APPLICATION OF ARTICULATION TESTING 355 



vary considerably depending on the manner in which the receivers are 

 held, care should be taken that this factor is controlled. 



Simulation of Typical Noise Conditions 

 It is essential to have apparatus available to supply noise of various 

 kinds, in order to simulate typical conditions under which telephone 

 circuits are used. A dependable and convenient source is provided 

 by phonograph records. 



As a source of room noise a single record is used, except in special 

 investigations. The material on the record is a combination of street 

 noise as heard through a window, speech from a number of persons 

 talking at once and other common noises. Violent changes in mag- 

 nitude, such as slamming doors, are eliminated from the record in 

 order to avoid making the test results dependent upon the purely 

 fortuitous coincidence of such peaks with the test syllables. If tests 

 are desired with particular types of room noise, special records of such 

 noise can be used. 



A number of records of line noise have been prepared in connection 

 with work on specific projects and are now available for general testing. 

 They include records of noise due to inductive interference from power 

 systems, radio static, resistance noise and several forms of crosstalk. 



Automatic Analysis of Data 



The advantages of mechanical apparatus which analyzes and records 

 the data of an articulation test become evident when it is considered 

 that in a single test eight callers each call a list of 66 syllables, in each 

 case to four observers. There are, accordingly, 8X66X4 = 2112 

 syllables observed, comprising 6336 sounds, to be analyzed. Since in 

 each case the test is ordinarily repeated at least once and in critical 

 cases several times, the time required to correct and analyze the data 

 when written records are used becomes an important consideration. 

 This is particularly true when extensive programs of tests on commer- 

 cial and experimental telephone circuits are contemplated, since many 

 factors may require variation. Automatic equipment to perform the 

 analysis makes it possible to deal with such situations economically. 



The time saved by such automatic equipment is important in itself, 

 but there are also other reasons which make it very desirable. The 

 articulation testing method, if precision is desired, requires careful 

 supervision and strict adherence to the details of the testing routine. 

 This is greatly facilitated when the engineer in charge can be provided 

 with the test result within a few minutes after the last syllable is 

 called. Inconsistent data permit early discovery of circuit trouble, 



