[linux-audio-user] jack latency & MIDI latency

Patrick Shirkey pshirkey at boosthardware.com
Tue Sep 30 14:36:00 EDT 2003

Antonio wrote:
>  From a few days I'm trying to use my pc as a synth to improve the 
> sounds of my keyboard.
> I've installed low-latency kernel, alsa, jack, muse, rosergarden... etc 
> from planet ccrma.
> I can't start jack with the period (-p) under 512 frames (with my SB 
> Audigy) and a number of periods (-n) under 2. That mean that I have a 
> latency of at least 23.2 msec (from qjackctl).
> Does this latency affects the sound I hear when I play my MIDI keyboard 
> with a synth (for example muse)?
> Does anyone knows if the Audigy (player) can go under 512 of the -p 
> parametre in jack?
> With windows & cubase VST I can get a latency of about 10 ms that it's 
> good for my purpose, while 20 msec
> Can I play directly in to alsa to reduce the latency (jump the jack 
> latency)?
> I think that the only way to get the so claimed 2.9 msec latency is to 
> reduce to 64 the numbers of frames, but I don't know if is it possible 
> with my sound card and if my actual limit of 512 is due misconfiguration 
> or is the real limit for the Audigy.
> Anyone want correct, suggest, help me is welcome.

The problem is mostly the audigy but also that jack is not designed for 
badly designed hardware like usb audio devices. There are some 
workarounds to allow for better latency which are in process at the 
moment. Currently they are in the TODO list and not being worked on IIUC.

If we are lucky Jack O'Quin will whip it out in the near future but 
don't hold your breath. (JACK developers are notoriously picky about 
what they spend there time on and workarounds for poorly designed 
hardware are not on the top of the list)

USB devices require a period size > 2 and provide fixed sized packets 
which basically fly in the face of good audio design. It seems that the 
audio companies are lying when they promote usb devices as being able to 
accomplish professional low latency. Probably 2.9msec is never going to 
be possible with usb hardware.

You may be able to get better results with alsa native but then you will 
not be able to take advantage of jacks routing possibilities.

With current JACK I cannot get good results from my usb quattro unless I 
use a period size of 1024 and can only use two i/os. It's pretty 
frustrating :/

Patrick Shirkey - Boost Hardware Ltd.
Http://www.djcj.org - The Linux Audio Users guide

Being on stage with the band in front of crowds shouting, "Get off! No! 
We want normal music!", I think that was more like acting than anything 
I've ever done.

Goldie, 8 Nov, 2002
The Scotsman

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