Frequency Shift Telegraphy — Radio and Wire Applications* 



By J. R. DAVEY and A. L. MATTE 



Frequency shift telegraphy is described and compared with amplitude modula- 

 tion telegraphy under various conditions found in radio and wire transmission. 

 Experimental data are given to demonstrate the influence of various design fac- 

 tors on the over-all performance under these conditions. It is shown that the 

 most outstanding characteristic of the frequency shift method is its ability to 

 accept large and rapid changes in signal amplitude. Frequency shift telegraphy 

 thus proves to be of great advantage for use in the H.F. radio range. Frequency 

 shift telegraphy also shows an advantage over amplitude modulation telegraphy 

 with respect to noise. For applications where the level variations are small or 

 slow the advantage of the frequency shift method over amplitude modulation 

 is relatively small. 



Introduction 



TOURING World War II, single-channel and multichannel frequency- 

 -*-^ shift radio telegraph systems proved of the utmost importance in pro- 

 viding the Allied Powers v^ith a v^orld-wide automatic printing telegraph 

 network for handling with precision, secrecy and dispatch the unprecedented 

 volume of traffic engendered by a war of global extent. It is expected that 

 the next few years will witness a greatly expanded application of this 

 method of operation by commercial telegraph companies and others in- 

 terested in long distance telegraphy. 



Frequency Shift carrier telegraphy (FS) may be applied to any carrier 

 telegraph circuit, but, as will appear below, it provides particularly striking 

 advantages in H.F. radio transmission. For some other radio frequency 

 ranges and for wire line operation the conditions are such as to limit the 

 advantages of the FS method. The main advantages of the FS over the AM 

 method are a greater ability to accept rapid level changes, which results in 

 better stabihty and lower distortion, and an improvement in signal-to-noise 

 ratio, which permits a reduction in carrier amplitude. It is therefore of 

 particular importance where automatic printing is desired over H.F. radio 

 circuits. When it is necessary to transmit through very high noise levels, 

 low speed AM signaling with aural reception of an audio beat note remains 

 the superior method. 



FS is a form of frequency modulation in which signaling is accomplished 

 by shifting a constant amplitude carrier between two frequencies represent- 

 ing respectively the marking and spacing conditions of the telegraph code. 

 Frequency variations in FS telegraphy correspond to amphtude variations in 



♦PubUohed in A. I. E.E. Transactions, Vol. 66, p. 479, 1947. 



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