818 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



the system for the more stringent television requirements. Furthermore, 

 line connecting equipment consisting of branching filters and additional 

 equaUzation is required. Branching filters are used to separate the 

 telephone and television bands so that they can be transmitted to their 

 respective terminal equipment. These are combined high-pass low-pass 

 structures compHcated by strict requirements on stability of the gain- 

 frequency and delay-frequency characteristics. Delay equalization for 

 the Une sections must also be provided in the television branch and a 

 large part of this is combined with the branching filters. Other com- 

 ponents required for long television systems are adjustable gain and 

 delay equalizers and associated amplification. 



The same type of equipment is used as that described for the other 

 repeaters except that a number of additional transmission panels are 

 required to mount the additional amplifiers and regulators associated 

 with the equalizers. Two 11 '-6'' bays are used to contain the equipment 

 for one through coaxial. One bay contains the receiving equipment 

 which precedes the line switch. The other bay contains the transmitting 

 equipment (transmitting amplifier and hybrid) and any line connecting 

 equipment for combined systems. Fig. 18 shows a typical main repeater 

 installation. 



3.4 AUTOMATIC SWITCHING* 



In order to preserve transmission in the event of the failure of a com- 

 ponent of the system and for transmission maintenance purposes, one 

 coaxial in each direction is operated as a standby. An automatic switch- 

 ing system is provided to permit substitution of the standby fine for any 

 of the working lines. The lines are switched at the input to the trans- 

 mitting ampUfier and at the output of the receiving amplifiers and 

 equalizers. (See Fig. 14). 



At the receiving end of a switching section, equipment is provided 

 whose function is to recognize failure of a working line and initiate the 

 switching circuits. Information as to the transmission conditions of 

 the system exists in the pilot regulators, the output of which controls 

 a sensitive relay with high and low hmit contacts. The operation of one 

 of these relays provides the switching system with the information that 

 transmission has failed or been seriously impaired. It is necessary to 

 make a switch as rapidly as possible in the case of a total failure in order 

 to reduce the effect upon the transmission circuits. As the relays take 

 appreciable time to operate, the receiving switch equipment is designed 



* Material written by P. T. Sproul. 



