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Degrading calls: connecting Asterisk through JACK with Puredata

Dennis Guse

Sometimes it is necessary to get access to the audio of a VoIP telephony connection (more precisely SIP/RTP). My reason for this is quite simple: for laboratory studies on subjective quality of telephony, I need be able to introduce degradations like noise and speaker echo.

This is a short howto (for Ubuntu 14.04) to let Asterisk run in proxy mode (directmedia=no), route the audio via JACK to Puredata and back. In Puredata then audio can be modified as needed.

  1. Install software:

sudo apt-get install asterisk qjackctl puredata
  1. Setup realtime for group audio (optional)

    If you need realtime for JACK, then enable it.

  2. Configure Asterisk

    Configure one extension (phone number) that:

  3. answers the call
  4. enables JACK for the caller to callee channel,
  5. then dials the callee,
  6. and on answering enables JACK for the callee to caller channel using a post dial macro.

The i(…) and o(…) tell asterisk to connect directly to the defined JACK ports - thus PD should already be running.

extensions = {
    default = {

	["_XX"] = function(c, e)
	    app.dial("SIP/" .. e, nil, "M(jack)")
    ["macro-jack"] = {
	["s"] = function(c, e)

Reload the configuration:

asterisk -rx "core reload"
asterisk -rx "module reload pbx_lua.so"

ATTENTION: If JACK is not running and the asterisk JACK application is thus failing to connect, it fail silently by just emitting a warning and audio is passed on directly.

  1. Start JACK and Puredata

    All applications connecting to a JACK server running as user X need also to run as X. As Asterisk is running as user asterisk simply start JACK and Puredata as user asterisk.

###JACK Start JACK either directly or using qjackctl as user asterisk. If you need realtime, enable it properly.

sudo -u asterisk "jackd --no-realtime -d dummy"

Dummy is used as backend as I do not need local sound output from JACK - if you want it use Alsa instead.

###Puredata Start Puredata as user asterisk and connect to JACK.

sudo -u asterisk bash pd -jack YourPatchFileHereOrNot

Then just enable DSP or do automatically.

Puredata version before 0.46 Overwriting the HOME-variable is necessary for Puredata version less than version 0.46, which will be included in Ubuntu 15.04. Otherwise the UI is not loaded as reading the config-files fails and the UI process stops.

sudo -u asterisk bash
> HOME=/tmp
> pd -jack YourPatchFileHereOrNot

Have fun.

Known issues:

  1. Asterisk Version prior 13 are limited to 8000Hz for interaction with JACK - see here.
  2. If JACK is not running Asterisk just ignores it and the call continues without.
  3. Due to the large numbers of buffers involved a relatively high delay is to be expected - a guess is ~80-100ms.