EFFICIENT CODING 735 



message, to produce a reduced signal having more nearly independent 

 symbols, but (in return) a highly peaked simple probability distribution 

 which allows savings with Shannon-Fano coding on a symbol-by-sym- 

 bol (fc = 1) basis. 



The simplest member of this class is the monogrammer. It is basically 

 merely a re-ordering device. The operation may be best understood by 

 the following example. Suppose someone supplied us with English text 

 encoded into a quantized pulse signal as follows: 



Xow the letter frequencies in English are shown in Fig. 3. Merely to 

 sa\'e average power in our channel we might wish to convert this signal 

 into one in which the pulse height is not alphabetical, but in which the 

 most common symbol is sent as a pulse of zero height, the next most com- 

 mon as a pulse of unit height, etc. In other words, we would like the fol- 

 lowing representation : 



The device shown in Fig. 4 will accomplish this translation. The orig- 

 inal signal is applied to the vertical deflecting plates of a cathode ray 

 tube. The rest position of the spot corresponds to "space", i.e. no pulse. 

 A pulse one unit high deflects the spot to A , a pulse two units high de- 

 flects the spot to B, etc. 



Now in front of these spot positions we place a number of light at- 

 tenuating filters. In front of the "space" position we place an opaque 

 mask. Hence when the spot is deflected to "space" the photocell receives 

 no light and no pulse is sent. In front of the "E" position we place a 

 mask having one unit of transmission. So although E is received as a 

 pulse 5 units high, it is sent as a pulse of unit height. In front of the 



