A TWELVE-CHANNEL CARRIER TELEPHONE SYSTEM 123 



proper carrier frequency to restore the voice frequency message. Be- 

 cause of the low level at which demodulation takes place, the demodu- 

 lator is followed by a single-stage amplifier to produce the level 

 desired in the voice frequency circuit. The gain of this amplifier is 

 adjustable over a moderate range. 



MODULATOR 



FROM 

 4-WIRE - 

 TERM SET 



FROM 

 CARRIER 

 SUPPLY 



FROM (-_ 



II OTHER J 

 MODULATOR 1_ 

 CIRCUITS ^ 



TO 

 •GROUP 

 MODULATOR 



COMP 

 FILTER 



COMP 



FILTER 



TO 



II OTHER 



DEMODULATOR 



CIRCUITS 



± 5DBGAIN 

 ADJUSTMENT 



DEMODULATOR 



FROM 



GROUP 



DEMODULATOR 



Fig. 3 — Channel modulator and demodulator. 



The combination of a single modulator and a single demodulator 

 and associated equipment shown in Fig. 3 is called a "Modem" and 

 two of these are mounted on a single equipment panel. Nine of these 

 panels, sufficient for one and a half type J systems, or eighteen con- 

 versations, mount in a single relay rack bay of standard height. 



Carrier Supply 

 The carrier frequencies 64-108 kilocycles are all derived as harmonics 

 of a 4-kc frequency produced by a tuning fork controlled oscillator. 

 This frequency is applied to an easily saturated coil to produce a 

 sharply peaked wave which is rich in odd harmonics. Even har- 

 monics of 4 kilocycles are obtained by rectification in a copper-oxide 

 unit of part of the odd harmonic output. Odd and even harmonics 

 appear in separate circuits from which each frequency desired is 

 separated by a quartz crystal filter. Frequencies as high as the 121st 



