British Kinematography (1950)

November, 1950 sandvik and veal : films for television 147 continuously. The only significant break in the scanning process is the interval during which the beam returns from the bottom of the picture to the top. During this interval, 1/750 second, every 1/60 second, one for the lace and one for the interlace, the picture is blanked out. Since it is not practical mechanically to advance the film during this short interval, some other method must be used. One common method is the use of short light flashes, so timed that the film picture is projected on the pickup tube mosaic for only 1/1200 second, every 1/60 second. These flashes occur during the retrace time. Since the pickup tube stores the charge, the mosaic can be scanned during the interval between the flashes. The flashes are provided by a rotary shutter placed in the light beam. A small slit on its periphery admits the light flash. The shutter is driven at a speed of exactly 3,600 r.p.m. by a special 3-phase synchronous motor which locks in the correct phase relation with the cycle of operation in the television system. The cycle Of operation is illustrated diagrammatically in Fig. 5. The required television frame rate of 30 per second is obtained by illuminating one motion-picture frame twice and the next frame three times, alternately. In order to employ this 2-3-2-3 scanning sequence successfully, the pulldown time must be shorter than that commonly used in standard projectors, as illustrated in the diagram. Transfer Characteristics. The combined or the over-all transfer characteristic of the Iconoscope and the Kinescope can be determined by a graphical method similar to that illustrated in Fig. 4. Owing to the dependence of the Iconoscope characteristic upon the illuminance and to the bewildering maze of knobs and controls at the camera, the monitor, and the receiver, each of which affects the picture quality, a transfer characteristic must be associated with a set of conditions. A typical transfer characteristic of log output current as a function of the log illuminance in foot-candles on the Iconoscope mosaic is shown in the lower right-hand quadrant in Fig. 6. A linear video amplifier is represented by a straight line in the lower left-hand quadrant. This relates the log input voltage to the control grid of the Kinescope with the log output signal of the Iconoscope. The brightness of the Kinescope screen is a power function of the applied signal voltage ; the exponent varies between two and three, according to the particular type of Kinescope tube. The relation between log luminance in foot-lamberts and the log grid voltage of a Type 10BP4 Kinescope is shown in the upper left-hand quadrant. The combined transfer characteristic is shown in the upper right-hand quadrant. The conditions were selected" so that a highlight illuminance of 60 footcandles on the pickup tube mosaic was reproduced at a luminance of 20 footlamberts on the Kinescope screen. This screen luminance is considerably higher than that normally used for either 16mm. or 35mm. film projection. The luminance level at which the Kinescope is operated depends upon the light level in the room, and to a great extent upon the preference of the observer. There is considerable evidence to indicate that many observers prefer to view an image of rather high brightness. Tone Compression. The combined transfer characteristic in Fig. 6 shows considerable compression of the luminance scale in the highlight region. Although the particular set of conditions used to obtain this curve shows no compression at low luminance values, it is generally recognized that shadow detail suffers from inadequate tone scale. In other words, the television reproduction, like the normal photographic reproduction, introduces some compression at both ends of the luminance scale, that is, both in the highlights and in the shadows. It has been found