TELEVISION BY PULSE CODE MODULATION 39 



a quantized input. The encoded signal is subsequently translated into the 

 conventional binary code. 



The coder and quantizer are probably the most important parts of the 

 terminals of a PCM system. The following discussion of the two types of 

 coding tubes will illustrate how they function and show how the new tube 

 can function at the greater speeds necessary for television. 



Consider the code plate on the left side of the next figure (4) . This plate 

 gives the conventional binary code as discussed in connection with the first 

 figure. In a time-division coding tube a point beam is used. It is deflected 

 vertically by the output of the sampler. After the beam has settled down 

 to its proper position, which corresponds to the quantized signal amplitude 

 it is swept across the code plate. An output collector is used which covers 

 the back of the code plate. If the beam goes through a hole in the code plate 

 a pulse is produced; if it is stopped by the code plate no pulse is produced. 

 By this means a code group is produced on a time-division basis for each 

 sample. 



As long as the beam does not fall on the edge of a hole, this arrangement 

 functions satisfactorily. Now consider the case where the beam sweeps 

 across the set of edges corresponding to the amplitude 15.5. It is seen that, 

 by a slight misalignment of the code plate and the horizontal deflecting 

 plates, the beam could produce either the code group corresponding to 31 

 or to depending upon the way the deflection axis is tilted with respect to 

 the code plate. This would result in an error equal to one-half of the total 

 amplitude range of the system. Corresponding errors of smaller magnitude 

 are possible for other levels. In all cases this type of error results for signals 

 which have amplitudes one-half way between values permitted by the code. 



Errors of this sort can be avoided by quantization of the signal before the 

 coding. This is accomplished in the earlier tube by using the output from a 

 mesh of grid wires in a feedback arrangement. The wires of the grid overlap 

 the edges of the holes in the code plate. When the beam hits one of these 

 grid wires, a current is fed back into the input which causes the signal am- 

 plitude to change in such a way as to move the beam between the grid wires. 

 After a short interval the beam settles down in a quantized condition. Then 

 the beam is swept across the code plate. If the beam tends to become mis- 

 aligned during the deflection process, the feedback from the grid wires cor- 

 rects this condition and an accurate code is produced. Because of the time 

 required for the feedback process, this method of quantization limits the 

 number of samples that can be coded in a given time. 



Another factor which limits the speed of this type of coder is the time re- 

 quired to sweep the beam across the code plate. 



It is apparent that the time required to register the code could be reduced 



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