150 BELL SYSTEM TECHNICAL JOURNAL 



Typical Circuit Combinations of Public Address 

 System and Lines 



Following are a number of typical combinations of the public ad- 

 dress system and telephone lines. The combinations by means of 

 which one-way service may be rendered, are given first, following 

 which certain combinations for giving two-way service are discussed. 



By one-way service is meant service in which no provision is made 

 for anyone in the distant audience to talk to the place where the 

 speaker is located. Two-way service provides for speakers at either 

 of two or more points addressing all of the other points. This is 

 similar to the two-way service rendered by regular telephone circuits. 



Fig. 3 shows the circuit arrangement which would be used when a 

 speaker at one point in a city, for example, at his office, is to address 



T " Non Loaded Cable 



V 

 Fig. 3 — One-Way Connection to Point in Same City. 



an audience at another point in the same city. A high quality close 

 talking transmitter T, together with a fixed gain single-stage amplifier 

 A, are provided at the point where the speaker is located. This com- 

 bination is designed to deliver to the line the same amount of power 

 as a commercial type substation set. Connecting this point with the 

 point at which the audience is gathered is a non-loaded cable circuit. 

 To correct for the distortion in this cable circuit, an attenuation 

 equalizer E is provided. The apparatus at the point where the audi- 

 ence is located is the equipment of the public address system without 

 the transmitter and its associated amplifier. In Fig. 3, B is the ampli- 

 fier for delivering sufficient power to the group of loud speaker pro- 

 jectors indicated by R. A volume indicator V associated with the 

 amplifier B is used in maintaining constant the volume of sound 

 delivered from the projectors. 



Fig. 4 shows the circuit combination required when a connection 

 is to be established to a distant city where the loud speaking receivers 

 are located. In the city where the speaker is located, connection is 

 made to the toll office by means of a non-loaded cable circuit equip- 

 ped with an equalizer similar to Fig. 3. A volume indicator Vi is asso- 

 ciated with the amplifier C\ at the toll office to enable proper adjust- 



