RADIO EXTENSION LINKS TO TELEPHONE SYSTEM 



629 



and resistance, and by virtue of connections to vary the amplification 

 of the two ampHfier tubes. The louder signals therefore put more 

 positive bias on the grids of the amplifiers than the weaker ones and 

 the amplifier tubes will amplify the stronger tones more than the 

 weaker. There is thus delivered to the output the original signal but 

 with very much improved signal-to-noise ratios. 



In the use of the Compandor on circuits having noise it has been 

 found possible to produce signal-to-noise improvements as high as 

 30 db. Average improvements are 15 to 20 db. The improvement 

 depends upon the amount of noise present. 



Returning again to Fig. 5, there are certain elements labeled 

 "Calling." 2 The particular configuration indicated with calling de- 

 vices attached to the telephone line in the first diagram and to a receiver 



LINEAR 

 RECTIFIER 



OUT 



Fig. 11 — Expander part of the compandor. 



in the second diagram are the particular calling arrangements used for 

 communicating with boats at sea. The fishing boats, such as men- 

 tioned previously, and private yachts, do not find it expedient to have 

 an operator listening at all times for calls nor do they like to have a 

 loud speaker operating continuously delivering all and sundry com- 

 munications to the people on a boat. It is desirable that means be 

 provided so that the boat may be called by having a calling mechanism 

 available. The system is indicated in Fig. 12. In this system the 

 receiver on the boat must be operated continuously and must be 

 connected to the selector and bell circuit so that whenever the correct 

 calling signal comes in the bell will ring. At the shore station the 

 calling is accomplished by sending out certain combinations of 600 

 cycles and 1500 cycles as indicated in this figure, the various com- 

 binations being chosen by the telephone dial which actuates a relay 

 switching one or the other audio frequency onto the transmitter. 



