266 BELL SYSTEM TECHNICAL JOURNAL 



AM telegraphy (CW) ; thus the signal transitions in FS are represented by 

 frequency-time transients, while in the AM case they are amplitude- time 

 transients. Since AM telegraph is the more common system, a discussion of 

 the FS method involves numerous comparisons between the two systems. 

 The merits of a telegraph system must be judged on its ability to combat 

 the various adverse conditions encountered in the transmission medium and 

 in the terminal apparatus. In general these adverse conditions involve 

 variations in amplitude, frequency, and phase of the signals and the presence 

 of extraneous signals and noise. 



In the course of the development of a number of FS radio teletypewriter 

 systems, a large amount of information concerning the characteristics and 

 design parameters of such equipment has been obtained. It is the purpose 

 of this paper to abstract therefrom selected data which will furnish a step- 

 by-step comparison of the FS and AM methods. Typical terminal arrange- 

 ments are described and the effects of varying certain design factors are 

 illustrated by experimental data. Although the material presented applies 

 largely to H.F. radio telegraph, much of it is of a general nature and with 

 proper interpretation apphes to other frequency ranges and transmission 

 mediums and to cases in which the telegraph modulated carrier may be a 

 sub-carrier or one of several sub-carriers. 



General Discussion 



Sideband Energy Distribution 



The difference between FS and AM signals as regards distribution of side- 

 band amplitudes is illustrated by the following two equations for a carrier 

 of frequency (m/Itt modulated with unbiased square wave dots of fre- 

 quence p/lir. 

 For AM (On-off) keyed carrier of unity ampUtude^: — 



e = 0.5 cos Oil -\- - [cos (w -j- p)t -j- cos (co — p)/] 



TT 



— — - |cos (oj -\- d>p)t -\- COS (co — ^p)t\ 



+ — [cos (oj + Sp)t -f COS (o) - Sp)t]. . .etc. (Ij 

 57r 



For FS keyed carrier of unity amplitude^: — 



