On Fri, 22 May 2009, Takashi Iwai wrote: > At Thu, 21 May 2009 14:44:27 -0400, > Jon Smirl wrote: >> >> I've been reading Lennart's article on glitch free audio.... >> >> On my scatter/gather DMA hardware I generate an interrupt at the end >> of each period. On the last period this causes ALSA to issue a >> trigger(STOP). However, there is a latency from when that interrupt is >> generated and when ALSA does the trigger(STOP). During that latency my >> hardware starts playing the next buffer (which contains stale data). >> Playing stale data causes a noise burst. I fixed this by programming >> my DMA hardware to not overrun appl_ptr. >> >> There's another solution. ALSA already knows the stream ends >> mid-period and pads out the partial period with silence. If it simply >> padded out the next period with silence too the overrun problem would >> become non-audible and there would time to process trigger(STOP). >> >> If padding out two periods at end of stream is possible, it then >> becomes possible for me to turn off interrupts in my audio driver. > > This can be another interesting feature. For example, you could add > a soft-mute padding to avoid a click noise at the end instead of a > whole silence. > > But, this kind of hack really depends on the hardware implementation. > If a hardware has no DMA but require explicit writes, adding two > periods is too much. Just a few samples would be enough. I agree to leave such things to lowlevel drivers. > Or, in the case of normal PCI DMA transfer, how can it be stopped? > That is, the stream is still running although it's claimed to be > stopped. If a period size is big (say over one second), the stop at > the next interrupt doesn't sound good. I would suggest to improve drain() logic in kernel with snd_pcm_update_hw_ptr() calls and using proper timeouts for schedule_timeout(). Note that this situation were handled with dead and removed pcm tick API - although it required assistance (setup) from application. Jaroslav ----- Jaroslav Kysela <perex@xxxxxxxx> Linux Kernel Sound Maintainer ALSA Project, Red Hat, Inc. _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
