508 BELL SYSTEM TECHNICAL JOURNAL 



bands of the marking and spacing frequencies were not completely separated 

 by the sending filters and a shift in carrier frequency affected the two sets of 

 sideband components dissymmetrically, thus causing bias and distortion. 



Bias at 120 Words per Minute 



The bias resulting from carrier frequency change in the various arrange- 

 ments at 46 d.p.s. or 120 w.p.m. was in general worse than at half this speed 

 because the received wave shape was more rounded. The distortion due to 

 carrier frequency variation, as shown in Figs. 10 and 11, consisted mainly of 

 bias, except in the two-source arrangements, as explained above. 



Other Considerations Relating to Single-Sideband Arrangement 



The single-sideband arrangement with — 6 db spacing carrier was found to 

 have the lowest distortion at high signal speeds. Consequently it was 

 thought desirable to study this arrangement further in order to see what 

 might be done to improve its stability during carrier frequency variations, 

 and to select an optimum location for the average carrier frequency. First, 

 curve A of Fig. 12 was plotted showing the distortion at 160 w.p.m. resulting 

 from a change in carrier frequency. Then it was assumed that a level com- 

 pensator might be provided. In order to simulate the effect of'such a device 

 without actually constructing one, the line loss was adjusted manually to 

 keep the r.m.s. marking carrier power constant at the receiving filter output. 

 Curve B of Fig. 12 shows an improvement in the change in distortion vs. 

 carrier frequency under this condition. Large variations in bias still per- 

 sisted in spite of level compensation, due to variations in shape of the 

 envelope of the received carrier signals caused by various amounts of quad- 

 rature component and sluggish in-phase component^ depending on the loca- 

 tion of the carrier frequency. In order to simulate the effect of a level com- 

 pensator providing automatic bias adjustment, the relay bias current was 

 adjusted to give zero receiving bias at each setting of the carrier frequency, 

 and also the receiving filter output was maintained at a constant value as 

 before. The results are given by curve C in Fig. 12 which shows a consider- 

 ably increased tolerance to carrier frequency changes when the arrangement 

 was stabilized in this manner. This curve indicates that the carrier fre- 

 quency could be increased about 17 cycles before the distortion started to 

 increase rapidly, and could be reduced about 5 cycles before a gradual in- 

 crease in distortion began to appear. 



It has been shown that for a certain ideal filter^ a suitable location of the 

 carrier frequency for the single-sideband method is at a point either at the 

 upper or lower side of the band where the loss is 6 db with respect to that in 

 the middle of the band. It can be demonstrated that the envelope of the 

 received wave is the same when the carrier frequency is set at either of these 



