Conditional Vertical Subsampling - A Technique to Assist in the Coding of Television Signals

The technique of reducing the horizontal-sampling frequency ("subsampling") in the moving parts of the television image has been pre787 788 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , APRIL 1972 viously described. 1 It was found that the frequency could be halved without visible degradation for most object speeds. For slow speeds the degradation was visible but not objectionable. Combining this technique with that of conditional replenishment 2 has proved particularly effective, 3 because in periods of fast movement, conditional replenishment by itself becomes uneconomic because so many picture elements (pels) change significantly from frame to frame. An obvious question to ask is whether a similar advantage results from subsampling vertically. In an interlaced television scan format, the idea of halving the verticalsampling frequency can be confusing because we must distinguish between fields and frames. A diagram showing the vertical position of lines for successive fields is shown in Fig. 1. One method of halving the vertical-sampling frequency is to sample every second line in each field and to replace the unsampled lines by interpolating the values of vertically adjacent elements in the same field [e.g., an unsampled line with coordinates y = 2 and t = 2 is replaced by an average of lines with y, t coordinates (0, 2) and (4, 2)]. This has been tried and the degradation is subjectively objectionable. The second method is to sample alternate fields so that in stationary pictures the vertical-sampling frequency is halved.