TRANSATLANTIC RADIO TELEPHONY 345 



quencies in each case, then the signal-to-noise ratio will be the same,^^ 

 provided the resupplied carrier is in frequency synchronism in both 

 systems and in addition in phase synchronism with the suppressed 

 carrier in the two-sideband system. ^^ 



When receiving single-sideband transmission the carrier suppressed 

 at the transmitting station is resupplied in the radio receiver. Since 

 this carrier will demodulate both sidebands with equal efficiency, the 

 opposite sideband must be eliminated before demodulation to prevent 

 the noise in this sideband from appearing in the voice-frequency out- 

 put. It the noise power in either sideband is p, then the noise power 

 without opposite sideband suppression is 2p and if we reduce the 

 noise power from the opposite sideband to O.lp the total received noise 

 will be reduced 



The maximum possible reduction in noise is 3.01 TU,^^ so that for 

 engineering purposes a 10 TU suppression of the noise in the opposite 

 sideband may be considered adequate. For other reasons to be 

 brought out later in this paper the opposite sideband loss must be 

 greatly in excess of this value. 



Provided the resupplied carrier used to demodulate the single side- 

 band suppressed carrier signals is large relative to the signal magni- 

 tude ^^ at that point in the circuit where we choose to supply it, the 

 only other requirement is that its frequency be correct. Since a dis- 

 placement of the resupplied carrier 50 cycles above or 20 cycles below 

 the zero of the equivalent voice-frequency band is sufficient to give an 

 appreciable decrease in speech intelligibility, its frequency should be 

 maintained within the smaller of these two limits or within plus or 

 minus 20 cycles of the correct value. It is interesting to note that an 

 absolute variation of only one-tenth of this amount can be observed 

 on music and that speech naturalness is similarly affected. 



5. Frequency and Frequency Band to be Received. To utilize the 

 power available at the transmitter most effectively, it is essential to 

 transmit only those frequencies contributing most to received intel- 

 ligibility. The energy ot speech lies largely below 500 cycles while 

 the frequencies most important for intelligibility lie between 400 and 

 2,600 cycles. ^^ By limiting the band transmitted to speech frequencies 

 above 400 cycles some saving is obtained in the transmitter power 



^^J. R. Carson, "Selective Circuits and Static Interference," Bell System Tech. 

 Jour., 4, 265; April, 1925. 



^' W. H. Martin and Harvey Fletcher, "High Quality Transmission and Repro- 

 duction of Speech and Music," Trans. A. I. E. E., 43, 384; 1924. 



23 



