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.
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.
So, it's an interesting idea and I also thought of that. But, the implementation isn't that straightforward.
BTW, regarding the problem of trigger(STOP) timing: I think the proper solution is to change the sequence in ISR. The current code flow in ISR for a "queue-up style" hardware is like: - update h/w and queue the next chunk - call snd_pcm_period_elapsed(): in snd_pcm_period_elapsed, - calling pointer callback - update hw_ptr - call trigger(STOP) if needed
Actually, the trigger(STOP) condition should be checked before queuing the next chunk so that the trigger won't be called after the new chunk. Not sure which change would be most feasible, but there should be a room to improve the code flow there.
Takashi