RADIO EXTENSION LINKS TO TELEPHONE SYSTEM 631 



and opened in 1927. Single sideband has since been applied on short 

 wave circuits to London and to Honolulu. 



Single sideband has the theoretical advantage of 9 db in signal-to- 

 noise ratio over double sideband with carrier transmission; 6 db is 

 secured from the utilization of all of the energy in one sideband and 

 3 db comes from reduced noise in the reduced band width of the 

 receiver. Tests have shown that the 9 db signal-to-noise improvement 

 is secured in practice. 



The application of single sideband to the short-wave circuits en- 

 countered a number of difficulties. One of the bigger difficulties was 

 that of resupplying the eliminated carrier. In order that speech 

 received over single sideband circuits be truly normal and be recog- 

 nizable as the voice of the talker, the carrier must be resupplied within 

 20 cycles. If it is supplied more than 20 cycles out of position the 

 speech will be intelligible in varying degrees but it is impossible to 

 recognize the voice even of one's best friend. To resupply the carrier 

 within 20 cycles when the radio frequency is 20 MC means that the 

 transmitter and the beating oscillator at the receiver individually 

 should not vary more than 10 cycles, and 10 cycles out of 20 MC is one 

 part in 2 million. The frequency of either oscillator must therefore 

 remain constant to better than one part in 2 million if voices are to 

 be recognized. It is of course possible to build oscillators which are 

 more stable than this. However, such oscillators at present appear 

 to be in the laboratory rather than the commercial class and so it 

 has been found desirable to adopt a different means for maintaining 

 this frequency of the resupplied carrier at the right value. This is 

 accomplished by transmitting a small part of the original carrier and 

 then at the receiver use this small or vestigial carrier as it is known to 

 actuate a mechanism which will supply a local frequency exactly in 

 synchronism with it. The resupplied carrier can therefore be main- 

 tained well within one part in 2 million, in fact it can be maintained 

 within one cycle in 20 megacycles. 



In producing a single sideband other difficulties are encountered. 

 In the present state of the art it is difficult to eliminate one sideband 

 and leave the other, except at relatively low frequencies. In the long- 

 wave transoceanic circuits which operate on 60 to 70 kc the elimination 

 occurs at 30 kc and a second modulation shifts the remaining sideband 

 to the desired position. This gives good selection of the desired side- 

 band and provides flexibility in the final positioning of the sideband. 



In operating at high frequencies, which may be as high as 22 MC, 

 it has been found desirable to reach the desired point not with two 

 steps in modulation but with three steps. This is indicated in Fig. 13. 



