CARRIER TELEGRAPH .METHODS 523 



The superiority of the two-source arrangements over the on-oflf arrange- 

 ment may be accounted for by differential recombination of the detected 

 waves, and by the greater amount of sideband power transmitted with two 

 separate carriers. 



As shown in Fig. 19, the tolerance to noise of the narrow-band two-source 

 arrangement at 60 w.p.m. was less than that of the normal-band two-source 

 arrangement with limiter because of the greater distortion obtained when 

 operating at this speed without noise. Also, the tw'o-source arrangement 

 illustrated b}' curves B and C in Fig. 19 appears to have been made worse 

 at low noise values by use of a limiter. Actually the limiter probably did 

 reduce the noise power, but the improvement in distortion obtained thereby 

 was greatly outweighed by the distortion increase which occurred without 

 noise when the limiter was added, as explained above under the heading 

 ''Distortion vs. Speed Tests", and illustrated in curves C and D of Fig. 6. 



The single-sideband arrangements were both more sensitive to noise 

 than the on-off arrangement. In the case of the single-sideband arrange- 

 ment with no spacing carrier the presence of quadrature- component in- 

 creased the marking bias; and in order to have zero bias on reversals it was 

 necessary to increase the d-c bias current in the receiving relay, which re- 

 duced the effective marking current and therefore made the arrangement 

 more susceptible to interference occurring during the marking intervals. 

 In the case of the single-sideband arrangement with —6 db spacing carrier 

 the amount of sideband power transmitted was less than in the on-off ar- 

 rangement and consequently noise was more troublesome. 



Distortion- at 120 Words per Minute 



Further tests were conducted at 120 w.p.m. on the arrangements having 

 linear receiving detectors, and results are shown in Fig. 21. The increase 

 in speed caused an increase in distortion on all arrangements at a given 

 noise level, but the increase in speed has less effect on the single-sideband 

 arrangements than on the others having the same loss characteristics, at 

 medium and high noise levels. On account of the narrower sidebands 

 transmitted on the two-source, normal-band frequency-shift, and on-off 

 arrangements, an increase in speed was accompanied by a greater decrease 

 in amplitude of received signal and therefore by a greater increase in noise- 

 to-signal ratio than in the case of the single-sideband arrangements, each of 

 which used a wider sideband. Consequently, at a given noise-to-signal 

 ratio, the speed increase was accompanied by a greater distortion increase 

 on the former group of arrangements than on the latter. 



As was previously found at 60 w.p.m. the frequency-shift arrangement 

 with limiter was less sensitive to noise than the arrangement without, when 



