LAKKIEK SYSTEM l()l< I'ROGKAM I k'.WS \f/SSf()\ 17.^ 



tor limiter across the out{)ul and a 78-kilocyfli' crystal from the oulj)Ut to 

 the input, yihasod for positive feedback. 



TvPK K Bkanchinc; C'lkrurr 



liecause of the operatin<i; recjuirements of a racho broadcasting network 

 there is need for complete switching llexil)iHt\-. On a national network 

 there may be scores of intermediate points, where the program must be 

 tapi)ed for local broadcasting and may originate in the case of special events. 

 If, to obtain this tle.xibility, the network were made up of short carrier links, 

 bringing the program down to audio frequencies at the end of each link, 

 there would be in some cases, between the originating studio and the most 

 distant broadcasting station, 50 to 100 or more links in tandem, involving 

 double that number of band filters. Terminal phase and attenuation 

 distortion, however, are proportional to the number of links. By means of 

 advanced filter and equalizer designs, the present system has been made 

 suitable for about 10 to 13 links in tandem. Additional arrangements 

 therefore are needed at intermediate points to serve local broadcasters, 

 without breaking in on the through program transmission. The branching 

 circuit serves this need. End branching circuits which split off a program 

 circuit from a carrier message route are needed for some network branches 

 and are less elaborate than the through branching circuits which provide full 

 switching service at intermediate points on a main trunk route. 



The flexibility of the through branching circuit is illustrated in the follow- 

 ing functions which it performs under remote control: 



1. Provides a receiving leg on a reversible through program circuit. 



2. Sphts the network to provide independently reversible hnks in each 



direction with the same or different program material on each link. 

 These functions are performed with negligible reaction on the associated 

 through-message circuits. A block schematic for one direction of trans- 

 mission on a type K system is shown in Fig. 5. 



For splitting the network a band elimination filter^' blocks frequencies 

 in the program assignment {M~4-i kilocycles) while passing the remaining 

 message frequencies. As network rearrangements are made during the 

 program switching interval transmission may be rerouted through the phase 

 simulating network'' which is substituted for the BEF when the program is 

 to go through instead of being blocked. The simulation of phase, which 

 must be close in order to avoid disturbance of voice-frequency telegraph 

 superimposed on any of the message channels, extends over all but the two 

 channels adjacent to the program. The transfer from one transmission path 

 to the other is accomplished by a chain of make-before-break relays in such 

 a manner that transmission on the message channels is virtually unaffected. 



Junctions of transmission circuits are made with resistance hybrid con- 



