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

From: Ralf Mardorf <ralf.mardorf@email-addr-hidden-dsl.net>
Date: Thu Apr 29 2010 - 12:17:31 EEST

michael noble wrote:
> So ok, I was able to confirm by having someone try it out for me (not
> on my linux machine right now) that Tim and of course Paul, you are
> both correct in that a JACK client in a send/return loop adds
> additional latency. So now I'm left with the obvious question of "why?".
>
> What is the difference, in terms of latency, between the following
> signal chains in JACK?:
>
>
> 1) A/D -> A -> B -> A -> D/A
>
> and
>
> 2) A/D -> A[B as plugin] -> -> D/A
>
> and
>
> 3) A/D -> A -> B -> C -> D/A
>
> Why is there additional latency added to client B in 1) but not 3),
> especially in the case where C could be another instance of A?
>
> And also, could the following section the JACK faq perhaps be altered
> to provide some clarification here:
>
>
> Doesn't use JACK add latency?
>
> There is /NO/ extra latency caused by using JACK for audio input
> and output. When we say none, we mean absolutely zero. The only
> impact of using JACK is a slight increase in the amount of work
> done by the CPU to process a given chunk of audio, which means
> that in theory you could not get 100% of the processing power that
> you might get it if your application(s) used ALSA or CoreAudio
> directly. However, given that the difference is less than 1%, and
> that your system will be unstable before you get close to 80% of
> the theoretical processing power, the effect is completely
> disregardable.
>
>
> I understand that this explicitly says "using JACK for audio input and
> output", but the question itself is a little broader than that. To me
> it implies that the entire JACK graph adds no additional latency. If I
> can be pointed to a clarified explanation of the matter I'd happily
> provide some documentation if no one else has the time or inclination.
>
> regards
>
> michael

I also wonder why JACK's zero-copy implementation could cause issues for
audio apps. There was an issue for Qtractor, anyway, until today I
believed that there is no latency when using JACK in any way, excluded
connections regarding to the sound card.
I never noticed a latency, but maybe it's because I used this insert
connections for time related FX only, e.g. flanger and delay.

Interesting to read about latency I never was able to hear.

Ralf
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Received on Thu Apr 29 20:15:02 2010

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