CARRIER TELEGRAPH METHODS 485 



?>. Single-sideband lelegrapliy has an advantage of providing somewhat 

 higher speeds without increasing the band width. Whether it holds 

 much promise for any general application in multi-channel systems 

 utilizing narrow bands and moderate signal speeds is questionable in 

 view of certain difficulties. For a single-channel high-speed circuit, 

 single-sideband telegraphy might be found worth while from the stand- 

 point of economical use of the frequency spectrum. 



4. Certain other arrangements tested possessed some characteristics which 

 have advantage under particular conditions. For example, two-band 

 arrangements may sometimes be conveniently obtained by combining 

 existing on-off arrangements. These two-band arrangements are capa- 

 ble of furnishing high-grade service over radio circuits subject to severe 

 fading. The use of a single source of carrier instead of two sources on 

 a two-band arrangement results in a substantial transmission improve- 

 ment. The performance then is comparable to that of a single-band 

 frequency-shift channel occupying the same frequency space. 



A more complete discussion of the results is given under the heading 

 "Summary of Results", at the end of this paper. 



Explanation of Terms 



The following is intended to explain what is meant by certain terms used 

 in this paper. They apply specifically to carrier telegraph operation in the 

 voice range but, in general, they could also apply to radio telegraphy. (It 

 will be appreciated that various other combinations of the instrumentalities 

 involved in the present discussion could be used.) 



Channel 



A telegraph channel is a path which is suitable for the transmission of 

 telegraph signals between two telegraph stations. In the present discussion 

 the term "channel" is restricted to mean one of a number of paths for simul- 

 taneous transmission in different frequency ranges as in carrier telegraphy, 

 each channel consisting of an arrangement of carrier telegraph equipment 

 designed for the transmission of one message at a time, in only one direction. 



On-Off Method 



This, the most common form of amplitude modulation, is the same as 

 "continuous wave" in radio telegraphy. It is a method of signaling over a 

 channel utilizing a single carrier frequency, normally located at the center of 

 the transmission band of the channel filters. The presence of carrier current 

 on the line corresponds to the marking condition of the channel, and its 

 absence, to the spacing condition. .A Fourier analysis of the line current 



