580 BELL SYSTEM TECHNICAL JOURNAL 



the possibility of compensating for the effect of the aperture by 

 putting in an electrical network with frequency transmission charac- 

 teristics the inverse of those so determined. Within the range of 

 important frequencies it turns out that the effect of the aperture is 

 the same as that of a network which changes merely the relative 

 amplitudes of the frequencies into which the picture signal may be 

 analyzed. Neglecting constant multiplying factors, the relative varia- 

 tion over the frequency range for a square aperture is given by the 



factor — — ^ and for a round aperture by — , where, as 



before, T is the maximum time for the aperture to pass a given point 

 and Ji is the Bessel's function of the first order. The derivation of 

 these factors is given in Appendix II. On Fig. 17, Curve 1 gives the 

 relative values of the equivalent transfer admittance for the square 

 aperture and Curve 2 for an inscribed circular aperture, both in case 

 of a 50-line scanned picture which is square and sent 16 times per 

 second. T then is equal to 1/40,000 sec. 



In the system as set up for demonstration the image is rectangular 

 with the vertical and horizontal dimensions in about the ratio 5 to 4. 

 The circular aperture is about \}i times 1/50 of the vertical height 

 and the scanning is done 17.7 times a second. T is then 3.53 X 10"^ 

 seconds and Curve 3 gives the corresponding frequency characteristics. 

 Curve 4 shows that a square aperture of the same area as the circular 

 aperture for Curve 3 gives a fairly good approximation to Curve 3. 

 Curve 5 gives the combined effect of the two circular apertures, 

 sending and receiving, corresponding to Curve 3. Curve 6 is Curve 5 

 plotted in terms of T U on the right hand scale. 



An inspection of this last curve indicates that this frequency 

 attenuation characteristic of the aperture introduces a considerable 

 loss at 15,000 cycles and leaves little of the signal components above 

 20,000 cycles. To see if an electrical circuit of characteristics inverse 

 to those of the aperture would materially improve the resolution of 

 the image, the circuit, ^ which, together with its frequency charac- 

 teristics, is shown in Fig. 18, was inserted between the sending and 

 receiving amplifiers. It was designed to compensate for most of 

 the aperture distortion and its phase distortion was made small 

 below 20,000 cycles. On the fan-shaped test pattern of Fig. 19 a 

 noticeable improvement was observed, the black and white angles 

 being resolved closer to the tip of the pattern. In the case of faces 

 the improvement appeared to be very little but could be detected 



1 This is a constant resistance type of corrective network or equalizer. ^ See 

 Chap. XVIII, "Transmission Circuits for Telephonic Communication," K. S. 

 Johnson. 



