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Power source is disconnected, tried every configuration, still has a high noise floor.Doubt it has to do with a driver. Probably plain HW. Surprising as Apple makes really nice HW. Did you try with power source disconnected?
Mac REW Version: 5.30.9, Windows REW: 5.30 Beta 1Are the settings the same for both REW setups? Same versions of REW?
Yes, both was set to mono mode via the volume slider (yes it works on mac as well, Cosmos ADCiso is the "record interface")Is the Input volume control slider set properly so that both set-ups are in mono mode? (Or stereo - your choice.)
Is the Mac internal microphone turned off?
DAC and ADC harmonics sometimes behave differently on selected Sampling Rate, though both are running the same 88.2kHz. Also maybe because of the slightly different REW versions.It's also puzzling that the HD2 is noticeably better on the Mac than on the Windows computer. Of course, most of the other harmonics are close to being buried in the noise on the Mac, so there's no way to tell about them.
BTW, that icon of a camera in the upper left of REW allows you to capture the screen and settings you might choose without using a screen capture utility.
No "add dither" on the checkbox of Generator interface. I also used another laptop as the generator just to make sure that mac isn't changing any generator settings.This almost looks like the signal was truncated/dithered to 16bits (at the sending end maybe?).
Yes, one need to make sure the settings in Audio Midi Setup for the device is what you want.... easy to miss...This almost looks like the signal was truncated/dithered to 16bits (at the sending end maybe?).
Well, as you say, the right output channel is always the right input channel regardless of mode, and has no bearing in noise floor.It looks like you are sampling the right channel of your ADCiso, at least with the Mac.
Averaging is a "boxcar" LP filter. LP filtering the noise has the effect of smoothing the noise floor so you get more of an averaged value. However averaging is not the same as reducing the noise per bin (then averaging/smoothing or not)....noise averaging has been a default method to reduce noise for many many decades.
Excellent point!To check the noise floor or the ADC, it's neither needed nor favorable to actually measure a device.