172 



BEI.L SYSTEM TECHNICAL JOV RN A L 



SO connected to the transmission relays that, when no control signal is 

 applied, both supervisory relays are released and the transmission relays 

 maintain the circuit condition established at the last reversal. 



A two-stage, tuned, feedback-stabilized amplifier is used to raise the 

 level of the 78-kilocycle receiving control signal selected from the receiving 

 high-frequency line by a narrow-band crystal filter. A copper-oxide recti- 

 fier converts the amplified signal to d-c. to operate a sensitive relay connected 



TO AUDIO 



FREQUENCY 



PROGRAM 



CIRCUIT 



■t— Hi PROGRAM 



^_ TRANS- 



L_-~| MITTING 



FROM J, K OR L 



RECEIVING 

 GROUP CIRCUIT 



— *■ TRANSMITTING 

 — •- RECEIVING 



TO J, K OR L 

 TRANSMITTING 

 GROUP CIRCUIT 



Fig. 4 — Schematic of reversing and control circuit. 



to the receiving supervisory relay. The supervisory relay, besides control- 

 ling the transmission circuits, also sends on a control signal as a d-c. simplex 

 on the audio-frequency pair leaving the program terminal. 



The same 78-kilocycle amplifier used for receiving the control signal is 

 also used as an oscillator to generate the high-frequency control signal in 

 the transmitting direction. The transmitting supervisory relay, under the 

 control of a d-c. control signal coming in on the audio-frequency pair, dis- 

 connects the receiving control signal rectifier and connects instead a varis- 



