The Cine Technician (1953-1956)

134 CINE TECHNICIAN A Technician's Notebook September 1955 Transmitting Film by T.V. (Part 2) IN this month's Notebook we conclude the exposition of the technique of transmitting film by television, taken from an article by WALLACE S. SHARPS, Manager of TV, Film and Radio Department, Smee's Advertising Ltd., which we reproduce by permission of the Editor of The British Journal of Photography. WHAT I wrote last month was " intended to provide a basis for a description of the methods of transmitting photographic material. Film scanners are of two types : (a) those which use a photoconductive type of tube, such as the ' Staticon,' and (b) those which use a ' flying spot.' The Staticon tube is also used in the Pye industrial camera and this is the basis of a number of dual live and film closed circuits that have been installed in the offices of advertising agents and commercial television film producers. However, because the flying spot system is used purely for the transmission of photographic matter, I shall restrict myself on this occasion to a description of its operation. Flying Spot A Cintel 35mm. flying spot scanner (of the type supplied to the B.B.C. ) is shown in Fig. 1 and it will be noted that there are two sets of film spool chambers. This provides for the running of an unmarried sound track in synchronism with the picture film being scanned. On the left of the picture a cathode-ray tube can be seen and this is basically of the same type as used in a domestic receiver. The electron gun inside the tube is directed upon its face and produces the line pattern known as the ' raster ' which appears to be a continuous pattern because of the persistence of human vision and the image retention properties of the internal phospor coating. The film runs in front of this tube and so a spot of light is continually passing over the film surface. The light passes through the film and falls on to a simple condenser which collects the light and directs it on to a photo-electric cell. Thus, type of film projector the film is projected when stationary and the shutter seals the light off while the film is being pulled down. Now, it fc J Fig. 1. Traction unit of Cintel 35mm. flying spot teleoine equipment. Photograph by courtesy ol Cinema-Television Ltd. a highlight on the film will allow a great amount of light to pass and so cause the photo-electric cell to emit a high signal. The flying spot scanner has the considerable advantages of using a normal type of tube and photocell and producing a very highquality picture. While camera tubes are subject to variation, even when the utmost care is taken in their construction, the receiver type of tube that can be used in this scanner can be obtained at a suitable standard relatively easily. The tube cost is also much less and maintenance is not unduly difficult. One knows that in an ordinary will be realised that the flying-spot scanner does not retain an image in the same way as do camera tubes, and so the difference in time between successive scans must be very small. Although it is theoretically possible to make an intermittent projector that will permit very rapid pull-down, there are a number of practical mechanical difficulties. It has therefore been suggested that the film be allowed to run continuously with appropriate corrections in the scanning mechanism, and it is this method which has been adopted in the machine illustrated. In order to accomplish this, use has been made