502 



BELL SYSTEM TECHNICAL JOURNAL 



in a variety of sentences, or a simple list of words chosen at random 

 and inserted in a common phrase. 



In the routine operation of the transatlantic channels, the operators 

 record a value of "circuit merit" which is a composite figure rep- 

 resenting the operator's judgment of the commercial value of the 

 circuit. All three of these types of test were used in comparing the 

 single and double-sideband systems. 



All observations were made on 9,790 kc. with an audio-frequency 

 band of from 250 to 2,800 cycles. The carrier during single-sideband 

 transmissions was 10 db in amplitude below one of two equal side 

 frequencies which loaded the transmitter to its maximum amplitude 

 capacity. Only a single tone was used to modulate the transmitter 

 when measuring signal-to-noise ratios. The degree of modulation of 



50 



40 



30 



< 20 



O 



in 



Z 

 < 



ID 



y 10 



10 20 30 40 



DOUBLE SIDEBAND SIGNAL-TO-NOISE RATIO IN DECIBELS 



50 



■"ig. 9 — Plot of signal-to-noise ratios on double sideband^vs. signal-to-noise ratios on 



single sideband. 



the double-sideband comparison signal was 45 per cent when tone 

 modulation was used. Speech modulation was made the same as for 

 two tones for both double and single-sideband transmissions. Direc- 

 tional antennas were used for both transmitting and receiving. Suc- 

 cessive observations were made on single and double-sideband trans- 



