[pulseaudio-discuss] massive clipping on USB devices when using combined output

[Jost-Philip Matysik]

Hi there!

In another matter I noticed some strange behaviour and am unsure whether
it's a bug or just misconfiguration:

I have 4 soundcards attached to pulseaudio to supply music to the
entire appartement. 

1 onboard Intel HDA configured for analog 7.1 output
2 Terratec USB cards configured for analog stereo out
1 Creative USB card configured for digital stereo out.

Each of the cards seperately works great. It's also no problem to
listen to one source on one card and a second source on another card.

But when redirecting a stream (say: Amarok) to the "simultaneous
output" option and thus activating it on all four cards, I get massive
distortions on all my USB cards!
The audio coming from my internal 7.1 card is still crystal clear, but
the USB outputs suddenly suffer from clipping (resulting in high
frequency noise) whenever bass-intensive audio is played.
High and mid ranges still sound fine, only bass playback is distorted.

When I redirect the stream from simultaneous output back to just one of
the usb devices the distortions immediately vanish.

Reducing the device volume has no effect, but I can eliminate the
distortions on simultaneous output by reducing the application volume
(on the "Playback"-Tab of pavucontrol) to 75%.



I suspect there's a problem with bass redirection overloading the
2.0 outputs for the usb interfaces (since mids and highs are unaffected
and not all my soundcards have a sub, but I have it activated in the
server to utilize the 7.1 setup in the living room).
On the other hand bass output levels are not noticably elevated 
during simultaneous output, so it might just be coincidence.

Has anyone else noticed this behaviour and might even have an
explanation for it?