Certain Factors Affecting Telegraph Speed' 



By H. NYQUIST 



Synopsis: This paper considers two funclaineiital factors enteiing 

 into the maximum speed of transmission of intelligence by telegraph. 

 These factors are signal shaping and choice of codes. The first is con- 

 cerned with the best wave shape to be impressed on the transmitting 

 medium so as to permit of greater speed without undue interference either 

 in the circuit under consideration or in those adjacent, while the latter 

 deals with the choice of codes which will permit of transmitting a maxi- 

 mum amount of intelligence with a given number of signal elements. 



It is shown that the wave shape depends somewhat on the type of 

 circuit over which intelligence is to be transmitted and that for most 

 cases the optimum wave is neither rectangular nor a half cycle sine wave 

 as is frequently used but a wave of special form produced by sending 

 a simple rectangular wave through a suitable network. The impedances 

 usually associated with telegraph circuits are such as to produce a fair 

 degree of signal shaping when a rectangular voltage wave is impressed. 



Consideration of the choice of codes show that while it is desirable to 

 use those involving more than two current values, there are limitations 

 which prevent a large number of current values being used. A table of 

 comparisons shows the relative speed efficiencies of various codes pro- 

 posed. It is shown that no advantages result from the use of a sine wave 

 for telegraph transmission as proposed by .Squicr and others- and that 

 their arguments are based on erroneous assumptions. 



Signal Shaping 



SK\'KRAL different wave shapes will Ih' assumed and roniparisnii 

 will be made between them as tn: 



1. Excellence of signals delivered at ihf distant entl oi the circuit, 

 and 



2. Interfering properties of the signals. 



Consideration will first be given to the case where direct-current 

 impulses are transmitted over a distortionless line, using a limited 

 range of frequencies. Transmission over radio and carrier circuits 

 will next be considered. It will be shown that these cases arc closely 

 related to the preceding one because of the fact that the transmitting 

 medium in the case of either radio or carrier circuits closely apjirnxi- 

 mates a distortionless line. Tclcgrapli\- i>\vv i)rdinar\- land liiu - 



'Presented at the Midwinter t"i)iuiiiliciM of tin- .\, 1. K. E.. I'hil.idclplii.i. I'd. 

 February 4-8, 1924, and reprinted from ilu- jourii.il of llu .\. I. K. K. \'ol. 43, p. 

 124, 1924. 



' A. C. C'rehore and <;. O. .S(|uier. ".\ I'radic.il Transniilttr Ising the Sine 

 Wave for Cable Telegraphy; and .Measurements with .Mlernating Currents upon 

 an .Atlantic Cable." A. I. K. K. Trans.. Vol. X\ll, 1900, p. 385. 



(i. (). Squier. "On .\n Cnbroken .Mternaling Current for Cable Telegraphy." 

 Proc. Phvs. Soc, Vol. X.WIl, p. .S40. 



C. O. Squicr. ".\ .Meth(Kl of Transmitting the Telegraph .Mphabet Applicable 

 for Radio, Land Lines, and Submarine Cables." Franklin Inst., Jl., \'ol. 195, May 

 1923, p. 633. 



324 



