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

Ralf Mardorf ralf.mardorf at alice-dsl.net
Thu Apr 29 09:17:31 UTC 2010

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.


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