U MULTICHANNEL MODULA TION SYSTEM 



noted that the channel samples thus collected are not ''held" at this stage; 

 i.e., each sample does not remain constant in potential during its assigned 

 interval, but rather changes to follow the wave form of the audio signal in 

 the corresponding channel. 



The multiplex signal is supplied to an instantaneous compressor, which 

 employs silicon rectifier elements to give an input-output characteristic of 

 the general form indicated within its block (Fig. 7). To understand the 

 purpose of this device, we must recall the discussion of quantizing noise 

 given earlier. There it was found desirable to provide a tapered distribution 

 of step heights in the staircase-like quantizing characteristic, thus devoting 

 a considerable number of small steps to the treatment of background noise 

 and low-level signals. Although coders have been devised which inherently 

 deal with signal amplitudes in this graded manner, it has been found more 

 practicable in the present system to apply amplitude compression to the 

 samples before coding, and to divide this compressed amplitude range into 

 uniform steps in the coder. The result is a tapered step distribution with 

 respect to the original uncompressed scale of amphtudes, details of the dis- 

 tribution being determined by the shape of the compression characteristic. 



It may be well to note here that this method can be used in reverse at the 

 receiver, with the decoding performed on an equal-step basis, and the re- 

 sultant samples passed through a complementary instantaneous expandor. 

 If the compression and expansion are truly complementary, the overall 

 characteristic relating amplitudes of input and output samples will be linear 

 except for the tapered array of quantizing steps (Fig. 4b). 



Incidentally, no added band width in the transmission path of this system 

 is required to accommodate the instantaneous compandor action. 



After compression, the multiplex signal is delivered at low impedance to 

 the inputs of two coders. In Fig. 7 the switch analogy is called upon again 

 to illustrate the routing of alternate samples to the ''odd" and "even" coders, 

 and concurrently the storage of these samples on "holding capacitors" to 

 keep them unchanged during the coding operation. Here the switches 

 rotate at 48,000 revolutions per second, each one closing six times in a com- 

 plete 8-kilocycle frame. The contact segment is drawn as a short arc to 

 indicate a brief closure, actually lasting about 5 microseconds and occurring 

 while the switch of the collector is in contact with a single segment. When 

 the circuit is thus completed from a particular channel to the holding ca- 

 pacitor, the vohage on the latter very rapidly assumes and then follows, 

 for the remainder of the 5-microsecond interval, the potential of the com- 

 pressed version of the channel signal. When the circuit opens, the latest 

 state of charge, which is essentially an "instantaneous" sample, is left on the 

 capacitor, and is thus held for about 16 microseconds — until the next closure. 

 These sampling operations occur alternately in the two coders. 



