Radio age research, manufacturing, communications, broadcasting, television (1941)

ESSENTIAL PARTS OF A TELEVISION SYSTEM WHEN A FILMED SCENE IS TO BE TRANSMITTED AND REPRODUCED. image orthicon and film sensitiv- ity. At two foQft-lamberts, both pickup tube and film can record pictures. At lower scene bright- nesses, only the pictures trans- mitted by the image orthicon are present. The fact that motion pic- tures are viewed at less than one- tenth the brightness at which the original camera recorded them is in support of the increased sensi- tivity of the eye over film. The discrimination of the eye (for half-tones and resolution) is enhanced when looking at brighter pictures. This means that both television pictures and motion pic- tures must improve their quality hand in hand with their bright- ness. Because television pictures, in order to be viewed in moderately lighted surroundings, are likely to be brighter than motion pictures, the quality of the television pic- ture should exceeed that of the mo- tion picture. This burden of im- proved quality is passed on mostly to the television pick-up tube, in that improved quality needs more illumination in the original scene or, preferably, more sensitivity in the pick-up tube. A particularly interesting problem arises when the original scene to be picked up is not as bright as the picture re- produced on the kinescope. For this picture to be judged satisfac- tory, the pick-up tube performance must exceed the performance of the eye in the same proportion as the kinescope brightness exceeds that of the original scene. Much confusion has been gen- erated by comparing the "limiting resolution" of motion pictures with the number of lines of a television picture. The conclusion has usually been that a motion picture is two or three times, as the case may be, better than a television picture be- cause the limiting resolution of film is 1,000 to 1,500 lines and the num- ber of lines in a television picture is only 500. This conclusion is at least misleading. The limiting resolution of a picture is not as im- portant in the eye's judgment as is the response, or signal-to-noise ratio, for 500 lines and below. A more valid comparison of television and motion pictures, based on signal-to-noise ratios, places the capabilities of a 500-line television picture close to the resolution of present 35 millimeter motion pic- tures. Major Problem in Film Industry The evaluation of the graininess of motion pictures has been a major problem in the film industi-y as evidenced by the many technical papers on this subject throughout the last quarter century. The tele- vision art is faced with the same problem in the evaluation of noise in television pictures. Only very recently has there been an appre- ciation that the complete treatment of this problem requires a knowl- edge of the properties of the eye as thorough as the knowledge of the properties of the motion picture or television picture. It is of consid- erable help that the eye appears to be limited by fluctuation phenom- ena in much the same way as film or television pick-up tubes. One might expect, if the eye is limited by fluctuation phenomena, to be able to "see noise"—that is, "visual noise"—similar to the noise in a television picture or graininess in a motion picture. The writer is convinced that such visual noise is readily observable at very low levels of scene brightness, around 110,000 foot-lamberts. A white surface then seems to take on a fluctuating grainy appearance. Other conclusions have resulted from the treatment of the eye in the same terms as pick-up tubes and film. For example, the large range of dark adaptation (the abil- ity of the eye to re-adjust itself to see scenes 10,000 times below nor- mal brightness levels) can be in- terpreted as arising from a "gain control" mechanism in series with the retina and brain and not un- like the volume control knob on a television receiver. Further, exist- ing data on the eye suggest that the gain control operates selec- tively more on blue light than on red light. The accurate comparison of the performance of film and pick-up tubes with the performance of the eye cannot but enhance one's ad- miration for the nicety of design that has gone into the human eye by chance selection. At the same time, one has firm grounds for ex- pecting even to surpass the per- formance of the eye by deliberate design of pick-up devices. FRADIO AGE 29'