622 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



O 



Fig. 9 — The 311A equalizer and autotransformer on the left are replaced in 

 the 500C and D set by the Vi and V2 varistors on the right, 



parts which have been ehminated — the 31 lA equalizer and auto-trans- 

 former. On the right are shown the Vi and V2 varistors which provide 

 essentially the same performance as the replaced parts. 



Another advantage of the change in design, though a difficult one to 

 evaluate, is that of dependability. The silicon carbide element has an 

 extremely long life and in this respect is inherently better than the type 

 of equaUzer which involves the use of a heated tungsten filament. 



OPERATION IN NOISY LOCATIONS 



Since the previous paper on the 500 set was published,^ a study has 

 been completed on the comparative performance of the 500-type set 

 and its predecessor, the 302 type, in locations where room noise is 

 severe. This is of particular interest, because the sidetone characteristics 

 which have been discussed play an important part in transmission per- 

 formance under such conditions. 



For many years the securing of improved performance of the tele- 

 phone set under noisy conditions has been a serious problem. It has been 

 established that receiving impairment in a noisy location is far more 

 limiting than the transmitting impairment resulting from the noise 

 picked up at the noisy location and delivered to the distant party. This 

 is due to the fact that the signal to noise ratio in the transmitting 

 direction is improved by the natural tendency of the speaker to raise 



