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SQ/AR2412 Traces
Nathan Riddle:
Finally got around to getting traces on A&H's SQ/AR2412.
Looking at the AR2412 traces I might see why SQ sounds slightly better than the 48kHz GLD/Qu/stage boxes. The phase wrap @ 20kHz & unstable performance at 20kHz is interesting. Not sure how performance hindering it is, but it's something.
SQ - Local - Main L - 0.52ms (Purple)
SQ - TB - Main L - 0.42ms (Teal)
SQ - Local - Mix - 0.48ms (Red)
SQ - Local - Grp - 0.44ms (Blue)
AR2412 - SQLocal - Out - 1.13ms (Gold)
AR2412 - SQTB - Out - 1.02ms (Green)
AR2412 - Local - Out - 1.48ms (Red)
AR2412 - Local - SQOut - 0.88ms (Purple)
Note, times are relative; I have no idea what the delay of my measurement setup is.
Scarlet 2i2 1-out > DUT
Scarlet 2i2 2-out > 2-in
DUT > Scarlet 2i2 1-in
1-out > Scarlet 2i2 2-in
Mark Wilkinson:
--- Quote from: Nathan Riddle on July 19, 2018, 10:05:40 AM ---The phase wrap @ 20kHz & unstable performance at 20kHz is interesting. Not sure how performance hindering it is, but it's something.
--- End quote ---
FWIW, I've recently found when trying to use Smaart for purely electrical measurements, that it seems kinda impossible to know what's really going on with phase of the DUT at the very end of the spectrum.
Because I think any measurement also includes sound card non-linearity.
And maybe even sample timing zero points with sound card and DUT don't completely line-up.... I dunno...
I just know that even at a 96kHz sampling rate, 1 sample is worth 70 degrees of phase at 20kHz.....so how good can 20kHz phase measurements be when both the souncard and DUT are digital and when there's no sync lock between them?
It's like there's a 35 degree +/- average error at 20kHz built in, if I'm thinking straight.......
Plus, Smaart doesn't to my knowledge allow sound card calibration for the card's non-linearites.
Coincidentally, yesterday I asked on the Rational Acoustics support forum, if there's a way to incorporate a sound card calibration.
I had been measuring some amp outputs and all the amps were showing the same tailing response up high.
So I checked the soundcard itself with REW. (which does soundcard calibrations)
Turns out, the tailing was in the sound card (UMC404) and not the amps.....
Which means the tailing would show in acoustic measurements as well doesn't it ?
Nathan Riddle:
--- Quote from: Mark Wilkinson on July 19, 2018, 02:53:20 PM ---FWIW, I've recently found when trying to use Smaart for purely electrical measurements, that it seems kinda impossible to know what's really going on with phase of the DUT at the very end of the spectrum.
Because I think any measurement also includes sound card non-linearity.
And maybe even sample timing zero points with sound card and DUT don't completely line-up.... I dunno...
I just know that even at a 96kHz sampling rate, 1 sample is worth 70 degrees of phase at 20kHz.....so how good can 20kHz phase measurements be when both the souncard and DUT are digital and when there's no sync lock between them?
It's like there's a 35 degree +/- average error at 20kHz built in, if I'm thinking straight.......
Plus, Smaart doesn't to my knowledge allow sound card calibration for the card's non-linearites.
Coincidentally, yesterday I asked on the Rational Acoustics support forum, if there's a way to incorporate a sound card calibration.
I had been measuring some amp outputs and all the amps were showing the same tailing response up high.
So I checked the soundcard itself with REW. (which does soundcard calibrations)
Turns out, the tailing was in the sound card (UMC404) and not the amps.....
Which means the tailing would show in acoustic measurements as well doesn't it ?
--- End quote ---
You might be onto something. And I won't pretend I know anything about all this. But my thought process is as such:
Purely considering just the results of measurements we can easily see 20kHz changes in phase/magnitude of small (in the degrees) by moving the mic minutely. How is that possible if 1 sample is truly 70 degrees @ 20kHz?
20kHz is extremely flat for a straight wire, and less so for various components, amps being some of the flattest and sound boards exhibiting some small phase distortion/difference.
Specifically the AR2412 (older purple one) measurements:
Using no smoothing or averaging one can pretty quickly discern the differences (noise floor maybe) in magnitude at 20kHz I think that's what I felt would be most noticeable audibly. Since who can tell there's a phase wrap at 20kHz when moving an inch changes the relative phase completely?
I dunno either...
Mark Wilkinson:
--- Quote from: Nathan Riddle on July 19, 2018, 03:38:31 PM ---
Purely considering just the results of measurements we can easily see 20kHz changes in phase/magnitude of small (in the degrees) by moving the mic minutely. How is that possible if 1 sample is truly 70 degrees @ 20kHz?
--- End quote ---
Hi Nathan, I stand ready to be corrected too ...you ask a good question..
Here's my thinking... first, pls check my numbers.....
96kHz sampling means 1 sample (1 sec/ 98,000) =0.0104 ms.
20kHz sine wave has a 360 degree period of (1 sec /20,000) = 0.05ms.
So, 1 sample @ 98kHz for 20kHz = (.0104/.0500)x 360 =75 degrees.
While we're at it, 75 degrees of 20kHz = (1130ft/20,000)x(75/360)= about 1/8 inch distance.
Ok, it seems to me, Smaart's time resolution has to be thought of as having two aspects...
One is the timing relationship, mathematically determined that probably? is continuous between the reference signal and the measurement signal.
But is seems that timing relationship is bound to being calculated off a discrete number of processing samples, represented by the delay finder time to the nearest sample. (Note you can't adjust the delay finder less than 1 sample's worth of time)
So a small 1/8 mic movement can make for a small phase movement, because the math is continuous around whatever discrete time difference has been determined. But Smaart has to recalc constantly to the nearest sample, when more than a sample's movement is detected. So it's kinda a hop and calc situation....
I've often seen on traces that tail up or down at 20kHz, that I simply can't get zero phase at 20kHz using MTW and it's 48kHz sampling.
Bumping delay finder between two samples....it's either tail up, or tail down...unless I move the mic a tiny bit, and get lucky on the hop location.
Did I make any sense at all ?
edit: changed 98kHz typo to 96kHz
Hayden J. Nebus:
--- Quote from: Mark Wilkinson on July 19, 2018, 02:53:20 PM ---FWIW, I've recently found when trying to use Smaart for purely electrical measurements, that it seems kinda impossible to know what's really going on with phase of the DUT at the very end of the spectrum.
Because I think any measurement also includes sound card non-linearity.
And maybe even sample timing zero points with sound card and DUT don't completely line-up.... I dunno...
I just know that even at a 96kHz sampling rate, 1 sample is worth 70 degrees of phase at 20kHz.....so how good can 20kHz phase measurements be when both the souncard and DUT are digital and when there's no sync lock between them?
It's like there's a 35 degree +/- average error at 20kHz built in, if I'm thinking straight.......
Plus, Smaart doesn't to my knowledge allow sound card calibration for the card's non-linearites.
Coincidentally, yesterday I asked on the Rational Acoustics support forum, if there's a way to incorporate a sound card calibration.
I had been measuring some amp outputs and all the amps were showing the same tailing response up high.
So I checked the soundcard itself with REW. (which does soundcard calibrations)
Turns out, the tailing was in the sound card (UMC404) and not the amps.....
Which means the tailing would show in acoustic measurements as well doesn't it ?
--- End quote ---
If you allow device aggregating, you can snap a TF of your I/O device with a loopback cable (OP to IP) and stim with the analyzers onboard generator, selecting the Gen as the reference. On v7 for me this always devolves into clock slip, but it works long enough to get a good TF of op and ip stages together. If they ain't flat enough for ya, use other IO.
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