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Digital distortion using loopback module - Raspberry Pi
Digital distortion using loopback module - Raspberry Pi
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Old 9th April 2020, 07:42 PM   #1
DarpMalone is offline DarpMalone  Canada
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Digital distortion using loopback module - Raspberry Pi
Default Digital distortion using loopback module - Raspberry Pi

Hi all,

I'm running PulseAudio Crossover Rack software on Raspian Buster & I'm experriencing horrible digital distortion when I loopback audio in PulseAudio from source back to a sink - but only when using samplerate's over 88.2k.

This happens whether I'm looping back from the input from my HiFiBerry DAC+ ADC / from a cheap USB Soundblaster input or a S/PDIF input... The strange thing is though that it plays perfectly for a few seconds, then the distortion kicks in. It's not a digital saturation type of sound but more like R2D2 "bleep-blrrrp-bdleep" almost as though something's falling out of sync after 30s of playback.

I initially thought that it was because I'm running out of CPU but this happens whether I'm using the Pi2B+, 3B or 4. Even the 2B manages loopback @ 88.2K albeit with a few minor glitches here and there.... This is not a huge problem obviously but more of a nuisance.

I'm puzzled because even the Pi3 plays 192k flawlessly with tonnes of headroom whether playing local audio/video sources, streaming DLNA sources, Network streams using PA-Prefs, Bluetooth Audio sources from my phone or laptop. It only falls apart when monitoring from a loopback source, and only when over 88.2k.

Does anyone know why this occurs?

Thanks,
Darp

Last edited by DarpMalone; 9th April 2020 at 07:45 PM.
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Old 10th April 2020, 07:03 PM   #2
xorcz is offline xorcz  Czech Republic
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Maybe it should be in the paxor thread? I use the rack (paxor) with LR4 filters for a 3-way and on Pi3 had to go down to 48k sampling rate. Later I moved paxor to J4105 board, which can maintain 192kHz. I found out sound degradation (starts with crackling to stuttering) happens if there is >80% load on the CPU. The network (I do not use wlan) steaming is demanding. I also used pulseaudio native tunnel. But even on recent Ryzen 5 i can get crackling if there are more (even local, so no network) sources of stream (typically I play audacious and have youtube in firefox - multiple).
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