I just started playback of a flash video inside Firefox over the UC as system-output and playback was broken again (not shrieking, but scrambled like reported before in another thread).
Ableton Live was still open in the background (set at 176.4 kHz) and when I started playback of a single audio track the output was scrambled, too.
After changing the sample-rate via Live Firefox remained silent even after closing Live. Changing system-output to onboard or the FF400 works, but changing it back to the UC still results in silence. I manually set sample-rate back to 176.4 kHz via Fireface settings and Firefox plays back again, but still scrambled.
General system-sounds work with different sample-rate, but scrambled. Restarting Live to check also results in scrambled sound.
1. Sending a sinusoid tone through the now scrambled driver shows that the scrambling itself happens at a fixed frequency. This frequency of the scrambling depends both on the sample-rate and buffer size being set. The frequency is mostly rather low (considerably below 40 Hz I'd say), but some combinations of buffer/sr produce a scramble tone well in the audible range over 50 Hz.
2. Setting the buffer size in Live to over 1000 samples results in clean and unscrambled playback at single and double speed (but not at quad speed)! But: Scrambling start with CPU load being much lower than the UC would usually work with, like 30% at 44.1 kHz with the simple sinusoid test-tone of Live whereas usually you could pull it all the way upto 80% (maximum Live offers as test-load).
3. Connecting a Kore 1 Audio USB controller reveals that in contrast to my former reports the Kore's playback is *not* scrambled this time. Even at 64 samples it allows more CPU load than the "broken" UC driver at 1024 samples. It has problems going all the way up to 80% though (even at 128 samples).
I will restart the computer now to reset the driver an measure the performance of the Kore again in order check whether it has been affected only slightly or not at all.
4. The breakup/scrambling of the driver seemed to happen "without a cause". Even though there has been the shrieking before, after my last fix of the shrieking I did nothing other than writing some posts on some forum. Live and AU Lab were left alone in the background and no system-sounds were played back.
But: I know that OS X turns off the speakers of the MBP 30 seconds after the last system-sound has been played back. Maybe when the UC is set as system-sound output it also is turned off (or at least OS X is trying)?! Maybe that lead to the problem?