[LAD] JACK Graph Internal Latency? (was Re: A small article ...)

[michael noble]

hi all,

On Thu, Apr 29, 2010 at 7:05 AM, Tim E. Real <termtech at rogers.com> wrote:

>
>
> Just a request: Would be awesome as a DSSI plugin so that we
>  don't have to use rakarrack in a 'send and return' loop within our apps,
>  which causes latency due to the round trip...
> Tim.
>
>
This raises a question that I've had for a while regarding latency in the
JACK graph. I may have even asked it in the past, but if I did I either
wasn't satisfied with the answer or have forgotten it completely. Either
way, I'm unable to find it in the archives.

My understanding is that connections between applications in the JACK graph
should add absolutely no additional latency to the signal path. Latency is
of course introduced at the A/D and D/A conversion stage of the graph, which
is to be expected. This is the latency figure that jack reports - the
latency introduced at every A/D or D/A stage. The only additional latency
introduced is by processing internal to any applications, for example by the
use of sample blocks as in the case of convolution.

Am I correct in this? If so, then I don't understand Tim's statement above,
as there should be no difference, in terms of latency, between using
Rakarrack as a plugin (if it were possible) within an application, or
placing it in an external send/return loop in the JACK graph.

If my assumption is not correct, then I'm actually highly confused as to
what the value of JACK is at all. If latency were introduced at every
additional JACK signal routing, then even a simple routing like the
following:

A/D -> Rakarrack -> Jackrack -> Ardour -> D/A

would have no less than four multiples of the internal JACK latency. This
would quickly become unworkable in more complex JACK graphs (for example
asymmetrical graphs would have signal chains running with different internal
latencies). This would make having application interconnects a pointless
exercise in frustration for the most part. And actually, from experience,
this is not what seems to happen at all.

Feel free to correct my admittedly limited technical understand of how
latency is handled internally to the JACK graph...

-michael
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