broken sound from realtek ALC888 using phonon

[Stefano Crocco]

This issue started some months ago. Unfortunately, being quite busy at the time, I couldn't investigate it properly, and now I don't know exactly when it was and whether I just upgraded something on my system.

The problem is the following: usually my sound card (realtek ALC888) works correctly, but every now and then, it produces a "broken" sound. To be more precise, it seems to linger on the beginning of the sound, endlessly repeating it. If this description isn't clear, I uploaded an example of one of such broken sounds, together with the same sound reproduced correctly (for those using KDE 4, it's the sound used by phonon when you press the "Test" button in SystemSettings). Here's the broken one, and here's the correct one (I've only recorded the first three seconds of both).

When sound gets corrupted this way, it stays so until I reboot the system (logging out and in again doesn't restore it). When this started to happen, I thought there was something at boot time which prevented ALSA from working correctly because it seemed that this issue would only happen when I started the system: if sound worked correctly then, it would go on correctly until I next rebooted. Later, I started noticing sound stopping working correctly in the middle of a session, so I began thinking that the cause could be another one. I use KDE 4 as my desktop manager, with the default xine backend for phonon, so I tried downgrading xinelib, switching from 1.1.18.1 to 1.1.17, but that didn't help. I also tried the experimental VLC backend for phonon but the sound corruption remained (actually, there was a small improvement with the VLC backend: with the xine backend, when the sound corruption started, I couldn't close my KDE session: I clicked the Log Out (or Halt or Reboot) button in KDE, but nothing happened. The only way out was to kill X using the Ctrl+Alt+Backspace key sequence. With the VLC backend, I can close the session normally).

I don't know what to do to solve this issue. I tried changing the kernel version (currently I'm using gentoo-sources 2.6.34-r1, but the issue happened also with at least version 2.6.33) and checking the kernel settings, but everything seems all right. Looking through the threads in this forum didn't reveal anything which could be related to this, except this thread, which lists several things to check when having sound problems. I made the tests it suggests but couldn't find anything strange on my system.

I'm not a sound expert (actually, in about seven years using linux this is the first time I have troubles with it) and don't know what else to check. I tried looking at dmesg and at the files in /var/log for hints but I couldn't find anything.

In case it can help diagnosticating the problem, I list here the sound-related kernel modules I've loaded:

• snd_seq
  
• snd_seq_device
  
• snd_hda_codec_realtek
  
• snd_hda_intel
  
• snd_hda_codec
  
• snd_pcm
  
• snd_timer
  
• snd
  
• snd_page_alloc
  

I'll be grateful for any hint on how to solve (or at least investigate) this issue. If you need more information, please let me know and I'll provide them.

Thanks in advance
_________________
Stefano