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

REW 5.30 Beta 4
MacOS 14.1.1
Mac Mini Intel 6-core 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:

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

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.

Hi Brad,

Johns suggestions seem sound, though nothing definitive was ever discovered. I haven't run any sweeps at those x2 or x4 sample rates in a while, probably a good time to re-test.