IMPROVED CIRCUIT FOR TELEPHONE SET 625 



and sidetone noise is directly lowered. The level of the received speech 

 is unaffected. Therefore, the signal to noise ratio in the receiving direc- 

 tion is improved. The transmitting level is not too adversely affected, 

 because the tendency of the subscriber to increase his talking level when 

 the room noise level is high largely offsets the lower eflficiency of the 

 transmitter to speech sounds. These effects are borne out in the applicable 

 curve of Fig. 10 which shows that an improvement of approximately 

 5 db in trunk results from the shunting of the transmitter. 



Carrying this experiment further and acoustically shielding the local 

 transmitter completely from noise, the lower curve of Fig. 10 was ob- 

 tained. This is equivalent to shorting the transmitter as is done in 

 the earlier "push-to-listen" types of telephone set, except that it does 

 not introduce any electrical effects in the circuit. This shielding provides 

 about 4 db additional discrimination against noise. This, then, indicates 

 the limit to which we can go in reducing the effects of room noise on 

 received speech by operating on the transmitter element alone. 



From the data presented in Fig. 10 it is evident that the 500-type 

 telephone set provides a significant improvement when the subscriber 

 is carrying on a telephone conversation under noisy room conditions. 

 While it has been indicated above that the greatest gain of the 500-type 

 set over the 302 type under high room noise conditions is with respect 

 to received transmission, it is also better in transmitting from noisy 

 locations. This is because of the less disturbing character of the trans- 

 mitted noise and because the signal to noise ratio is improved by having 

 the transmitter closer to the subscriber's Ups, which is a feature of the 

 new handset. 



Where noise conditions in the field are severe, the 500-type set will 

 provide material improvement. Typical of such conditions are noisy 

 business locations, telephone stations which are located in power plants 

 and near loading ramps at airports. Where the noise is particularly 

 severe, the provision of a 40-ohm shunt resistance on the transmitter 

 of the 500-type set offers still further improvement and is recommended 

 for application by the local telephone people. 



CONCLUSION 



The simplification of the 500 set made possible by the use of silicon 

 carbide varistors, with its attendant reduction in manufacturing and 

 maintenance costs and increase in life represents a significant step 

 forward in station set design. A valuable characteristic of the 500-type 

 set is the substantial improvement in transmission performance under 

 conditions of severe ambient noise. 



