"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."
but of course, if you just run pd, you don't need it and you can set
latency into pd, I forgot that. I mostly interface pd with other soft
( ardour or any recorder, midi sequencer, some plugin via jack-rack or
calf etc...), and I plug midi controllers, but you can do it in
qjackctl without running jack (alsa tab in connection window)...
Practically speaking - on systems I used until now I could always
achieve lower buffer sizes using pd standalone and thus get lower
round trip delays too.
I couldn't tell by ear which config was faster, Pd alone or Pd plus
JACK.
maybe I'm wrong, but I thing only jack can provide rt...
you can run pd with the -rt flag, without jack. fo me it has proven
to be far more stable at the lowest possible latencies than using
jack. but this depends on your setup too. I don't use jack when I
don't need it.
Martin
as said, it depends also on the setup you are using. assuming you get pd
to run with alsa or oss and a buffer of 256 samples @ 48kHz you have a
minimum delay of 256/48000 s = 5.3 ms. now add a bit for the soundcard's
own buffer for in- and output and you have a delay of slightly more than
5.3ms. This would be the case for a stand alone pd.
Now you use jack and use 256 samples as buffer. This will cause pd to
use the 256 samples buffer too and no latency is introduced (this is
what the jack homepage states too).
However this never worked for me without tweaking my system for hours. I
always had to use a slightly increased buffer, for example 3 * 128 or 2
* 256 samples to run stable. This might depend on the kernel's real time
capabilities (don't know) and also the setting of irq and application
priorities (tried and succeeded). Ideally with a proper configured
system you can use the same buffer size in jack as in pd stand alone.
But since lots of people don't know how to configure it properly you
will almost always end up with an increased buffer size.
Also note, the default setting of jack which works in almost all systems
is 2 * 1024 samples -> the buffer size is 42ms. It's likely any
'realtime' tools uses smaller buffers by default when using oss / alsa.
(for example the dj mixing tool xwax.)
Thes's nothing wrong about using jack, but you will have to spend more
time if you want to make it bulletproof against xruns.
Martin
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