British Kinematography (1950)

*^f^»enc» Libra,, November, 1950 sandvik and veal : films for television "Wl Branch 143 means of supplying sequences or scenes that are either difficult or impossible to obtain with the television studio camera. Fundamental Limitations. The present discussion, however, will deal primarily with some technical aspects of motion-picture film characteristics for television. The performance of any picture pick-up device depends upon several basic properties of the medium or characteristic features of the system, namely : (a) Sensitivity. (b) Resolution or rendition of picture detail. (c) Image inhomogeneity or random fluctuations. (d) Tone reproduction or transfer characteristic. These properties of motion-picture films and of television-system elements have been discussed extensively in the literature. Each of these properties depends, to a lesser or a greater extent, upon the others. The relative sensitivities of motion-picture film and of the television camera tube are of practical importance primarily when, in case of adverse lighting conditions, one has the choice of using either medium. Resolving Power. A comparison of the detail-rendering abilities of the two media, on the basis of the limiting resolving power of the film and the number of television scanning lines, is of little significance. Detail which is just resolved under special viewing conditions corresponding to limiting resolution, probably does not contribute appreciably to picture sharpness. The criterion of satisfactory picture sharpness is that the image structure be sufficiently well resolved so that it can be seen easily under normal viewing conditions. The vertical resolution in a television picture is not generally equal to the number of scanning lines within the picture area since it depends on the composition of the subject. It has been shown that, on the average, 64 per cent, of the picture elements are reproduced correctly in the scanning process. Moreover, the vertical and the horizontal resolution in a television system are not necessarily equal. This depends on the over-all effective band width of the television system, that is, the frequency-response characteristic of the camera tube, the video amplifiers in the camera, the transmitter, and the receiver. The frequency-response characteristics can be modified by appropriate electrical network in the video amplifiers. Thus, as the response of the high frequencies, which carry the fine image detail, is increased, the rate at which the resolution of the fine image structure fails can be modified accordingly, as it approaches a limiting value. , The picture sharpness which can be obtained in a well-adjusted closed-circuit television system using a flying-spot scanning system and a band width of 4J Mc/s, probably approaches that of average commercial quality 35mm. motion pictures. At present the quality of some pictures on the home television receiver is more nearly comparable to that of 8mm. motion pictures. When comparing the picture sharpness of the two media, or the detail that the eye can see, there appears to be one notable difference, namely, the viewing distance relative to the size of the picture. A motion picture can be viewed with comfort, without annoying disturbances owing to graininess, at a distance equal to about twice the width of the picture. The preferred viewing distance has been found to be about 3 J times the picture width. A limited survey indicates that a critical observer prefers to view a television picture at a distance equal to about eight times the picture width. Since this is more than twice the distance that the average eye fails to resolve the scanning lines, the amount of detail that the eye can see is reduced proportionately.