342 BELL SYSTEM TECHNICAL JOURNAL 



presence of the weak signal, while the solid one represents the effect of 

 the large signal alone. The curves of this figure were taken with a 

 bias of + 0.5 volt, which was found not to be critical. 



The results of the experimental observations made with this de- 

 tector were entirely in accord with theory, as will be discussed later, 

 while similar observations made with a zero bias gave results which 

 differed considerably from those predicted by the theory of the linear 

 rectifier. Lack of the small bias caused a considerable departure from 

 linearity, as was plainly evidenced by the fact that when it was absent 

 the audio-frequency output due to the two carriers was by no means 

 independent of the magnitude of the larger. 



The tuned circuit in the mixing amplifier was so broad as to have an 

 entirely negligible effect on the fidelity of the radio receivers. 



Listening Conditions 



In studying the effects of noise background some observations were 

 made in the open laboratory, and a greater number in a partially 

 deadened room 10 feet x 10 feet x 10 feet. The sound-proofing of 

 this room was sufficient to keep out street noises and other extraneous 

 disturbances of moderate intensity. 



In determining the dependence of a given effect upon the magnitude 

 of the carrier ratio, there was recorded that value of the ratio at which 

 the effect was just perceptible. 



Results of Experimental Work 



A number of observations have been made with the intention of 

 obtaining practical data on the characteristics of reception in the 

 presence of a noise background, and with the purpose of checking the 

 theoretical predictions already given. It has been pointed out that the 

 flutter effects depend upon the type of detector which is employed and 

 upon the ratio of the two carriers. If a square law detector is used the 

 effect should be very nearly independent of the magnitude of the noise 

 level, so long as it is within reasonable limits and does not either over- 

 load any of the equipment (including the ear of the listener) or fall so 

 low as to be hardly noticeable. On the other hand, if a linear detector 

 is employed, flutter effects should increase with the noise level. In 

 either case the modulations of the two stations play no important part 

 in determining the flutter effects except in so far as high modulations 

 may temporarily mask them. 



As a result of these considerations it was decided to employ un- 

 modulated carriers for the greater part of the work. In order that a 

 suitable level might be chosen, the strong carrier was first adjusted to 



