APPLICATION OF ARTICULATION TESTING 351 



When the measuring device is located in the circuit tested a sudden 

 increase in the reading may indicate that the caller has increased his 

 intensity or, on the other hand, he may have spoken in the desired way 

 but the transmitter may have had a momentary change in efficiency. 

 In the first case the caller should be instructed (except in special types 

 of tests) to lower his intensity; in the second case the variation is 

 simply one of the factors affecting the result which should be known 

 but not compensated for. 



To direct the caller as to the intensity of his speech, independently of 

 variations in the circuit tested, an arrangement known as the caller's 

 control circuit has been developed. This is essentially a high quality 

 circuit which is inserted between the caller's lips and the transmitter of 

 the circuit to be tested. 



Normally the caller's control circuit is so operated that the output of 

 the artificial mouth ^ which terminates it is a faithful copy both in 

 intensity and frequency (between 100 and 7000 cycles per second) 

 of the output of the caller's voice. It contains a gain control, however, 

 so that if desired the output of the artificial mouth may be adjusted 

 independently of the caller's intensity. Such a control is desirable, 

 for example, in testing the load characteristics of certain circuit 

 elements, since if a marked change in intensity is made by the voice 

 itself, it is accompanied by distinct changes in the characteristics of the 

 voice. 



An essential part of the caller's control circuit is an automatic device 

 for measuring the magnitude of the caller's speech wave and for indi- 

 cating to the caller whether or not he is maintaining the desired value. 

 There are two objectives for the caller to meet: The average intensity 

 for the list should be the desired value, and the deviations from the 

 average during the list should be small. An average obtained by 

 calling the first half 10 db high and the second half 10 db low would 

 evidently be undesirable. The caller is instructed to avoid abrupt 

 changes and, when trained, is very successful in doing so. Some devia- 

 tions from the desired value are inevitable, however, and the caller is 

 informed of these by a system of signal lamps in the calling booth, 

 which may be seen in Fig. 1. 



The automatic volume indicator which controls the signal lamps is of 

 a special type. Instead of indicating a separate measurement for each 

 test sentence the volume indicator of the control circuit is arranged to 

 show the algebraic sum of the deviations measured from the desired 

 value. As long as the center lamp alone is illuminated the caller 

 knows that his intensity has been maintained correctly up to that point 



- "A Voice and Ear for Telephone Measurements," A. H. Inglis, C. H. G. Gray 

 and R. T. Jenkins, B. S. T. J., XI, p. 293, April, 1932. 



