TELEVISION BY PULSE CODE MODULATION 



41 



the original in terms of the rectangular waves. The second digit gives a sec- 

 ond-order correction to add to the first digit, and the third digit gives a 

 third-order correction to add to the first and second digits. 



The rectangular waves, of course, are the envelopes of the pulses that are 

 transmitted over the various digit channels. Because the respective values 

 represented by the various digits are once and for all known, it is not neces- 

 sary that the amplitudes with which the pulses are transmitted be equal 

 to the values which they represent, but they may to advantage be sent with 

 the same amplitude in all of the digit channels. At the decoder the relative 



-'1^^- 



KTn.-'I^I 



1 ST DIGIT 



I I I I I I I 



2 ND DIGIT 



3 RD DIGIT 



R^ 1^ ^ 1^?^ 1^ 1^ ^ [x1 N [XI P?^ [^ ^ N 



I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 

 Fig. 5 — Rectangular wave approximations. 



amplitudes of the digit channels are restored according to the coding con- 

 vention and the results added to obtain the rectangular wave approxi- 

 mation to the original wave. It is seen that the first three digits give a fair 

 approximation to the original wave. More digits, of course, would improve 

 this approximation. 



In general terms, from this point of view, the coder is an analyzer which 

 determines the best approximation to the information wave in terms of a 

 series of rectangular waves of decreasing ampHtudes. The decoder is a syn- 

 thesizer which approximates the original wave by adding the rectangular 

 waves obtained from the coder. The coding convention allows the derived 

 rectangular waves to be transmitted with the same amplitude for all the 



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