[NOT SOLVED, I just found out the problem]
It's beyond my understanding, it didn't crossed my mind that this wireless keyboard could interfere with another USB device and cause such a problem. I used it in the last two years on my Rpi3 without any problems and when I bought the Rpi4 I've just hooked it in an USB port and left it there.
A few minutes ago I said to try one more time the RPi4 from the ground, without HifiBerry Digi+ Pro attached and I…
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I just bought a Raspberry Pi 4 8GB and I installed LibreELEC 9.2.
I configured the audio output device via USB (shown as ALSA: DigiHug USB Audio, Analog) because I use an external USB DAC. The problem is that all audio files are resampled at 48kHz, no matter they are 44.1/16, 96/24 or 192/24. "Output configuration" is set to "Best match"and "Limit sampling rate" at 192kHz (maximum sample rate supported by my DAC). I tried also with ALSA: DigiHug USB Audio, S/PDIF as audio output device, with same result, the audio files are resampled at 48kHz.
With LibreELEC 10 Beta I don't have audio output at all over DigiHug USB Audio S/PDIF.
On my Raspberry Pi 3B+ I don't have this problem, the audio files are sent at their original sample rate to the DAC with the same settings ("Best match" and limit sample rate 192kHz).
I know that if I use "Best match" at output configuration and the software volume in Kodi is set 100%, the audio output should be passed unaltered, bit perfect, and this is my goal.
Does anyone knows what is happening?