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
