364 BELL SYSTEM TECHNICAL JOURNAL 



Such a method is, of course, somewhat cumbersome, and requires 

 a large amount of data to iron out random variations and individual 

 peculiarities of little general interest. But as the fundamental rating 

 method, supplemented by laboratory test, it has been systematically 

 used in studying the value of the anti-sidetone circuit and in selecting 

 instrument characteristics. 



Supplementing the repetition observations, it has been found useful 

 in service rating to obtain data on speech levels delivered to the line 

 for each condition observed. This has been done with the volume 

 indicator, a vacuum tube voltmeter so designed that the reading is 

 approximately proportional to mean syllabic voltage.* The informa- 

 tion thus obtained is useful not only in analyzing the results of service 

 tests but also in determining typical values for speech levels, necessary 

 for laboratory tests. 



Laboratory tests are of two general types: objective measurements, 

 and subjective tests. Transmission measurements cover a wide field 

 with objectives ranging from the physical analysis and study of dif- 

 ferent designs, to the determination of overall performance character- 

 istics of structures and systems. It is these latter tests that we are 

 more particularly interested in here, as most descriptive of the physical 

 properties of importance in providing telephone transmission service. 



Subjective tests in the laboratory may be said to be midway between 

 physical measurements and field performance tests. Made under 

 controlled and somewhat artificial conditions, they indicate quanti- 

 tatively the capabilities of a telephone system in transmitting articulate 

 speech under the particular conditions of the test. They cannot, of 

 course, indicate the relative probability of occurrence, and hence 

 importance, of these different conditions, nor predetermine how well 

 the subscriber will avail himself of the capabilities provided. 



Consideration of some of the results of investigations in both labora- 

 tory and field will do much to explain the rather large transmission 

 improvement realized by the introduction of the new sets in actual 

 service, particularly if examined with the conditions of a direct con- 

 versation as a basis of comparison. 



The Station Circuit 



There are two characteristics of the new station circuit of particular 

 importance from a transmission standpoint. 



Reduction of Sidetone 

 The first is the anti-sidetone induction coil through which the 

 transmitter and receiver are coupled to the line. This coil comprises, 



