PUBLIC ADDRESS SYSTEMS 



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introduces no appreciable distortion over the range of frequencies 

 required for good reproduction of speech, but it must be understood, 

 that this quality was obtained only at the sacrifice of sensitiveness, 

 the latter being in the order of 1-1 000th that of the transmitter used 

 at telephone stations in the Bell system. With the multi-stage 

 vacuum tube amplifiers available this low volume efficiency is not 

 serious, and in fact we are using this transmitter for what is known 

 as distant talking, i.e., the speaker may be at a distance of five or six 

 feet from the transmitter. This is, of course, necessary in any trans- 

 mitter suitable for public addiess work as it is not possible to greatly 

 limit the movement of the speakers, nor can they be required to use a 

 hand transmitter. It might be well to point out that this sacrifice in 

 volume efficiency in order to gain high quality is possible at the trans- 

 mitting end of the system, but not at the opposite end where the 

 electrical power is transformed into sound waves and propagated, 

 as the device at this point must be capable of handling large amounts 

 of power with minimum distortion. 



Referring to Fig. No. 7 which is a cut-away view of this push-pull 

 high quality carbon transmitter, the granular carbon chambers will 

 be seen. The electrical path through each of these variable resistance 

 elements is from the rear carbon electrode through the carbon granules 



Fig. 7. 



to a gold-plated area on the diaphragm itself. The resistance of this 

 path is about 100 ohms and as the two buttons are in series for the 

 telephone currents, the transmitter is designed to work into an im- 

 pedance of 200 ohms. The double button construction almost com- 

 pletely eliminates the distortion caused by the non-linear nature of 

 the pressure-resistance characteristics of granular carbon. 



