This document briefly describes the screen readers and self-voicing application extensions available in the Linux environment. It serves as a central starting point from which interested users, system administrators and software developers can explore further. I make no attempt here to assess any of the software mentioned. My criterion for inclusion is that each program has acquired a sustained community of users who are blind or vision-impaired, and that it continues to be the focus of development efforts. Projects which are no longer maintained or which have not gained substantial support within the user community are not included.
A second purpose of this publication is to orient the reader to speech and braille access under Linux. There are features which distinguish Linux, as a member of the UNIX family of operating systems, from other environments for which assistive technologies have emerged - principally, a highly capable textual interface relevant to all aspects of the system, together with diversity and freedom of choice in the selection of components at many levels. Challenges and opportunities for further development are identified, particularly in relation to graphical user interfaces, but speculation is kept to a minimum.
I welcome your comments and suggestions for improvement. Please direct any correspondence to firstname.lastname@example.org.
This material is intended to be read by users, system administrators, software developers and other interested parties. Familiarity with the Linux operating system, but not with assistive technologies used by people with disabilities, is assumed. Only a cursory explanation of the latter is offered, however, and interested readers are invited to investigate assistive technologies further by examining the capabilities of the software packages described here, and by searching the Web for relevant materials.
By a screen reader I mean a program designed to enable a user with a disability to interact with the operating system and its applications via synthetic speech or a refreshable braille display.
What is distinctive of a screen reader is that it retrieves information from the operating system about the user interface presented on screen. It may also intercept the user's keystrokes to provide feedback and to establish keyboard commands specific to the screen reader which facilitate interaction, for example by enabling screen contents to be reviewed interactively. The screen reader then presents a braille or spoken user interface with which the user interacts with the underlying system or application software.
Note: a refreshable braille display is a tactile device supporting
an array of plastic pins that can be raised above a reading surface
under the control of the host computer to form braille dot patterns.
These patterns, typically comprising a single line of text, can then
be read by the user. Braille displays also provide buttons, switches
and other controls (depending on the design of the individual
product) with which the user can
navigate the contents of a
screen or window. Some models also include a braille keyboard,
allowing text to be typed directly in braille.
By a self-voicing application extension I mean an extension to a Web browser, editor or other application that constructs a speech-based interface with which the user can interact. Self-voicing extensions differ from screen readers in that they have access to high-level data and functionality of the application which is customized; they do not operate within the user interface layer of the underlying system. As a result, they typically offer richer speech interfaces than screen readers, while taking advantage of auditory icons and distinctive voice characteristics to enhance the efficiency of interactions.
By Linux I mean any GNU/Linux distribution. Much of the software described below can be run on other UNIX-like operating systems as well. On the other hand, most of the software discussed is not supported by operating systems based on the Linux kernel that differ substantially from GNU/Linux distributions, for example Android. In general, distributions intended to be installed in embedded systems are beyond the scope of this review.
Fundamentally, Linux supports two broad types of user interface: some programs operate in a terminal, for example a virtual console or pseudo-terminal, whereas others run in a graphical environment provided by a display server. Certain programs can operate equally in both environments. For example, GNU Emacs (see below) can be run either in a terminal or as a graphical desktop application. In general however, it is best to classify user interfaces under Linux as either graphical or terminal-oriented, then to consider the accessibility of the operating system and application software from this point of view.
As can be seen in the descriptions below, most screen readers for Linux operate in a terminal; they are therefore intended to make terminal-oriented system utilities and applications accessible. Since a wide variety of computing tasks can be carried out within the terminal environment, which, moreover, offers powerful and flexible tools unmatched by graphical user interfaces, providing access to the terminal is enough to enable a user to perform many functions and to do so efficiently.
In parallel with command line tools and terminal-oriented applications, Linux offers a rich and diverse heritage of graphical user interfaces, together with a variety of desktop environments. Access to these interfaces allows a user to perform tasks that cannot be undertaken, or which it would be inconvenient to complete, using terminal-based software. Since the construction of assistive technologies capable of yielding a speech or braille interface to the terminal is essentially a solved problem, the focus of effort has largely shifted to making graphical applications and desktop environments accessible. This is not to suggest that terminal-oriented screen readers are no longer under development; to the contrary, they continue to be fine-tuned. The point, rather, is that the most complex challenges for accessibility in the Linux environment, as indeed in computing generally, are related to graphical user interfaces.
Though terminal applications are generally amenable to non-visual
access, there are certain practices, such as the use of highlight
bars rather than the system's cursor, which can create difficulties.
Such problems have usually been solved within applications
themselves rather than in the assistive technologies, for example by
show cursor option which ensures that the
location of the system's cursor is updated appropriately. This
feature can be found, for example,
(The latter additionally offers a
single column folder list
option convenient to users of text-to-speech screen readers.)
Similarly, the Mutt e-mail client
braille friendly option to enhance accessibility
by placing the cursor on the first line of text whenever the body of
a message is displayed.
Since most terminal-based software available for Linux is very accessible to screen reader users, including almost all command line programs, it would be superfluous to provide a list of accessible applications here.
Historically, two technical approaches have been developed to make graphical user interfaces accessible to users of screen readers. Both techniques are intended to solve the problem of obtaining the necessary information from the operating system and applications with which to build a braille or spoken interface. Unlike a terminal, consisting essentially of a rectangular array of characters that a screen reader can easily interrogate, a graphical interface is a composite of text (in various fonts) and images (such as icons) drawn on the screen. The primary means of input is a pointing device, typically a mouse or a touch-sensitive display. The difficulty for a screen reader, accordingly, is to collect details of the graphical interface from which to construct a spoken or braille rendering with which the user can interact.
The first approach to be developed involves constructing a
off-screen model, a database describing the
graphical interface derived by intercepting low-level functions of
the operating system, for instance graphics and font rendering,
window operations and so forth. This technique has proven to be
difficult to implement reliably and unsuitable for providing
higher-level information, such as the logical structure of a
document, needed for effective non-visual interaction. For these and
other reasons, research and software development efforts in
accessibility now concentrate on the deployment of accessibility
APIs (application programming interfaces), which are widely regarded
as a superior alternative to off-screen models. Indeed, under Linux,
access is provided exclusively by accessibility APIs; off-screen
models have never been used. In this respect, Linux is similar to
most contemporary operating systems: off-screen models are to be
found only in assistive technologies originally developed prior to
the widespread use of accessibility APIs.
Thus the second, and most widely implemented, strategy is to design an accessibility API, a programmatic mechanism which makes available the role, state and content of each component of the user interface to a screen reader or other assistive technology. The API provides functions with which user interface components of different types, such as text input fields, check boxes and menus, can be disclosed to assistive technologies. As will be emphasized in the discussion of Orca below, in order to be effective, such an API must be implemented correctly in custom user interface components provided by applications as well as in libraries of UI components used by system and application software. Since the API is the exclusive means by which Orca and other assistive technologies access the graphical interface, programs that do not implement it are inaccessible.
Graphical interfaces under Linux are characterized by diversity. No single UI library or desktop environment has ever been dominant, and there are signs that patterns of cooperation and competition among developers, software projects and vendors are likely to intensify the tendency toward diversity. While affording more options to users and system administrators, the plurality of UI libraries, desktop environments and applications presents a challenge for developers of assistive technologies: there is no system-wide library of UI components, nor a dominant suite of applications, on which to focus attention.
In the description of Orca, I give a non-exhaustive list of UI libraries, desktop environments and applications that support the accessibility API. The adequacy of this support varies greatly, and obviously the details even with respect to a single component change as bugs are fixed and regressions introduced. For this reason, I do not evaluate the accessibility of individual desktop environments and applications here. Widely used applications with well-resourced communities of developers are typically the most successful in implementing support for accessibility.
The general trend, however, is toward greater accessibility and improved quality of implementation. This can be seen in the extent to which bugs reported by users and developers are corrected, and in ongoing efforts to enhance accessibility across a range of desktop environments and applications. Complex applications such as Web browsers and office suites have also implemented the accessibility APIs of several operating systems. This is typically done by creating an accessibility API internal to the application, which is then mapped to the accessibility API of each supported operating system. Accessibility-related improvements within the application can thus benefit users of all supported platforms.
The advantage of self-voicing tools is that they have access to all of the facilities available to extensions of the application with which to construct an effective spoken interface. They are not confined to information and functionality exposed by an accessibility API. Although speech-based tools have so far been the subject of extension development, it would be possible to create self-brailling extensions and likewise other types of assistive technology along similar lines.
If a system is composed of several layers, where each successive layer serves as a platform for the development of applications in the layer above it, then the question emerges as to where in the hierarchy any given assistive technology should be written. Self-voicing applications are the result of pushing assistive technology development up the hierarchy, for example from the operating system to the Web browser. The potential of this strategy is yet to be fully realized. Its benefits, on the other hand, are demonstrable, as ChromeVox and Emacspeak show.
This section summarizes each of the software projects in turn,
ordered alphabetically. Most of the tools discussed here can be
obtained from widely used Linux distributions such
Linux, Debian, Fedora,
and their respective derivatives. There also exist special-purpose
distributions such as Vinux
and Sonar intended to be used
by people who are blind or vision-impaired. These distributions
activate screen readers by default and may also carry additional
accessibility-related enhancements that have not propagated
upstream software projects.
Note: several distributions can be installed independently by a blind or vision-impaired user via speech or braille access software activated early in the installation process. These distributions include, notably, Arch Linux (via a customized installation image), Debian (see this wiki page) and Ubuntu (see this guide). Some specialized distributions intended to be used primarily by system administrators for recovery and maintenance purposes also include access tools. A popular example is Grml (see this page for details).
For the most part, the screen readers and self-voicing extensions enumerated here should be regarded as complementary rather than competing projects. BRLTTY, for example, gives access to Linux virtual consoles, but may also be invoked by Orca to extend this access to graphical desktop environments. Although Speakup offers access to terminal applications in general, and consequently may be used with Emacs, a superior spoken interface can be experienced by running Emacspeak. As these cases illustrate, each tool has its own capabilities and strengths, knowledge of which empowers the user to make the most of the Linux environment.
BRLTTY is a screen reader that provides braille access to Linux virtual consoles. In conjunction with Orca (see below) and an included API, BRLTTY offers access to graphical desktop environments as well. BRLTTY supports hardware from most refreshable braille display manufacturers, including all of the best-known vendors of such products. Drivers for the hardware are integrated into BRLTTY itself, and an attached braille device can often be detected automatically when BRLTTY is run. In this case, the operating system becomes immediately accessible to the user without any prior configuration of BRLTTY.
Serial, USB and Bluetooth interfaces may be used; see the documentation for further explanations. BRLTTY offers contracted and uncontracted braille in a variety of languages. Braille keyboards, as incorporated into certain displays, can also be used for input.
BRLTTY Home Page: http://mielke.cc/brltty/
ChromeVox is a self-voicing extension to the Chrome and Chromium Web browsers. Under Linux, it can be used with any text to speech software supported by Speech Dispatcher. With ChromeVox, the user can navigate and interact with Web sites via a sophisticated spoken interface. In effect, it is an assistive technology implemented as an extension to a browser. As of this writing, it is not packaged by any Linux distribution and must therefore be installed separately from the operating system.
ChromeVox Home Page: http://code.google.com/p/google-axs-chrome/
Emacspeak is a self-voicing extension to GNU Emacs. It supports both text to speech software and hardware-based synthesizers attached via a serial interface. A speech server for each synthesizer is included.
It is important to appreciate that, with its many extensions, Emacs is much more than a text editor: among other functions, it can serve for example as a calendar and diary, a calculator, an e-mail client, a file manager, an IRC or XMPP client and a basic Web browser. Moreover, it provides customized editing functionality for a wide variety of document formats (including HTML, XML and LaTeX) and programming languages (C, C++, Python, Perl, Ruby, Haskell, Lisp, Bourne Shell and many more).
Emacspeak creates a highly customized speech interface for Emacs and many of its extensions, taking advantage of the author's research into speech and auditory interaction. Moreover, extensions specific to Emacspeak are provided to carry out common desktop computing tasks, such as searching the Web and reading electronic books. In sum, Emacspeak aims to be a speech-based desktop environment integrated into Emacs.
Emacspeak is packaged by several Linux distributions, including Debian. However, as of this writing, these packages are not kept up to date. Prospective users may prefer to download Emacspeak directly from its project site. Many Emacspeak enthusiasts choose to run it directly from a checked out copy of its Subversion repository, downloaded to a suitable location under the user's home directory. The documentation should be consulted for details regarding installation and configuration.
svn checkout http://emacspeak.googlecode.com/svn/trunk
Orca is a screen reader for graphical desktop environments, principally GNOME. It has also been used, with varying degrees of success and support, under Xfce (version 4.10 and later) and Ubuntu Unity. Officially, Orca is part of the GNOME project.
Orca provides non-visual access via both speech and braille interaction. It can work with any text to speech software supported by Speech Dispatcher and with any braille display supported by BRLTTY (described above). Whereas earlier versions of Orca also offered screen magnification, this functionality is now supplied separately by Gnome-Shell Magnifier.
In order to be compatible with Orca, desktop environments and applications need to implement the GNOME Accessibility API. (The GNOME Accessibility Developers Guide should be read for an explanation and examples of usage.) Moreover, it is important that the user be able to interact with the user interface by means of a keyboard alone, as discussed in the GNOME Human Interface Guidelines. Thus, desktop environments and applications which do not support the GNOME Accessibility API are inaccessible to users with disabilities who require a screen reader. Further, if the API is partially or incorrectly implemented by one or more UI components, barriers to access are likely to arise. These difficulties can range from inconveniences to obstacles that entirely preclude the use of an application by screen reader users.
Well known applications supported by Orca include Mozilla Firefox, Mozilla Thunderbird, LibreOffice, Pidgin and Eclipse. The GNOME Accessibility API, required by Orca, is implemented by popular UI libraries such as GTK+, Qt (version 4.8 and later) and Clutter. this overview of the accessibility API implemented in Qt. Java applications built with UI components that implement the Java Accessibility API are supported. Support for the GNOME Accessibility API is also available to Mono Common Language Infrastructure (CLI) applications that implement User Interface Automation; see this architectural overview for a description.
It should be noted that access to Java applications requires the Java ATK Wrapper to be installed. This can be obtained from Linux distribution repositories or directly from GNOME. Access to Qt 4.8 applications requires the AT-SPI Accessibility Plug-in for Qt to be installed. As of version 5.0, this component has been integrated into Qt and therefore no further installation is necessary. Application authors should refer to this documentation for an overview of the Qt accessibility API.
Orca includes support for a variety of languages. Contracted braille, for example English Grade II, is available.
Orca Home Page: http://live.gnome.org/Orca/
Speakup is a screen reader for Linux virtual consoles. It consists of modules loaded into the Linux kernel. As such, Speakup can be invoked early in the boot process. Thereafter, all interactions in virtual consoles are accessible to the user.
Speakup is compatible with text to speech software such as Espeak and with other TTS systems supported by Speech Dispatcher. In addition, Speakup can operate hardware-based speech synthesizers connected via a serial interface. Speakup is widely available from Linux distributions. Furthermore, it is well suited for incorporation into installation media, rescue images and other environments in which resources are tightly constrained.
Efforts are under way to prepare Speakup for inclusion in the
mainline Linux kernel. Currently, it resides in the
directory of the kernel, in which device drivers and other
components not yet ready for full integration are developed. Despite
this status, Speakup has been used intensively to perform desktop
computing and system administration tasks for many years, and, like
other projects described here, has a vibrant community of users.
Speakup Home Page: http://www.inux-speakup.org/
Yasr is a screen reader for Linux virtual consoles. Its principle of operation is similar to that of Tmux and GNU Screen: Yasr allocates a pseudo-terminal and then invokes a shell, for example Bash. All user interactions within the pseudo-terminal are accessible via Yasr's speech interface. Yasr is typically invoked after the user has logged into the operating system.
Yasr works with TTS software supported by Emacspeak speech servers or by Speech Dispatcher. Certain hardware-based speech synthesizers, connected via a serial interface, are supported directly. Yasr is packaged by Debian.
Yasr Home Page: http://yasr.sourceforge.net/