Two-way audio allows students to interact with the instructor. Telewriters and electronic pens and tablets are variations on this technology. A page is scanned by a FAX machine at one end, transmitted, and printed at the other end in about 45 seconds. The cost, after the initial equipment investment, is that of the telephone call. FAX is used for important, high-quality images such as conference papers, contracts, design plans, and graphics.
FAX machines can also interface with microcomputers and can double as copy machines.
Freeze-frame or slow scan video can best be described as an electronic slide show transmitted by telephone lines. Transmission takes a few seconds to more than a minute. Compressed video uses a telephone data circuit. The video signal is compressed by a picture processor to eliminate redundant electronic information. A moving video signal appears instantly at the receiving end, but rapid motion may be jerky or blurred.