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BELL SYSTEM TECHNICAL JOURNAL 



shift the program curve downward. Just how far it should be dis- 

 placed is very hard to determine, as the amount will depend upon the 

 type of program on the undesired station. Observations have indi- 

 cated that the shift should amount to at least 7 db. The dashed curve 

 in Fig. 10 has been drawn 7 db below the solid curve. 



40 



35 



10 



-I 30 

 lij 

 m 



a 



^20 



2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 



NOISE LEVEL IN DECIBELS BELOW 30% EQUIVALENT MODULATION 



Fig. 10 — Relative importance of program interference and flutter interference. 



It will be noticed that with a band width of 3500 cycles the flutter 

 curve crosses the program curve at a noise level equivalent to about 

 2 per cent modulation. (In every case the noise level was measured 

 with the 7000 cycle band, regardless of what band was to be used in the 

 listening tests. This should be kept in mind throughout the present 

 discussion.) This means that at equivalent modulations of more than 

 2 per cent the flutter effect would be more objectionable than the 

 program interference. However, at high noise levels, say 5 to 10 per 

 cent, the listener would be sure to reduce the band width of his receiver 

 to considerably less than 3500 cycles and this would reduce the relative 

 importance of the flutter. Nevertheless, at very high noise levels the 

 flutter is more important than the program interference. If the un- 

 desired station were to employ abnormally low modulation the program 

 interference would be decreased and the relative importance of the 

 flutter increased. 



It is evident that the dependence of the flutter on band width, and 

 the different reaction of individual observers as to what type of inter- 

 ference is the more objectionable, renders it impossible to make a definite 

 statement as to the exact values of carrier ratio and noise field which 

 will make the two types of interference equally important. But we 

 can draw the useful conclusion that in cases of excessive noise, such 



