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

one picture period previously. Figure 6(d) shows the lines recorded during the last scan, and Fig. 6(e) the picture join obtained. The effect of afterglow is to blur the edges of the join. It will be appreciated that, to obtain such a perfect join, the shutter angle and velocity stability of the shutter have to be substantially perfect. In the Eastman Kodak television-recording camera for American standards, the shutter is driven by a separate synchronous motor and the shutter edges are stoned down on test to produce an exact shutter angle. Even so, the picture join could be detected on some early American recordings.10 Figures 6(f), 6(g) and 6(h) show a proposed solution to this problem by the use of a fading shutter.8 In this method, the shutter is caused to fade in the lines gradually, instead of occulting them. One picture period later the lines are faded out, the fade-in zone of the picture overlaying the fade-out zone. An alternative method of achieving a 2food picture join is to employ blanking of the recording monitor.11 In this case, the cathode-ray tube is normally biased Deyond beam cut-off. When the film is n the gate and ready for exposure, the camera sends a trigger pulse which •aises the cathode-ray-tube bias to the correct figure for exactly one picture Deriod (the correct number of lines being counted electrically), after which the cathode-ray tube is again biased back and the film is pulled down. There is no shutter on the camera. With this system, lines recorded just prior to film pull-down are not exposed to the afterglow and tend to be under-exposed. As the joint occurs only on alternate frames, the variation in density tends to cause 1 2cycle flicker. The effect can be reduced by reducing the pull-down time and phasing so as to allow additional exposure time after each television frame. The picture joins produced by this system can be undetectable unless movement has occurred during the picture period when there is a discontinuity at the join. (4.2) Interlacing In order to record an interlaced picture by means of a continuous-motion process, it is necessary that the film and scanning spot should move in such a fashion that after an interval of 0.04 sec the scanning spot is in the correct position relative to the film within a small percentage of the line pitch. Even with a 405-line picture, this implies a high degree of velocity stability for the film movement. The difficulties of eliminating sprocket-hole velocity modulation (or "flutter") and slow-speed velocity modulation (or "wow") are well known to motion picture sound-recording engineers, but in their case the integrated displacement error over any period of time is relatively unimportant, providing the period is long. Much work on the stabilization of film velocity for television purposes has been carried out in connection with flying-spot telecine machines, and it is a tribute to the engineers concerned that substantially 50:50 interlacing with a picture float of less than a line pitch has been obtained. It should be noted that it is the error integrated over the 0.04-sec period that is important, and advantage has been taken of this in selecting the rotation speed of the stabilizing flywheel used in the latest B.B.C. continuousmotion recorder already described, where sub-harmonics of 25 cycles were carefully avoided. Owing to the nature of the film base, the linear dimensions of motion picture film are variable with its age and the humidity and temperature of the storage conditions. With triacetate base, used for noninflammable film, the sprockethole pitch of raw stock is usually within 0.1% of the nominal dimensions, a shrinkage having occurred since perforation. This shrinkage may rise to 0.2% or more if some months are allowed to elapse between perforation and use.12 W. D. Kemp: Television Recording — Abstract 375