576 



BELL SYSTEM TECHNICAL JOURNAL 



5. Comparison of the Out-of-Focus Amotion Pictures with 

 A 240-Line Television Image of Known Characteristics 



The motion picture machine was arranged to project out-of-focus 

 pictures onto a screen set up beside the cathode ray receiving tube of a 

 laboratory television apparatus ^ of excellent design. Duplicate films 

 were run in the two machines, and the images were made equal in size 

 and approximately equal in color and brightness. Special low-pass 

 filters in the video circuit limited the frequency band without transient 

 distortion, and permitted the trial of three different band widths. 

 The conclusion was reached that the nominal band width of the video 

 circuit, expressed in cycles per frame period, was equal to 1.3 times the 

 number of figures of confusion in the frame area. 



LENS 



FILM 1^ PLANES ) 



APERTURE 



SCREEN 

 7.3" X 5.9' 



X 



r-i. 



u 



36" 

 55.6 



/ 



59.0 - 

 -55.6- 



Fig. 6 — Essential dimensions of the motion picture optical system as used for 

 the correlation with a 240-line television image. For this case a^ = 1.00 square inch. 



36 X 



y = 



h 



55.6 55.6 + X 

 = 1.45 y. 



A group of observers compared the two images, each observer being 

 allowed to adjust the focus of the projection lens until he judged the 

 images to be equal in sharpness. The distribution of lens positions, 

 in terms of microscope scale divisions, was found to follow a normal 

 error curve fairly well, and the median value for the group was used in 

 computing the sizes of the figure of confusion. The external aperture 

 shape was always square. 



Since the television film scanner had been designed without regard 

 for the unused space between frames on sound film, it became necessary 

 to modify some of the dimensions of the out-of-focus projection system 

 in order to make the two images equal in size. Figure 6 shows the 

 modified dimensions. Comparison with Fig. 7, which gives the di- 



