Cinema Canada (Apr 1979)

TEd4d NEWS FLYING SPOT SCANNERS FILM POST-PRODUCTION ON VIDEOTAPE 3 In England and Europe, as well as some other parts of the world, the television system operates at 25 frames (50 fields) per second, instead of 30 frames (60 fields) as in North America. The use of this lower frame rate greatly simplifies the scanning of motion picture film. Existing films shot at the standard rate of 24 frames/sec. can be speeded up slightly in the telecine transport to match the television scanning frequency, while films being made specially for television can be shot at 25 frames/sec. and then played back in telecine at the same rate. The ability to reproduce films in the television system without the need for frame rate conversion enabled equipment manufacturers in England and Europe to take an entirely different approach in designing telecines, as compared with the North American practice. The outcome was the development of what is known as the flying spot scanner. Cathode Ray Tube as Light Source In the flying spot scanner a cathode ray tube (CRT) is used as the light source, instead of a tungsten lamp as in motion picture projectors. The cathode ray tube is similar in most respects to a small television picture tube, in that an electron beam is driven back and forth inside the tube, exciting a phosphor layer coated on the inner surface of the flat face plate, and producing a uniformly illuminated raster. A lens focuses the rapidly moving spot of light on the tube face at the plane of the film in the gate of the film transport mechanism. The light passing through the film is collected in an optical system which makes the red, green and blue separation, and then directs these three light beams into photomultiplier tubes where the video signals are generated. If the eye could act quickly enough it would see a tiny, rapidly moving spot of light sweeping back and forth across the face of the CRT, but as the entire frame scan takes place in one twenty-fifth of a second, the eye sees what appears to be a uniformly illuminated rectangle. Since the television system must react very quickly in order to trace out picture information, the system “sees” the rapidly moving spot of light. When film is being held stationary in the gate of the telecine, a picture frame is actually scanned by the spot of light from side to side and top to bottom. From this brief description it should be easy to see that the color and intensity of the spot of light will be modified (modulated) by the film image as it passes through the film. Then, in the following optical system, after color separation has taken place, the output signal/levels from the three photomultiplier tubes will rise and fall in relation to the intensity of the light modulations. Two Television Fields from Each Film Frame. This method of reproducing film in the television system is basically much simpler than the North American practice of projecting films into a. television camera. But in practice it is not possible just to scan the film frames one by one, because the telecine output must be in the form of two interlaced fields for each film frame. Rank Cintel in England has been making flying spot scanners for many years, utilizing a continuous film transport and a twin-lens optical system in which the scanning for two consecutive fields on the face of the CRT is imaged on the film at two different positions in its travel. The continuous motion of the film contributes about half of the required height of vertical scanning, and a rotating shutter allows light to pass through alternate optical paths. This most ingenious system has been utilized most successfully by broadcasters in England and Europe, giving excellent picture quality. The flying spot scanner has a number of important advantages. First and foremost, color separation takes place after the film images have been scanned, thus eliminating altogether any possibility of color misregistration and the annoying color fringes sometimes seen in pictures from _ vidicon telecines. The pictures from film obtained in flying spot scanners wére for a long time so much better than the pictures by Rodger J. Ross from live television cameras that an entirely different approach to film reproduction was taken in _ television centres operating on the 25-frame scanning standard. For the most part, manual operation of telecines has been the normal practice, although in recent years some European broadcasters have gone over to automatic signal level control to save operating costs and some have been installing cameratype telecines to take advantage of the greater programming flexibility and lower equipment costs that multiplexed telecine chains offer. The Flying Spot Scanner in the United States and Canada Many attempts. have been made by equipment manufacturers to adapt the flying spot scanner principle for use by television stations operating at 30 frames/sec. but without noticeable success. North American broadcasters have become so much attached to the camera-type telecine, valuing especially its versatility and flexibility and its ease of operation with automatic signal level control, that. any other system for reproducing film had little chance of adoption. Those who saw in the flying spot scanner the possibility of producing much better television pictures from film were confronted with the additional handicap of frame rate conversion — it turned out to be very difficult to devise a practical system for obtaining 60 interlaced television fields per second from film running at 24 frames/sec. Rank Cintel adopted a method in their flying spot scanner known as “jump scan”. With this method the scanned portion of the raster on the CRT was shifted electronically into five different positions for every two film frames, to obtain the necessary five television fields, or 2 1/2 fields per film frame, But it was ver difficult to entirely suppress the 12-cycle flicker that resulted from slight differences in the brightness of the raster in the different positions on the face of the CRT. Digiscan System of Frame Rate Conversion All of these problems have been Cinema Canada/31