By Dave Phillips
Most Linux users know that a server provides services, whether on one machine or on a network. The purpose of a server is to eliminate the need to directly access these services at the application level. For example, an X server manages access to and control of the video services of the computer's graphics chipsets, relieving user-level application developers of the burden of programming for those services directly.
An audio server manages access to the capabilities and services of the installed audio devices. These audio devices include soundcards, on-board audio chipsets, and any other audio hardware (such as telephony hardware, combined A/V cards, television, and radio boards).
The graphic Linux desktop depends upon X for its graphics and video services. Thus, your favorite KDE or GNOME applications include routines for accessing those services through the X application's programming interface. Programmers can code for the hardware via a generalized API instead of having to directly address the hardware. Alas, the sonic Linux desktop lacks a single standardized solution for serving audio resources system-wide. Instead, a variety of solutions have appeared, including the artsd, esd, NAS, and JACK systems.
For triggering system sounds, listening to CD and DVD audio, and simple recording, the demands on an audio server are relatively light. Managing several audio streams at this level requires no sample-accurate synchronization, nor is there a need for a highly flexible client routing system. Most users simply want to be able to play recorded audio without blocking other audio streams. Artsd and esd are sound servers designed to meet these requirements for the KDE and GNOME desktops. The Network Audio System (NAS) is an alternative network-friendly client/server audio delivery system intended to serve as an audio equivalent of the X server. Within their limits, Artsd, esd, and NAS all function effectively. However, none of these servers provides sample-accurate synchronized I/O of multiple audio data streams, nor were they designed for performance within low-latency systems. If your audio needs require these high-end capabilities, you have ventured into the domain of professional audio systems, and now you need to know JACK.
| Regarding Soundcards |
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The audio industry distinguishes between consumer and professional grade audio devices. Consumer-grade devices include PCI and USB audio interfaces, on-board chipsets for integrated desktop and laptop sound support, and more advanced hardware such as Creative's SB Live! and Audigy cards. These devices normally provide channels for a master volume control, PCM and CD audio output, and inputs for microphone and line-level signals. The master volume, CD, mic, and line channels are self-explanatory. The PCM channel is a general digital audio playback channel providing volume control for programs playing WAV, AIFF, OGG, MP3, and other soundfile types. Depending on the audio chipset, these basic services may be expanded to include channels for on-board synthesizer output, digital audio connections, surround sound channels, and bass/treble tone controls. Software mixers such as alsamixer poll the audio hardware for its capabilities and configure the mixer to display the available channels and switches. Thus, my laptop's CS4232 audio chipset supplies little more than the basic services, while my desktop machine's SBLive provides a much larger array. Professional-grade audio boards such as the RME Hammerfall or the M-Audio Delta cards are designed to satisfy different needs, providing higher-quality audio connectivity such as AES/EBU and balanced 1/4" plugs, a greater number of audio I/O channels, higher sampling rates, and hardware synchronization capabilities. These professional-quality cards may or may not include hardware MIDI connectivity, and they do not usually include consumer-grade amenities such as an on-board synthesizer or a connector to your CD drive's audio output. The distinction between the grades is obscured by some more advanced devices intended for the consumer market, and it is certainly possible to achieve truly high-quality results from some of the more modern soundcards. However, for truly professional requirements you'll need professional-grade audio hardware. |
Developer Paul Davis has created one of the most remarkable pieces of open-source audio software, the JACK Audio Connection Kit, better known as just JACK. JACK is specifically designed for systems tuned for low-latency and high demand. Professional audio recording systems cannot afford audible delays and dropouts (known as xruns), and such systems are expected to support the synchronous operation of multiple clients in a low-latency environment.
JACK comes with features such as:
JACK's primary task is the management of multiple audio data streams, coming from and going into a variety of applications with synchronized I/O. JACK requires an audio system - it is not a replacement for an audio system such as ALSA or OSS/Linux. JACK does not supply soundcard drivers, nor does it access hardware directly; instead it depends on a low-level audio layer to handle that communication, which in Linux is either ALSA or OSS. JACK doesn't care about the underlying hardware, it simply wants to manage the streams coming into and out of your devices.
JACK is available as a basic package in the AGNULA/Demudi and Planet CCRMA audio-optimized systems. A source tarball of the latest public release is available from the JACK website. The site also provides instructions for building JACK from CVS sources for those who want to keep up with the latest development. No special build requirements are needed for JACK itself beyond Erik de Castro Lopo's libsndfile audio file I/O library.
According to the JACK FAQ, you must have a recent Linux kernel (2.4 or higher) with the tmpfs file system turned on. Most modern distributions will have this turned on by default, but you can check for it by running cat /proc/filesystems. The FAQ also states that you must mount a shared memory filesystem on /dev/shm, advising that the following line be added to /etc/fstab:
shmfs /dev/shm shm defaults 0 0
The FAQ further notes that you may have to create the /dev/shm directory yourself.
After unpacking the sources, simply enter the new JACK directory, read the README for up-to-date instructions, then invoke ./configure --help to see the available configuration options. JACK is built with the familiar autotools utilities, so for most users the compile process is as easy as running ./configure [your options here]; make; make install.
Installing JACK from an RPM or other package also requires no special support. Follow the basic installation procedure for your system, and voila, you have a fresh JACK system ready for use.
The JACK server is launched with either jackd or jackstart. The JACK manual page (man jackd) tells us that jackd invokes the JACK server daemon and that jackstart is used when using JACK's built-in support for realtime capabilities. All options are the same for either invocation. For most users working on systems with a patched 2.4 kernel, jackstart will be the preferred method of starting the server. Users working with 2.6 kernels should use jackd.
Here's a relatively simple beginning :
jackd -R -d alsa -d hw:0
In this example, JACK has been started with realtime capability, acknowledging the ALSA back-end and addressing the first hardware device in the audio system. The -d hw:0 switch is actually unnecessary; the hardware selection always defaults to hw:0 anyway. Obviously, you would use a different number for a different card or chipset in a system with multiple sound devices.
Here's a slightly more complex example for my SBLive soundcard :
jackstart -R -d alsa -d hw:1-p 512 -r 48000 -z s
Once again, we see the realtime and ALSA options. The device selector is numbered hw:1 because the SBLive is the second card in that particular machine. I've added options for the buffer size (-p), for the JACK sample rate (-r), and for the audio dithering option (-z). Note that the -p option sets the software buffer size. As Jack O'Quin points out, this is the buffer size seen by all JACK clients.
See the box titled "JACK Options" for more on command line options for JACK.
| JACK Options |
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When you first meet JACK, you may be confused by some of its options. This brief summary will help you find your way around. First the parameter settings:
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We have already seen JACK in action at the command prompt. However, when working in an X environment it's nicer to have a GUI for JACK's setup and configuration, and thanks to developer Rui Nuno Capela, we have the wonderful QJackCtl (Figure 1). This most helpful utility provides an all-in-one graphic interface for configuring and controlling all of JACK's operations. In addition to the convenient setup dialog (Figure 2), QJC supplies an audio connections panel for JACK clients and a set of basic JACK transport controls (if you want to use QJC as the JACK transport master). QJC further supplies messaging and status display panels, controls to start and stop the server, and play/pause controls for JACK's transport control system.
QJC also includes a MIDI connections panel for ALSA sequencer clients, letting users manage audio and MIDI connectivity from a single control interface. You can save and load your total connections graph as a Profile in QJC's Patchbay (Figure 3). The Patchbay's operation isn't quite automatic, but it is a real time-saver if your connections are many and complex.
QJC is my favorite standard tool for controlling JACK, but there are at least two other GUIs for managing JACK connectivity. Dave Robillard's Patchage is a patchbay for both JACK audio and ALSA MIDI connectivity via its unique visual interface (Figure 4). Matthias Nagorni's QJackConnect is a nice JACK-only QT-based patchbay, but it appears that project development is on hold.
JACK support has become an expected feature in new Linux audio software. As a result, the list of implementations has become too lengthy to print here, but its domains of implementation include hard-disk recording systems (Ardour, ecasound, Wired), drum machines/rhythm programmers (Hydrogen), software sound synthesis environments (Csound5, SuperCollider3), audio/MIDI sequencers (Rosegarden, MusE, seq24), soundfile editors (Snd, Audacity, mhWaveEdit, ReZound), and standalone softsynths (AMS, Om, ZynAddSubFX). Other significant JACK-savvy projects include the LinuxSampler and Specimen sampler projects and the various schemes for supporting VST/VSTi audio plugins under Linux (these schemes also require the WINE software). Linux media playback systems such as MPlayer, XMMS, and AlsaPlayer also provide JACK support.
Readers should note that these applications vary in the scope of their JACK support. Some use only its audio connectivity, some use only partial implementations of its transport control, and a few already take more complete advantage of JACK's features. Please consult the documentation for any JACK-aware application to determine the extent of its support.
The basic JACK package includes a number of useful command-line tools such as jack_connect/jack_disconnect (manages client connections), jack_metro (a configurable metronome), jack_lsp (lists JACK ports, their connections and properties), and jack_transport (manages JACK transport control status).
JACK has also inspired a wave of cool tools and utilities. Bob Ham's JACK-Rack is a very useful container for LADSPA plugins that lets you build a virtual rack of audio processing modules with MIDI control of plugin parameters. Steve Harris's JAMin is the result of a collective effort by Linux audio professionals to create a pro-quality stereo mastering interface based on LADSPA audio signal processing plugins. Timemachine is another treat from Steve Harris. It's essentially a recorder that always maintains a buffer of the last ten seconds of recorded material. When fully armed, Timemachine writes the buffer to disk and continues recording in realtime. Fons Adriaensen's JAAA (JACK and ALSA Audio Analyser) is a professional-grade signal generator and spectrum analyser designed for accurate audio measurement. And just to show that there's no absolute need for a fancy GUI, Florian Schmidt's jack_convolve is a JACK-based command-line convolution engine, very handy for creating high-quality reverb effects and other interesting sounds.
URLs for all these and other JACK applications are listed on the Linux Sound & MIDI Software site [9].
Figures 5 and 6 show off JACK in two typical uses here at Studio Dave. Figure 6 illustrates the simpler use in an audio-plus-MIDI network combining the seq24 MIDI sequencer, the QSynth soundfont-based synthesizer, and the JACK-Rack, all operating on my PII 366 Omnibook and its humble Crystal Sound CS4232 chipset. Figure 5 demonstrates a more ambitious set of routing and connections with JACK managing I/O.
There's little more to say about using JACK in these scenarios. Once I've configured JACK, its performance is completely transparent. All I have to do is make my connections and make my music.
Programming with the JACK API is beyond the scope of this article. Interested readers can find excellent instructional material in the JACK source code (see simple_client.c in the example_clients directory) and on various Web sites. James Shuttleworth's tutorial at http://www.dis-dot-dat.net/index.cgi?item=/jacktuts/starting/ is a well-written introduction to adding JACK to a simple audio application, Lewis Berman has contributed a PDF on writing a JACK audio recorder at http://userpages.umbc.edu/~berman3/, and of course, the JACK API can be read and studied in the well-commented jack.h header file.
If you build JACK yourself and you have the doxygen software installed, you can generate JACK's developer documentation. This documentation is also available on the JACK website, but it is out of date as of September 15, 2005.
In 2004, JACK won a well-deserved Bronze award in the Merit Awards granted by the Open Source Initiative. At that point, JACK's development was at version 0.9x. As I write this article, JACK is now at version 0.100.0, heading steadily towards its 1.0 release, and the future is looking good for JACK.
Stephane Letz has successfully ported JACK to OSX. Support for OSX has become common in new Linux audio software. Incidentally, an implementation for Java has already appeared.
MIDI musicians are familiar with time code implementations not currently supported by JACK, and a coordination of synchronization capabilities would be most welcome. Some work in that direction has already begun, so it is likely that a blend of MIDI and JACK will evolve.
JACK's attractions may seem irresistable, but it may not be the best solution for common desktop audio services. Unlike ALSA, JACK is not planned for inclusion with the Linux kernel sources, so its presence in any Linux distribution results from a decision made by the distro's producer. Also, JACK is not so transparent to the user as the artsd and esd servers, and more configuration is required for obtaining best performance. Nevertheless, JACK is a very flexible system and may yet become the de facto audio server for the Linux desktop.
For the more professionally inclined JACK is a godsend. Its performance stability has already been tested and verified in high-demand real audio world applications, and its applicability can be seen in an expanding suite of increasingly powerful JACK-based programs. JACK's API has paved the way for a new wave of high-quality Linux audio applications. Whether you need rock-solid audio system performance for Ardour or you just want to have some fun routing the output from XMMS, you need to know JACK.
| INFO |
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[1] The JACK Home Page: http://jackit.sourceforge.net
[2] ALSA Soundcard Matrix: http://www.alsa-project.org/alsa-doc/ [3] Patchage: http://www.scs.carleton.ca/~drobilla/patchage/ [4] QJackConnect: http://www.suse.de/~mana/jack.html [5] QJackCtl: http://qjackctl.sourceforge.net/ [6] Interview with Paul Davis at Builder.com: http://builder.com.com/5100-6375-5136755.html?tag=tt [7] The Low-latency Mini-HOWTO: http://www.djcj.org/LAU/guide/Low_latency-Mini-HOWTO.php3 [8] Florian Schmidt's notes on building a low-latency 2.6 kernel: http://tapas.affenbande.org/?page_id=3 [9] The JACK page at linux-sound.org: http://linux-sound.org/jack.html |