820 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



to operate directly from the dc output of the regulator associated with 

 the 7,266-kc line pilot. This permits complete switch operation in about 

 15 milliseconds. 



Upon receipt of information from the regulators that one or more of 

 the pilots have gone out of limits, the switch initiator signals the trans- 

 mitting switch control equipment at the transmitting end of the switch- 

 ing section as to which line has failed. Signalling is accomplished by the 

 use of tones in the 280 to 296-kc range which are transmitted over all 

 coaxials in parallel in the reverse direction. Use of the coaxials for trans- 

 mission of these signals obtains maximum speed of operation of the 

 switch. All paths are used in parallel to preclude failure of the switch if 

 one channel in the opposite direction would be inoperative. 



The transmitting switch control equipment causes the transmitting 

 end of the standby line to be switched in parallel with the line in trouble 

 and then signals the switch initiator that this has been done. This verifier 

 tone actuates the line switch at the receiving end to complete the switch. 



When the trouble clears, the switch is released as the initiator checks 

 every minute to see if service on the working line can be restored. In 

 the event of a prolonged trouble, the switch can be locked manually 

 and the initator will no longer attempt to restore to normal. Release of 

 the switch is accomplished by the transmission of a release tone to the 

 transmitting end while a checking tone returned by the transmitting 

 end indicates completion of release and readies the switch initiator for 

 further switching. 



For maintenance purposes, manual operation of the switching equip- 

 ment is provided. In effect, manual switches are made by simulating a 

 failure. Alarm features are provided to indicate to the operating personnel 

 failure of the coaxial system or failure of the switching equipment. 

 Care has been taken in the design to insure that failure of the switching 

 equipment in no way affects transmission except by removing the pro- 

 tection afforded by the presence of the switching facility. 



One ll'-6" bay is required for the switch control equipment for each 

 direction of an 8-coaxial system. The Une switches are mounted in the 

 miscellaneous bay of the main repeater lineup. 



3.5 TERMINALS* 



Television terminal equipment, which is required to modulate the 

 video frequency signals to and from the high-frequency line, is described 

 in a companion paper* and therefore, will not be discussed here. 



♦ Material written by C. G. Arnold. 



