240 BELL SYSTEM TECHNICAL JOURNAL 



single program, switches or reversals may have to be made to change the 

 originating point temporarily. To provide for these rapid changes, special 

 operation and special switching and reversing equipment are required at 

 many points along the network. Much of this equipment is under remote 

 control from selected points. 



The greater portion of the switching of program circuits is done at about 

 25 points throughout the country on the major networks. On the average 

 more than 25,000 switching operations per month are performed at these 

 25 points. During the busy hours of any typical evening there may be 

 something over 500 men on duty at all of the offices about the networks. 



At points where switching requirements are simple, the switching equip- 

 ment consists m.erely of a few keys. At the larger points where the switching 

 requirements are complex, the switching equipment consists of elaborate 

 relay and control arrangements. These are so designed that it is possible 

 to set up in advance the circuit com.binations required for the ensuing 

 program period without disturbing the program.s in progress. The actual 

 switching operation takes place at the instant the monitoring attendants 

 signal the the receipt of the last of selected cues, and not before then. This 

 type of arrangement affords a maxim.um of protection against error, as it is 

 possible to check the presetting for the next switch or make a last minute 

 change if necessar}' any time before the switch has been made. 



Figure 4 shows a picture of such a switching arrangement in use at 

 Omaha, Nebraska for one broadcasting company. At this point 13 circuits 

 used in various trunk and branch sections of two networks are connected 

 to the switching equipment. These are grouped in various combinations to 

 take care of as many as five simultaneous programs. A maximum of five 

 cues might, therefore, be involved in a switch at this point. 



The operation and maintenance of the networks are carried out by a 

 special organization under centralized authority and trained in the applica- 

 tion of uniform methods and procedures found by experience to be produc- 

 tive of best results. Transmission is mionitored continuously at strategic 

 points about the networks. In order to facilitate the activities of this group 

 many thousands of miles of intercommunicating telephone and telegraph 

 circuits are provided full time for their use. 



A picture of a monitoring position in the program transmission office at 

 Washington, D. C. is shown in Fig. 5. It will be noted that the monitoring 

 attendant is using an individual headset. This is of a special high fidelity 

 type and is used to avoid the confusion that would result from attempting 

 to monitor a number of different programs sim^ultaneously with loud- 

 speakers. Loud-speakers are available, however, for supplementar}'- checks 

 of quality whenever required. 



Accurate transmission measuring equipment is necessary at the various 



