CARRIER SYSTEMS ON TELEPHONE LINES 577 



amplification of the two amplifiers. There are also other require- 

 ments which these filters must meet which are discussed later. 



The amplifiers are the same as used for group amplification purposes 

 at the terminals. Each consists of a two-stage reactance-coupled 

 vacuum tube circuit having four tubes in parallel push-pull connection 

 in the output circuit. The carrying capacity of this amplifier with 

 the standard plate voltages is about one watt in the output, and the 

 overall amplification or gain including incidental filter losses is about 

 30 TU. Where gains greater than 30 TU are necessary in the 

 higher frequency group provision is made for the addition of an 

 amplifier stage ahead of the unit shown, which adds approximately 

 15 TU gain. At the same time provision is made for the addition 

 of greater directional filter selectivity. 



An important feature of the repeater circuit is the equalizer which 

 is connected ahead of the amplifier. Because the line circuit attenu- 

 ation varies with frequency and is greatest at the higher frequencies 

 it is necessary that the amplification introduced at a repeater point 

 be varied with frequency. The amplification introduced by the 

 amplifier unit itself is substantially uniform with frequency. The 

 equalizer network, however, by introducing a loss which is a minimum 

 at the highest frequency of transmission and which increases for the 

 lower frequencies makes the overall repeater amplification a function 

 of frequency and in general proportional to the line attenuation which 

 it is designed to overcome. 



A typical overall gain characteristic of the repeater !s shown in 

 Figure 7. The adjustment of the exact amount of gain desired at 

 any time is made by the potentiometer at the input of the amplifier. 



Pilot Channel. As noted previously, the attenuation of open-wire 

 circuits of substantial length is affected by weather conditions. This 

 makes it necessary to make occasional gain adjustments throughout 

 the system. The extent of these adjustments is determined by means 

 of the pilot channel, which provides a visual indication of the trans- 

 mission levels of the carrier system in both directions of transmission 

 without interfering with the speech currents over the channels them- 

 selves. It is, in effect, a separate constant frequency carrier channel 

 allocated between certain speech channels in each transmission group. 



The operation of the pilot is relatively simple. At each repeater 

 point and receiving terminal there appears a meter for registering the 

 output level of the amplifier. The pointer of the meter is expected 

 normally to rest on the zero or normal level layout of the system. 

 If a change in the attenuation of the line circuit causes a departure 

 in the transmission level, the meter reading shows a corresponding 



