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I'm guessing he didn't understand the question. [UPDATE: I was thinking "he" here was a Digidesign support guy but a sharper-eyed pal hipped me up that it was an APOGEE support guy. So, you know, he should be up on Apogee issues, for sure, but I guess he can be forgiven for being less so on PT ish.]
Since your compressor is, presumably, an analog device, we can assume we don't have to compensate for any latency from it.
So, therefore the only latency we need to measure is the time it takes for a signal to get out of Pro Tools LE and through your converters out into the analog world and then turn around and come back in through the hardware into your track.
Of course, it bears remembering that EVERY overdub you do involves this particular latency, since what you hear (what's coming out the converter's DA output) is your time reference for laying in overdubs.
At any rate, it's pretty easy to calibrate this -- but remember the amount of time the converters take changes with the sample rate (although it's not absolutely proportional in most cases), so you need to recalibrate for different sample rates.
The typical calibration is the so-called "loopback test" and it's easy.
Just connect one of your analog outputs on your converter to an analog input. (Make SURE you do not have "input monitoring" turned on or you could end up with a supernasty feedback loop.)
Now, take a sound with a sharp, easily identifiable transient (typically a "ping") and play that out your looped patch cord and back into an input, recording the analog copy of the ping.
Now, by zooming in, locate the exact sample value of the original ping and the exact value of the copy. The difference between these is the "tracking misalignment" between existing tracks and new overdubs.
ANY overdub will be subject to this misalignment unless otherwise corrected.
Since the misalignments are typically not huge amounts, you may not notice it. IIRC, an 001, being PCI-based, has a very short misalignment, less than 2 ms, I think. Th 002 and Mbox (especially) have considerably higher misalignments.
With overdubs the issue is whether or not this misalignment is a musically significtant value (and then there are us sticklers who think that everything should line up, period).
But with effects sends/returns you may end up blending dry with returning wet (not as likely in the case of a compressor, of course) and that may cause phase issues.
In the ASIO world, audio drivers are supposed to report their hardware latency and many DAWs try to use those reported latencies to compensate. But in actual testing, it appears that this compensation often leaves quite a bit of accuracy to be desired, so many DAW makers have begun offering some form of individualized compensation that can be adjusted to breach the gap.
Mackie Tracktion has an automated ping loopback calibration. I understand that Cubase/Nuendo has such an automated comp but there seems to be debate about how well it works. Cubase users may still have to tinker their numbers even after calibrating, according to some. And -- according to some reports -- Cubase's automated compensation doesn't work when a driver has a "negative offset." (It reports a false value that makes compensation try to push it too far forward.) Sonar (and Logic, I'm told) both have manual compensation slots where you can enter an adjustment value after you have "hand calibrated" it with the loopback test described above.
So, it appears that these unadjusted for or poorly adjusted for hardware latencies affect many of us to varying degrees and at least some of our DAW makers have tried to give us the tools to compensate for them -- but that they don't always work as well as we'd like -- when we have them at all.
And, of course, the very same latency that affects the round trip of audio going out into the analog world for external FX loops and back in is the same latency that will affect any overdub/new track in relation to previously recorded tracks.
PS... the reason I say the Digi tech support probably didn't understand your question is because I ORIGINALLY became aware of this whole issue by reading complaints on the DUC (Digidesign User Conference bb) about tracking misalignment with 002s and Mboxs when they first came out. It's not exactly news. (I was feeling pretty smug until I tested my own PCI based converter at the time and found an approximately 4.5 ms misalignment! At that time, my DAW didn't even have a sample-value accurate nudge command so to compensate I would have had to calibrate a MIDI tick value for each tempo and adjust by that. I chose to ignore the gap but when I switched to a MOTU 828mkII whose WDM/KS drivers had a nearly 8 ms misalignment, I simply had to get proactive. I recognize that that's the time it takes sound to go about 7 feet [correction: make that about 9 feet. Wasn't awake yet!] but it was, nonetheless, throwing off my timing in some stuff, particularly hand percussion.)
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