Measuring impedance in REW (MacOs Mojave)

[rodelius]

Hi all,
Although I have used REW on several occasions, I have never tried to measure impedance.
Yesterday I got down to it and built not one, but two jigs to be able to do the measurements. After calibrating the levels and performing the calibration, I started to measure, but I see that putting a resistance as a circuit load, the impedance measurement becomes erroneous from approximately 1kHz. Same thing happens with both measurement jigs and no matter what Rin I specify for the soundcard...

Any ideas on what can be the cause?

I'm using a Fast Track Pro Interface and Mac Os Mojave. Tried using High Sierra and got the same results.

Any help will be appreciated.

Rod

 

[EarlK]

( From what I've observed since this feature was implemented ) >> the most common user-error is having the leads reversed.

OTOH, maybe your truly ancient soundcard is somehow to blame .

 

[rodelius]

Thanks for replying.

Also thought that might be the cause at first, but I went through everything and also built two different jigs... My soundcard is rather old, yes, but it seems to be working fine in FuzzMeasure, albeit with limited sample rate and bit depth. What puzzles me is the fact that it seems to work fine up to 1kHz.

 

[John Mulcahy]

Perhaps there is a very high latency from output path to input path. Try making a normal (not impedance) measurement of a loopback connection and see if that works or is truncated at high frequencies. If latency is an issue it might be helped by reducing the buffer sizes in the REW soundcard preferences. Also make sure the sample rate in REW and the sample rates for the default formats of the interface input and output in Audio Midi Setup are all the same.

 

[rodelius]

Hi John, thanks for replying.
Apparently, the soundcard is working fine (see jpg). I deleted the previous calibration and made a few normal measurements using the same (left) output and the left and right inputs.

Then I also swapped the channels (cable and REQ soundcard prefs) and tried to do a new calibration. The problem above 1kHz persists.

I've been wanting to buy a new interface for some time now, but first I'll try to borrow some other interface to see if it could be due to a problem related to the operating system.

Again, thanks.

 

[John Mulcahy]

The soundcard loopback measurement impulse response shows an extreme clock rate error, typical of cases where the interface is not operating at the selected rate - your selected rate is 44.1 kHz so the interface may be supplying data at 48 kHz. Try using 48 kHz in REW and check the sample rates for the default formats of the interface input and output in Audio Midi Setup.

 

[rodelius]

Dear John, thank you very much for taking the time to check out this particular problem. Indeed, the input and the output were at two different frequencies. Unfortunately in Mojave it is no longer possible to select 48kHz for input and output, so I had to use 44.1. For what I need, it seems to work perfectly.
All the best,
Rod