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Technical Review

Early development of the BBC’s colour television service

This is an extract from “Twenty-five Years of BBC Television”, BBC Engineering Division Monograph No 39: October 1961

By Sir Harold Bishop, C.B.E., F.C.G.I., M.I.E.E., M.I.Mech.E. Director of Engineering, British Broadcasting Corporation

This is document was made available to the general public in 1961 and is reproduced here with thanks to the BBC.

6.2 Colour Television

The BBC has been carrying out research and development on colour television since the resumption of the television service after the war. In the early post-war years an experimental sequential system with mechanical colour separation was developed, and a considerable amount of research was carried out on the fundamentals of trichromatic colorimetry as applied to television.

By 1953, the consensus of opinion was (as it still is) that no public colour television service could be contemplated unless it were compatible, i.e. the transmissions were of a form which would enable existing monochrome receivers to produce black-and-white pictures. It seemed unlikely that compatibility would be achieved with any system which was not effectively simultaneous, and the first report of the Television Advisory Committee pointed out that the impossibility of increasing the channel spacing in Band 1 (41-68 Mc/s) would necessitate a fully compatible system if colour transmissions were made in this band. Towards the end of 1953 the BBC Research Department began the development of an adaptation of the American N.T.S.C. fully compatible simultaneous colour television system to the British 405-line standard.

On 7 October 1954, the first 'compatible' type of colour television picture was radiated from the medium-power transmitter at Alexandra Palace. The pictures included slides and 16-mm motion pictures, and the details of the standards employed on this occasion differed little from those employed regularly from 1955 until the present time. On this historic occasion only one colour television receiver, so far as is known, displayed the pictures, but there was a fair-sized audience viewing the compatible black-and-white pictures in their homes on normal domestic television receivers. Although many hundreds of tests were subsequently necessary to prove the point, it seemed to the observers of this first transmission, which was a co-operative effort of the Research Department of Marconi's Wireless Telegraph Co. Ltd and the BBC Engineering Division, that in this adapted N.T.S.C. system there existed a standard capable of providing excellent colour pictures and compatible ones of good quality.

During the winter of 1955-6 a regular series of transmissions was radiated from the medium-power transmitter at Alexandra Palace, with the primary purpose of testing the compatibility of the pictures on a comparatively large sample of domestic receivers. Again, only slides and pictures from 16-mm motion film were used, this time the equipment being of BBC design and manufacture. In the meantime Studio A at Alexandra Palace had been equipped with a single three-tube colour camera of Marconi design, and the first occasions on which colour pictures including scenes from the studio were broadcast occurred on 3, 4, and 5 April 1956 during a special demonstration for delegates of Study Group IX of the C.C.l.R., who were visiting this country as part of a world-wide assessment of the state of development of colour television. The programme of this demonstration was probably the most comprehensive which has yet been given, and was as follows:

Colour pictures were first shown using three separate full-bandwidth links to convey the R, G, and B signals, in order to show the performance of the R.C.A. 21-in shadow mask tube independently of the N.T.S.C.-type coding and decoding system. Pictures were then shown after passing the signals through the coding and decoding circuits in the following alternative forms:

1. Composite video signal over a single cable link (to estimate the effectiveness of the Band-sharing system).

2. Chrominance and luminance over separate cable links (to estimate whether the transmission of luminance in a separate channel might improve picture quality).

3. Sound and composite vision signals transmitted by radio on the Band I, Channel 1 frequencies. (To observe whether any deterioration of picture quality or mutual interference between sound and vision were noticeable.)

The effects of varying the bandwidth of the 'I' or in- phase component of the chrominance signal, of impulsive and random noise interference and of varying errors in subcarrier phase, were also demonstrated.

In all cases where a 'Y' or luminance signal was available the picture was also presented on a black-and-white receiver having the same picture size.

By the autumn of 1956, Studio A at Alexandra Palace had been equipped with a second experimental colour camera and, a little later, a 35-mm Cintel film scanner was installed to supplement the slide and 16-mm film scanner. With this equipment and with the enthusiastic help of a small group of programme staff, an ambitious and comprehensive series of programmes was broadcast, this time from the Crystal Palace transmitter, in the winter of 1956-7 and was observed in people's homes on specially developed experimental colour receivers and also, of course, on a large number of black-and-white domestic sets. The details of this series of tests and the results obtained therefrom have been fully described in Monograph No. 18. On 30 and 31 January 1957, a special programme was broadcast and shown to a large audience of Members of both Houses of Parliament on six receivers installed in a room in the House of Lords.

During the winter of 1957-8 a further series of experimental programmes was broadcast from the studio at Alexandra Palace and was seen by a rather bigger audience on colour receivers than in the previous year. At the conclusion of these tests in 1958 the studio at Alexandra Palace was dismantled and the cameras installed temporarily in a van which carried out two outside broadcasts. The slide and film-scanning equipment was moved to the Lime Grove Studios whence a regular series of transmissions outside normal programme time has been given, beginning in the autumn of 1958 and continuing with only short breaks to the present time.

The BBC demonstrated colour television to the public for the first time at the National Radio Show at Earls Court in 1961. Live transmissions from a glass-sided studio as well as film transmissions were demonstrated on six 21-in. monitors. Each colour monitor had a 21-in. black-and-white monitor alongside it to demonstrate compatibility. Only a small minority of the public or of dealers had seen any colour television previously, and both groups were favourably impressed.

6.3 Television Broadcasting on UHF

In 1956 the Television Advisory Committee recommended that transmissions should be carried out in the ultra-high frequency bands (Nos. IV and V) to assess the potentialities of these bands for television broadcasting, and experimental transmitters for both bands were in- stalled at the Crystal Palace television station. The Band IV tests were carried out with a low-power transmitter radiating square-wave-modulated signals at 495 Mc/s, but the Band V installation included transmitters for both vision and sound, with a vision transmitter power of about 10 kW and an e.r.p. of 125 kW from a helical aerial at a height of 690 ft. The transmitters could be modified to work either on a 405-line system (with a.m. sound) or a C.C.I.R. 625-line system (with f.m. sound), the vision carrier frequency being 654-25 Mc/s in both cases. In addition to the BBC a number of other organizations participated in these field trials, which were the most comprehensive series of UHF television propagation tests yet conducted in any country. The trials are fully described in another BBC publication, and the mobile field-strength measuring laboratory developed by the BBC is described in an earlier BBC Engineering Monograph."

The main conclusions of the trials which applied equally to a 405-line or a 625-line service were that over a terrain such as south-east England, the first-class service area of a Band V transmitter with an e.r.p. of 1,000 kW would be comparable in size to, but more irregular in shape than, that given by the present Band I transmitter with an e.r.p. of 170 kW at 45 Mc/s.

The second-class service area obtained would also be of approximately the same actual size as that given by the Band I station but, on the higher frequencies, the topography would materially modify the shape of the area with respect to the present Band I coverage, with consequent changes in the numbers of viewers served.

Owing to the greater dependence of the Band V signal on topography, the contours of both the first- and second- class service areas would be more irregular in the case of Band V, and this would increase the number of transmitters needed to give national coverage.

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