EFFICIENT CODING 



737 



that the system can be designed to have a lower peak power capacity. 

 The signal out of the monogrannner can be remapped into binary digits 

 using a Shannon-Fano code, pulse by pulse. However, this could have 

 been done ecjually well with the original signal merely by rearranging the 

 code groups in the coder tube. It is when we extend the principle to di- 

 gi-ams and trigrams that the potentialities of the system become evident. 

 We can easily take account of the influence of the preceding message 

 symbol. To do this we apply the signal to the vertical plates as before, 

 and to the horizontal plates we apply the signal delayed by an amount 

 eciual to the time l^etween successive pulses as shown in Fig. 5. Thus the 

 beam is deflected vertically by the present message symbol, and horizon- 

 tally by the previous message symbol. Whereas before we used a single 

 colimm of optical filters chosen in accordance with the simple probabil- 

 ities of the letters, we now have 27 columns, one for each letter and one 

 for the space. The filters in each column are chosen in accordance with 

 the conditional probabilities which apply when the corresponding letter 

 was the previous symbol. For example, in the "Q" column (last letter 

 Q), and the " U" row (present letter U) the mask would be opaque, since 

 U is most common after Q. In general, the transmission of cell ij, in the 

 i^^ column and / row, is proportional to the rank of the entry for pi(j) 

 when the entire distribution (conditioned on i) is ordered in a monotone 

 decreasing sequence. The amplitude distribution of the output pulses 



PHOTO 

 CELL 



Fig. 5 — The "Digrammer." 



