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1949 TV PICKUP FOR TRANSPARENCIES 141
the time each slide is positioned. The actual change-over time then is less than 0.2 second, and the effect of an instantaneous change-over is imparted to a casual viewer. The friction of springs contacting the slide being positioned dissipates the momentum imparted to the slide. Coasting is further minimized by the sine motion of the plunger, which decelerates to zero velocity at the end of the stroke; and indexing of the slide is controlled entirely by the dimensions of the linkages. A mechanical brake held in contact with a cam raised on the rim of the crank disk by spring tension prevents excessive coasting of the mechanism after the motor is de-energized. The brake is released by a solenoid which is wired in parallel with the motor for simultaneous operation. A limit switch depressed by cam action at the end of the forward stroke completes the cycle.
To supplement the instantaneous change-over, an automatic control circuit has been developed to fade the output video signal of the scanner to black, whereupon a slide change is made and a fade-in of the next slide follows. Either of two rates of fading can be selected, and the switch accomplishing the selection, the switch initiating the cycle, and all other operating controls can be remotely located.
An attribute of the flying-spot device is the possibility of attaining good geometrical linearity; as little displacement of a point from its true position on the raster as 1 per cent of the picture height is possible with production units. The evenness of illumination is good. Shading is almost entirely due to the optical system and is not objectionable even when using a negative slide, in which case the gamma corrector exaggerates intensity variations. Resolution of 500 lines in the horizontal direction, measured by RMA methods, is readily attained. Vertical resolution is limited to about 450 lines by the number of active scanning lines in the raster.
The use of color slides for reproduction in black and white is entirely feasible and preliminary tests do not reveal adverse effects despite the fact that the cathode-ray-tube output and the phototube sensitivity are largely confined to the green portion of the spectrum. Because of the density of color slides, the signal-to-noise ratio is inferior to that obtained by the use of good black-and-white slides.
Since the raster is sharply focused in the plane of the cathode-raytube screen, transparencies may be placed on the face of the tube with only slight defocusing resulting from the thickness of the glass. In this way, two transparencies may be superimposed. An effective device utilized by WABD, New York, is that of a special clock made