|B.B.C.'s Big Day||TV Arrangements||International TV||Conversion|
Mobile Equipment in Holland for the Coronation Relay.
READERS will see from our route map of the Coronation relay to Europe (following
page) that the British 405-line signals are converted to the continental 625-line
standard at Breda in Holland. The converter equipment, designed by the Philips Research Laboratories at Eindhoven, is basically the same as that used by the B.B.C. at Cassel last year for changing from French to British standardsa c.r.t. monitor displays the incoming picture and this is viewed by a camera working on the new standards. The situation is rather different at Breda, however, in that the pictures are going in the opposite direction and are being converted from a low number of lines to a higher number of lines.
The trailer in which the equipment is installed.
Moreover, the equipment is a good deal smaller than the B.B.C.'s, and is actually installed in a trailerwhich also contains a reserve converter (in case of breakdowns) and a quantity of monitoring and test gear. This trailer is stationed outside a church known as the Grote Kerk in Breda, and from it cables run up the side of the building to the centimetre-wave transmitting and receiving equipments which are mounted on the steeple.
The smallness of the converter has been achieved mainly by the use of a c.r. tube with a screen diameter of only 5in to display the incoming picture. Normally, with a screen of this diameter, the definition would not be very good because of the relatively large size of the spot; but the tube is actually a flying-spot scanner with very small spot and has a definition of 1,000 lines. The camera has an image iconoscope pick-up tube, and this is fitted with a mask at the edge of its viewing window to provide a black reference for the 625-line signal.
As in previous converter equipments, the monitor c.r. tube uses a long-persistence screen as a means of light storage. Without this, the camera pick-up tube would tend to act as a simple photo-cell and would respond to the instantaneous variations of intensity of the light spot. Thus it would produce a spurious waveform corresponding to the 405-line vision signal, and this would beat with the normal 625-line signal to give a completely meaningless output. With the long-persistence screen, however, a large component of unmodulated light is introduced, so that the intensity variations of the spot are made negligible in comparison and have little or no effect on the pick-up tube.
At the same time, of course, the persistence must not be made long enough to preserve one picture into the next picture period, otherwise blurring would occur with moving images. Actually the decay characteristic of the phosphor is such that the brightness of a point on the screen falls to about fths of its original value by the end of one frame period.
Another important point is that the scanning beam of the camera is arranged to <( read " the picture at a more-or-less constant time interval behind the " writing " spot of the c.r. tube. (This is possible because, although the line periods of the two systems are different, the frame periods are the same.) If this were not done there would be a phase drift between the two scanning systems, and sometimes the camera would be "reading" the picture while it was still bright from the spot and sometimes while it was fading out a long way behind the spot, and the result would be that the outgoing picture would fluctuate in brightness. The two scanning systems are actually locked together by synchronizing the camera waveform generators with the frame sync pulses of the incoming 405-line signal.
Since the conversion is from a low number of lines to a higher number, it has been necessary to "fill in the gaps " in the 405-line picture by spot-wobbling. Without this device, the. scanning lines of the camera would sometimes coincide with those of the 405-line picture and sometimes fall between them, and an interference pattern would appear on the outgoing picture.
Recently Wireless World had an opportunity of seeing the converted pictures at Amsterdam, after they had been transmitted from Lopik, and we were agreeably surprised by their quality. Inevitably there was some degradation, but not enough to worry the average viewer, and we have seen worse on receivers in this country.
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22nd September 2004