770 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



It is of interest to mention that "slope" prediction is equivalent to 

 two "previous value" predictors in tandem. Three or more "previous 

 value" predictors in tandem are ecjuivalent to a binomial weighting 

 of the previous values of the signal to form a predicted signal. 



For example, the prediction for three "previous value" predictors in 

 tandem is given by 



Op = 3»Sl,o — 3*^2,0 ~t~ *J3,0 



For four "previous value" predictors in tandem 



Sp = 4aSi,o — 6o2,0 ~[~ 4*S3,n — *S4,0 



As can be seen from the above equations, further extension of "previous 

 value" tandem operation results in a heavier weighting of picture ele- 

 ments further and further from the point to be predicted. For most 

 pictures this leads to greater errors. 



^^Previous line'' prediction, shown in Fig. 7, would be expected to be 

 similar to previous value prediction, since a picture would presumably 

 have approximately the same correlation vertically as it does horizon- 

 tally. This would be the case except that our interlaced scanning system 

 makes the previous line signal some 28 per cent further away from >So,o 

 than the closest horizontal sample, *Si,o . The error signal, e, is given by 

 where T is a line time. The error output has a maximum peak amplitude 

 of twice the input. 



The filter characteristic can be expressed as 



■ \2 ~ "2~J 



F(co) = 



_2sm-J 



Fig. 6 — Example of "slope" prediction. Here the next signal value is assumed 

 to lie on a straight line that intersects the two previous signal values. 



