[LAD] JACK and ALSA elucidations

Paul Davis paul at linuxaudiosystems.com
Wed Mar 12 12:12:38 UTC 2014

On Tue, Mar 11, 2014 at 11:21 PM, Gianfranco Ceccolini <
gianfranco at portalmod.com.br> wrote:

> And where does the 16 (from ALSA: use 16 periods for capture) affects?

As Jeremy noted ... JACK asks to use "N periods, or the nearest number of
periods it can". You apparently have a device that can only do 16 periods,
so when JACK asks for 2, it gets 16.

> 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?

no, not at all. the total buffer size is always ACTUAL periods * period
size. So as Jeremy said, it will buffer 2048 samples. During each traversal
of that buffer, ALSA will wake the client 16 times, every 128 frames.

> If I used -p256 / -n2 instead ALSA would wake the client each 32 frames?

I have no idea why you think that. As I tried to explain, the period size
(256) is the interval between when ALSA wakes the client.
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