PRODUCTION OF TELEVISION SIGNALS 



579 



Unless one effect is much more easily remedied than the other, 

 the knee of the curve appears a reasonable point to select for operation. 

 At the knee NTk = lirfcTk = tv and Tk = 1/2/c. At this point the 

 total lag is not much greater than that due to the frequency restriction 

 alone and is l//c or twice Tk. That is, at this point, the additional 

 lag in the time of rise of signal due to the restricted frequency range is 

 equal to that due originally to the aperture, though the additional 

 lag due to the aperture is not much greater than that due to the 

 frequency restriction alone. For a square aperture in a square picture 

 of 2500 elements sent 16 times a second T = 1/40,000 of a second, 

 and fc = 20,000 cycles at the knee of the curve. The point on the 

 curve where the effect of frequency restriction introduces a sluggishness 

 in following light changes comparable to that introduced by a square 

 aperture is the same frequency as that arrived at as the upper limit 

 to useful frequencies by considerations from still picture transmission, 

 in the introductory paper by Mr. Ives. Its value is equal to one half 

 the number of picture elements. 





> 



-2 



4000 



12000 20000 



FREaUENCY IN CYCLES 



28000 



36000 



12 =) 



20 



Fig, 17 — Equivalent transfer admittance of various apertures 



It has furthermore been found possible to determine ideal electrical 

 transmission characteristics or equivalent transfer admittances of 

 circuits which produce exactly the same distortions as various types 

 of apertures. While it appears impossible at present to construct a 

 physical circuit which will produce such characteristics over the whole 

 frequency range, the problem is not difficult if we limit ourselves to 

 the most important frequency band. This is of interest as it points out 



