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“STOP-SCAN EDGE DETECTION SYSTEMS OF TELEVISION BANDWIDTH REDUCTION”
by William K. Pratt
June 1965
A family of systems is developed to code television pictures such that the bandwidth, or time, required for transmission is reÂduced significantly compared to transmission by conventional pulse code modulation. The systems have application for spacecraft teleÂvision communication where the reduced bandwidth enables a reducÂtion in transmitted power. Data from optical detection and tracking sensors may be processed at a significantly increased rate with the systems. Also, for a fixed communications bandwidth, the television bandwidth reduction systems permit an increase in the number of television channels that can be relayed by an earth orbiting satellite or microwave relay station.
The major informational content of a television picture as judged by a human viewer lies in its outline, or edges, which occupy only a small area of the total picture. A bandwidth reduction is realÂized by transmitting video information suitably coded and time redis -tributed only at the edge positions in a picture. Edges are formed by subtracting the intensities of adjacent picture elements along a teleÂvision line. If the difference signal exceeds a threshold value an edge exists. The position of each edge is coded as the number of elements scanned since the previous edge occurrence.
The time redistribution is performed by a stop-scan picture information gathering and display process at the coder and decoder eliminating the need for high speed buffer storage units. In the stopÂscan process, camera scanning proceeds at a uniform rate; when an edge is detected, scanning is halted. At the next allowable edge transÂmission time, the video information is transmitted, and scanning reÂsumes. The display of video information at the decoder follows an inverse stop-scan process.
In this report the psychophysical properties of image viewing related to television systems are investigated to determine the relaÂtionship between television design parameters and communications bandwidth. Information theoretic bounds of bandwidth reduction and the optimum selection of system parameters are established. StaÂtistical image measurements are performed to determine the probÂability of occurrence of edges. The implementation of an operational prototype of the basic stop-scan edge detection system is described. Viewing tests are performed to verify the quality.of pictures proÂcessed by the basic system.