Dear Hifiberry,
I'm setting up my freshly arrived setup (made of a RaspberryPi4 + DAC+ADC Pro) which I would like to use as a data acquisition for a scientific microphone setup. In particular, for this application, knowledge of the frequency response of the system and accurate frequency characterization of the electronic noise is important.
So, I started taking a look at the white noise on an empty recording with the DAC+ADC Pro, using, in particular:
"arecord --format=S32_LE --rate=192000 --channels=1 --duration=15 background.wav"
After that, I first analyzed the collected data to measure the ADC dynamic range (which I found around -118 dB) and the ENOB (about 18.5 bits). Then, with MATLAB, I performed some spectral analysis, and found a cutoff at 22 kHz and higher frequencies, although I set the acquisition sample rate at 192 KHz. Here a spectrogram of the 15s of data collected above. 
Looking around in alsamixer I found nothing that could pinpoint me to a frequency cutoff. I thought the DSP profile might have an influence, if a lowpass FIR or IIR filter for human audio applications is implemented, so I tried a few different profiles, with no result.
Could you help me with some understanding, and perhaps suggesting me how to remove (if possible) this 22 kHz cutoff to get the full 192 ksamples / 2 = 96 KHz bandwidth of the ADC?
Thanks
