Sound Engineers Guide to Jackd

This page attempts to collect in one place various issues surrounding the design and abilities of Paul Davis's wonderful Jackd ( from the point of view of a technical user. It is not an introduction or a usage howto. Thanks to all the developers who have contributed to this fantastic example of FOSS development. Jack is already a great success and will undoubtedly go on to even greater things! hardware support: ----------------- Hardware support is good due to its usage of Alsa as a back-end. There are some gaps, and its possible that these gaps may get bigger in the future due to consumers accepting increasingly unhelpful behaviour from manufacturers. As well as Alsa's direct access to soundcard hardware, Jack also can be used with the dmix plugin to share the card with other non-Jack applications, albeit at the expense of some audio quality. In addition, Jackd supports some firewire devices such as from Edirol and Focusrite. This does not use Alsa, but is done directly using FFADO ( audio quality: -------------- Being designed for discerning audio usage, Jackd doesn't suffer from any audio quality issues. It uses 32bit floating point throughout, which is considered adequate for the vast majority of work. Dithering options are available if required. Other than dithering, jack does not modify the signal in any way. In terms of timing accuracy, due to the blocked nature of audio processing, event timing can be up to 10ms out (4096, 44.1k), eg in the case of a tempo change. latency: -------- In most cases Jackd offers equal or better latency than competing systems. Recent patches for 2.6 kernels can achieve latencies of around 1ms over and above the inherent soundcard latency. This round trip soundcard latency can be as high as 4ms. For comparison, each metre you are from the speaker adds another 3ms. Typical Jackd latency is about 6ms. This is for a system with 2 periods, each of size 128, running at 44.1kHz. system integration: ------------------- Currently jackd is usually started and stopped manually. However, libjack does have a recently added function to automatically start it on demand, which some jack apps make use of. Running Jack as a system service started automatically at startup is easy if desired. This approach is not officially encouraged, as in some situations it raises security and stability issues and prevents other applications from accessing audio hardware. Non jack apps can be used via the Alsa Jack-plugin ( This allows properly behaved Alsa applications to record and playback through Jackd. These apps see Jack as add addition 'device'. Other audio backends such as Pulseaudio and Gstreamer can use Jackd as a backend/sink. Older methods are the LD_PRELOAD hack using the libjackasyn from Guenter Geiger. And the bio2jack that non-callback based apps such as mplayer can use. These wrappers do not and cannot support jack-transport. scalability: ------------ Because of the context switching, there is increasing overhead as the number of clients is increased. The figure of 20 clients has been mentioned as a rough maximum. This might be a practical limitation for some uses, such as constructing a modular synth. controllers: ------------ there are a few applications to help with inter-connections and stopping/starting. gui's: -qjackctl: -patchage: -qjackconnect from Matthias Nagorni: command line: -jack_connect and jack_lsp can be used for command line patching. To use bash completion, see this email from Florian Schmidt: dbus: there is an exciting new patch to Jackd from Nedko Arnaudov that allows you to control jack via dbus. Still very experimental atm. Search for "jack dbus and logs improvement". non-realtime graph execution: ----------------------------- A fairly standard feature of music production software is to be able to render (parts of) the graph at faster than realtime, often to reduce the amount of dsp required to 'run' a project. This has now been implemented by switching to 'offline' mode. With the addition of jackmidi, midi can now still be used in this mode. All hardware i/o is of course muted. This would break any clients that used a kernel/cpu clock. multiple soundcards: -------------------- multiple cards on the same machine can be used if they are synced together. This can be achieved by either using a central word-clock or by feeding a digital audio feed from one card into another and putting it into slave mode. In order for jack to access them, they must be abstracted to a single 'device' using an alsa config file. If you dont mind some quality loss due to resampling, you have other options: Alsainout is a recent jack client that will make a resampled connection to an alsa device. ( You can also run a separate unsynced jackd server for each soundcard and use jack-diplomat ( to transfer between them. multiple jack servers: ---------------------- It is now possible to run more than one server on the same machine concurrently using separate soundcards. Nothing is synchronised, each server has its own separate process graph and transport state. A single client app can access each server. Netjack or jack_diplomat can be used to route signals between them. transport: ---------- see excellent detailed documentation at Jackd offers centralised transport control. Transport changes in one client are reflected simultaneously in all others. Eg, a client can issue the command: "go to bar 21 and start playing". Time is specified in either jack_frames, bars/beats/ticks or smpte time. The ticks resolution (eg 1920 ticks per beat) is specified by the timebase_master client. Timebase master: although jackd counts time while the graph is running, the 'transport master' provides tempo and smpte-formatted information. (despite its name the timebase_master does NOT really control the timing of the system.) Timebase_master can either be jackd (the default (but note that jackd only provides frame numbers.)) or one of its clients. tempo changes: Currently jackd cannot do tempo changes. Tempo map information is confined to one client only (the Timebase Master). Because of this, other clients cannot reliably operate with tempo changes as they do not know when the tempo change occurred. Also, for example, other clients will not be able to give visual feedback on the tempo map. The tempo info needs to be stored in the server, accessible by all. This is not complicated, but just needs the details to be worked out. It would make a nice little project for someone to make a patch for this. Currently, with tempo only being updated once a period, there is a potential error of about 4096/44100, or 10ms. Seamless looping support is sometimes talked about. There is a utility app called jack_trans2midi which will convert jack transport information to midi clock. slaving: -------- Being able to slave to an external device is an absolutely essential feature for professional situations, which is currently not supported by Jack. The Ardour and Rosegarden clients can act as MTC slaves. Ardour can sync to both MTC and MMC, and apparently it works very well, but other clients do _not_ get synced even if Ardour is the timebase master. In theory, Rosegarden _can_ be a timebase master when slaved via MTC, but apparently this has not been properly tested. Jackd cannot currently act as a positional master for external systems, but Ardour can output mtc. Work was being done on a software smpte generator, but afaik it never came to fruition. There was previously some discussion of implementing the sgi UST/MSC timestamping, as used by some more video oriented applications, but nothing concrete is currently planned. Usage of OpenML ( or MLT ( has not been discussed, afaik. MMC can be used using jackctlmmc: There is no support for such things as VITC, video sync, 9-pin RS422 video controllers. contains code for a syncing jack to timecode. "It includes a simple JACK tool that displays incoming LTC and can encode jack-transport to LTC." (also check out the gstreamer 'gst-launch' app, with a chain of rtp and a codec, using jack as a source/sink) multimedia: ----------- Luis Garrido's video player Xjadeo ( can be used to sync a video with jack-transport. Lives ( as of 1.3.3 also has jack transport sync capability. There are very few possibilities for syncing to other media types such as swf/smil/sdl running on the same machine. This is not the fault of Jackd of course. Various comprehensive multimedia frameworks have been proposed but so far come to nothing. Patches are available to incorporate the transferal of video streams into Jackd itself. See: For example, you can use Ekiga to have Jack-powered video conferencing! 64 bit audio wordlength: ------------------------ There is some benefit in working at higher wordlengths, and as processing power gets cheaper, it is becoming increasing common, often inside particular processors. Cakewalk software uses fully 64 bit processing, and it is also common in mastering applications. For comparison, recent Digidesign hardware works at a mixture of 24/48/56bit integers. Currently Jackd only supports 32bit audio wordlength, and while implementing 64bit probably wouldnt be difficult (especially as the x87 and SSE support 64 bit operations), there is little demand from current Jack users. 64bit cpu's: ------------ Jackd fully supports 64bit cpu's. Lots of linux audio applications still do not run in 64bit mode, but as of version 0.114 (with further mods in 0.116), Jackd compiled as a 64 bit application can now connect 32 bit clients and vice versa. scrubbing: ---------- Scrubbing of the graph is not possible. As far as i am aware there is no demand for this feature. plugins: -------- jack has support for in-process plugins. Plugins should be able to do anything that a regular client can do, but without the context switch. However, except for the examples with the jackd source (inprocess.c and intime.c), there are currently none. (Note that LADSPA plugins are plugged into jack clients and not directly into jack.) remote clients / distributed processing: ---------------------------------------- Use of more than one box is fairly essential for execution of even moderately complex projects. It currently seems very unlikely that any reasonably-priced general purpose (ie, open) pci dsp cards will be forthcoming from hardware manufacturers. It's also useful as a way of routing audio between different rooms, removing the need for specialised audio cabling, and overcoming spdif cable length restrictions, and possible clock sync and jitter problems. netjack: Netjack designed to have fully (synced) slaved soundcardless jackd's as 'processor farms'. Consists of a jack client for the master box, and a 'network' jack driver for the slave. It features single-period roundtrip latency, transport sync, latency compensation, and support for slow-sync clients. For use with unsynchronised systems, there is optional asynchronous resampling and bitrate reduction for slow networks such as the Internet. The latest version has support for celt, a high-compression constant-latency lossy codec ( 24 channels are reported to work reliably at a latency of 5.8ms over 100Mb ethernet. However, because its designed to be low latency, there is very little additional buffering, so its important to keep the network segment free of other traffic, especially X messages, so for serious usage, an additional ethernet card is required. Udp is used for network transport. Netjack is packaged within Jackd itself. Authors: Torben H, Dan Mills, Robert Jonsson. jackdmp: The netjack support in jack2 currently doesnt support Transport, but does support automatic host discovery. icecast: There is a working icecast ( jack client (now included with the icecast distribution) which can be used to transmit the compressed output of a jack node to a remote machine. I dont know if it can do more than that. Presumably the remote sink, slaves to this stream, ie there cannot be a jack graph on the remote end. This method is utilised for example in the gtk InternetDj application ( varispeed: ---------- Jack can be varisped by syncing your audio hardware. Note, there is some confusion over the term 'varispeed'. Due to the change in sample frequency this is only useful in certain situations (eg when the output is feeding a dac or resampler). In some ways the dsp equivalent, of timestretching/pitchshifting/resampling is not true varispeed, although it is sometimes convenient to refer to it as such. pitchshifting: -------------- (realtime pitchshifting of one or more Jackd nodes) Not supported. While a nasty thing to have to do, pitchshifting is often useful, even if just for auditioning purposes. It has been inconclusively discussed on the mailing list (FIXME: refs?). For best results this would have to be done at individual nodes on the Jack graph. It seems unrealistic to expect all clients to support this, but it could theoretically be done by Jack itself. It could be tricky to work out which nodes to pitchshift, to avoid signals being shifted twice. It seems unlikely that this will be done, even in the medium term. time stretching: ---------------- Not supported. Tempo change without changing pitch. Similar situation to pitchshifting only slightly more complex and with less demand. driver switching: ----------------- This could be useful for freeing up hardware for other uses without disrupting the graph. status: Probably working now, but i havn't checked. disconnections: --------------- Occasionally clients get booted from the graph for being too slow. There is no official policy on what clients should do in this situation. Some clients automatically reconnect. Potentially, this can be handled by a session manager such as LASH. A workaround is to run jackd with the timeout switch, but this isnt always 100% successful. In addition, Jackd will sometimes commit suicide, via the Watchdog. This is not configurable. hardware monitoring: -------------------- There is support for zero latency monitoring on some RME and Maudio cards. on-the-fly sample-rate/buffer-size changing: -------------------------------------------- this would prevent having to restart jackd and reconstruct the graph in order to make some mode changes. Buffer size changing could be particularly useful for changing from a 'recording' to a 'mixing' mode with higher latency. status: not sure but i believe buffer-resizing was added in v0.99 routing changes: ---------------- This cannot be done without an xrun. (The jack audio thread stops while the new graph order is recalculated.) There is a client available to do realtime switching on pre-configured nodes. sessioning support: ------------------- LASH ( offers the ability to save/retrieve your complete audio/midi setup - including Jack connections - in one operation. status: still work to be done in this area, but things look promising with recent renewered interest, and incorporation of dbus. 'framedrop'/soft mode: ---------------------- Normal operation of jackd enforces strict obediance from clients. If they are late in providing data, they are booted. This is to ensure that the running project is executed 100% or not at all. This is inappropriate in any kind of performance situation, such as a live event, or a studio engineer playing a project for a client. Jackd's 'softmode' option allows that clients can be late in processing data, and will not be booted from the graph for doing so. However, this causes the whole graph to be delayed and an audible glitch is likely to result. Stephane Letz has implemented this feature as part of the alternative Jack implemtation, Jackdmp (jackd-multi-processor). ( Jackdmp calls this "asynchronous activation mode". It adds an extra 'process-cycle' to allow for failed clients. contains a technical paper with the details. It works by timeslicing each period into multiple parts, and each cpu core is allocated all the nodes for one of the timeslices. Reverse Play: ------------- status: not implemented, not important. looping: -------- extremely important for music production. Requires clients to made aware in advance that there is a discontinuity in the middle of a frame. Loops need to be able to go forward, not just backward. status: not fully implemented. scheduled for post v1.0. midi: ----- Recent versions of Jackd include timestamped midi. Jackmidi provides tight synchronisation of midi to the audio, gives the possibility of using midi in freewheeling renders, and simplifies application development. Jackmidi is in all recent versions of jack 1 and 2. -jackmidi doesnt have its own routing. Everything that the client sends to an output port will be sent to all clients that are connected to that output port. Because of the healthy state of kernel scheduling, support is only provided for real-time(ish) messages - you cannot pre-queue them (beyond the current process block?). -routing to and from alsa midi: Jackd includes support for this from version 0.108 onwards using the -X alsa driver switch which connects alsaseq devices to jackmidi. Alternatively there is the A2jmidi daemon by Nedko Arnaudov ( which reportedly has better timing. -apps that are currently able to use jackmidi: Muse (1.1) (, Linux-Sampler (, Non-sequencer (, Ingen (Om), Specimen, ZynAddSubFX/zynjacku, Azr3, Jack-keyboard, Jackmixer, Machina, Ardour-3, Epichord (, Bristol (, Jacker (tracker) (, Yoshimi (, zynjacku (, Specimen sampler (, Qsynth, Jack-smf-tools ( All LV2 and DSSI hosts support jack midi, so any synth plugin (eg Whysynth) will work. -patching works with Patchage and QJackCtl. jack2 (jackdmp) --------------- Jack2 is being maintained in parallel to Jack1 A major advantage of Jack2 not mentioned above, is that connections and disconnections should be glitchless. dither ------ Jackd can only dither the output to the soundcard. In offline mode, i presume that the output is usually still floating point. given that 32bit fp is not enough resolution to do technically accurate dithering to 24bit int, the subjective performance of the dithering algorithm should be subjected to some critical appraisal, but i dont know if this has been done. the future ---------- It looks like version 2 of Jackd may run as a true always-running system daemon with the ability to switch soundcards (backends) on the fly. Various control options will be available, including the recently added Dbus interface. author: Tim Orford, 2003-8. This document can be used under the terms of the GPL v3 license.