7G0 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



obtain a well-defined black-on-white curve of p(x) versus x on a loga- 

 rithmic probability scale. The logarithmic scale has the advantage of 

 making the curve shape independent of exposure length and giving uni- 

 form relative accuracy over the entire range. 



Fig. 9 shows some typical results obtained by means of the "proba- 

 biloscope." The two small curves are distributions of two different still 

 pictures. The left-hand end corresponds to black, the right-hand end to 

 peak white; the blanking intervals (slightly blacker than black) cause 

 the peaks at the extreme left. (The signals did not contain any synchro- 

 nizing pulses.) The tall and slender curve at the right of Fig. 9 is the 

 distribution of errors resulting from previous-value prediction of one 

 of the pictures in Fig. 4. The peak corresponds to zero error which is 

 seen to be most probable, as it should be if the prediction criterion is 

 good. Increasingly larger errors are increasingly improbable or rare. 

 The six decades of probability density spanned by the curve were ob- 

 tained in three separate exposures and subsequently joined, since stray 



02468 10 02468 10 



AMPLITUDE, X, IN ARBITRARY UNITS 



Fig. 9 — Typical probability distributions as obtained from the probabiloscope. 

 Curves at left are for video signals; right-hand curve is for difference between 

 video signal and delayed replica. 



