540 BELL SYSTEM TECHNICAL JOURNAL 



In teletypewriter toll circuits of one or more sections the over-all 

 distortion is made up of increments from a number of sources. Ex- 

 perience has shown that in genera! the over-all distortion of a particular 

 signal element is equal to the algebraic sum of the individual incre- 

 ments. For each specific piece of equipment or element of the circuit 

 the sign and value of the distortion cannot be predicted exactly as they 

 depend upon facts which vary with individual cases. However, 

 representative values of the maximum distortion experienced in a 

 period of moderate length with miscellaneous signals for different 

 types of circuit and equipment may be determined with fair accuracy. 

 Experience and probability theory indicate that the most probable 

 value of the over-all distortion of a telegraph circuit may be computed 

 by taking the square root of the sum of the squares of the corresponding 

 values for the various component parts of the circuit. With this as a 

 basis coefficients have been established for individual telegraph circuits 

 of the various types employed in the TWX transmission system. 

 These coefficients are, in general, proportional to the square of the 

 maximum distortion experienced with severe signal combinations under 

 comparatively unfavorable conditions of circuit adjustment, weather 

 conditions, etc., taking into account what is known about the general 

 stability of the particular facility concerned. An estimate may then 

 be made of the transmission impairment to be expected in service w^ith 

 a teletypewriter circuit made up of a number of sections of various 

 types by adding the coefficients of the component parts. For con- 

 venience the value of the coefficients has been so chosen that satis- 

 factory operation normally will be obtained over a connection if the 

 sum of the transmission coefficients for the subscriber lines, switch- 

 board circuits, and toll lines involved does not exceed 10. 



Using these coefficients the entire transmission system is designed 

 to provide satisfactory transmission between any two subscribers or 

 combinations of subscribers. It is found that subscriber lines less than 

 about 5 miles in length contribute little or no distortion to the over-all 

 connections. Those up to about 35 miles may contribute distortion 

 so as to warrant allowing a coefficient as high as 1.0 or 1.5, and for 

 those up to 50 or 60 miles the coefficient may be as great as 3.5 or 4.0. 



The following discussion assumes that the subscriber lines have a 

 coefficient of not more than 1.0 or 1.5 from the subscriber station to 

 the jack connected to the teletypewriter toll line at the switchboard, 

 leaving for the toll links of the connection a maximum coefficient of 

 about 7.0 or 8.0. In the case of intra-area connections involving 3 

 toll links, a permissible coefficient of 8.0 for all the links of the con- 

 nections would, of course, permit a coefficient of about 2.7 for each 



