The Cine Technician (1953-1956)

118 CINE TECHNICIAN A Technician s Notebook August 1955 Transmitting Film by T.V. A S it seems highly probable that ■t* a good many of us are going to be involved to a greater or lesser extent in the production of material for the television screen we are devoting this and next month's Notebooks to an exposition of the technique of transmitting film by television, taken from an article by WALLACE S. SHARPS, Manager of TV, Film & Radio Department, Smee's A Jvertising Ltd., which we reproduce bv permission of the Editor of The British Journal of Photography, in which it originally appeared. T^ARLY television transmitters ■*-* were based on the use of mechanical devices, such as scanning discs, the origin of which dates back to 1884. With the invention of the electronic tube known as the " iconoscope," purely electronic methods of transmission were developed. With all the vast technical advances made since the pioneering days, the basis of television remains the same, in that a scene is broken into small portions in the camera and can be reassembled in the receiver, the image having been transmitted by radio waves through the ether. on to the target, causing a secondary emission of electrons to take place. These are collected by the target screen of wire mesh and a positive charge is left on the image side of the target that bears a direct relationship to the tones of the optical image. Then the electron gun at the rear of the tube sweeps across the other side of the target, so that the image is scanned. The electrons from this beam remain on each part of the target in sufficient quantity to neutralise the positive charges on the other side and the residue electrons are returned in a beam to the electron multiplier. Therefore, the returning beam will be weakest for the highlight parts of the optical image and strongest for the shadow portions. The electron multiplier increases the strength of the signal which is then reversed to a positive and passed via amplifiers through the air and into your receiver. Scanning Beam The scanning beam in the British system sweeps across the target in such a way that it traces out 405 lines in one twenty-fifth of a second. This is shown in Fig. 2. Our system is to use a 2-to-l interlaced scan, so that the target is scanned twice (two fields) for each complete picture (frame). The first OBJECT TO 8E LIGHT SENSITIVE SCANNING TELEVISED PLATE MESH .TARGET BEAM ELECTRON GUN SOCKET PRONGS :i> CAMERA LENS RETURNING BEAM CARRYING IMAGE TO BE TELECAST ELECTRON MULTIPLIER !•" i K . 1. Sectional view of the image orthicon camera tube showing how light from an object is converted into electrons and then into electrical signals by the action of an electron gun. Diagram by conrtesx of Greenberg, New i'ork. The camera tube in general use today is the image orthicon, and a sectional view of one is shown in Fig. 1. In this tube, the image is focused by the camera lens on to a light-sensitive ( photocathode) plate. This plate emits electrons in proportion to the amount of light that falls upon it and these pass field is shown by the solid lines in Fig. 2, the second set of scanning lines being shown broken. It will be noted that at the end of each line, the scanning beam moves to the extreme left in order to commence the next scan. For various reasons it has been found most expedient to use an odd number of lines and so one field begins, and the other ends, with half a line. The vertical definition is limited by the number of lines in each Fig. 2. A 2-to-l interlaced scan, as used in the British system. Solid lines represent the first field, broken lilies the second. frame and the horizontal definition depends on a number of factors that determine the size of the spot produced by the scanning beam. Because of this, there is a loss of definition when the brightness control on a receiver is turned up and so the size of the spot on the fluorescent screen is increased. The reason for using an interlaced system of two fields per frame is due to the need to reduce flicker on the image viewed. A field frequency of 25 per second is obtained by using a sequential scan in which each line from top to bottom is traced in order. However, this frequency is doubled with the interlaced system that scans alternate lines of the frame for each field and so flicker is reduced to a point where it is not too noticeable at ordinary viewing distances. At any moment of time only one spot is on the screen but, because of the retcntivity of the human eye and the persistence of the fluorescent image, the impression is given of continuous movement. The ratio of the picture width to height is 4:3 and this is known as the aspect ratio. The ratio can be varied at the receiver or transmitter, and if either is wrongly set up. a distorted picture results. It is important to boar this ratio in mind when producing material of any type for transmission. (To be continued)