350 BELL SYSTEM TECHNICAL JOURNAL 



frequency is subaudible. If there is a moderate noise background 

 present, it will tend to mask the program and will therefore permit 

 of somewhat higher interfering field strength. However, if the inter- 

 ference is raised beyond a certain level, dependent upon the received 

 band width, flutter effects will become pronounced. This will not be 

 true with a linear detector and manual gain control, but in practice 

 radio receivers which have linear detectors almost invariably have 

 automatic volume control. 



If the noise level is very high it may mask even rather loud program 

 interference, and under such conditions the flutter effect is likely to be 

 much the most serious source of trouble. This condition is of practical 

 occurrence in outlying areas where a degraded service must be toler- 

 ated continually. In such regions shared channel broadcasting is 

 limited in usefulness primarily by the flutter effects, and in extreme 

 cases, by distortion of the desired program due to the heterodyning 

 action of the interfering carrier. Both of these types of disturbance 

 would be eliminated by synchronizing the carriers of the two stations, 

 and it seems likely that control of the carrier frequencies to within 

 ±0.1 cycle might definitely extend the limits of the lower grade service 

 areas of shared channel stations. 



Acknowledgment 



I wish to acknowledge my indebtedness to Mr. J. E. Corbin for his 

 assistance in carrying out the experimental work which has been 

 reported in this paper. 



