TRANSMISSION FEATURES OF NEW TELEPHONE SETS 367 



terms of net effective transmitting and receiving loss, as determined 

 by service observations.^ Their data, shown in Fig. 5, are relative 

 to the sidetone of a reference set. The heavy solid lines are the 

 original experimental data, the dotted extensions to these curves 

 being extrapolated. 



UJ o 



z < 



iiJ UJ _5 



Z§ 



~ t- 



O 9 



LU 

 U. 



u- -10 



•20 



0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 4 — -Circuit efficiency of anti-sidetone circuit vs. sidetone circuit. 



In addition to the original ordinates, others are shown which are 

 of interest. They are based on the results of loudness balance tests, 

 and while not perhaps of great precision, do approximately indicate 

 the relationship of the sidetone of a telephone conversation to that 

 for direct speech, and illustrate the differences in the effects of sidetone 

 for transmitting and for receiving. 



On each curve are indicated the average sidetone value of the 

 standard sidetone and the new anti-sidetone set, each, as before, with 

 the new transmitter and receiver. There is also shown the range of 

 sidetone for each type of set, within which practically all service 

 conditions will fall. This indicated range takes into account not 

 only variations in sidetone balance due to line impedance variations, 

 but changes (with loop resistance) in battery supply to the trans- 

 mitter. It should be noted that in only a few cases is the absolute 

 sidetone of the anti-sidetone set on the worst sidetone conditions, as 

 high as or higher than that of the sidetone set on the best sidetone 



