MULTICHANNEL MODULATION SYSTEM 



35 



With this arrangement the beam, suddenly turned on, moves either 

 upward or downward from its initial "unquantized" position to the nearest 

 position of stable equilibrium. Quantization is completed in less than a 

 microsecond. Thereafter, as the beam is swept horizontally across the tar- 

 get area, it remains pressed upw^ard against the lower surface of its guiding 

 wire. Quantization is thus maintained until the end of the sweep, when 

 blanking occurs. The margins against hopping across wires, while quanti- 



Fig. 20. ''Television picture" of the aperture plate of a coder tube. 



zation is in effect, are related to the amount of loop gain in the feedback path. 

 In the present system feedback measuring about 20 decibels is provided, and 

 will counteract changes which, without feedback, would move the beam two 

 grid wires in either direction from the initial position. 



This coding process, based on the electron beam coding tube and making 

 use of a two-dimensional permanent layout of the code, is more straight- 

 forward than the various coding processes w^hich depend upon counting or 

 sequential comparison. Accordingly it leads to higher speeds and greater 

 circuit simplicity. 



Figure 20 illustrates the coding accuracy obtained. This photograph of 



