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

[Lee Revell <rlrevell@joe-job.com>]

On Tue, 2005-08-09 at 16:49 -0400, Paul Davis wrote:
> On Tue, 2005-08-09 at 22:17 +0000, Salvatore Di Pietro wrote:
> 
> > Moreover, using jack_bufsize, or resizing buffer within Ardour, causes 
> > complete muting of output sound (bit not clients death, as it was 
> > before), until I restart jack.
> > Notice that this "complete muting" only affects the output ports, i.e. I 
> > can continue recording with qarecord, or ardour, I just cannot hear (and 
> > monitor) anything anymore until I restart jack and its clients.
> 
> 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.

Does this sound reasonable?

Lee



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