CARRIER AND SIDE-BANDS IN RADIO TRANSMISSION 97 



increases with the degree of modulation, k, or, as will be discussed 

 more fully later, with the ratio of carrier to side-band. 4 



Single Side-Band Transmission 



So far it has been assumed that the wave applied to the detector 

 is identical with that produced by the modulator, a condition seldom 

 encountered in practice. For, in addition to the undesired modifica- 

 tions which the modulated wave undergoes because the transmission 

 characteristics of practical circuits are not ideal, there are other 

 changes which when properly made yield distinct advantages. These 

 intentional changes will be discussed first. 



It will be remembered that any component of the signal can be 

 reproduced by the combination of the carrier with either side-band. 

 Hence it is unnecessary to transmit both side-bands. Suitably 

 designed electrical filters make it possible to transmit one side-band 

 and effectively suppress the other. 5 This makes possible a very 

 great saving in the frequency range required per channel. It is of 

 particular importance for long wave radio telephone transmission 

 where the width of a single side-band is so large a fraction of the total 

 frequency range available that the number of independent channels 

 is at best very limited. The intensive development of a limited 

 frequency range by the use of single side-band transmission has prob- 

 ably progressed farthest in connection with carrier telephony over 

 wires. Here commercial service is being given over circuits on which 

 the carrier currents of adjacent channels are separated by only 3,000 

 cycles. It is obvious that the transmission of both bands would 

 nearly double this separation, thereby halving the number of channels 

 per circuit. There is, of course, no reason why similar savings may 

 not be effected in the field of radio transmission. In addition to this 

 major advantage there is an incidental improvement in the quality 

 of reproduction, for the distorted components resulting from beats 

 between components of the two side-bands, that is, the sums of the 

 signal frequencies, are eliminated. 



Carrier Suppression and Homodyne Reception 



The other important modification has to do with the so-called 

 " unmodulated " component of carrier frequency, C cos p t, in equation 



* A similar form of distortion generally occurs in modulation, resulting in new 

 components being produced in the frequency range of the side-band. 



5 For a description of such filters see the Colpitts and Blackwell paper referred to 

 above. 



