I just bought an Alesis Core 1. (USB audio interface.) A few times I have returned to the computer and found that the sound output has become heavily! distorted / metallic / crackling. There is silence (as it should be) when no sound is playing. After unplugging the usb cable and plugging it back in, the sound is OK. This is PulseAudio 4.0 on Ubuntu Studio 14.04.
Actually, this happens even when I'm using the computer, but then the crackle/distortion is lighter. So maybe it isn't related to screen dpms.
Intuitively, this may be a timing issue, where the timer of the soundcard and pulseaudio are drifting apart. Just guessing.
I don't think it's a clock drift issue. Pulseaudio runs at the sound card rate. When the system clock is used, the usage is adapted to any differences compared to the sound card clock. The audio may become corrupted if pulseaudio writes too close to the point in memory where the hardware is reading. (An aside: I don't understand the mechanism of that - I would understand a single glitch, but I don't understand how the audio can become permanently corrupted, and the resulting distortion is weird.) I don't know if you have this problem, but "metallic" sounds similar to what I've experienced on an internal HDA card long time ago. When this happens, you could record from the sink's monitor source to verify that the recorded audio is clean. If it's clean, the distortion happens in lower layers of the audio stack. Here's the command to use (all on one line): parecord --device=alsa_output.usb-Alesis_Core_1-00-C1.analog-stereo.monitor --fix-rate --fix-format --fix-channels > test.wav (I got the device name from the "pactl list" output in your other bug, 99112.)
I got lots of light-to-medium crackles (sounds like buffer underruns) from html5 playback in Chrome. I captured the sound using the command line you gave me, except it had to be parec. The sound on the recording was fine.
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