"Timing offset" warning when calibrating REW with UR22C and Amp

[atharva]

First, a huge thanks to John for making this wonderful software available to us.

This is my first time doing this. I've got the UR22C connected to a Macbook Pro M1 and am using a NAD class AB amp with a voltage divider as in the following thread except with 47K and 22K resistors and a 9.6R sense resistor.

I can't post links so had to do this

https://www.avnirvana.com/threads/sound-card-calibration-and-impedance-circuit.6810/

I've got 2 questions.

1. As per post #8 of the thread John says the only calibration required is the Impedance calibration. However the Impedance measurement screen has a place to upload Cal files for the UR22C. So am confused.

2. If I should, the sound card calibration step suggests to route the Output of the interface back to the Input. How is this to be done when using an amp and a voltage divider network?

3. I just skipped the sound card calibration and moved on to the open circuit calibration but I get the following warning with a 96KHz sample rate. (Midi rate also set to 96Khz)

What could be causing this? If I move up to 192Khz I get an offset of 1+ samples and REW says it can't use that measurement.

If I proceed with the short circuit and reference calibration I get the following measurements. Does this look ok?

Open Circuit...

Short circuit...

Reference cal with 32.2R resistor...

Thanks!

 

[John Mulcahy]

The cal files button shouldn't be shown for impedance measurements, I've fixed that for the next build. Soundcard cal is not used for impedance measurements. Some cards have a timing offset between channels, though it tends to be some fraction of the sample interval and so remain the same at different sample rates. The combination of the divider and some input or stray capacitance might be causing some phase shift, could see how the offset changes if you reduce the divider resistances. Your cal results look fine though.

 

[atharva]

Thanks for the clarification John. I tried simulating the error when uping the sample rate to 192K. But I just couldn't. The offset just changed from 0.14 to 0.16. I am positive that I got an error saying the offset is too large. I think it was something like 1.16.

Regarding calibration I suppose it is needed (useful?) when doing frequency measurements? In that case can you confirm that the way to do it would be to short the Rsense (speaker is disconnected) and do a measurement?

 

[John Mulcahy]

Yes, you could short Rsense if you plan to measure frequency response with the same setup. There is a guided process to measure the interface and create the calibration file on the soundcard preferences.

 

[atharva]

Thanks John.. The steps in the preferences screen said to create a loopback and I just wanted to confirm that when doing this with a voltage divider circuit and an amp that the way to create the loopback is to short the Rsense. Thanks for confirming that.

My calibration measurement distortion looks like this.. Is that too much distortion?

 

[John Mulcahy]

No, looks fine.

 

[atharva]

Thanks again John.