[HELP] Choppy sound at Background and Playback with TDM400P


I have set up a system with an ISDN AVM C4 (4xBRI P2P) and a Digium Wildcard TDM400P (4xFXS speak 4 internal phones). Asterisk is at 1.2.4, zaptel at 1.2.3 and chan_capi at 0.6.3. The kernel is 2.6.13 (SuSE OSS 10.0)

When Asterisk plays voice-prompts through either Background or Playback, the sound sometimes is interrupted, sometimes accellerated sometimes small parts are missing. This is regardless of the channel that is used (SIP, ISDN, Zap) to play back the voice-prompt. The disturbances occur at different point im times if the same voice-prompt is listened to multiple times.

During regular conversations (regardless which channels) or listening to Music-On-Hold, this does NOT happen. MOH is configured to use the format_mp3.

The system load is usually around 0.0 - 0.6 % even with 6 simultaneous calls. This is an AMD64 with 1 GB RAM. So load is not an issue.

I have tracked down the problem to the zaptel driver being the issue. If I unload the zaptel drivers, the problem goes away. The moment the zaptel drivers are loaded, the problem comes back; even if chan_zap is not loaded within asterisk.

Has anyone an idea what the issue could be? Anyone seen similiar behaviours?

With best regards,