A TWELVE-CHANNEL CARRIER TELEPHONE SYSTEM 127 



to minimize interchannel crosstalk. The group modulators are of the 

 doubly balanced bridge type which aids in suppressing some of the 

 unwanted modulation products. Following the first group modulator 

 and also following the first group demodulator are coil and condenser 

 type 400-448 kc band filters which reject the unwanted products and 

 pass the band of frequencies containing the twelve channels. Between 

 this filter and the second group modulator on the transmitting side of 

 the terminal, an intermediate amplifier is used in order to keep the 

 level of the group transmission above danger of noise. Following the 

 second group modulator and also following the second group demodu- 

 lator are low-pass filters which cut off frequencies above about 160 

 kilocycles, to suppress unwanted modulation products. From the 

 output of the receiving low-pass filter the twelve-channel group, 60-108 

 kilocycles, passes through a two-stage "auxiliary" amplifier to bring 

 it to the desired level. 



The carrier frequencies for group modulation and for group de- 

 modulation are derived from the same 4-kc tuning fork controlled 

 oscillator that supplies carriers for the twelve-channel unit. From 

 the circuit in which appear the odd harmonics of 4 kilocycles, the 77th, 

 85th and 121st harmonics, that is, 308, 340 and 484 kilocycles, are 

 selected by carrier supply filters and separately amplified by two-stage 

 amplifiers to produce the powers required for group modulation. 

 Outputs from these amplifiers are fed to individual frequency busses 

 capable of supplying the group modulators and demodulators for ten 

 systems. An emergency carrier supply for these frequencies is also 

 provided, with arrangements for switching rapidly from the regular to 

 the emergency circuits. 



Terminal Amplifiers 



As indicated on Fig. 5, the transmitted twelve-channel group, now 

 transferred to the proper frequency range for line transmission, goes 

 from the low-pass filter at the output of the second group modulator 

 to a transmitting terminal amplifier which is similar in most essentials 

 to the amplifiers of the line repeaters. The twelve-channel group, 

 arriving from the line, passes through a regulating amplifier arranged 

 and controlled to compensate for variations in equivalent of the 

 adjacent line section before passing to the first group demodulator. 

 Similar regulating amplifiers are used at all repeater points. 



Filters 



At terminals and also at repeater points, two kinds of filter sets 

 are required. One kind is used in the line to separate the type J 



