23 Feb
2016
23 Feb
'16
6:40 p.m.
I do understand that JACK is designed to be
completely
synchronous. But a good pipelined architecture, can take multiple
synchronous processing chains running independently
(asynchronously to each other), and then merge them at the output
end.
That implies (a) extra CPU load (b) possible loss of quality.
Yup. No risk and no
resources, no gain :-) But given what I have (and
what is within reach with a stack of RPis), it seems well worth it to try.
What I want to do is the equivalent using JACK at the synchronous
level, so that I take advantage of more of my computing power.
Eventually I will want to do exactly the same using four or five
RPi-compatibles, with just one of them having the audio output;
Doing this correctly would imply using a single word clock distributed
to all your R-Pi's. But I doubt that you're going to do that. Instead,
you're going to try to distribute the "JACK clock".
You have two choices:
(1) try to use one of the implementations of netjack, which
effectively distributes the "JACK clock" across the network
(2) the "other" zita tool, zita-njbridge, which is a client that
sends/receives audio over a (local) network and resamples as
necessary. You will still need to run JACK on each R-Pi and decide
what to use as the clock via your choice of backend. No clock sync is
assumed.
Indeed, that sounds like my starting place. I have to admit that I like
the zita-njbridge approach more, because that way, since I am definitely
resampling before it hits the audio hardware, it is only a very simple
and short chain -- IP inputs to zita-njbridge to hardware -- which has
to be synchronous with the physical audio chipset. So the DSP % usage
on the hardware-connected chain becomes low, because it is as simple as
it is, and each of the chains in action have a lot less also,
distributing the work carefully. If zita-njbridge treats zero input in
the stream as silence, the overall benefit should be enormous :-)
--
Jonathan E. Brickman jeb(a)ponderworthy.com (785)233-9977
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