SINGLE SIDEBAND TRANSMISSION 71 



larger component to approach the resultant. Consequently, adding 

 the dark current causes the resultant to become more like the in-phase 

 component, thus reducing distortion due to the quadrature component. 



In half-tone pictures many of the transitions are in small steps. 

 The quadrature component for small steps is frequently small com- 

 pared to the total in-phase component. By reasoning similar to that 

 in the previous paragraph, it follows that distortion due to quadrature 

 component at small steps, is apt to be negligible. The quadrature 

 effect in half-tones is also reduced by the fact that some of the changes 

 are gradual. 



The aperture effect has not been mentioned explicitly above. The 

 aperture effect may be considered as being equivalent to a certain 

 frequency characteristic and it may be assumed that the filter charac- 

 teristics shown, include it. Incidentally, it is found that the aperture 

 does not greatly affect the relationship between the in-phase and 

 quadrature components. 



While it may be expected that the quadrature component should 

 have similar effects in picture transmission and in television, it is 

 perhaps desirable to point out that there are important points of 

 difference such as the presence of motion in the television images and 

 the difference in response characteristics of a television screen and a 

 photographic surface. It is not therefore an inevitable conclusion 

 that television images will be as little affected as picture transmission 

 images by the quadrature component. 



Experimental 



The conclusions are confirmed by certain experimental transmissions 

 which were made over a picture machine employing a single sideband 

 system as described by Reynolds.^ The system makes use of 100 lines 

 to the inch and has a total bandwidth of about 1000 cycles. The speed 

 of the spot of light over the picture is about 20 inches per second. 

 Two specimens of printing of different sizes were transmitted. A 

 portion of each specimen one centimeter wide, after transmission, is 

 shown in Fig. 15 enlarged to about five times its original size in order 

 to avoid interference between the half-tone pattern and the picture 

 pattern. Figure 15 should be viewed at about five times the normal 

 reading distance. Group (a) was transmitted on a single sideband 

 basis with the dark current reduced practically to zero. Group (b) 

 was similarly transmitted, excepting that the dark current was 14 db 

 below the maximum current. Group (c) shows a double sideband 



^ Loc. cit. 



