178 



BELL SYSTEM TECHNICAL JOURNAL 



Fig. 7 the line bridging equipment is mounted in one standard cable duct 

 type bay 19" wide and 11' 6" high. The equipment for each direction of 

 transmission is mounted on the bay in a group consisting of a modulator 

 and transmitting amplifier, blocking filter and by-passing network with 

 switching relays, and demodulator and demodulator-amplifier. The rear 

 or wiring side of this equipment is also provided with three separate covers. 

 The apparatus or front side of two of the panels, modulator and demodulator- 

 amplifiers, is equipped with parallel vacuum tube sockets so that the 

 vacuum tubes can be removed from the circuit for testing purposes without 



J EXPANDED L 

 n SCALE r 



2000 3000 4000 5000 6000 

 FREQUENCY IN CYCLES PER SECOND 



Pig 8 — Typical 10-link attenuation and delay distortion characteristics. The length 

 of this circuit was 7300 miles. 



interfering with service in the same manner as is done on K2 carrier tele- 

 phone equipment.^^ 



The necessary carrier supply for this equipment is obtained from regular 

 standard carrier supply bays used for other broadband carrier equipment. 

 The presence of undesired residual harmonics of 4-kilocycles from this 

 carrier supply necessitates modification of the carrier circuits at several 

 points to provide additional suppression to undesired components which 

 would otherwise appear as 4 or 8-kilocycle tones in the program channel. 



System Performance 



The longest commercial network circuits now in operation are in the 

 order of 7000 miles long, including the transcontinental backbone route 



