On 15.11.2010 23:06, Raphael Raccuia wrote:

Le 15. 11. 10 22:21, Pierre Massat a écrit :

Hi,
I have installed Ubuntu 10.04 LTS, plus some ubuntu studio stuff (the audio packages and the plugins). I also tried the rt-kernel. It didn't work.
But i am amazed, blown away, baffled, etc. Because the generic kernel does have some crazy rt capabilities indeed. I guess the ubuntu studio packages must have created the audio group and jack must have written the proper things in the limit file, but still, it works surprisingly well. Jack can run in rt with a latency as low 4 ms without any xruns, although it crashes.It works just fine at 5.33 ms. Even pd itself work with an extremely low latency (I'd say below 7 ms), I'm assuming that's because it was configured to run in rt during the install.
I don't even know if i need JACK anymore.

jack don't add latency, and it's a powerfull sound server... you can connect pd to the outputs of your sound device and/or other sound/midi softwares, then you can save a patchbay to recover your patch... it's one of the most interesting stuffs on linux.

it does add latency in most cases. you specify the amount of latency by using different period sizes and numbers of periods settings. But you can use very small buffers on some systems with good audio hardware, so they have the same size you would use inside pd in standalone (64samples). In this case you will not have more latency using jack than using pd standalone. (I'm also only 99.9% sure about this, beware.).

"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