The following appeared in the March 1938 edition of Television and Short-Wave World magazine.
REMARKABLE results in colour television were demonstrated by the Baird Company at the Dominion Theatre, Tottenham Court Road, last month. The demonstration was remarkable in several ways and it was the first of its kind ever given, though in 1928 Mr. Baird showed a picture a few inches square in colour at the British Association meeting in Glasgow. This, however, was transmitted by wire.
In this latest development the size of the picture is 12 ft. by 9 ft. and the transmission by radio from the South Tower of the Crystal Palace, a wavelength of 8.3 metres being used. Another special feature of the demonstration was that the colour is exceedingly good and on the whole more pleasing to the eye than are the latest colour films. No claim was made that the definition came up to the standard of the ordinary transmissions and Mr. Baird clearly stated that this was regarded as a preliminary experiment. Even so, the demonstration was most impressive and the results came as a surprise to those who witnessed it.
The present apparatus is shown by the photographs and drawings. It transmits a 120-line picture, the scanning at both transmitter and receiver being by mechanical means. The transmitter consists of a mirror drum with twenty mirrors inclined at differing angles revolving at 6,000 r.p.m. These mirrors reflect the scene to be transmitted through a lens, causing an image to be formed on a rotating disc with 12 concentric slots at different distances from its periphery. By this means the field given by the 20-line drum is interlaced six times to give a 120-line picture repeated twice for each revolution of the disc. Each of the slots is covered with a light filter, blue, green and red being used alternately, the effect of this being to transmit alternate lines of the picture corresponding to a blue-green image and a red image.
At the receiving station a similar device is employed, the rotating drum in this case being much larger (12 in. in diameter in place of the 8 in. drum at the transmitter).
Light from a high-intensity arc lamp is concentrated on the moving aperture in the disc and yields sufficient light to fill a screen 12 ft. by 9 ft. The projected picture could be seen from all parts of the Dominion Theatre, which has a seating accom- modation of 3,000.
The programme included impersonations of a military officer, a shiek and a Turk. Two ladies exhibited various coloured hats, and the programme concluded with the White Ensign and a coloured photograph of the King. The actual objects televised were afterwards inspected at the Crystal Palace, and it was apparent what excellent reproduction had been obtained.
There are some very interesting features about this apparatus and a very large number of practical difficulties had to be solved before it could be put into successful operation. One trouble was in the construction of a mirror drum that at the high speed of revolution would not distort the mirrors or burst. Depending upon the method of securing the mirrors to the drum, they would either bend outwards and become convex or inwards and become concave until a suitable method of con-struction had been developed. The fire regulations of the L.C.C. had also to be overcome and this necessitated the entire receiver and projector being enclosed in metal cases with only a small peephole through which the arc crater could be observed. This latter, by the way, takes a current of 150 amps. and the heat generated makes water cooling essential.
|Schematic drawing of the Baird colour television transmitter.||Schematic drawing of the receiver.|
The general layout of both transmitter and receiver will be clear from the schematic drawings and it will be seen that the drum and disc are directly coupled together by means of reducing gearing, one motor being used to drive both. The Kerr cell used for light modulation is of the ordinary type with an aperture about 3/16 in. square. Other types of cell using both rays were tried, but it was found that gains obtained in one direction were more than lost in others and that the ordinary type gave the best results. Actually the transmitter is mounted on a bogie of much the same type as is used for the Emitron camera and it is therefore reasonably flexible in use. The entire apparatus is a beautiful piece of work and reflects great credit on the production engineers of the Baird Company.
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26th December 2006