A number of redundancy reduction techniques are used in a coder that is about eight times more efficient than simple PCM. The coder is capable of transmitting Picturephone® signals at an average rate of one bit per picture‐element (2 Megabits per second). When there is movement in the scene, most transmission time is devoted to the parts of the picture that change significantly. The data are generated irregularly but the data flow is smoothed prior to transmission in a buffer that holds about one frame of data. The redundancy reduction techniques used and the behavior of the coder are discussed both from an intuitive and from a statistical viewpoint. The positions of elements that change are signaled by addressing the first element of a run of changes and marking the end of the run with a special code word. The changes of luminance are transmitted as frame‐to‐frame differences using variable‐length code words. When rapid motion makes the buffer more than a quarter full, only differences for every second element are transmitted, the values of the intervening changed elements being set equal to the average of their neighbors. If the buffer continues to fill, the threshold that determines which changes are significant is raised from 4/256 to 7/256 of the maximum signal value. When violent motion causes the buffer to fill completely, replenishment is stopped for about one frame while the buffer empties. Subsampling and raising the threshold are not objectionable because viewers rarely detect the small impairments introduced in moving images. Observers are critical, however, of small impairments in stationary scenes. Thus, to maintain high quality in stationary areas, the entire picture is forcibly updated every three seconds by transmitting 8‐bit luminance values for three lines of every frame. A record of the coder's behavior is available as a 16‐millimeter movie film.
This paper describes an experimental method for encoding television signals which takes advantage of the frame‐to‐frame correlation to reduce transmission bit rate. The technique encodes only those elements that change between successive frames instead of encoding every element of every frame. We have demonstrated the method in real‐time using the head‐and‐ shoulder view of a person in animated conversation as the picture source, such as is likely to be encountered in a visual communication system. An average transmission rate of one bit per picture element gives quality comparable with standard eight‐bit PCM transmission.
This paper describes some techniques for efficient coding of two‐tone (black and white) facsimile pictures. These techniques use the two‐dimensional correlation present in spatially close picture elements to change the relative order of transmission of elements in a scan line. This ordering increases the average length of the runs of consecutive black or white elements in the ordered line, making the data more amenable to one‐dimensional run‐length coding. We describe several variations of the ordering scheme, which differ in complexity and coding efficiency and evaluate their coding efficiency. For a variety of 8‐1/2 inch by 11‐inch typed documents, road maps, and circuit diagrams scanned with 200 lines/inch, these techniques reduce the bit rate by 30 to 50 percent over and above the one‐dimensional run‐length coding along a scan line; for single‐spaced typed material with 100 lines/inch, this reduction is about 25 percent. We compare one of our techniques with a two‐dimensional compression technique recently proposed by Preuss. We show that our technique results in an entropy about 10 to 18 percent lower than that obtainable through Preuss' technique.
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