Journal of the Society of Motion Picture Engineers (1930-1949)

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four times the definition could be realized, but it would be quite unnecessary and uneconomic to do so. The generally accepted standard seems to comprise a lens and negative-stock combination having a limiting resolution under best conditions of about 40 lines/mm on the axis, and some 30 lines/mm over the whole field of a frame (22.05mm X 16.03 mm). After processing, the release print has a limiting resolution of about 25-30 lines/mm on the axis. This is not a very high standard of definition, and a single35mm frame projected statically to normal screen dimensions generally appears fairly soft. Under running conditions, however, "dynamic resolution" makes its effect apparent and helps to produce an impression of adequate sharpness. The mechanism of the dynamic-resolution effect lies in the fact that surface noise is random and adds from frame to frame in quadrature. The image, on the other hand, is repetitive and therefore tends to add arithmetically over a number of frames; moreover, the sharpness of edges is improved because a random succession of film grains, as it were, scan them and sharply delineate them. For the choice of standards of electronic-image definition to give results comparable with motion-picture film produced by normal methods, it is necessary to consider the order of resolution required and that realizable in the present state of electronics. So far as image dissection is concerned, the only variable quantity is the number of lines, since the picture repetition frequency is fixed by motion-picture standards at 24 frames/ sec. The decision regarding the number of lines controls many factors, of which the bandwidth of the system, the signal/ noise ratio and the size of the scanning spot at both camera and reproducing tube are of cardinal importance. It is well known that, for a given number of lines, there is a calculable bandwidth which must be used in order to produce definition which is equal in both vertical and horizontal directions. It is worth remembering that the use of many more lines than the available bandwidth justifies can result only in progressive deterioration of the picture detail, since the detail frequency increases as the square of the number of lines. The effect of increasing the number of lines, however, has a meretricious appeal, because of the finer resultant structure of the picture, but, whilst easier on the eye, it has no advantage for photography, where the linear structure is going to be eliminated in any case by one of the known expedients and out-and-out detail resolution is all that counts. Considering, in the absence of anything better, a direct translation from optically produced film-definition standard to television, the following assembly of facts is arrived at : The resolution of a normal motionpicture negative has been assessed, at best, to be about 40 lines/mm, which represents 80 television-picture points per millimeter. Since the frame is 22.05mm wide, the definition along the line is equivalent to a total of 80 X 22.05 = 1,764 picture points. This, however, is based on photographic limiting-resolution values, so that it seems possible, in the light of the foregoing arguments, that appreciably less television picture points would suffice to produce a picture of acceptable sharpness. In this connection, Kemp* has suggested that it would be permissible to introduce a factor C, of which he considers the value to be about 0.75, to compensate for the more rapid decay of response of the photographic system with increasing fineness of detail, as opposed to the maintenance of a high level of television modulation up to the frequency of cut-off. Application of this factor gives the definition along the lines as the equivalent of 1,764 X 0.75 = 1,323 picture *W. D. Kemp, "Television recording," J. Inst. Elec. Engrs., [London], 99, Part III A, No. 17: 115-127, 1952. 452 December 1952 Journal of the SMPTE Vol. 59