366 BELL SYSTEM TECHNICAL JOURNAL 



the receivers in series-parallel), has a high input impedance, so that 

 it may be bridged across the receivers used in the telephone sets. 



Phonographic Sources of Noise 



Both the room noise and line noise phonographs are controlled by 

 the automatic data recording system, so that the reproducers are auto- 

 matically set down on the records at the beginning of each list and 

 lifted and returned to the starting position at the end. As may be 

 seen from Fig. 5, the output of the room noise phonograph, after 

 passing through various controls, is reproduced by loudspeakers, of 

 which four are located in the observing booth, and one in each of the 

 sidetone booths. In this way the receiving end transmitters and the 

 observers are both exposed to the same noise. Because of the highly 

 absorptive character of the walls of the testing booth it is necessary 

 to use equalizing networks in the reproducing amplifier in order to 

 insure that the frequency distribution of the reproduced noise is that 

 desired. Throughout the test the electrical volume supplied to the 

 loudspeakers may be checked by a volume indicator located at the con- 

 trol board. 



The output of the line noise phonograph is applied to the circuit 

 tested through a high impedance bridging coil or through a resistance 

 network ordinarily at the middle of the trunk. Line noise magnitudes 

 are adjusted to the desired value with the help of a circuit noise meter 

 and are then continually checked throughout a test by means of the 

 control board volume indicator. 



Automatic Data Analyzer 



Two different systems of automatic analyzing equipment have been 

 designed and used. The first, on which active development work was 

 initiated in 1930, was used in the routine laboratory testing of com- 

 mercial telephone circuits from 1931 to the early part of 1933. The 

 present system, simpler in design and embodying a large number of 

 improvements, was then installed and has been used since that time.* 



As pointed out before, the present data handling machine embodies 

 a number of the parts and operating principles of standard printing 

 telegraph systems. The testing lists are previously prepared in the 

 form of perforated tapes, ^ of which a large number are available. 



^ Another type of analyzing equipment for articulation testing has been described 

 by J. Collard, Electrical Commutiication, X, p. 140, January, 1932. 



^ In making up such tapes it is necessary to apply a code to the various articulation 

 sounds since the keyboard of the tape perforator contains only the standard English 

 alphabet. Each syllable appears on the tape as three consecutive sets of perforations, 

 one for each sound. Additional perforations are used in some portions of the tape to 

 control various functions of the automatic recording apparatus. Two lists of 66 

 syllables are recorded on each separate tape to make possible comparison tests on two 

 difTerent systems with a minimum of delay. 



