100 BELL SYSTEM TECHNICAL JOURNAL 



constant of proportionality and the negative amplitude indicates a 

 reversal of phase during half of the audio frequency cycle. Such a 

 variation may be represented by the expression 



i = K C S cos {qt + o) cos p t. (9) 



Taking the sum of these expressions for all the components of the 

 signal gives the second term of equation (3) which was shown to rep- 

 resent the side-bands. 



In estimating the power saved by carrier suppression the com- 

 parison should be made with a system transmitting the carrier which 

 has been so adjusted that the power is used to the best advantage. 

 So far as a single signal component is concerned this would call for 

 making the carrier and side-band equal, as their product would then 

 be a maximum. This, however, would imply that the distorting cur- 

 rents from the interaction of two side-band components would be as 

 large as the signal currents themselves. That is to say, quality 

 considerations require that the major part of the transmitted power 

 be in the carrier component. Quantitative data on the relation be- 

 tween the ratio of carrier to side-band and the quality of transmission 

 has been secured in the laboratories of the Western Electric Company, 

 and it is hoped it will be published in the near future. Briefly, the 

 results indicate that the good quality which is obtained when the 

 carrier component is large falls off very rapidly as the magnitude of 

 the carrier component is reduced so as to approach that of the side- 

 band, the latter being measured when a sustained "ah" sound is used 

 as the signal. Under these conditions the side-band is sustained at a 

 value about equal to the maximum occurring in ordinary speech. 

 That is to say, even the peak power in a carrier suppression system is 

 less than the carrier component alone in an ordinary system adjusted 

 to give the same side-band. From these considerations it appears 

 that there has been a tendency to attach undue significance to "com- 

 plete modulation," as a more or less unique and ideal condition of 

 operation. For nothing revolutionary occurs as the carrier is de- 

 creased thru the value corresponding to that condition. The dis- 

 tortion due to interaction between the side-bands is present for larger 

 values of carrier and continues to increase progressively for smaller 

 values. The exact degree of modulation to be permitted therefore 

 depends upon the standard of quality to be met. In a carrier sup- 

 pression system the degree of modulation, k, approaches infinity more 

 or less closely depending on the completeness of the suppression. 



In addition to making possible the use of carrier suppression, 

 homodyne reception presents other advantages. It furnishes a ready 



