In this package of jackd, the 'dummy' and 'sndio' drivers are supported.
The 'dummy' driver is mostly useful for testing jackd itself. The
'sndio' driver uses sndio(7). To start jackd with default driver
parameters, use the command 'jackd -d <driver>'.
By default, jackd runs in full-duplex mode with a relatively low
latency. This may not be exactly what you want, and it may not be
possible with all hardware. Therefore, some configuration options may
be necessary to effectively use jackd. The 'sndio' driver options can
be viewed with the command 'jackd -d sndio -h'. Below are some tips
and caveats to consider when using jackd on OpenBSD.
Some audio devices do not support full-duplex operation. If jackd
errors out on startup with a message about opening a full-duplex
device, this is likely the problem. If the device only supports
playback, then the driver needs to be configured to use 0 input
channels. For example 'jackd -d sndio -i 0' starts jackd in
half-duplex play-only mode.
By default jackd uses 2 periods (blocks) of 1024 frames each. This
means there will be at most 2048 frames in the playback buffer. Some
slower or loaded machines may not be able to keep the playback buffer
from underrunning with these conditions. Jackd is designed to use 2
periods, so if underruns are observed, it is best to first increase
the period size with the driver's -p option. The period size should
be a power of two. The number of periods can be increased with the
-n option. For example, 'jackd -d sndio -p 2048 -n 4'.
The sndio driver uses 44100 Hz sampling rates by default. Some
devices may not support the default sample rate. Or it may be that
most of the audio that will pass through jackd is based on a
different sample rate. Avoiding sample rate conversion will use
fewer CPU cycles and perserve audio quality. The -r option can be
used to change the sampling rate as in 'jackd -d sndio -r 48000'.
The realtime option -R is commonly used on other operating systems.
However, this option is inconsequential on OpenBSD.
It is possible to run jackd on top of aucat(1) with the 'sndio'
driver. However, the aucat server must be running at a higher than
default scheduling priority. The aucat(1) manual has an example for
doing this with renice(8). Note that jackd will use aucat's block
size as it's period size. The following command sequence could be
used to run jackd over aucat.
$ aucat -l -b 1024 -z 1024
$ sudo renice -20 `pgrep -x aucat`
$ jackd -d sndio -p 1024 -n 2
There is currently no MIDI driver in jack that works on OpenBSD.
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