A TWELVE-CHANNEL CARRIER TELEPHONE SYSTEM 137 



message band lies between 60 and 108 kilocycles. The pilot frequency 

 is 84.1 kilocycles which is obtained by modulation of 88 kilocycles, 

 from one of the output taps of the channel supply of that frequency, 

 with 3.9 kilocycles derived from a tuning fork oscillator. This modu- 

 lation is performed in a copper-oxide bridge similar to the channel 

 modulators and the desired product is selected by an 84-kc. carrier 

 supply filter. The output of 84.1 kilocycles is sufficient to supply 

 pilot current for ten terminals in the office. A sharply selective crystal 

 band elimination filter is inserted between the output of the twelve- 

 channel terminal and the point where the pilot source is bridged on the 

 circuit to eliminate any current near the pilot frequency which would 

 interfere with the small pilot current that is sent out to control the 

 system. 



The two group modulation processes alter this pilot frequency of 

 84.1 kilocycles so that it appears on the line as 59.9 kilocycles in the 

 west to east directional band, and as 116.1 kilocycles in the east to 

 west band. Correction in accordance with the magnitudes of these 

 mid-group currents in the two directions is satisfactory over all twelve 

 channels under ordinary conditions. In the case of ice or snow the 

 channels at the edges of the directional frequency groups may not be 

 properly adjusted. Additional pilot frequencies will probably be 

 needed ultimately to care for such unusual conditions. 



Regulating Amplifier 

 Figure 9 shows the circuit of the regulating amplifier, and above this, 

 the circuit of the pilot channel receiving equipment which controls it. 

 Current enters the regulating amplifier circuit from the left, coming 

 from the receiving directional filter through a shielded transformer and 

 the pads and building-out networks used for equalization. At the 

 terminals the circuit includes also the basic equalizer. Last in the 

 circuit leading from the line to the regulating amplifier is the regulating 

 network which consists of a series of three equal networks having a 

 total loss of 20 db at 140 kilocycles in the east to west direction and 

 15 db at 84 kilocycles in the west to east direction. The network 

 loss increases with frequency in the same way as the difference between 

 dry and wet weather attenuation of the line. The two terminals of 

 the regulating network and the two junction points between the three 

 networks are brought to four sets of stator plates on an adjustable 

 condenser. The rotor of this condenser, which has about the same 

 area as one set of stator plates, is connected to the grid in the first 

 stage of the regulating amplifier. Rotation of the condenser therefore 

 applies, to the grid of the first tube, a voltage which decreases con- 

 tinuously as the condenser rotates from left to right. 



