Screen reader

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[9] 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.[10][11]

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.[12]

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.

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[13]
• Apple Accessibility API[14]
• AT-SPI
• IAccessible2
• Microsoft Active Accessibility (MSAA)
• Microsoft UI Automation
• Java Access Bridge[15]

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 the developers of screen readers, but fails when applications do not comply with the accessibility API: for example, Microsoft Word does not comply with the MSAA API, so screen readers must still maintain an off-screen model for Word or find another way to access its contents. One approach 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 (e.g. Microsoft Word and Active Accessibility).

Some programs and applications have voicing technology built in alongside their primary functionality. 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 telephone services allow users to interact with the internet remotely. For example, TeleTender can read web pages over the phone and does not require special programs or devices on the user side.

Virtual assistants can sometimes read out written documents (textual web content, PDF documents, e-mails etc.) The best-known examples are Apple's Siri, Google Assistant, and Amazon Alexa.

A relatively new development in the field is web-based applications like Spoken-Web that act as web portals, managing content like news updates, weather, science and business articles for visually-impaired or blind computer users. Other examples are ReadSpeaker or BrowseAloud that add text-to-speech functionality to web content. The primary audience for such applications is those who have difficulty reading because of learning disabilities or language barriers. Although functionality remains limited compared to equivalent desktop applications, the major benefit is to increase the accessibility of said websites when viewed on public machines where users do not have permission to install custom software, giving people greater "freedom to roam".

This functionality depends on the quality of the software but also on a logical structure of the text. Use of headings, punctuation, presence of alternate attributes for images, etc. is crucial for a good vocalization. Also a web site may have a nice look because of the use of appropriate two dimensional positioning with CSS but its standard linearization, for example, by suppressing any CSS and Javascript in the browser may not be comprehensible.

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.[16] 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.[17]

Some screen readers can read text in more than one language, provided that the language of the material is encoded in its metadata.[18]

Some screen reading programs also include language verbosity, which automatically detects verbosity settings related to speech output language. For example, if a user navigated to a website based in the United Kingdom, the text would be read with an English accent.