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BELL SYSTEM TECHNICAL JOURNAL 



It is frequently necessary to test very poor systems where the 

 standard lists giving an articulation of a few per cent, are not satis- 

 factory. The vowel consonant lists are somewhat more satisfactory 

 under these circumstances. Lists of sentences have also been found 

 to be very useful for such purposes. The sentences were of the 

 interrogative or imperative form containing a simple idea. They 

 were designed to test the observer's acuteness of perception rather than 

 his intelligence. Tests were made with these sentences and the 

 standard lists on various circuits, involving carbon transmitter circuits 

 and various filter systems. The data are shown in Fig. 11. The 



60 



ir 20 

 o 



20 



40 60 



SYLLABLE ARTICULATION 



80 



100 



Fig. 11 — Discrete sentence intelligibility vs. articulation 



sentences were considered to be understood if the observer either 

 recorded the sentence correctly or recorded an intelligent answer. 

 As stated earlier, the percentage correctly observed is called the 

 discrete sentence intelligibility. 



It will be seen that for changes in distortion, the changes in the 

 discrete sentence intelligibility will be small for systems having 

 syllable articulations greater than 30 per cent, but very large for 

 systems having syllable articulations below 20 per cent. It is for 

 systems in this latter class that these test sentences are useful. A 

 case in point is the measurement of the degree of secrecy obtained 



