452 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



room noise/''^ The new technique was based on fundamental studies of 

 speech and hearing, including a very extensive series of articulation tests 

 on different combinations of lines and loops and telephone sets, in which 

 the Hne portions were modified by electric wave filters to transmit various 

 frequency-band widths, and distortionless attenuators] controlled the line 

 loss. Different representative types of subscriber loops were included in the 

 tests. The then standard deskset telephones were used (No. 337 transmitter. 

 No. 144 receiver, and 46 induction coil) and also special telephone sets using 

 experimental types of transmitters and receivers having ideal, fiat, fre- 

 quency-response characteristics. In comparing complete systems having dif- 

 ferent types of lines, but otherwise similar, the computed articulation- 

 ratings agreed sufficiently closely with the ratings determined from tests to 

 warrant substantial confidence in the experimental use of the computation 

 technique in the exploratory loading development studies. 



17.3 New Higher Cul-of Loading Systems 



The theoretical studies as appHed to an exchange plant using the stand- 

 ard deskset telephones showed that a desirable improvement in the trans- 

 mission intelligibihty of complete connections could be obtained by using 

 the new higher cut-off loading systems which are described in general 

 terms in Table VIII. As discussed later, large economies also resulted in 

 the design of new plant, and in the rearrangement of old plant. The loading 

 designations used in the table are in accordance with a simplified system of 

 designations which was adopted in 1923. The letter-component is a symbol 

 for the spacing, and the number signifies the inductance. Prior to the intro- 

 duction of these new loading standards, medium loading-spacing (M) was 

 about 8775 ft. It was changed to 9000 ft. to facilitate coordination with the 

 other types of loading in the layout of the cable plant. The "D" spacing 

 was an entirely new spacing. 



The decision to standardize the particular loading systems of Table VIII 

 naturally involved extensive plant cost-transmission studies. These were 

 directed to determining the maximum utilization of the new cheaper cables 

 previously mentioned, and the most advantageous ultimate uses of the 

 higher-grade, more expensive cables already in use. Practical considerations 

 of economy dictated that the new series of loading standards should include 

 systems which could use available loading coils and existing loading vaults 

 in the important underground cable plant. These matters were also of great 

 importance in the gradual rearrangement of the existing exchange area 

 loading at a minimum expense to comply with the new cut-off standards. 



While the improvement in intelligibility was one of the factors influenc- 



<') Comprehensive information on Dr. Fletcher's researches is given in Reference (33). 



