580 BELL SYSTEM TECHNICAL JOURNAL 



transmitted through the directional filter to the line. The attenuated 

 pilot and sideband currents pass from the first section of the line into 

 the receiving directional filter of the first repeater and enter the 

 amplifier. The pilot channel indicator circuit is bridged across the out- 

 put of the amplifier, and is tuned to discriminate very sharply against 

 all but current of the pilot frequency. This circuit has a high im- 

 pedance relative to the line, so that only a very small percentage of 

 the pilot current is drawn from the line at a repeater point. The 

 remainder is transmitted through the outgoing directional filter and 

 over the subsequent section of the line. 



That portion of the pilot current which enters the indicator circuit 

 is amplified and rectified in the vacuum tube detector, and the output 

 current is read on a d.-c. milliammeter. As stated above, this meter is 

 calibrated to read in TU above and below a mid-scale position which 

 represents a normal transmission level to which the system is initially 

 adjusted. 



Entering the receiving terminal of the carrier system, the pilot and 

 speech currents pass through the directional filter and are amplified. 

 As at the repeater, the pilot indicator circuit is bridged across the 

 output of the amplifier. At this terminal, in addition to showing 

 level, the output of the indicator actuates an alarm circuit which 

 operates when the transmission level at this point varies from normal 

 for a set interval of time by more than a prescribed amount. This 

 delay action in the operation of the alarm provides selectivity against 

 slight interference into the pilot channel from currents on the other 

 channels of the system and thereby insures that the alarm indicates a 

 definite level change. 



The pilot channel thus insures that the high-frequency portion of 

 the system is continuously checked with the exception of the individual 

 channel band filters and modulator and demodulator units. These, 

 however, are particularly stable in operation and require no unusual 

 attention in maintenance. Of course, the overall check is made at 

 only the pilot frequency in each direction. Variations of line equiva- 

 lent caused by weather changes increase in magnitude with frequency. 

 Therefore, corrections must be made in the gain relations of the 

 individual channels whenever these weather changes are great. Fortu- 

 nately the corrections follow a fairly definite relation with variations 

 of pilot level and are ordinarily made by the terminal attendants on 

 the channel potentiometers controlling the demodulator gain by 

 reference to a table. This table shows the relations between the 

 required gain changes at the three channel frequencies in terms of 

 changes at the pilot frequency. 



