Screen reader

A screen reader is a software application that attempts to identify and interpret what is being displayed on the screen. This is then presented to a blind user as speech (by text-to-speech) or by driving a braille display. Screen readers are used by people with little or no functional vision: people with some vision often use screen magnifiers.

Types of screen reader

CLI (text) screen readers

In early operating systems such as MS-DOS which employed a Command Line Interface (CLI), the screen display consisted of characters mapping directly to a screen buffer in memory and a cursor position. Input was by keyboard. All this information could therefore all be obtained from the system by hooking the flow of information around the system and reading the screen buffer and communicating the results to the user. This was relatively easy to engineer. Screen readers were developed for early personal computers such as the BBC Micro .

GUI screen readers

Off-screen models

With the arrival of graphical user interfaces (GUIs), the situation became more complicated. A GUI has characters and graphics drawn on the screen at particular positions, and as such there is no textual representation of the graphical contents of the display. Screen readers were therefore forced to employ new low-level techniques, gathering messages from the operating system and using these to build up an "off-screen model", a representation of the display in which the required text content is stored.

For example, the operating system might send messages to draw a command button and its caption. These messages are intercepted and used to construct the off-screen model. The user can switch between controls (such as buttons) available on the screen and the captions and control contents will be read aloud.

Screen readers can also communicate information on menus, controls, and other visual constructs to permit blind users to interact with these constructs. However, maintaining an off-screen model is a significant technical challenge: hooking the low-level messages and maintaining an accurate model are both difficult tasks.

Accessibility APIs

Operating system and application designers have attempted to address these problems by providing ways for screen readers to access the display contents without having to maintain an off-screen model. These involve the provision of alternative and accessible representations of what is being displayed on the screen accessed through an API. Examples include AT-SPI, Microsoft Active Accessibility and the Java Access Bridge. Screen readers can query the operating system or application for what is currently being displayed and receive updates when the display changes. For example, a screen reader can be told that the current focus is on a button and the button caption to be communicated to the user. This approach is considerably easier for screen readers, but fails when applications do not comply with the accessibility API: for example, Microsoft Word does not comply with the Microsoft Active Accessibility API, so screen readers must still maintain an off-screen model for Word or find another way to access its contents.

Screen readers can be assumed to be able to access all display content that is not intrinsically inaccessible. Web browsers, word processors, icons and windows and email programs are just some of the applications used successfully by screen reader users. However, using a screen reader is, according to some users, considerably more difficult than using a GUI and many applications have specific problems resulting from the nature of the application (e.g. animations in Macromedia Flash) or failure to comply with accessibility standards for the platform (e.g. Microsoft Word and Active Accessibility).

Self-voicing applications

Some programs provide audio output (i.e. speech) so that they can be used by blind people or people who cannot see the screen. These programs are termed self-voicing and can be a form of assistive technology if they are designed to remove the need to use a screen reader.

Some web browsers illustrate this type of program. Opera offers built in text-to-speech support but is not specifically designed for use by a blind person: the speech is supplementary to the sighted user interface. IBM's Home Page Reader, on the other hand, is designed to be wholly usable by a blind person with a keyboard or Braille display and no screen reader running.

Screen reader products

Many different commercial screen readers are available, with different approaches and features. For example, some screen readers can be scripted. Scripts are written for individual applications and users so the behaviour of the screen reader can be tailored to the application and user. A person's choice of screen reader is dictated by many factors, including cost — screen readers can cost many hundreds of U.S. dollars — and the role of organisations like charities and schools. Screen reader choice is contentious and strong opinions and preferences are common.

Most screen readers are available as commercial products. Major screenreaders include:

Other screenreaders, such as Virgo from Baum, are sold as part of a package with a Braille display, allowing users of the Braille display to access their computer. Screen magnifiers often come with some rudimentary speech, such as reading items under the mouse cursor.

Recent operating system developments have included some built-in screen readers: Microsoft Windows 2000 and XP come with a very limited built-in screen reader called Narrator which works through Active Accessibility (and so can be used for MSAA applications such as Windows Explorer and Notepad but not for Microsoft Office or Internet Explorer). Apple has built VoiceOver, a much more fully-featured screen-reader, into Mac OS X.

Finally, there are Open Source screen readers at varying levels of ability, including the Linux-based and powerful Emacspeak ^* and the less developed BRLTTY and suse-blinux, for unix text consoles and gnopernicus for unix graphical applications exporting the AT-SPI interface. There is also the CLC-4-TTS Suite which works on multiple operating systems (Windows, Mac, and Linux), although it is currently limited to just reading for the Firefox web browser.

References

Assistive technology | Screen readers

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