MULTICHANNEL MODULATION SYSTEM 37 



device. The time constant / = RC is such that, during any single pulse 

 interval, whatever charge is on this capacitor decays precisely 50% in ampli- 

 tude. It follows that the charge remaining at some chosen instant after 

 the arrival of a complete code group consists of contributions of all its pulses, 

 weighted in a binary manner. That is, if we define the contribution of a 

 pulse in the final digit position as ^, then contributions of -^ , ■§-, xVj A? 

 ■^ and y^sj respectively, are made by pulses in successively earlier posi- 

 tions. Any value from to xf J"? in steps of xt8> niay thus be produced. 

 Of course the digit holes in the aperture . plate of the coder are laid out to 

 make this straight-forward scheme workable. Since samples of low-level 

 audio signals are coded near the center of the aperture plate, the corre- 

 sponding decoded values lie in the neighborhood of J. 



The basic Shannon decoder, then, comprises the resistance-capacitor cir- 

 cuit, means for supplying it with precisely controlled units of charge at 

 precisely determined times, and a sampling and holding circuit (repre- 

 sented by switch B or B' in Fig. 10) to measure and store the decoded 

 potential which is fleetingly present across the capacitor at a regularly 

 recurring instant T, following the final pulse position. The scheme 

 employed to inject the identical charges involves a regulated source of 

 current and a gate to admit this current to the resistance-capacitor circuit 

 under control of the regenerated PCM pulses. Two successive slicing opera- 

 tions and careful gating, as described earlier, make these pulses more than 

 adequately uniform. 



The wave form sketches of Fig. 21 show three typical decoding cycles. In 

 the first, a single pulse in digit position 7 produces a decoded amplitude of 

 J; in the second, a pulse in position 1 gives y^q; and in the third, pulses at 

 2, 5 and 7 provide a decoded value of ^. It may readily be verified that 

 the provision of an idle channel interval between operations (following from 

 the alternate use of two decoders) allows the residue of one decoding opera- 

 tion to decay to a negligible value (never larger than xJs" of ^ single step 

 height or "quantum") by the time of the next consecutive sampling. 

 Experimentally, interchannel crosstalk from this source is virtually non- 

 existent. 



In the foregoing it has been emphasized that precise timing is required for 

 this basic Shannon decoder. Although the necessary accuracy was actually 

 obtained without great difficulty in early tests, a modification has also been 

 introduced which eases the requirements to a very marked extent. This 

 scheme, devised by A. J. Rack, employs a damped resonant circuit in con- 

 junction with the resistance-capacitance elements in a manner illustrated 

 by Fig. 22. The natural frequency of the resonant circuit L, C2, R2 is made 

 equal to the PCM pulse rate, and the time constant of the damped oscillation 

 {t=2R2C2) is matched to that of the circuit Ri, Ci. The same charging 



