30 BELL SYSTEM TECHNICAL JOURNAL 



the cathode current to F2, cannot occur as long as the grid of either Vi or the 

 added tube is positive with respect to the critical slicing potential. Tripping 

 does occur whenever this limitation is removed, and thus the desired gating 

 and slicing functions are performed concurrently. 



In the experimental system this circuit is used to regenerate the PCM 

 pulses at the common output of the two coders, and again at the input of 

 each decoder to slice the received pulses and to sort out code groups of odd 

 and even channels. 



Coders. The method of binary coding used in this system was originally 

 'suggested by F. B. Llewellyn. It employs a novel electron beam tube. 

 This device, pictured in Fig. 16, carries out simultaneously the two functions 

 of quantizing and of coding. The tube is about 10| inches long and 

 2\ inches in diameter. It has the 128 combinations of the 7-digit code 

 laid out permanently as holes in an "aperture plate," and translates sample 

 amplitudes from the form of beam deflections directly into PCM pulse sym- 

 bols. Figure 17 shows one of these tubes in its socket on the rear of a coder 

 panel, and above it a rectangular permalloy shield which covers the coding 

 tube of the other coder serving the same 12-channel group. 



As shown schematically in Fig. 18 the coder includes, in addition to the 

 coding tube, a sampling and holding circuit which sorts out the odd (or even) 

 channels from the input multiplex signal, push-pull amplifiers for vertical 

 and horizontal beam deflections, and simple arrangements for blanking, 

 focusing and centering. Within the tube are shown, from left to right, a 

 conventional electron gun, vertical and horizontal deflection plates, a rec- 

 tangular '^collector" for secondary electrons, a "quantizing grid," the 

 "aperture plate" and finally a "pulse plate." Figure 19 shows the target 

 end of the tube, as viewed from a point near the gun. "Digit holes" in the 

 aperture plate, laid out in accordance with the desired binary code, may be 

 seen behind stretched parallel wires of the quantizing grid. One may count 

 64 narrow holes separated by equally narrow bars of metal in the left-hand 

 vertical column, 32 holes in the next column, 16 in the next, and so on for 

 seven columns. There are 129 grid wires uniformly spaced and accurately 

 aligned so as to mask the upper and lower edges of every one of these holes 

 when viewed from the geometric "point of origin" of the beam. 



Stored audio samples from the sampling and holding circuit provide poten- 

 tial for the vertical deflection, with zero at the center, positive amplitudes in 

 the upper half, and negative amplitudes in the lower half of the target area. 

 A sawtooth sweep provides the horizontal deflection. The beam is blanked 

 while deflection potentials are being changed to move it upward or downward 

 from one sample amplitude to the next. When first restored, the beam 

 strikes in the left-hand unperforated region of the aperture plate, and is then 

 swept linearly across from left to right. Electrons which pass through the 



