relative volume of the piano versus the other instruments.

Here’s a weird problem I don’t understand.

This works OK for a tune recorded at the beginning of the rehearsal (I leave both devices on all the time). Here’s an example:

However, for a tune recorded towards the end of the rehearsal, it does not work:

Apparently, the “time base” is off somehow, so that even if I line up the tracks, they very soon get out of sync.

Can anyone shed any light on this?

Using XP and Audacity 2.0.0.

Sound cards and other digital audio devices have an on-board “clock” signal to tell the device when to record or play the next audio samples.
On very expensive devices these clock signals are very accurate, but in professional studio set-ups they go one step further and wire multiple devices together so that they share the same clock signal. This is the only way to guarantee synchronisation between different digital audio devices.

If your two devices are keeping good time when they are cool and recently turned on, that is lucky. If they drift out of sync, that is (sadly) not surprising.

One easy, non-electronic way around this is to provide sync points. “2:30PM, Mark” and clap once. That gives you a concrete sync point to go back later and match things up again. You’ll figure out the drift rate and how often you need to do that. On a motion picture shoot using film, this happens about every 11 minutes of running time.
Screen shot 2012-07-29 at 9.26.19 PM.png
Yes, that’s what that is and that’s what it does.
“Camera Mark” [Bang!].

Thanks, guys. You’ve come to the same conclusion that I have.

However, I’m still puzzled as to how far off the sync is for that second recording.

It’s difficult to tell for sure what is going on in that second recording, so I was “best guessing”.
When you listened to each track individually (before mixing them), did they both sound OK, or did one sound noticeably too fast with clicks and/or jitters?

OK, two claps. Exactly ten minute apart – or longer – not using the computer clock. Figure out which one is off. If you play your cards right, they’ll both be off in different directions.

It should make no difference at all, but is one set to 44100, and the other 48000? One is Music CD standard and the other is video sound.

I’m going with the Zoom is probably very close. Those things are every popular and compulsive engineer friends of mine love theirs. The engineering definition of a successful piece of equipment is you can’t read the dials any more because the paint rubbed off and in any event you can’t see it right now because it’s out on a job.

The time base in your computer is likely to be floor sweepings.

You should do some kind of test like that. Concrete numbers are always good when diagnosing a problem from multiple time zones.