MULTICHANNEL MODULATION SYSTEM 



together with phase linearity. For the same purpose in a frequency-division 

 system, filters are used to select and to combine channels. To avoid crosstalk 

 the filters must be sufficiently selective, and amplitude non-linearity must be 

 held within limits. 



In the present system, the pulse code is delivered by the coder as on-off 

 pulses in time sequence. It is therefore natural to organize the pulses of the 

 different channels so that they appear in sequence, thus forming a time-division 

 multiplex. Most types of coder require an appreciable length of time, after 

 delivering the pulses of one channel, to prepare for coding the next. This 

 preparation time may be afforded conveniently, without introducing gaps in 

 the pulse train between assignments of consecutive channels, by providing two 

 coders, which take turns at the channels in each time-division group. 



As the number of channels increases, evidently the time interval which can 

 be assigned to each channel must be reduced since all of them must be fitted 

 into one period of an 8-kilocycle wave. Similarly the allowable duration of a 

 code or digit pulse becomes shorter as the number of time -division channels 

 in a group is increased. Then too, pulses tend to become more difficult to 

 generate and transmit as their duration decreases. For these reasons it is 

 desirable, and eventually it becomes necessary, to restrict the number of chan- 

 nels included within a time-division group. Frequency division may then be 

 used to aggregate several time-division groups. 



For our purposes groups of 12 channels are multiplexed by time division. 

 With seven digits per channel, each group has a capacity of 672,000 pulses per 

 second. To combine eight of the groups for a 96-channel system we again have 

 a choice between frequency division and time division. The equipment of 

 Fig. 1 is laid out to accommodate either procedure. In the first case each 

 group is assigned a carrier for amplitude modulation, as used in actual tests 

 to be described. For the second case the pulse durations would be cut down 

 by a factor of eight, and the pulses from the different groups interlaced. Here 

 the supergroup would have a capacity of 5,376,OCO pulses per second. 



In carrying out coding operations, and in multiplexing on a time-division 

 basis, various control pulses are required which differ in repetition frequency, 

 in time of occurrence, and in duration. These may be generated from a stable 

 base frequency oscillator through the use of harmonics or, alternatively, of 

 sub-harmonics. In a time-division system which requires a variety of flat- 

 topped waves for switching operations, the use of sub-harmonics fits naturally. 

 Frequency step-down circuits of the multivibrator type produce waves either 

 approximating the desired forms directly, or requiring only simple circuits for 

 reshaping. In contrast harmonic generation requires filters for component 

 wave selection, more and more elaborate in structure as the order of the wanted 

 harmonic goes up. Then, after selection, the harmonic has to be amplified 

 ^nd limited or otherwise shaped for switching purposes. Generally speaking 



