Journal of the Society of Motion Picture and Television Engineers (1950-1954)

taken of interlaced television pictures which show 188f-line structure. (4.3) Loss of Definition on Movement A television programme derived from a live scene provides 50 images/sec of the subject, but two of these are recorded on each film frame, since the film runs at 25 frames/sec. When the film is retransmitted, it is unlikely that the two scans will be reproduced in the correct order. In practice, owing to imperfect framedeflection linearity, portions of both scans are transmitted in different zones of the picture height. The use of spotposition modulation (see Sec. 4.7) to eliminate the line structure on the recorded film makes it even more difficult to separate the scans. This is illustrated in Fig. 7. Four consecutive television scans of a subject consisting of a vertical line moving horizontally at a rapid rate are shown in column (a). Television scans 1 and 2 are recorded on film frame 1, and scans 3 and 4 on film frame 2, shown in column (b). Column (c) represents the retransmission of the recording when spot modulation is used. It will be seen that a doubling of the image has occurred. When the rate of movement is less than that shown an irregularity on vertical edges is produced. The doubling of images on fast movement is more objectionable to the eye than the blurring produced by the normal motion picture camera. With television cameras involving storage, however, the doubling is not so apparent since the original images do not have distinct edges. (4.4) Optical Efficiency and Resolution Optically compensated continuousmotion systems require a movement of the image during exposure, and this necessitates a greater coverage for the lens. To keep the image in focus during its travel, the film is usually moved round a curved path, the lens being designed to have a spherically curved field, of radius equal to that of the gate. A second cylindrical lens is usually necessary to flatten the field in a plane transverse to the direction of movement. The above considerations do not make for high resolution, particularly as the use of mirrors often necessitates additional lenses to collimate the light and so prevent double images. It is difficult to obtain a high aperture with such systems. The B.B.C. continuous-motion system described gives an effective aperture of about //3.O. The Kemp-Duddington 16mm camera has the merit of using the same lens for both images. The effective aperture is about //3.3. The resolution is quite satisfactory; it is possible to photograph an optical test pattern of 1000 picture elements per picture height with good depth of modulation. (4.5) Film Stability The defect known as picture float has been referred to in Sec. 4.2, and a continuous-motion film-traction mechanism has been described (see Sec. 3.1.3). It has been found possible to reduce this to small proportions, although it is always present to some extent in continuousmotion recordings. More rapid variations of picture position, known as "jump" or "bounce," are due to lack of registration of one or more film frames with the sprocket holes. Picture jump can occur, for instance, in the mirrordrum type of recorder when one or more mirrors are incorrectly set, or in intermittent recorders when each film frame is not properly registered. Correct registration is of paramount importance, and it is normal film practice to have separate register pins apart from the claw mechanism used to advance the film. The Kemp-Duddington camera follows this procedure, being provided with a lifting gate and registration pins integral with the camera aperture. Very good film stability is obtained by this arrangement. In cameras where the pull-down is fast, such as the Eastman Kodak for American standards W. D. Kemp: Television Recording — Abstract 377