TRANSMISSION OF INFORMATION . 555 



Fitting the Messages to the Lines 



To facilitate this discussion it seems desirable to introduce and 

 explain a few terms. For transmitting a sequence of symbols various 

 sorts of media may be available, such as a wire line, an air path, 

 as in direct speech, or the ether, as in radio communication. For 

 convenience we shall group all of these under the general name of 

 "line." Each such medium is generally characterized by a range of 

 frequencies over which transmission may be carried on with reasonable 

 freedom from distortion and external interference. This will be 

 called the "line-frequency -range." Similarly the symbol sequences 

 corresponding to the various modes of communication such as telegraph 

 and telephone, will be designated as "messages." Each of these will, 

 in general, be characterized by a "message frequency-range." This 

 may be thought of as being determined by the frequency-range of 

 that line which will just transmit the type of message satisfactorily, 

 or we may think of it as that part of the frequency scale within which 

 it is necessary to preserve the steady state components of the message 

 wave in order to permit distinguishing the various symbols as they 

 appear in the transmitted wave. 



When we set up practical communication systems it is often found 

 that the message-frequency-range and the line-frequency-range do 

 not coincide either in magnitude or in position on the frequency scale. 

 If then we are to make use of the full transmission capacity of the line, 

 or lines, we must introduce means for altering the frequency-ranges 

 required by the messages. Two such means are available, which 

 together offer the theoretical possibility of accomplishing the desired 

 end of making the message-frequency-ranges fit the available line- 

 frequency-ranges. 



The process of modulation so widely used in radio systems and in 

 carrier transmission over wires makes it possible to shift the frequency- 

 range of any message to a new location on the frequency scale without 

 altering the width of the range. This follows at once from the well- 

 known fact that the steady state description of the wave which 

 results from the modulation of a carrier wave by a symbol wave 

 includes a pair of side-bands in each of which there is a component 

 corresponding to each steady state component of the original wave. 

 The frequency of each component of the side-band differs from the 

 carrier frequency by the frequency of the corresponding component 

 of the symbol wave. The elimination of one of these side-bands 

 results in a wave which retains the information embodied in the 

 original symbol wave and occupies a frequency -range of the same width 



