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measuring system latency, output to input


atari800

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Attempting to measure the round-trip latency of an outboard effect unit, so I figured that first I'd measure the system roundtrip latency of the Macbook itself. Plugged a cable from the Macbook analog out to the analog in and recorded a test track out of Logic. Ended up with 112 samples, which is about 2.6ms at 44100khz sampling rate.

 

However when I patched in the effect unit -- a Lexicon PCM 42 set to 00 ms delay, run 100% wet -- the test track is actually coming out a few samples *ahead* of the original in Logic. I have no plugins installed or region delays specified. I/O buffer is 128 samples.

 

Confused, I ran the Macbook self-test again and this time it was almost perfectly aligned with with the original, neither early nor late.

 

Any idea what's going on??

 

Here's a picture of the bizarreness for your enjoyment.

 

Thanks in advance.

 

(Pun intended.)

latency.jpg.06d3989450a4de338888d27f08a29497.jpg

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Great pun!

 

Not wishing that you should endure any further delay in providing you an answer... I think you should run your original test -- output to input -- at least 4 times in a row and see if the recorded waveforms of each subsequent recording are in sync. I anticipate that they won't be.

 

Use a buffer size of 128, sample rate 44.1K like you did before. No plugins. No PDC. Start your source (which should be some kind of audio track, not a loop) at bar 2 but initiate playback from bar 1. Record at least four tracks of the looped-back recording. Post back with your results!

 

-=sKi=-

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Oh, I love being correct! :mrgreen:

 

LOLOLOLOL!!

 

Yup. My guess is that the built-in audio drivers of your Mac do not establish a consistently-sized record buffer. So every time you go into record, the recording delay is going to be different. And I'm not at all surprised to learn (from your original post) that sometimes the recordings are early.

 

Or, it could be that the record buffer is consistent, but that it reports a different delay amount to Logic each time you come out of record.

 

Either way, it's a royal PITA that you can't get consistent results using the built-in audio driver. You might want to invest in a "real" interface so that you can record live music and have consistent results. With your current system, every time you play back a part that was recorded live, it's going to be a total crapshoot as to whether or not the part will end up being early, late, or on the beat.

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Of course the Lynx Aurora 16 is down at the studio, this was just a quick and dirty from the kitchen table.

 

Actually, a Lynx Aurora, with the LTFW card, as an interface will very likely display the same or worse timing characteristics. These are "no-drivers-required" interfaces and use OSX's built-in firewire audio support, which is notoriously unpredictable timing-wise.

 

If this kind of operation is important to you, look into RME, Apogee or Metric Halo interfaces which come with their own drivers.

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It is an LT-FW card.

 

I remember the '90s -- like it was yesterday. The barrage of third-party plugin standards, the add-on sound cards, the proprietary drivers that perpetually went out of sync or needed updating, the custom system extension sets to avoid conflicts (internet while recording, you must be crazy), the floppy disk authorization keys, the SCSI IDs. I practically have a prison tattoo that says Sound Manager. And man I gotta tell you, I am so happy not to install drivers anymore. On an OS that doesn't crash, no less. Hell sometimes I unplug my audio interace during a session. Just because I *can*.

 

They *ain't* sending me back, not over a lousy 2.6 milliseconds!

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Great post!

 

BTW, my prison tattoo is the outline of a 5-pin MIDI plug with "OMS" emblazoned around the circumference in a gothic font with blood dripping from the letters, flanked on either side by anguished angels and Ruebenesque cherubs committing suicide, all in the foreground of a Hieronymous Bosch-style scene of apocalypse...

 

Now...

 

For my rebuttal...

 

Don't napalm the significance of 2.6 milliseconds. And don't you gowon and say, "I love the smell of delay in the morning" or some other curt(z) response. 2.6 ms is enough of a delay to kill the groove of whatever you record, particularly when the delay is constantly going to vary. Once you start using a system which has a fixed/predictable delay time (unlike the bloody built-in audio drivers) you can set Logic's recording delay to compensate. Then, parts you record will come back with the exact feel that they had when they were played. Othewise, it's a crapshoot.

 

And honestly, the difference between them days and these days in terms of drivers and so on is like comparing apples and (agent) oranges.

 

Or something like that.

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Great post!

 

BTW, my prison tattoo is the outline of a 5-pin MIDI plug with "OMS" emblazoned around the circumference in a gothic font with blood dripping from the letters, flanked on either side by anguished angels and Ruebenesque cherubs committing suicide, all in the foreground of a Hieronymous Bosch-style scene of apocalypse...

 

Ski... you did time?!!! Wow!! How many milliseconds were you sentenced to? .... and your crime? .... I dare not ask! :shock:

 

:wink:

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Yeah man, I had a brush with the music police a few years ago. The judge wanted to sentence me to for a rash of "alleged" crimes, including:

 

• one count of being "maliciously overbearing" for insisting that two dotted quarters followed by an eighth is not the same feel as a triplet

• one count of fraud for calling a piece I wrote in 7/8 a waltz (some people don't know how to take a joke)

• 12,987 counts of composing while under the influence of indecision

• 900 counts of aggravated nitpicking at violinists' tunings during sessions

• 12 counts of harmonic crimes against humanity

• 1 count of "menacing" for getting mad at a flutist when she asked if she could play a 16th note quintuplet as four straight 16th notes. See, she couldn't play the part right and then tried to justify it by telling me, "what you probably meant to write was..."

 

...and that's all I can remember before the thorazine they gave me in the psych ward kicked in.

 

But eventually, the judge had pity on me and let me off with a "slap on the wrist" -- a sentence of 75,000,000,000,000 milliseconds of community service, serving as a moderator on Logic Pro Help.

 

[sigh]

 

So here I be.

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