The first type translates text into speech using the same set of rules all English speakers use. All words are not translated correctly in this type of system because of the incredible number of exceptions to the rules in the English language. The second type of synthesizer digitizes the human voice, records it, and plays it back on demand. This results in a human sounding voice, but the number of words or phrases available to the user are very limited because this approach takes up a large amount of computer memory. Many of the devices described in this guide work independently of a computer system but can also be included in a system to add to the range of information and abilities the user with disabilities can access.
Some devices use the standard interfaces provided on the computer.
For example, joysticks and the mouse generally connect to a standard computer port which is either built in or readily available. Others require very special interface devices. Simple switches require a special switch interface that accepts the plug of the switch. Most switch producers also make a switch interface. Some interface devices are designed to accept more than one type of input device.