MULTICHANNEL MODULATION SYSTEM 



33 



beam is blanked, and is simultaneous with the application of the succeeding 

 audio sample. 



The wires of the quantizing grid are used to guide the beam so that it can 

 illuminate only the particular row of apertures which correspond to the 

 initial vertical deflection. Without this feature, erroneous codes would be 

 produced when the beam straddled the edges of apertures or crossed from 

 one ampHtude level into another, as a result of electrode misalignment or 

 possible slight drift in potential of an applied sample. The guiding action 

 (basically proposed by W. A. Harrison and applied to the PCM coder at the 

 suggestion of G. Hecht) is obtained by means of feedback from the quan- 

 tizing grid to the vertical deflection amplifier. 



COLLECTOR 



QUANTIZING 

 GRID 



-AUDIO SAMPLES 



FROM 



COMPRESSOR 



APERTURE 

 PLATE 



SWEEP 



Fig. 18. Functional schematic of the coder. 



The feedback signal is actually a current taken from the positively biased 

 collector, which draws to it secondary electrons from the grid wires. The 

 portion of the beam current striking the grid varies as a cyclic function of 

 the vertical deflection. It follows that for some spot positions the value 

 /X|S in the feedback loop is positive, for others negative; hence with proper 

 amplifier design there is a stable and an unstable region associated with each 

 wire. 



The spot can come to rest (vertically) only within one of the stable regions. 

 In order to locate it consistently near the center of such a region, and thus 

 gain equal margins against ''hopping" upward or downward across a wire, 

 a "quantizing bias" is introduced into the vertical deflection amplifier, along 

 with the feedback and the signal samples. This bias is a current of opposite 

 polarity from the unidirectional feedback current, and of magnitude equal 



