Video telephony is full-duplex, real-time audio-visual communication between or among end users. The idea of the video telephone, also called the videophone, is about as old as the telephone itself.
The primary challenge facing developers of the video telephone is the fact that full-motion, high-resolution video data requires far more bandwidth than audio data. This is true whether the signals are analog or digital. The bandwidth of a video signal can be minimized by using the lowest image resolution that will give acceptable results, by settling for grayscale rather than color imagery, and by transmitting non-moving images at intervals of several seconds rather than a continuous, full-motion image. A video signal of this type, which is a form of slow-scan television (SSTV), can be transmitted and received over ordinary copper telephone lines.
Broadband Internet solutions such as DSL (Digital Subscriber Line), cable, and land-based wireless make it possible to transmit and receive video data at higher resolutions and more rapid refresh rates than is the case with the ordinary telephone system. Broadband satellite solutions can also work, although the latency inherent in geostationary-satellite systems produces an image and voice delay that is objectionable to some users. Even with the best broadband technology available to consumers today, the image quality is modest, resembling the picture on a small fast-scan television (FSTV) receiver.
The technology for SSTV video telephony has been widely available for years, but videophones are not in widespread use. Possibly the main reason is that most people do not want to be seen when they are on the telephone, unless it is for business reasons (videoconferencing). Another apparent reason is the cost of the hardware, which requires a display screen and a video camera. A third reason is the generally low image quality and slow refresh rate. As high-speed Internet access becomes more widely available, fast-scan television (FSTV) or high-definition television (HDTV) video telephony is likely to become more attractive for potential home and business users.