2 BELL SYSTEM TECHNICAL JOURNAL 



These experiments tested the ability of critical listeners to hear changes in 

 band width on direct comparison when listening to representative program 

 material. The purpose of this paper is to present the data from these tests. 

 Similar experiments have of course been done before. The excuse for this 

 paper is that the experiments represent a complete set of data and the analy- 

 sis of the data is believed to be in such form as to be useful in further con- 

 sideration of the requirements of program fidelity. 



The circuit arrangements used for the tests are shown schematically in 

 Fig. 1. The essential features are a source of program, a switch for con- 

 necting into the circuit either of two low-pass filters, and a high-quality loud- 

 speaker. Controls for adjusting levels, volume indicators, etc., are omitted 

 from the diagram. The arrangements included a signal visible to the 



_--- o- 



A B 



LI i 



Fig. 1 — Arrangement for testing program band widths. 



listeners in which one of the letters, A, B, or C, could be illuminated. On a 

 given test two of these letters were associated with the switch so that one 

 letter was illuminated for one position and the other letter for the other 

 position. The choice of letters among the three was varied more or less at 

 random for different tests. Low pass filters were available to provide cut- 

 offs of 3, 5, 8, 11 and 13 kc. When no filter was inserted the band was con- 

 sidered to extend to 15 kc. as this was about the upper limit of transmission 

 of the testing circuits and loud speaker. The lower limit of the transmitted 

 band for all conditions was approximately 40 cycles. 



In conducting a test, a group of observers listened to comparisons between 

 two of the available band widths, the conditions being switched every few 

 seconds until a sufficient number of comparisons had been made. The 



