MULTICHANNEL MODULATION SYSTEM 



21 



small division of the circle constitute a commutator bar which is activated by 

 the information of a particular code digit. Thus as the brush is stepped 

 around at a uniform rate it puts digit information on the line in proper se- 

 quence. In each complete revolution there appears a set of on-off pulses 

 constituting one frame. With twelve channels in the group, and seven 

 digits to each channel, one revolution of the brush arm covers the frame of 

 84 digit positions. The revolution period is 125 microseconds. Roughly 

 one and a half microsecond is allotted to each digit, and the brush steps 

 672, ceo times a second. The driving force for stepping is supplied by a 



HEAVY LINES- MESSAGE PATH 

 DASHED CIRCLES-MECHANICAL" DRIVE 



Fig. 9. Representation of the method used for framing, in relation to a sirgle 12-channel 

 time-division group. 



stable oscillator, its frequency determined by a quartz crystal. One of the 

 digit positions is taken away from its message duties and assigned for framing 

 purposes, as described in the last section. 



Our clock now is a one-handed affair with the brush arm for the hand. 

 Its scale has the customary twelve main divisions, but each main division 

 is divided into seven parts, one for each digit. Hour and minute markings 

 refer to channel and digit, respectively. 



At the receiving end we imagine a similar clock structure to be provided. 

 Input pulses go directly to the clock hand which is stepped around to dis- 

 tribute pulses to the twelve message channels, plus a framing channel which 

 takes up but a single digit space. Here the clock has no independent driving 

 source, since it must follow the transmitter. Accordingly, the basic 672- 



