364 BELL SYSTEM TECHNICAL JOURNAL 



control circuit, the other artificial mouth is disconnected, and the ob- 

 server's ampHfier is connected to the receiver at the same end of the 

 circuit as the transmitter in Booth II. Thus, when a sentence is 

 called it is picked up by the transmitter in Booth I while the trans- 

 mitter in Booth II is picking up room noise. At the end of the time 

 allotted to the sentence and the various measuring and recording 

 operations the automatic switches reverse the circuit simply by trans- 

 ferring the caller's control circuit to the artificial mouth in Booth II 

 and the observer's amplifier to the receiver at the other end of the 

 circuit. This reversal is repeated after each of the 66 sentences. 



Automatic Change and Agitation of Transmitters 

 Two motor-driven devices serve to change the transmitters used in 

 testing, agitate them in a uniform way and then center them properly 

 in front of the artificial mouths. This apparatus is under the control 

 of a dial at the control board so that it is unnecessary to enter the 

 sidetone booths to make the changes. The apparatus in the sidetone 

 booth is shown In Fig. 7. 



The transmitters to be used at each end of the circuit are mounted v 

 on circular plates, which may be removed as units from the rotating 1 

 devices. An unmounted disk holding a set of transmitters is shown \ 



beside the rotating device in the photograph. When direct compari- 

 sons between two different types of transmitters are to be made, two 

 transmitters of one type and two of the other are mounted on each 

 disk. 



Just before the calling of a list is started the engineer at the control 

 board manipulates the dial, which is merely a modified telephone set 

 dial, by which the transmitters to be used are selected. Following 

 the dial pulses a series of relays causes the transmitters in both side- 

 tone pickup booths to rotate through an angle of not less than 360 

 degrees, which has been found to supply adequate agitation. After 

 this the rotation continues until the desired transmitters are exactly 

 in front of the artificial mouths. In directly comparing two circuits 

 which make use of the same type of transmitters, the same trans- 

 mitters are used for two successive lists, but the rotating agitation Is 

 applied between lists. 



Automatic Measurement of Received Speech and Noise 



The apparatus used for the automatic measurement of speech and 



noise on the circuit under test is similar, except for the input circuit 



and recording circuit, to the automatic volume indicator just described, 



but is designed to handle a larger range in volume. A peak voltage 



