AN(, IMPROVED TELEPHONE SET 



247 



preconditioning and heat treatment, to maintain the improved modulating 

 performance better over longer periods. At the same time the rate of resist- 

 ance increase with age is markedly reduced. 



Along with these several factors contributing to increased volume output 

 of the transmitter, the response was altered in an attempt to provide more 

 nearly orthotelephonic overall response. 



To assure that these instrument gains would be realized in actual service 

 introduced one of the principal technical problems of the transmission de- 

 sign, the better control of sidetone. Without a better job on sidetone, much 



TRANSMITTER 



/input pressureA 

 from artificial 



y MOUTH 



/"output pressure'^ 



\\N 6 CC coupler J 



]^l^ 

 ^ 



TELE- H NO. 26 - ^'^^IL^^ 

 GAUGE ^"^^ 



PHONE 

 SET I— I LOOP 1—1 



CORD 

 CIRCUIT 



STEP BY _ 1000 FT 



STEP 



CORD 



CIRCUIT 



NO. 26 

 GAUGE 

 LOOP 



M TELE- 

 PHONE 



M SET 



RECEIVER 



LOOP RESISTANCE IN OHMS 

 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 



J—, 1 1 I ^_l L 



oc -20 



2000 



4000 6000 8000 10,000 12,000 



LENGTH IN FEET (NO. 26 GAUGE LOOP) 



Fig, 6 — Comparative sidetone levels. 



14,000 16,000 



of the value of the higher instrument efficiencies on long loops would fail of 

 realization because of resulting lower acoustic talking levels, and increase of 

 the masking effect on incoming speech of room noise picked up by the 

 transmitter. The solution adopted for the initial design lay in choosing a more 

 complex impedance to give the best overall balance over the frequency range 

 for the loop and trunk conditions with which it must function. The relative 

 sidetone of the two sets as a function of loop length for a typical circuit is 

 shown in Fig. 6. The solution has given a set with essentially the same side- 

 tone as the present set in spite of a ten db increase in instrument efficiencies, 

 thus assuring the full effective gain represented by this increase. 

 Typical loop loss characteristics for the 500-type set compared to the 302- 



