Performance Characteristics of Various Carrier Telegraph 



Methods 



By T. A. JONES and K. W. PFLEGER 



This paper describes laboratory tests of certain carrier telegraph methods, to 

 determine their relative advantages from the standpoints of signal speed, and 

 sensitivity to level change, carrier frequency drift, interchannel interference, and 

 line noise. 



Introduction 



MOST of the carrier telegraph methods mentioned below are well 

 known,^ '2 -^ -^ -^ but the selection of the best method for a particular 

 application is difficult without comparative tests on specific designs. It is 

 the purpose of this paper to record data taken during such tests and to ex- 

 plain the results so that they may be helpful to those concerned with the 

 selection of the optimum method for a given set of requirements. 



The conclusions here reached regarding methods of telegraph transmis- 

 sion do not necessarily apply to transmission of sound, pictures, or television, 

 because their requirements differ. In telegraph transmission it is important 

 that signal transitions be received at approximately the correct times, and 

 wave rounding is permissible. 



Computations for a square cut-off band-pass filter with zero phase distor- 

 tion^ show that the shape and duration of the transient in the received wave 

 are about the same for a sudden transition in both on-off and frequency-shift 

 arrangements (explained in the next section), when the total frequency shift 

 is not more than half the channel width. As telegraph distortion depends 

 largely upon the transient, one might therefore infer that, if the transients 

 are about alike, there is no particular advantage in frequency-shift over the 

 on-off method as far as signal speed is concerned. However, the computa- 

 tion for the idealized filter gives no assurance that a physical filter will per- 



1 H. Nyquist: "Certain Topics in Telegraph Transmission Theory", A. I. E. E. Trans., 

 Vol 47, pp. 617-644, April 1928. 



^H. Nyquist and K. \V. Pfleger: "Effect of Quadrature Component in Single Side- 

 band Transmission", Bell System Technical Journal, Vol. XIX, pp. 63-73, Jan. 1940. 



' E. H. Armstrong: "Methods of Reducing the Effect of Atmospheric Disturbances", 

 Proc. I. R. E., Jan. 1928, pp. 15-26. 



^ J. R. Carson: "Reduction of Atmospheric Disturbances", Proc. I. R. E., July 1928, 

 pp. 966-975. 



' F. B. Bramhall & J. E. Boughtwood: "Frequency Modulated Carrier Telegraph 

 System", Eieclrical Engineering, Vol. 61, No. 1, Jan. 1942, Transactions Section, pp. 36-39. 



• Fig. 3 of H. Salinger: "Transients in Frequency Modulationf , Proc. I. R. E., August 

 1942, pp. 378-383. 



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