Short answer:
Click the option "Enable D-Bus interface" in qjackctl. :-)

Long, descriptive answer:
Per default Debian has a pulseaudio config (/etc/pulse/default.pa), that tries to autodetect a running jackd soundserver:

 
### Automatically connect sink and source if JACK server is present
.ifexists module-jackdbus-detect.so
.nofail
load-module module-jackdbus-detect channels=2
.fail
.endif

But for this autodetection to work, jackd needs to be connected to the D-Bus.
http://www.freedesktop.org/wiki/Softwar.../#index9h2

Then in the pulseaudio Volume Control window (pavucontrol) a new "Jack sink" appears, which can be assigned to the "ALSA plug-in [Pianoteq 5].

The final routing is now:

Pianoteq --> Audio device type: ALSA --> Pulse Audio Sound Server --> pulseaudio-module-jack --> jackd (RT) --> soundcard.

cheers

@mwinthrop: So pulseaudio-module-jack is the required layer between PulseAudio -->|--> Jack?

If you want to use Pianoteq and hear sound from Youtube at the same time, then pulseaudio linked to Jack using pulseaudio-module-jack is a way to do this.  Not sure if there is another way.  If Pianoteq connected to ALSA is all you need, then perhaps all is good as it is.  I would again recommend looking at KXStudio.  If you were to decide to install it, it could make things easier.  It did for me when I was new to Linux.

Yes, that's clearer. I just can't find it represented in my real setup. Just for interest I installed pulseaudio, pulseaudio-module-jack and qjackctl on my workstation.

It is no problem to get sound from Pianoteq -->  Audio device type: ALSA --> Pulse Audio Sound Server --> hardware. (I expected that)

But I can't find an option to connect Pulse Audio Sound Server with qjackctl, as you described (neither in pulseaudio/pavucontrol nor in qjackctl). When qjackctl is running (with realtime option of course), the sound-output from Pianoteq via pulseaudio stops working. Maybe the layer "pulseaudio-module-jack" is not working or has to be configured??

Anyway ... it is not really important for me. I just wondered, if someone really has it working this way.

* Additionally pulseaudio-module-jack can be installed: Pianoteq --> Audio device type: JACK --> and then? is audio next routed to Pulse Audio Sound Server?? But I see no other options in qjackctl than without pulseaudio-module-jack? Can someone clarify this?

@mwinthrop: So pulseaudio-module-jack is the required layer between PulseAudio -->|--> Jack?

@delt: That seems to confirm, that 'pulseaudio-module-jack' is just the necessary glue between pulseaudio and jackd.

PS: Youtube of course can talk directly to ALSA and hardware, it is not mandatory to use sound-servers like pulseaudio or others in between.

Jack apps (such as pianoteq) --> jackd --> alsa kernel drivers --> hardware (This is where pianoteq/renoise/ardour/etc. "pro aduio" software sends audio)
Most apps --> pulseaudio --> jackd (This is where desktop apps ie. browser, and so on, send audio)
Alsa/old apps --> pulseaudio --> jackd (this is where all the other/obsolete apps will "normally" send audio by default.)

Not gonna pollute the forum with any more fancy ascii art here, but you get the general idea

Note that a lot of programs have the option to use one, several, or all of the above options. Also, i could get "slightly" lower latency by sending pianoteq directly to the alsa drivers, but that would completely occupy the sound card solely for pianoteq, and no other apps could play sound.

On my main computer i use this simple script to tell pulseaudio to run its signal through jack - it just needs to be run after jack is ready to process requests. It should be all you need.

For more advanced users: to get jack running at system boot time, ie. before any login prompt appears, i use this script, which i wrote for my particular setup, but if you want to use it and need help with it, just pm me and i'll do my best to help

/* EDIT */ Oops, i totally forgot that linux mint, and some "compact" 32bit distributions, for some reason don't include the two very small and necessary files for this to work: they are called "module-jack-sink.so" and "module-jack-source.so". I extrated them from ubuntu studio, 32 and 64 bit. (version 14.04) Here are the packages: 32bit, 64bit -- or if you prefer plain old tarballs, 32bit and 64bit.

On the question of how things are routing, please see my attempt to diagram it below:

                        Pianoteq -->|
                                           |--> Jack --> ALSA --> Soundcard  --> Speakers
Youtube --> PulseAudio -->|

where ALSA is the software that drives your soundcard, which outputs to your speakers.  Note Pianoteq and PulseAudio are both feeding into Jack (it is hard to draw).  Unfortunately, Youtube (I assume through your browser) does not know how to talk to Jack directly, but does recognize PulseAudio.  Jack is a program that is very good at combining sounds coming from different programs, but the programs have to be "Jack aware," which many audio programs are.  What I wrote down is consistent with Barafu's earlier post and is also one of the ways KXStudio can route the sound.

I'll try to sort it out:

* Most simple: Pianoteq --> Audio device type: ALSA --> Direct (audio)hardware. (I use this all day)

* Alternatively: Pianoteq --> Audio device type: ALSA --> Pulse Audio Sound Server --> hardware. (Personally I have no use for this and have not tested the latency).

* Additionally qjackctl/jackd can be installed: Pianoteq --> Audio device type: JACK --> hardware. (jack is a low latency audio server and router).

* Additionally pulseaudio-module-jack can be installed: Pianoteq --> Audio device type: JACK --> and then? is audio next routed to Pulse Audio Sound Server?? But I see no other options in qjackctl than without pulseaudio-module-jack? Can someone clarify this?

Thanks!

One way to do this is documented in detail on wikbooks. Personally i like to have my audio stuff independent of the graphic stuff (what's gotten into linux distros lately??) so i run it in a slightly different way, which also allows any user part of the group "audio" to use Pulse (but not jack). Feel free to PM me if you need more help/info.

(yeah, linux audio these days is a mess )

Do you have some positive experience with Pianoteq over "jack over pulseaudio" with "removed Realtime tick"?

Thanks

Lets fix the terminology first. ....

So yes, you can not share one sink between Pulse and JACK. However, you can make JACK to serve as a sink for Pulse. Just as you can not share one sink between Pulse and ALSA, and Pulse serves as a sink for ALSA sources.

So I need both pulseaudio and pulseaudio-module-jack, with jack taking the output of browser/youtube and pianoteq, feeding my (only ) sink, my speakers.

Thanks - I'll have a go at setting that up.

So yes, you can not share one sink between Pulse and JACK. However, you can make JACK to serve as a sink for Pulse. Just as you can not share one sink between Pulse and ALSA, and Pulse serves as a sink for ALSA sources.