A practical voiceband television system design.

Abstract

A need exists to send television on channels of limited bandwidth, particularly 3 kHz. Successive images must have adequate picture content yet be frequent enough to give the appearance of motion. A new coding scheme was devised to meet this need for video data compression. A 2400 baud system was designed and built. A hierarchy of transmission modes is used which allows the transmitter to choose the most efficient of 3 modes to send a line. One mode uses a variable-length Instantaneously Uniquely Decipherable code optimized for the statistics of some typical expected pictures. The picture size is 64 by 64 elements, each with 4 luminance levels. The transmitter has a stored line (129 bits) and a two-dimension model to predict the following line. After comparing the prediction to a fresh line the deviations from the prediction are sent. Then the receiver uses the same model to update a similar stored line to be displayed. Frame duration can range from .13 to 3.54 seconds as determined by the actual picture.