[linux-audio-user] Again on sudden and occasional Jack crackling sound

[Lee Revell]

On Tue, 2005-08-09 at 22:12 -0400, Lee Revell wrote:
> On Tue, 2005-08-09 at 16:49 -0400, Paul Davis wrote: 
> > as far as we can tell, this is an ALSA (kernel) driver issue. it does
> > not affect other backends, and does not affect (we think) every ALSA
> > supported piece of h/w. it would be nice for it to be either fixed or at
> > least tracked down conclusively.
> > 
> 
> OK, here's an idea.  Say there was an off-by-one error or some other bug
> in the ALSA middle layer that caused the reported value of the hardware
> pointer to be off by one.  So snd_pcm_avail_update would report 64
> frames available while there were actually 63.  If JACK gets scheduled
> within the space of one frame (quite possible especially with the RT
> kernel, it's a ~20us window at 48KHz) then we could overwrite the wrong
> part of the buffer - a frame or two ahead of the hardware pointer.
> AFAICT this would introduce a crackle, but no xruns would be reported.

Hmm, I think I might be on to something.  This sounds like the exact
same bug, but they can produce it without JACK:

http://www.winehq.org/hypermail/wine-patches/2005/08/0173.html

The WINE developer had the same idea:

      //* /*FIXME*/: snd_pcm_mmap_hw_ptr() should not be accessed by a user app. *//
      //*        It will NOT return what why want anyway. *//
      hw_ptr = _snd_pcm_mmap_hw_ptr(wwo->pcm);

  I thought, maybe it is not returning the correct information for 
  GetPosition, so that is why the crackling exists, since new samples are 
  off-position from what they should be in order to guarantee a smooth 
  sound

So in both cases we're operating in mmap mode and ALSA is returning a
slightly incorrect hardware pointer which leads to crackling.

Lee