36 



BELL SYSTEM TECHNICAL JOURNAL 



the screen of a test oscilloscope may be thought of as a television picture of 

 the aperture plate of the coding-tube. To produce the pattern an audio- 

 frequency sawtooth wave of full-load amplitude was applied to the sampling 

 and holding circuit at the input of a coder. The resulting sample ampli- 

 tudes, quantized by the coding tube and falhng into all the possible 128 steps 

 of the quantizing characteristic with uniform regularity, were used to ener- 

 gize the vertical deflection of the test oscilloscope. An ordinary synchro- 



REGENERATED 

 PCM PULSES 



TIME >- 4,/g^ 



Fig. 21. Shannon decoding circuit and waveforms. 



nized sweep provided the horizontal deflection. The square PCM pulses, 

 delivered by the coder via the pulse regenerator, were applied to the intensity 

 control. 



Thus the code pulses corresponding to each quantized amplitude were 

 made to appear as a row of blanks in a horizontal trace at the proper relative 

 height. This pattern is very useful in studying coder performance. 



Decoders. The decoding method is an impressively simple one originally 

 proposed by C. E. Shannon. In its basic form it employs a pulsed resistance- 

 capacitor circuit as illustrated in Fig. 21. Upon arrival of each pulse of the 

 code, an identical increment of charge is placed upon the capacitor of the 



