I am still quite new to Linux in general. I am learning a lot by just trying to use it, and trying not to move back to OSX just to do a simple task. So that this post might be for a person like me, who has struggled a lot on this ‘new’ way of patching sound in and out and configuring in a right way.
I have used Jack before in OSX when I was trying to do a virtual multichannel test, but there was a graphical interface, not at all complicated. There is also ‘system preference’ that OSX offers. When I change the soundcards, or using different inputs/outputs, I didn’t need to do much. But here, it might be a simple thing, but I was in panic.
So that I would like to share my experience if this can help anyone who is like me.

I have two different versions of linux that I am using in different laptop. 11.04/11.10.

Here I am focusing on 11.04, because 11.10 had less issues.

Jack versions and library installed : jackd, libjack-jackd2-dev, jackd2-firewire, jackd2, libjack-jack2-0, libasound2-plugins

First of all, I was at the first moment trying to use ‘qjackctl‘ that offers graphical user interface. But weiredly, the value that I have changed was not applied to the result.

Screenshot of qjackctl setup window

The default sample rate was set to 48000, and I wanted to change it into 44100. Even though I was doing it, it didn’t change.

Asking some people around, I found that pulsa audio can be the cause of issue, so I removed it and tried to use only ‘alsa.’

Jack setup

When I run scvim, I use a bash script in order to reduce any possible complications such as already-running-sclang. I saved it as ‘iscvim’ and fixed chmod to an exacutable, and put it into a path (i.g. /usr/local/bin.).

Here I include jack setup as well.

#!/bin/bash
cd ~
killall jackd
killall sclang
killall scsynth
killall scvim
killall sclangpipe_app
jackd -t 500 -d alsa -d hw:0 -r 44100 -p 1024 -s -S &
sleep 1
scvim

Here I set to use alsa as the driver, and device as hw:0, sample rate: 44100, period 1024, softmode and the last of -S is to cofigure card for 16bit. This configuration can differ from soundcard. I tried to use 32 bit, but my sound card only supports 32 bit-float. But this created problem with sc3, (creating distortion) and I assume that this is because of -float- (it could be supposed to be integer).

Check something like this when you run jack,

ALSA: final selected sample format for capture: 32bit float little-endian

or

ALSA: final selected sample format for capture: 16bit little-endian

In order to know what card you have;

cat /proc/asound/cards

In order to know what other configuration will be possible with your soundcard, use ‘jack_control,’ which shows you all possible options for your setup, and not difficult to use.

For Realtime Scheduling

This is followed by what ‘jack’ website suggests, and it works very well. (http://jackaudio.org/linux_rt_config)

Open /etc/security/limits.d/audio.conf

Then fix as below.

# Provided by the jackd package.

#
# Changes to this file will be preserved.
#
# If you want to enable/disable realtime permissions, run
#
# dpkg-reconfigure -p high jackd
@audio - rtprio 95
@audio - memlock unlimited
#@audio - nice -19

So far, no issue when I am using SCVim, and I run Jack when I need to use it.  Hope this helps.