▼ Menu

Colour Television

Wireless World, May, 1945.
Extras ▼


Discussion on ways and means.

In opening an informal discussion on colour television before the Radio Section of the IEE, L C Jesty introduced the subject by saying that it seemed inevitable that a colour television service would ultimately be established. Development should be directed towards (a) the agreement of the technical methods to be employed, particularly with regard to the colour analysis and synthesis of the picture, and (b) the standard of definition to be achieved before colour is introduced.

With regard to (a), the literature shows that the methods proposed for colour television have followed logically the same steps as already trodden in colour photography an d cinematography, but have not yet reached an equivalent of the elegant solution to the photographic problem known as the subtractive integral tri-pack technique. Television, however, being electronic and therefore practically inertia less and instantaneous, enables the older additive principles to be used more advantageously than in cinematography.

All the demonstrations of colour television so far-given, by Baird in this country, and Bell Telephone and CBS in America, have employed scanning processes embodying various colour sequences for analysis and synthesis. It is now taken for granted that the science of colour has established the necessity for a minimum of three primary colours for acceptable reproduction.

Scanning sequences can be classified under three heads: (1) Scanning each picture point; (2) scanning each picture line; and (3) scanning each picture frame, in the three primary colours. Of these (1) is the most attractive but the most difficult. It gives the minimum of difficulty in colour registration and fringing; allows the retention of the same basic scanning frequencies (line and frame) as the equivalent definition black-and-white picture; and allows the possibility of adding colour to an existing black-and-white -system, the existing receivers continuing to receive the picture in black and white. The difficulties with this system lie in changing the colour of the scanning spot at about three times the maximum video frequency of the black-and-white picture and in maintaining colour synchronism. At the other extreme (3) offers the simplification of changing the colours at only about three times the equivalent black-and-white frame frequency, but at the expense of three times the frame and line frequencies; it also suffers from the inability to add colour to an existing black-and-white system.

Additive Systems

Various methods from 'reseau' screens to moving filters have been proposed for producing the necessary primary colours. All additive colour systems result in a loss of sensitivity in the transmitter camera and loss of brightness in the received picture. These must be restored by improvements in cameras and cathode-ray. tubes. Additive systems fall into two main classes: those employing optical or electro-optical superposition of the colour images, and those employing sequential projection or scanning of the colours. The former suffer from errors of super-position of the images, giving colour fringes where the registration is inaccurate, but offer the possibility of using separate channels for each colour with corresponding advantage. The latter suffer from colour fringes on moving objects, owing to the time lapse between the presentation of the successive colours, but these lags can be made imperceptible provided the colour sequence is fast enough.

Electronic scanners give high relative accuracy in the location of picture points, but absolute accuracy is of a low order. Their use for the former method is therefore ruled out unless some auxiliary device is used for ensuring registration. The same argument applies to the use of a fixed 'reseau' with either scanning sequences (1) and (2) above.

It would appear, therefore, that the only immediately practicable system is the sequential-colour frame-scanning system (3) (above), unless some unpublished device has been perfected, such as a method of altering the colour of a fluorescent screen at will, or receiver picture storage, or the simultaneous transmission of all picture points instead of scanning.  [*] The Shadow-Mask tube arrived in 1950

With regard to the standard of definition, the additional information to be conveyed in a colour picture results in an increase in the video band-width of about three times compared with the equivalent definition black-and-white picture. In going. from a black-and-white to at colour picture with the same available bandwidth, it follows that the black-and-white picture will have about three times the number of lines of the colour picture. At low definition this would be very noticeable, but in the region of 400 lines or over the comparison in definition would become less obvious. A 405 line colour picture would require about three times the video bandwidth, and with vestigial sideband transmission about twice the ether space of the pre-war 405 line transmission. On this basis, a 500-600 line colour picture is not inconceivable as a long-term development. Should it be demonstrated, how-ever, that higher definition, say, 800 - 1,000 lines, is necessary on purely visual grounds, then it would seem that colour television is only a remote possibility until much greater experience of the higher transmission-frequency bands has been obtained.

During the course of his remarks, Mr Jesty gave demonstrations of the synthesis of white light from three primary colours. Sequential illumination of a set of snooker balls through a rotating three-colour filter served to show brilliant colour fringes when the balls were set in motion. Finally, the meeting was given the opportunity of comparing monochromatic and colour cine films of the same subjects projected simultaneously side by side.

In the discussion which followed, several speakers commented on the apparent improvement in contrast in the colour pictures, and it was agreed that less range of tone was required in colour than in a black-and-white system. On the other hand, the brightness level of an additive colour television picture would be less than that of monochrome, and there was need for further development to increase the efficiency of screen fluorescence. One speaker thought that a mechanical system of scanning might provide a solution: small high-speed motors were now available with a useful life of the order of 4,000 hours.

Colour Fringing

Colour fringing on moving objects was a serious defect of present frame-by-frame scanning methods. It could also be caused by hum in the receiver if the vertical scanning rate were not an integral multiple of the mains frequency and by fading in a propagation system depending on a network of radio links. Point-by-point scanning must be the ultimate goal, and one method by which this might be reached was to introduce transverse velocity modulation in the time base so that the wanted colour in each point would receive a longer period of illumination.

In a written communication, J L Baird expressed the opinion that point-by-point scanning did not offer sufficient advantages over line-by-line scanning to counterbalance the increased difficulties involved. With present frame-by-frame scanning methods a considerable reduction in colour flicker was obtained by increasing the number of inter-lacings (and consequently the frame frequency) for the same number, of lines. He thought it rather misleading to state that frame-by-frame scanning could not be added to existing black-and-white systems. A two-colour 600 line system (200 line frames at a frame frequency of 50 per sec., interlaced three times) could be used in the pre-war BBC 405 line system, and would be received as a 200 line black-and-white picture on existing receivers. A three-colour system was necessary for accurate colour reproduction, but, in his view, a two-colour system gave a pleasing and acceptable picture.

Other speakers held that colour reproduction should not be attempted until adequate definition was assured, and that the problem of colour should be set as a separate objective, not as an adjunct to existing systems.

In his concluding remarks, the chairman (H L Kirke) said colour television was not likely to become an established service for some years, but when it did it would be of great value, as there were many subjects which could not be adequately portrayed in monochrome. From the aesthetic point of view he thought the subtle improvement over black-and-white of pictures with ordinary sober colours was of greater value than the more striking effects of vivid colours.

Use browser back button to return.