Em 11/03/2014, à(s) 23:50, Paul Davis <[email protected]> escreveu:
> > > > On Tue, Mar 11, 2014 at 10:26 PM, Gianfranco Ceccolini > <[email protected]> wrote: >> >> On Mon, Mar 10, 2014 at 9:51 AM, Gianfranco Ceccolini >> <[email protected]> wrote: >> >> Paul. I’m really sorry for my lack of comprehension, but I still don’t get >> it. Is there a relation between the ALSA period with JACK’s period? >> >> they are identical. > How can that be? JACK says -p 128 samples and ALSA says 2 on the PC and 16 on > the BBB > > How does ALSA "say" anything? Sorry. I mean, JACK says ALSA uses that period size (ALSA: use 16 periods for capture) > > >> >> note: other audio APIs call this a "buffer". some, like ASIO, force a double >> "buffered" model where the total size of the hardware memory area used for >> transfers (what ALSA calls the "hardware buffer") is always twice the >> "buffer size". >> >> ALSA offers control over *both* the "period" (between interrupts) and the >> total hardware buffer size. This is unusual among audio APIs, except that >> JACK follows the same convention in its ALSA backend. You can specify the >> period size (-p) and the number of "periods" that make up the hardware >> buffer. > So if I use -p128 and -n2 it means the kernel will buffer 256 samples per > interrupt, is it? > > No. What it means depends on the precise details of the device. In an ideal > world, those settings mean that the DEVICE will buffer 128 samples before > issuing an interrupt, and that its total buffer size is 256. > > However, USB devices etc. do not work this way, so it translates more > generically to "ALSA will wake the client up whenever at least 128 samples of > data (and/or space) are available". > > The -n 2 parameter only affects the total amount of hardware buffering, not > the interval ("period") between when the application is woken. > And where does the 16 (from ALSA: use 16 periods for capture) affects? if I call jack using -p128 / -n2 and the opening message states that ALSA uses 16 periods for capture the sound card will buffer a total of 256 samples and ALSA will wake the client each 16 frames? If I used -p256 / -n2 instead ALSA would wake the client each 32 frames?
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