PRODUCTION OF TELEVISION SIGNALS 



577 



It is readily seen that if a square aperture passes with uniform 

 velocity over a part of the picture having an abrupt change from dark 

 to light the result is that we get a signal from the photoelectric cell 

 which, instead of building up in- 

 stantaneously, builds up linearly 

 during a time, T, Fig. 14, which is 

 the time required for the aperture 

 to pass a given point. ^ The net 

 effect is an apparent sluggishness 

 in the response of the system. The 

 dotted curve of Fig. 14 shows the 

 integrated illumination passing 

 through a circular aperture of a 

 diameter corresponding to the same 

 time, T, for the condition of Fig. 



13. Due to the simpler analysis the discussion will be carried out in 

 terms of the square aperture though the sluggishness due to the round 

 one is seen to be slightly less. 



Now this kind of sluggishness in response is quite similar to that 

 introduced in the electrical part of the system when the upper 

 frequencies are cut out or not transmitted as efficiently as the lower 

 ones. The effect of frequency limitation can be investigated theo- 

 retically in a fairly simple fashion if we make the ideal assumption 

 that all frequencies are transmitted without distortion up to a cut-off 

 frequency, fc, and extinguished beyond it. In Appendix I, it is 

 shown how the signal of Fig. 14 is affected by a frequency limitation 

 of this type. We can then plot a set of curves as shown on Fig. 15 



Fig. 14 — Elementary signal change as 

 distorted by a square aperture 



16 



20 



24 



Nt 



Fig. 15 — Elementary signal change as distorted by a square aperture and by ideal 



frequency restriction 



1 This effect of aperture distortion was pointed out in the paper "Transmission of 

 Pictures over Telephone Lines" by Ives, Horton, Parker and Clark, B. S. T. [.. 

 April, 1925. 



