STUDIES IX RADIO BROADCAST TRAXSMISSIOX 209 



tical problems. On the other hand if but a single side-band is trans- 

 mitted the difficulty is reduced to placing the carrier within a very 

 few cycles of its correct position. The allowable departure will 

 depend on a number of things but there is reason to believe that for 

 high quality transmission it must be very small, perhaps no greater 

 than two or three cycles. 



With the single side-band carrier suppression method, invented by 

 John R. Carson, the radiation is stripped down to the minimum which 

 will fully transmit the telephonic signals and this reduces to a minimum 

 the exposure of the signals to the ravages of selective fading. If the 

 spacing interval of the fading is relatively narrow as in the cases we 

 have examined hereinbefore, this form of transmission would not 

 fade seriously in average volume but would be subjected to a continual 

 changing of its frequency-amplitude characteristic, that is to say 

 individual frequency components would fade progressively as the 

 minima of the selective fading wandered back and forth across the 

 frequency range encompassed by the single side-band. If the spacing 

 interval of the fading were very large so that the minima were very 

 broad of if some other, at present unexplored form of fading which 

 covers a wide band at one time were acting, the signal would fade in 

 average volume but the range of its variation would be only the square 

 root of that of a carrier transmitted signal, since only the side-band 

 would fade and the locally supplied carrier would remain unchanged. 



The extent to which these theoretically drawn conclusions may be 

 realized in practical application is yet to be determined but we have a 

 few records bearing upon the matter which at least do not run con- 

 trary to them. 



All of the transmission tests where the radio signal was beat with a 

 local oscillator and the detected beat note observed, were equivalent 

 to single frequency single side-band transmission with carrier sup- 

 pression, the local oscillator functioning as the carrier suppressed at 

 the transmitter. In this case, for which a number of records have 

 already been shown, the detected signal is in proportion to the product 

 of the amplitudes of beating oscillator and received radio signal. The 

 phase of either does not affect the amplitude of the audio signal. Hence, 

 the only important modification of the original signal is the variation 

 in the amplitude resulting from selective fading. 



Unfortunately we have no records in which a direct comparison is 

 made between single side-band transmission with and without carrier 

 suppression but the case can be visualized from the record shown in 

 Fig. 12 or 13. Here each one of the frequencies recorded may be looked 

 upon as a single side-band frequency which has been detected through 



