[alsa-devel] VT1708 clicks & pops

[Takashi Iwai]

At Tue, 20 Sep 2011 15:05:34 -0400,
Forest Bond wrote:
> 
> Hi Takashi,
> 
> Thanks for your response.
> 
> On Tue, Sep 20, 2011 at 09:22:23AM +0200, Takashi Iwai wrote:
> > At Mon, 19 Sep 2011 19:14:13 -0400,
> > Forest Bond wrote:
> > > 
> > > On Fri, Sep 16, 2011 at 04:36:01PM -0400, Forest Bond wrote:
> > > > I have two boards with a VT1708:
> > > > 
> > > > * VIA EPIA EX15000G
> > > > * VIA VB8002
> > > > 
> > > > I'm using VLC to play audio via PulseAudio to the analog outputs.  The VB8002
> > > > sounds great, but the EX15000G has random pops and clicks in the output.  Both
> > > > machines are running exactly the same configuration (they are using the same
> > > > pre-built OS image).
> > > 
> > > Maybe this issue would be more approachable if I could produce it without
> > > PulseAudio involved?
> > 
> > Yes.
> 
> Heh, I guess I'm probably not going to be able to do that very easily.
> 
> > > Incidentally, these boards both produce a crackling/static sound when PulseAudio
> > > is playing and the PCM mixer is turned all the way down.  I have seen scattered
> > > reports of this in various bug trackers.  The problem does not occur when
> > > playing via alsa directly.  One thing I have noticed is that the crackling noise
> > > follows the rhythm of the music being played.  It actually sounds like the music
> > > has been heavily distorted (i.e. via extreme digital clipping) and then
> > > attenuated significantly.
> > > 
> > > It seems likely that there are driver issues that PulseAudio is exposing.  What
> > > can I do to track them down?
> > 
> > Is the problem related with the driver version?  You mentioned the
> > difference of drivers installed on your systems.
> > If it's a regression of the driver, it'd be easier to figure out.
> 
> No, the issue occurs with both driver versions.
> 
> > Other than that, I'd try to adjust PA, e.g. try tsched=0.  Usually
> > this kind of problem is because of inaccurate playback position
> > reporting from the hardware.  PA relies (too much) on the playback
> > position as default.
> 
> Using tsched=0 fixes the pops and clicks on the EX15000G.  Any idea why I might
> see problems on one board but not the other given how similar they are?

Do they have the same HD-audio controller chip (VIA one)?
Usually it's an issue of controller chip, not codec chips, so I wonder
if they behave differently even with a same controller.

> The crackling with PCM volume set to 0 persists with tsched=0.

Hrm, which PA version are you using?  I remember vaguely a buggy PA
SIMD operations in some old PA versions.

Also, when you mute the mixer (nor the PA's mixer), e.g. via
"alsamixer -c0", the noise goes away, right?

>  It seems
> unlikely that this isn't driver or hardware issue.  But fixing the pops and
> clicks without using tsched=0 is more important to me.  If you can provide
> suggestions as to where I might start looking, I would be happy to try and come
> up with a fix.

As mentioned, the usual problem is the inaccurate DMA position
reporting, especially when tsched=0 helped.  It means a problem of
HD-audio controller.  We have already many workarounds in the driver
code, but more investigations are needed in such a case...


Takashi