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Mike RaznickNYU | Digital Audio Processing IIFinal Research Paper

Digital Audio And The Linux Operating System

This paper explores the increasingly popular Linux operating system as a free, viable

alternative to other operating systems such as Microsoft Windows or Macintosh OSX for

the use of professional audio applications. Specifically, discussion will focus on the

history behind Linux as well as the varying layers of applications that support digital

audio within the Linux operating system.

In order to fully understand the success of Linux, it is important to define the word “free”

in the context of Linux and open-source development. Although “free” does imply that

software can be downloaded and used without any required monetary payment, the term

“free” in this case more importantly refers to “restriction-free”. In this sense, open-source

(open sharing of the source code) can be thought of as a type of shareware system where

“payment” consists of active development community participation, often leading to the

release of bug fixes and improvements back into the public/user community. “Any

motivated individual can contribute to the product's development and can inspect all

aspects of the underlying code, which is not possible with most commercial products.”

Open source therefore “relies on a volunteer development community's willingness to

share all improvements to the code with the rest of the world.”1

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Before Linux

Linux is an operating system that is similar in design and functionality to the UNIX

operating system. Linux was primarily created to provide its developers and users with a

free system that rivaled traditional UNIX systems. UNIX, the predecessor to Linux, was

first developed in the late 1960s at the Bell Laboratories research facilities with the

purpose of creating a “single, scalable operating system” that would exist “on all

computers”2 for the needs of the company. UNIX was written in the C programming

language. By the 1970s, C had become increasingly popular for

solving a variety of development tasks and was therefore

supported by a variety of computing environments. Because of

this, the latter part of the 1970s saw many types of computers

running ported versions of UNIX. While UNIX was distributed

widely and proved easily portable due to its C implementation,

ownership of the source code was retained by AT&T. This

meant that licenses for the use of UNIX had to be purchased at enterprise-level prices that

were clearly out of the range of most users and enthusiasts. Furthermore, these licenses

prohibited redistribution, the making of derivative works, as well as any sort of

evolutionary improvement of the code-base by a community of programmers.

In 1984, Richard M. Stallman, an employee at MIT’s Artificial Intelligence Laboratory,

launched the GNU project with the intent of developing a free operating system called

GNU (a recursive acronym for “GNU’s Not UNIX). The GNU project promised to allow

computer users the “freedom to run, copy, distribute, study, change and improve the

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software.”3 Stallman set out by programming a set of tools that were designed to work

without requiring further modification to an existing UNIX system. Additionally,

volunteer programmers were recruited to architect and construct additional tools

necessary for the project. Throughout the 1980’s, the set of GNU tools became widely

accepted by UNIX users throughout the world. The most important concept behind the

GNU project was that it provided the freedom for all interested parties to able to improve

a software program and then release these improvements to the public so that the entire

community could benefit, leading to added “stability, reliability, and maintainability of

the tools.”4

During this period, the University of California at Berkeley also commenced the design

and implementation of a new version of UNIX that maintained free distribution in the

academic world. This version of UNIX, which came to be known as BSD UNIX, also

treated the original version of UNIX created by AT&T as a design standard. However,

the range of application for BSD UNIX was limited by its license terms. Furthermore, all

hardware specific BSD code was considered proprietary and was therefore not released in

the distribution of the source code. This meant that a working operating system could not

be built for a variety of computers using BSD, which therefore was not useful as a

universal operating system for large-scale community development purposes.

History of Linux

Between 1989 and 1991, Linus Torvalds, a computer science student at the University of

Helsinki in Finland, developed a project where he adapted an existing tool known as the

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MINIX kernel. The MINIX kernel has been commonly used in academia as an example

for teaching courses in Operating System development. Without at first realizing it,

Torvalds found himself transforming the MINIX kernel into a usable kernel that was

adapted for UNIX on the Intel x86 processor, found in today’s PC home computers.

Torvalds, realizing that in order to increase the usability and eventual success of his

newly named Linux kernel, also adjusted his operating system design and architecture

decisions so that existing GNU components would be compatibility with his kernel. This

was important because for the first time, the free GNU components were bridged with a

new operating system that would additionally be free. Torvalds therefore created the

framework for a completely free software package that was also completely self-

contained.

In 1991, Linus Torvalds released the first version of Linux to the Internet community. At

the initial time of release, Linux, although “sketchy” in its implementation, existed as a

free software kernel for a UNIX-like operating system that was developed especially for

personal computers. Linux was “fully compatible with and designed convergent with the

large and high-quality suite of system components created by Stallman's Project GNU

and distributed by the Free Software Foundation.”5 Because Torvalds made the decision

to release the Linux kernel under the Free Software Foundation's General Public License,

any software engineer who was interested in contributing to the further development of

the Linux kernel could feel confident that their contributions would result in permanently

free software. This meant that any software written for Linux would not be subject to the

similar fate as past, proprietary products that were also widely available. Of equal

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importance was that all interested parties would have the opportunity to test and

scrutinize existing as well as new source code so that it could be shared, consistently

improved upon, and redistributed. Furthermore, the development of the Linux kernel

proved an early example where the Internet could successfully bring together many

thousands of part-time developers, resulting in a software development project involving

well over one million lines of code. This scale of unpaid collaboration was previously

unprecedented among such a geographically dispersed group.

The release of version 1.0 of Linux for the first time represented a usable, production-

level kernel. Version 2.0 was subsequently released in 1996, and by 1998 version 2.2 was

expanded to include support for a variety of machine architectures, in addition to the Intel

x86 processor family. During this time, GNU/Linux and Microsoft’s Windows NT

operating systems remained the only two operating systems that saw consistent gains in

market share. A Microsoft-based assessment of the credibility of Linux in October 1998

stated that "Linux represents a best-of-breed UNIX, that is trusted in mission critical

applications, and - due to it's open source code - has a long term credibility which

exceeds many other competitive OS's."6

Today, a variety of distributions of the GNU/Linux system, based on the kernel Linux

developed by Linus Torvalds, are in widespread use all over the world. “The number of

GNU/Linux system users is currently estimated to be around 18 million.”7 Some of the

more popular Linux distributions include: the Fedora Project, SuSe (pronounced “SUZ-

eh”), Linux, Mandrake Linux, Red Hat, Yellow Dog, Debian, Slackware and Licoris. A

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good source of information regarding the various distributions is the LinuxISO.org

website.

Linux Support for Digital Audio

As early as 1992, the first audio applications programming interface (API) was developed

for Linux by Hannu Savolainen. The Open Source System Interface (OSS/Free)

initially included support for the basic SoundBlaster-compatible devices. Supported

features included: the ability to perform PCM audio recording and playback, MIDI input

and output, as well as an audio device mixer. The OSS/Free kernel API was included as

part of the Linux distribution. Over the next number of years, OSS/Free provided

additional, functional support for a number of advanced features, including on-board

synthesizers and full-duplex recording. This was due, in part, to the donation of required

specifications by a few companies that specialized in digital audio. The majority of

professional digital audio hardware manufacturers, however, were unwilling to provide

access to relevant specifications and documentation that they felt were proprietary to their

product offerings. This resulted in Linux software applications developers not having the

means for architecting high-performance sound software that would be compatible with

professional audio hardware.

Hannu Savolainen once again assisted in bringing together the Linux community and

digital-audio hardware manufacturers by forming a company that would sign non-

disclosure agreements (NDAs) in order to provide Linux audio developers with the

information necessary to create a greater collection of professional-audio sound drivers.

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Drivers could then be released in the form of commercial software where certain

components existed as binary-only closed-source. This package, known as the

OSS/Linux commercial driver package, could be thought of as a direct descendent from

the OSS/Free API. OSS/Linux currently supports a number of professional audio boards

and chipsets. For example, drivers for the M-Audio Delta series of multi-channel audio

boards, support for the Creative Labs Sound Blaster Live and Audigy sound cards, as

well as drivers for the RME Hammerfall series of professional digital audio boards have

since been officially supported.

The next important event that significantly enhanced the Linux operating system in its

support for professional-grade digital audio applications occurred in 2002 with the

release of the Advanced Linux Sound Architecture (ALSA) API into the 2.5.x

development cycle of the Linux kernel. The ALSA sound driver was originally written in

order to replace the Linux kernel sound driver for Gravis UltraSound (GUS) cards. When

this replacement proved to be a success, the author started the ALSA project with the

intent of creating a generic driver that could be effectively used for any number of sound

chips, with a fully modularized design.

While ALSA is now compatible with the OSS/Free and OSS/Linux sound drivers, ALSA

additionally has its own interface that is improved over that of the OSS drivers. A

complete list of supported soundcards can be found by visiting the ALSA Soundcard

matrix web page at: http://www.alsa-project.org/alsa-doc/. Furthermore, a complete list

of ALSA-supported applications can be found on the ALSA Applications webpage at:

http://www.alsa-project.org/applications.php3. The ALSA sound drivers proved to be

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such a success that they have since replaced the OSS drivers in the Linux kernel.

The inclusion of the ALSA API and drivers into the stable Linux kernel releases meant

that sound and MIDI-based applications in Linux could be supported with professional-

level capabilities. While the OSS/Free kernel modules, and the OSS/Linux commercial

driver package have their relative strengths, the ALSA drivers and library are now

considered to be the standard set of audio and MIDI drivers for Linux. Notable features

included with ALSA are as follows: “support for audio interfaces from consumer-grade

sound cards to professional digital audio boards; multi-processor, thread-safe capabilities;

fully modularized drivers; a user-space library to simplify audio applications

programming and provide high-level functionality; and compatibility with the older

OSS/Free API. ALSA also supports serial port and USB interfaces. It is completely free

and open-source, with its code base licensed under the GPL (General Public License).”8

Furthermore, because ALSA supports professional audio hardware such as Echo Digital

Audio products, RME Hammerfall and the M-Audio Delta series multi-channel boards, it

is now possible to accomplish audio-related work with professional-grade hardware and

software under Linux.

The Modern Linux Sound System

The Linux sound system can currently be thought of as consisting of three layers, which

together allow for solving the problems inherent to the increased system performance

demands required by audio and video applications. These layers can be described as

follows: the kernel (defined as the innermost core component of the Linux OS -

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including the presence of ALSA; the kernel operates directly on the computer’s

hardware), the middle layer (consisting of additional components such as ALSA, JACK,

LADSPA), and finally the user space (including programs such as Ardour, Sweep, and

MusE). Development at each of these levels has been extensive and will be further

discussed.

In addition to its presence at the kernel level, ALSA has a significant presence in the

middle layer with its exceptional utilities and tools such as aconnect (a mechanism for

routing MIDI I/O between ALSA-aware programs) and ALSA-mixer (a text-mode

system audio mixer). As a general rule, it can be said that if an application supports

ALSA drivers for both audio and MIDI, it is most likely to exhibit excellent performance

as a standalone program. For MIDI, the ALSA Sequencer interface allows applications

that support this interface the ability to “publish” their inputs and outputs, allowing third-

party applications to connect to them. For example, if a MIDI-based drum machine and a

MIDI sequencer both provide ALSA Sequencer support, it would be possible to record

the events of the drum machine by simply connecting it to the sequencer. JACK, which is

discussed below, also supports this functionality.

The JACK Audio Connection Kit (JACK) is also an important component that exists

on the middle layer. The JACK audio server was designed specifically for purposes

related to professional audio work. The intent behind development of JACK was to focus

on two key areas: “synchronous execution of all connected clients as well as low-latency

operation. JACK can connect a number of different applications to an audio device, as

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well as allowing them to share audio between applications. A JACK client program can

run in its own processes (for example as a as normal, stand-alone application), or it can

run within the JACK server (for instance, as a plugin).”9 For example, any JACK-aware

application can connect its output to the input of another JACK-aware application. A

software synthesizer could therefore be used as a plugin within an audio-recording

program by connecting each component together using JACK.

JACK Interface

Examples of current JACK-based applications include the JACK Rack

(http://arb.bash.sh/~rah/software/jack-rack/), defined as an “effects rack” for the JACK

audio API. The JACK Rack can be populated with LADSPA effects plugins (LADSPA is

the Linux equivalent to VST or Audio Units technologies) and can then be controlled

using the ALSA sequencer. Additionally, for mastering purposes, JAMin, the JACK

Audio Mastering interface is designed to perform professional audio mastering of stereo

input streams. JAMin also uses LADSPA for digital signal processing (DSP). Finally, for

viewing meters in JACK, the JACK Meterbridge supports a number of different meter-

types that can be rendered using the SDL library and user-editable pixel maps.

Additional sound servers existing within the Linux middle layer include KDE’s Analog

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Real-Time Synthesizer daemon (arts) and GNOME’s Enlightened Sound Daemon

(esd), libraries such as libsndfile and libaudiofile, and plug-ins based on the Linux

Audio Developers Simple Plug-in API (LADSPA). LADSPA is supported by many

Linux audio applications. Roughly equivalent to the Audio Unit or VST plug-ins used in

most commercial digital-audio workstations, the LADSPA plug-in architecture allows a

single set of audio tools to be shared by all of the audio applications on a system. There

are well over 100 LADSPA plugins available that include applications for signal

processing techniques such as flangers, delays, reverbs, filters, phasers, as well as a full

assortment of additional processing tools. More information can be found at

http://www.ladspa.org/.

Finally, the user space layer of the Linux audio system presents full-featured digital audio

workstations such as Ardour. This program rivals similar commercial applications in

functionality, with support for 24 or more channels of 32-bit audio. “Ardour capabilities

include: multichannel recording, non-linear, non-destructive region based editing with

unlimited undo/redo, full automation support, a mixer whose capabilities rival high end

hardware consoles, lots of plugins to warp, shift and shape your music, and controllable

from hardware control surfaces at the same time as it syncs to timecode.”10 Ardour was

developed by Paul Davis and was designed for users familiar with the Pro Tools digital

audio workstation (DAW) model. Additional information and downloads can be found at

http://ardour.org.

Also present within the user space layer of the Linux audio system are MIDI sequencer

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programs such as MusE and Rosegarden. Both have many of the features included in

MIDI sequencers on commercial platforms, including audio-record and import

functionality as well.

Graphical editing window in Ardour

It should be noted, however, that these integrated audio capabilities are reported as “not

yet refined” to the level as what is currently available in the commercial applications on

other platforms. However, as with most Linux-based components, one can be relatively

certain that development will continue and that product quality will only improve over

time.

Rosegarden, in addition to the typical track, event-list, and piano-roll views, includes a

standout feature that allows the user to view their music in standard musical notation.

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Furthermore, Rosegarden additionally includes functionality for exporting music for use

by other Linux-supported applications such as Csound or LilyPond. Finally, Rosegarden

also allows the user to save its output as a Csound score. MusE also has a few notable

features that are not found in Rosegarden, including a mixer window as well as integrated

access to some of the software synthesizers available on the platform . Rosegarden can

Rosegarden MIDI and audio sequencer is designed similar to programs such as Cubase

be downloaded from the their dedicated website: http://www.rosegardenmusic.com.

MusE can be downloaded from http://muse.dyne.org.

Sweep, is a versatile audio program that can be used both in a serious production

environment and as a performance tool for fully digital DJs. Sweep makes for a unique

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case study because of its development being supported by the film animation studio

Pixar, who likely required a high-quality audio editor that could be used on Linux and

Sun UNIX workstations. This application is possibly the first to come into existence out

of a large movie/multimedia studio into the Linux audio community, and may present a

model that could be repeated in the future. As an application, Sweep can be thought of as

a conventional multi-channel audio file editor. However, the inclusion of a virtual stylus

rather than the traditional cursor makes the program quite unique when compared to most

other applications. This allows the user to “scrub” through a file to hear the exact location

where an edit should be made. However, “the virtual stylus, known as Scrubby, has been

programmed with the physics of a real turntable. Throwing the mouse to the left results in

a spin-back effect, decelerating Scrubby to a full stop.”11 Sweep supports a variety of

music and voice formats including WAV, AIFF, Ogg Vorbis, Speex and MP3.

Sweep is an audio editor and live playback tool for GNU/Linux

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Additionally, support for LADSPA effects plugins is included for multi-channel audio

file-editing.

Ecasound is another software package designed especially for multi-track audio

processing. It can be used for tasks ranging from audio playback, recording, and format

conversions, to multi-track effect processing, mixing, recording and signal recycling.

Ecasound supports a wide range of audio inputs, outputs, and effect algorithms. Effects

and audio objects can be combined in various ways and “their parameters can be

controlled by operator objects such as oscillators and MIDI-related controllers. A

versatile console-mode user interface is included in the package.”12

Notation software is also available on the Linux platform and continues to be enhanced

by an active development community. LilyPond, which takes a specially formatted text

file and converts it into printable music scores, creates professional looking scores that

are said to rival hand engraving. There are other programs that can also export files in the

LilyPond format.

Software synthesizers of interest include FluidSynth, AlsaModularSynth and

additionally, for building custom synthesizers, Csound, which is also built for Linux (in

addition to active Windows and Mac-based user communities) is available. FluidSynth

can load multiple SoundFont files and play them on 16 MIDI channels at once.

FluidSynth does not include a graphical interface. However, once SoundFonts have been

loaded, it is possible to choose among the loaded instruments using MIDI Bank Select

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and Program Change messages (or through command-line instructions). If SoundFont

files need to be created or edited, an editor called Swami is also available.

AlsaModularSynth, with its powerful capabilities, and lots of included example patches,

has a clean, intuitive graphical interface that allows the user to patch together modules in

any configuration that is desired.

It should be noted that there are a variety of other tools for musical applications such as

sample rate converters and CD burning software to name a few. Other notable categories

include applications working with CD technologies (rippers, burners, players), telephony

systems (see the Bayonne project), digital DJ tools, virtual drum machines (Hydrogen

drum pattern editor) MP3 and OGG audio compression software, MOD trackers (for

video game music), software synthesizers (ZynAddSubFX), as well as various work /

audio solutions, including network sound servers and streaming audio delivery systems.

Tuning a Linux Installation for Digital Audio

When Linux is first installed on a system, it must be patched/tuned so that it can support

professional-level digital audio tasks. This means that a low-latency patch must be

downloaded and applied to the Linux kernel. This is an extremely important step that is

necessary for setting up Linux to be used for digital audio applications. An un-patched,

un-tuned Linux 2.4 kernel can create latencies of up to 300 ms. Low latencies (defined in

digital audio terms as a “lag time” between when an event should occur and when it

actually occurs) are required for multimedia applications in order to achieve smooth

audio/video performance, even on the most powerful machines. Generally, most issues

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related to “jerky video/choppy audio” are software related and are mainly caused by the

OS scheduler. With the correct low latency patch (or combination of patches), Linux can

rival most desktop operating systems allowing latencies as low as 2.1 milliseconds. This

proves especially useful for real-time audio use. Specifically, patches from Andrew

Morton and Ingo Molnar have proven that the Linux kernel can be tuned to eliminate

performance bottlenecks associated with latency and bad performance, particularly when

scheduled processes remain active. Information regarding latency and performance of

Linux for digital audio use can be found at http://www.linuxdj.com/audio/.

Packaged Distributions

While the Linux 2.5.x development kernel has officially introduced ALSA as the new

kernel sound system (dispensing with the aging OSS/Free API), and has also introduced

Robert Love’s preemptive patch as a kernel configuration option (which gives certain

processes preemptive status at the Kernel level), the most convenient way to begin

learning about Linux audio projects is to install one of the turnkey systems optimized for

sound and music production: Planet CCRMA, Turn-Key Linux Audio

(http://lulu.esm.rochester.edu/kevine/turnkey/home.html), APODIO or ANGULA

(www.agnula.org) are a few of the most popular. They all provide an out-of-the-box

low-latency Linux kernel, ALSA drivers, JACK and LADSPA, as well as a host of sound

and music applications. Note that these are either full-fledged distributions or require

only a simple installation to upgrade an existing system.

APODIO 0.9 (http://cd.apodio.org) has very recently been released in it’s English

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(GNU/Linux) version distribution and is especially attractive because it can act as a live

bootable CD, containing major audio tools (under GNU/Linux) and a complete operating

system that is based on Mandrake 9.2. APODIO can therefore be used from boot, without

the need to install or make changes on the hard disk. This makes it particularly easy for a

potential user to try out. If desired, APODIO can then be installed directly to the hard

disk and run locally. Turn-Key Linux installs within an existing Mandrake 9.x

distribution by executing only a single script. This also makes for an easy installation for

users that may be unfamiliar or uncomfortable with command line interfacing.

Furthermore, this ensures a virtually transparent initiation into Linux as a multimedia

platform without the potentially frustrating experience of spending hours configuring the

system with correct versions of necessary software. The Turn-Key package was initially

put together as a way to provide students at Eastman Computer Music Center with the

same tools used in the studio for use on their home systems.

AGNULA is an acronym for “A GNU/Linux Audio distribution” and is also devoted

completely to professional and consumer audio applications and multimedia

development. Its goal is to offer two distributions: one will be Debian-based (DeMuDi)

and the other will be Red Hat-based (ReHMuDi). AGNULA has the Alsa Modular Synth,

Cecilia (a graphical user interface for Csound), Jack, jMax, LADCCA, Nyquist, and

TkECA among its offering. LADCAA is an acronym for “Linux Audio Developer's

Configuration and Connection API”. More specifically, “it is a session management

system for audio applications on the GNU/Linux platform. It understands both the JACK

audio API and the ALSA MIDI sequencer interface.”13

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Planet CCRMA (pronounced “karma”) is another Linux audio-based distribution and

has been packaged for release by Fernando Lopez-Lezcano, composer and system

administrator at Stanford University's Center for Computer Research in Music and

Acoustics. Lopez-Lezcano, who is responsible for the configuration and maintenance of

CCRMAs network of mostly Linux workstations, originally created the software package

to “mirror the Center's software so that students could run exactly the same system at

home as they did at the Center. This package eventually became what is now called

Planet CCRMA At Home (Planet CCRMA for short, or just the Planet).”14

Planet CCRMA installs on top of a default RedHat 7.3, 8.0, 9 or Fedora Core 1

installation, and provides three important features. First, a “tuned” Linux kernel that is

optimized for low-latency operation is included. Next, Planet CCRMA provides a

collection of interrelated projects which make up the professional-level Linux sound

architecture discussed above, including ALSA, Jack, and LADSPA. Additionally

included with Planet CCRMA are a substantial number of applications designed to take

advantage of this optimized audio foundation. Planet CCRMA, like the aforementioned

distributions represents a convenient installation option where applications created

separately are acquirable in a single download. Planet CCRMA can be downloaded from

the following Stanford-based website: http://ccrma.stanford.edu/planetccrma/software/.

More than 100 LADSPA plug-ins are installed with Planet CCRMA as well as programs

already discussed, including Ardour, MusE and Rosegarden, Csound, and Lilypond.

Software synthesizers such as AlsaModularSynth and FluidSynth as well as many more

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are also included. Other programs such as Pd (a program similar to Cycling 74’s MAX),

Snd (a powerful sound-file editor), Ecasound and many more are also included. A

complete list with descriptions can be viewed at

http://ccrma.stanford.edu/planetccrma/software/soundapps.html.

Will Linux Compete with Apple and Microsoft In Multimedia Production?

While it is clear that Linux is evolving into a platform that is worthy of serious

consideration by sound researchers, musicians, and multimedia artists, it is not yet found

in most professional studios. However, in the world of business applications, Linux has

continued to grow in status to become one of the major contenders as a platform of

choice. The reasons for this become quite obvious when it is considered that, while

Microsoft continues to release patches for its security flaws and worm attacks, Linux-

based companies like CodeWeavers are releasing software such as their CrossOver

Office 2.0 package (based on the Wine project), which now makes it possible for Linux

users to run a variety of Windows-based applications. For example, it is now possible to

view and run all types of QuickTime content, Shockwave-enabled web sites, as well as

applications such as Adobe Photoshop, Lotus Notes and Microsoft Office XP—including

Access. CrossOver also includes a Windows software installation tool that helps to ease

the potentially complicated process of installing and running Windows applications on

Linux.

Linux has been embraced by companies such as IBM, Novell, Oracle, Sun and Sybase

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and it is evident that most business software must now be Linux compatible in order to be

considered as a viable product. The current major drawback of Linux as a professional

audio system is its lack of support for commercial hardware interfaces. However, as

Linux continues to build strength and popularity, it has the potential to enter the audio

arena in a similar manner as in the business application arena. At the present time,

however, the state of Linux desktop audio applications is in its infancy, and “both

proprietary embedded hardware and software based on trade secrets aren't going to

disappear from studios overnight.”15 It is more likely that Linux machines will be

gradually introduced to perform specific tasks, which will increase their profile in the

professional sound industry over time or within specific niche fields.

References

13. AGNULA: LADCCAhttp://www.agnula.org/packages/ladcca/

Brooks, Jason, CodeWeavers' Latest Release Is Music to Linux Users' Ears, eWeek (May 5,2003)

Advanced Linux Sound Architecture (2002)www.alsa-project.org

10. Ardour: the new digital audio workstationhttp://ardour.org/

7. Estimating the number of Linux users, The Linux Counterhttp://counter.li.org/estimates.php

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The Free Software Definitionhttp://www.gnu.org/philosophy/free-sw.html

6. Halloween Document II (Version 1.4), Open Source (2004)http://www.opensource.org/halloween/halloween2.html

9. Jack Audio Connection Kithttp://jackit.sourceforge.net/

11, 15. James, Daniel, Open Source Music Software & The AGNULA Project, Sound on Sound(February 2003)

Krill, Paul, Lawsuit won't stop Linux, Oracle exec says, Infoworld, (June 06, 2003)  

Linux Audio Developer's Simple Plugin API (LADSPA)http://www.ladspa.org/

1. Masciarotte, Oliver, Free Beer, Mix, Nov 1, 2003http://mixonline.com/ar/audio_free_beer/index.htm

2, 4, 5 Moglen, Eben, Anarchism Triumphant: Free Software and the Death of Copyright, TheMultimedia Journal of new Music, 1999

3. Metts, Allan, Sounds from Another Planet, Electronic Musician (Jan 1, 2004)

Niccolai, James, Ellison says Oracle's 'whole business' to run on Linux, Computerworld (January31, 2002)

8. Phillips, Dave, Computer Music and the Linux Operating System: A Report from the Front,Computer Music Journal: Vol. 27, Number 4 (Winter 2003)

Phillips, Dave, Linux Audio Plug-Ins: A Look Into LADSPA, ONLamp.com (02/02/2001)http://www.linuxdevcenter.com/pub/a/linux/2001/02/02/ladspa.html

12. Phillips, Dave, Sound and Music Software, Freshmeat (December 15th 2001)http://freshmeat.net/articles/view/354/

14. Phillips, Dave, Visiting the New World of Linux Sound & Music Software, Linux Journal(January 07, 2004)

Raymond, Eric S., The Cathedral and the Bazaar, First Monday (1998)http://firstmonday.org/issues/issue3_3/raymond/index.html

Stallman, Richard, Richard Stallman's 1983 biography (1983)http://www.stallman.org/#humorous%20bio