How use PulseAudio and JACK?

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How use PulseAudio and JACK?

Many Linux systems now use PulseAudio as the default sound server, using it to handle all sound playback (media players, desktop alerts, web browsers and more). Here are a couple of basic observations about using JACK and PulseAudio together:

While PulseAudio is running, JACK cannot access the same soundcard that Pulse is using. Even though in theory ALSA provides mechanisms for this to be possible (e.g. “dmix” device access), they do not work well enough to support both of these systems reliably.

PulseAudio and JACK can appear to have similar goals to many people, and they wonder why its not possible to replace one with the other. However, beyond a very superficial similarity, they really do not have much in common:

PulseAudio is focused on desktop and mobile audio needs. It doesn’t try to address low latency usage, but does provide seamless device switching, network routing, global per-application volume control and lots more great stuff.

JACK is focused on the needs of pro-audio and music creation users. It offers the lowest possible latency, complete routing flexibility between applications and audio hardware, and all audio is always sample synchronized - apps don’t run ahead or behind of others. It doesn’t provide the smooth desktop experience that PulseAudio is aiming at.

Combining PulseAudio and JACK on the same machine can be problematic. There are several options, some of which leave PulseAudio and JACK as entirely separate systems with no audio flow between them. Others connect them so that audio from one of them can be heard via the other.

Option 1: don’t use PulseAudio with JACK

The most experienced and demanding users of JACK do not attempt to link PulseAudio and JACK. Many of them will not run PulseAudio at all, having either never installed it, removed it from their systems, or disabled it. They will generally route audio from other apps to JACK without using PulseAudio, via the techniques described in the “How do I route audio to/from …” section of the FAQ.

Option 2: use two different soundcards

Many users of JACK have computer systems with more than 1 soundcard. Typically there is the builtin soundcard, which is a cheap consumer grade device, often with a variety of limitations that make it less suitable for use with JACK. The user has subsequently added another audio device, either as an internal (PCI) card or an external unit connected via FireWire or USB.

Two ALSA-supported soundcards

All that is necessary in this case is to define the device that PulseAudio and JACK will use. Run the following command in a terminal window: cat /proc/asound/cards You will some output like this:

 0 [SB             ]: HDA-Intel - HDA ATI SB

                      HDA ATI SB at 0xfcef8000 irq 16

 1 [DSP            ]: H-DSP - Hammerfall DSP

                      RME Hammerfall DSP + Digiface at 0xfcff0000, irq 20

 2 [NVidia         ]: HDA-Intel - HDA NVidia

                      HDA NVidia at 0xfe57c000 irq 32

(This is from a machine with 3 soundcards: a builtin device, an additional 26 channel RME Hammerfall DSP device, and (unintentionally) an NVidia video adapter that also has an HDMI audio output.)

Each of these cards can be accessed using its name, which is contained within the square brackets []. Generally, the name will simply be the conjunction of “hw:” and the card name. So for example, "hw:DSP" refers to the RME Hammerfall DSP in the above list, and "hw:SB" refers to the builtin soundcard. The names in your system will vary.

Telling JACK which device to use

To tell JACK to use the RME Hammerfall DSP, you would typically do this either via the upper right of QJackctl’s setup dialog:

screenshot of qjackctl’s setup dialog

or on the command line, by passing -d hw:DSP to JACK’s ALSA backend.

Telling PulseAudio which device to use

Option 3: route PulseAudio to JACK while JACK is running

Please see this wikipage.

Option 4: suspend PulseAudio while JACK is running

If you start JACK using QJackctl, then you should go into its setup dialog and edit the field labelled “Server path”. Insert pasuspsender -- in front of whatever is there (probably just something like jackd). So for example, if you had “jackd” in the “Server path” field already, it should now say:

pasuspender -- jackd

This will cause PulseAudio to suspend itself while JACK runs, and restart when JACK exits (or crashes).

If you start JACK any other way, you will need to figure out how to make your method use pasuspender -- as the command that actually starts JACK.