TRANSMISSION FEATURES OF NEW TELEPHONE SETS 379 



2. Noise picked up by the transmitter and returned via the sidetone 



path and receiver to the listener's ear — termed "return noise." 



3. Circuit noise. 



4. Room noise picked up at the far end and transmitted over the cir- 



cuit. 



The different relative contribution of the "return noise" for the 

 two sets is of interest. The net result is a total noise in the telephone 

 ear, lower than the actual room noise for the anti-sidetone set, and 

 higher for the sidetone set. 



For the circuit and room noise conditions shown, the ratio between 

 received speech level and noise is about 25 db for the sidetone condi- 

 tion, and 35 db for the anti-sidetone. The corresponding ratio for air 

 transmission to one ear under the conditions shown is in the order of 

 20 db. 



Results of Laboratory and Field Performance Tests 



It has been seen that the new sets are superior in volume and in 

 minimizing the disturbances of noise. The frequency measurements 

 just discussed have indicated marked superiority also in the quality 

 of reproduction. 



One measure of the effect of this reduced distortion is by means of 

 the articulation test. Such tests have shown that for a typical tele- 

 phone system equipped with the new telephone sets, 95 per cent of the 

 letter sounds spoken into the transmitter are correctly understood by 

 the listener. With air transmission, 98 or 99 per cent of letter sounds 

 are correctly understood. The difference is almost entirely due to 

 the broader frequency band transmitted by the air path. 



With the final designs of the new sets, tests have been made by the 

 methods developed for determining "effective" transmission. ^ The 

 results of these tests have shown that the new sets under the conditions 

 of actual service provide a marked advance in transmission perform- 

 ance. The average total transmitting and receiving gain is about 

 15 db on the effective basis of transmission, as compared to sets of 

 the sidetone type used with the older type of instruments.^' 



Conclusion 



In general it appears that the notable transmission improvement 

 which has been achieved in the design of the new telephone sets, in 

 their freedom from distortion, higher effective volume, lower sidetone, 

 and general convenience in use, makes possible a closer approach to 

 the ease of a direct conversation than has hitherto been possible 

 commercially. 



