The Bell System Technical Journal 



Vol. XXIII January, 1944 No. i 



The Discernibility of Changes in Program Band Width* 



By D. K. GANNETT and IDEN KERNEY 



One of the factors that should be considered in determininj^ how 

 wide a transmission band is required for high fidelity broadcasting is the 

 ability of people to perceive the effects of restricting the band to 

 various limits, when listening to typical radio programs. Tests are de- 

 scribed in which this was directly measured. The tests were concerned 

 only with the physical ability to hear the differences in band width 

 and disregarded the question of the enjoyment or aesthetic apprecia- 

 tion of wider bands. It is concluded that changes in band width are 

 detectable about twice as readily with music as with speech; that one 

 must go from 8 to 15 kc. to obtain a change as readily detected as a 

 change from 5 to 8 kc; and that both these changes, for speech, are 

 just sufficient to have an even chance of being detected by listeners 

 having experience in such tests. 



THE question of how wide a frequency band it is necessary to transmit 

 to provide high fidelity broadcasting involves consideration of a num- 

 ber of factors. Among these are the limits of hearing of the human ear, the 

 spectra of program material, the aesthetic sensibilities of listeners, the effect 

 of room noise in studios and homes, and the acoustic properties of rooms. 

 A true engineering solution of the problem would attempt to assign nu- 

 merical values to each of these factors, and then to combine them in some 

 way to obtain a figure of merit versus band width. Sufficient information 

 to do this in a complete and satisfactory manner is not available, however, 

 and in practice the final answer is usually obtained by the exercise of judg- 

 ment, bolstered by such technical data as can be found on the component 

 factors. 



The first two of the above factors, the limits of hearing and the spectra 

 of program material, have been separately investigated and the results pub- 

 lished in the technical literature by a number of experimenters. Because of 

 the intangibles involved, however, even these two sets of data cannot 

 readily be combined, forgetting the other factors, with complete assurance 

 that their contribution to the answer is established. The authors, there- 

 fore, undertook a series of tests to measure directly their combined effect. 



* This paper is a publication, substantially without change, of a report prepared some 

 time ago before work non-productive to the war effort was suspended. 



