What is causing these strange THD vs Frequency results at 96k and 192?

[Chuck Zwicky]

REW 5.30 Beta 4
MacOS 14.1.1
Mac Mini Intel dual i7

I've been frustrated by these strange THD vs Frequency results when testing a variety of interfaces at 96kHz and 192kHz sample rates.

I've experimented with different buffer settings in the I/O device settings in the preferences, and with different sweep lengths and repetitions, but I cannot get a consistent reading.

I suspect that this is a synch error somewhere, I'm using a standard setup - 2 channels to the DAC output, looped into the ADC inputs using the loopback as the timing reference.

At lower sample rates with the same settings, no anomalies:

 

[John Mulcahy]

Probably dropouts or buffer overruns from other activity pushing REW into the background too long. Try fewer, shorter sweeps.

 

[Chuck Zwicky]

I had tried different sweep lengths, but no improvement - interesting that it's only happening above 88.2kHz

 

[John Mulcahy]

Are you setting the sample rate in the Audio Midi Setup utility to match the REW rate?

 

[Chuck Zwicky]

Yes, I'm checking the sample rates in REW, in Audio MIDI setup and at the interface driver software.
Normally I set the interface first (Audio MIDI setup automatically follows) then I set the sample rate in REW

Note: in REW after changing the sample rate, sometimes you need to toggle the I/O buffer setting (switch to a different setting and back again) before it will pass signal.