Recording Latency vs. Recording Offset
How recording latency can cause Recording Offset by Orren Merton
By Orren Merton (excerpted from _Logic Pro 8 Power_ by Orren Merton and Kevin Anker, ©2009 Course Technology PTR all rights reserved).
Imagine this scenario: You are using a 4/4 drum loop in which every beat is exactly on the bar. You start recording. The first beat hits at 1.1.1.0. You play the part you wish to record exactly on time. However, your recording latency is 441 samples. If Logic were to place the digital audio at the exact point in the Arrange that it appeared in Logic, there would be a recording offset, or a difference between when the source audio was recorded and when the digital audio was placed on the Arrange, of 441 samples.
Thankfully, Logic is extremely smart regarding recording offset. All audio interfaces report how many samples they take to do their processing to Core Audio. Core Audio then adds to that number how many samples it uses to process the audio and passes that information to Logic. Logic takes that number from Core Audio, adds the setting of its own audio buffer to it, and then places that digital audio on the Arrange at the correct spot. So using our earlier example, Logic would figure out that there was a recording latency of 441 samples and place the audio backwards at 1.1.1.0 on the Arrange, thus eliminating the record offset. Your performance is exactly in time with the drum loop!
However, most, but not all, audio drivers report the correct recording offset to Core Audio. In this situation, Logic does the only thing it can--it places the audio where it thinks it should, based on the inaccurate numbers it has to work with. If you play back what you just recorded and it sounds consistently off, as if your recording is consistently early or late, this may very well be the reason.
If you do discover that there is a recording offset, the first thing to do is to fire off an email to the manufacturer of your audio interface informing them of the problem. If you are an advanced user and have used the Logic Pro Sample Editor to count exactly how many samples of record offset there are, be sure to send that information as well. Hopefully, this will help the manufacturer write a new Core Audio driver that reports the correct sample delay.
You’re not simply left at the mercy of the manufacturer, however. In Preferences > Audio > Devices > Core Audio, Logic offers a Recording Delay slider. You can use this to manually adjust Logic’s record offset correction amount. So if, for example, you have determined that all your audio is being placed 140 samples early, you can use this slider to delay the placement of the audio by 140 samples. If you determine that the audio is being placed 140 samples late, you can use this slider to subtract 140 samples. If you’re not sure, just experiment with different amounts until you get as close to zero as you can.