Screen reader

Types

Command-line (text)

In early operating systems, such as MS-DOS, which employed command-line interfaces (CLIs), 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 be obtained from the system either by hooking the flow of information around the system and reading the screen buffer or by using a standard hardware output socket and communicating the results to the user.

In the 1980s, the Research Centre for the Education of the Visually Handicapped (RCEVH) at the University of Birmingham developed a Screen Reader for the BBC Micro and NEC Portable.

Graphical

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 therefore there is no purely 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 and/or shown on a refreshable braille display.

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. Existing APIs include:

• Android Accessibility Framework
• Apple Accessibility API
• AT-SPI
• IAccessible2
• Microsoft Active Accessibility (MSAA)
• Microsoft UI Automation
• 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 is to be communicated to the user. This approach is considerably easier for the developers of screen readers, but fails when applications do not comply with the accessibility API. One approach when the accessibility API is insufficient is to use available operating system messages and application object models to supplement accessibility APIs.

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, according to some users, using a screen reader is considerably more difficult than using a GUI, and many applications have specific problems resulting from the nature of the application (e.g. animations) or failure to comply with accessibility standards for the platform.

Customization

Most screen readers allow the user to select whether most punctuation is announced or silently ignored. Some screen readers can be tailored to a particular application through scripting. One advantage of scripting is that it allows customizations to be shared among users, increasing accessibility for all. JAWS enjoys an active script-sharing community, for example.

Verbosity

Verbosity is a feature of screen reading software that supports vision-impaired computer users. Speech verbosity controls enable users to choose how much speech feedback they wish to hear. Specifically, verbosity settings allow users to construct a mental model of web pages displayed on their computer screen. Based on verbosity settings, a screen-reading program informs users of certain formatting changes, such as when a frame or table begins and ends, where graphics have been inserted into the text, or when a list appears in the document. The verbosity settings can also control the level of descriptiveness of elements, such as lists, tables, and regions. For example, JAWS provides low, medium, and high web verbosity preset levels. The high web verbosity level provides more detail about the contents of a webpage.

Language

Some screen readers can read text in more than one language, provided that the language of the material is encoded in its metadata.