Re: [LAD] JACK and ALSA elucidations

On 03/12/2014 04:21 AM, Gianfranco Ceccolini wrote:
>
> Em 11/03/2014, à(s) 23:50, Paul Davis <paul@email-addr-hidden> escreveu:
>
>>
>>
>>
>> On Tue, Mar 11, 2014 at 10:26 PM, Gianfranco Ceccolini <gianfranco@email-addr-hiddentalmod.com.br> wrote:
>>>
>>> On Mon, Mar 10, 2014 at 9:51 AM, Gianfranco Ceccolini <gianfranco@email-addr-hiddentalmod.com.br> 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)

Hi Gianfranco,

Seems like you're mixing up number of frames (JACK -p option) and number
of periods (JACK -n option). -p128 means 128 frames per period while the
ALSA message normally refers to the -n setting (so 16 equals to -n16 in
your case).

>>
>>
>>>
>>> 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?

Not sure but I think 128*16=2048 frames are buffered in your case.

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

Probably not because the BBB audio cape driver apparently doesn't do -n2
but switches to -n16.

But I could be wrong and I'll risk some spanking ;)

Bye,

Jeremy

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