Sonar Workflow - Multisesson Tracking Loopback Latency

From Sonar

Introduction -- Sample Accurate Recording With loop-back Latency

Imagine you have a killer drummer playing along with a beat (an audio loop for example). You record their performance, and it sounds great while recording. Now when we play back, it sounds OK, but there is this feeling that the groove is gone, or the beat is no longer "in the pocket" like it was when recording. This is a common problem with DAW hardware and software on computers and is caused by incorrectly compensating for loop-back latency.

Loop-back latency is the time it takes for sound to play back out of a computer into the analog world and loop-back into the computer to be recorded again. This is a real and finite latency. If everything is perfect in the system, this loop-back latency is compensated for and the recorded track lines up perfectly with the playback track, giving us a sample accurate recording. That is to say that the loop-back latency still exists but the various tracks in the DAW software line up accurate to the sample. When the loop-back latency isn't perfectly compensated for we end up with a loop-back offset.

This problem is also often referred to as loop back delay, recording latency, recording offset, and recording timing delay.

List of Terms

Multisession Loop-backing: recording a new instrument while playing along with a previously recorded instruments or sounds in a multitrack environment.

Loop-back Latency: time delay caused by a the process of digital to analog and analog to digital conversions (DAC & ADC) and any functions of the ADC/DAC related hardware and their software drivers that cause time delay.

Loop-back Offset: when loop-back latency is not fully compensated for and therefore results in offsets between tracks when multisession tracking.

Sample Accurate Recording: when loop-back latency is fully compensated for, resulting in no loop-back offsets. The tracks line up perfectly with each other.

Concerns

Loop-back offsets on the order of a few milliseconds are nothing to be concerned about for standard multisession tracking since we can't tell when different instruments are off by that amount. So sample accurate recording isn't an absolute requirement. Much more than a few milliseconds and songs just don't sound tight. A bit more than that and something is obviously wrong. [Since this is all somewhat subjective no numbers were used.]

However, there are instances where sample accurate recording may be desired. Routing tracks out of the digital domain for external processing of some sort and then recording them back into the DAW software again could have adverse affects. Some examples of this are re-amping, or utilizing external effects processors, a reverb chamber, or a reverb plate.

Problem scenarios:

1) Several DAC to ADC conversions (or other delays of some sort) add enough overall loop-back latency that the final track is delayed from the rest of the tracks enough to be heard as a timing issue in relation to the other instruments.

2) The looped-back track is mixed in with the original track. Depending on the contents of the loop-back track, this mixing could add undesired phase distortion.

This is an issue that can happen with any DAW software. This is because it is generally caused by the drivers for the sound interface (soundcard) not reporting the true loop-back latency so the DAW software can compensate properly. There a number of reasons that this happens that we won't worry about here. The point is that you need to measure your DAW's actual compensated loop-back latency and correct for it if needed.