#1
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30-line mechanical television
This is about the first play ever produced on British television, The Man With the Flower in His Mouth, in July of 1930 using the Baird 30-line mechanical system. There was only one camera used, and since it was bolted to the floor, in order to go from one character to another, one actor would have to move off camera and let another one take his place. During this transition, one of the stage hands would place a board with a checkerboard pattern in front of the camera, because otherwise during the changeover the equipment would lose synch and the image disintegrate into a bunch of squiggles. My question is, why would it do that? And how would a checkerboard prevent it? (During the pre-production phase they had used a board of uniform colour, but found that it didn't work.)
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#2
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I think they were concerned that the receivers tuned in would lose sync if there wasn't a sufficient signal supplied to the phonic coils to keep the motors in step. Even in 1930 there was nothing like a checkerboard pattern to generate some good ol' sync buzz!
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#3
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I think that televiewers had to keep their hands on a speed control to actually keep the scanning disk in sync. If they broadcast a solid color card there would be nothing for the viewer to synchronize but with a pattern there was.
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Reece Perfection is hard to reach with a screwdriver. |
#4
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Yes, I have seen pictures of early mechanical televisions which had a hand-held control connected by a cable for synchronization, although I think that later they found a method for automatic control. David, could you explain what phonic coils are? I am not a technologist of any kind, and much of the terminology is unfamiliar to me.
Probably everyone already knows this, but in 1967 an engineer from Grenada Television in Britain restaged the Pirandello play using a combination of the original and contemporary equipment. He caught a number of lucky breaks: the original script was still in the BBC Archives, as were the title cards and artwork, and the same 78-rpm record for the music. Most importantly, the original director, Lance Sieveking, was still alive and agreed to direct it, coaching the actors in the style of the period. For these reasons, it is thought to be a very accurate reproduction. The identity of the actors is unfortunately unknown, and that has always struck me as a nice irony: given how poorly documented early television is, it's rich that the 1967 cast should be unknown because nobody bothered to take down their names. |
#5
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The idea of the phonic coils was to not use a sync pulse, but apply the whole video signal to a set of coils that acted on a toothed wheel attached to the motor shaft. As long as there was sufficient low frequency video contrast, this would hold the motor at the right speed - but the phase could vary.
Regarding sets with manual speed controls (and no synchronization means), they were found to be too difficult to adjust, such that the picture would only be in sync for a few seconds at a time. Modern experiments confirm this. Check out the Narrow Band TV Association: http://www.nbtv.org/ [edit] here's a picture of a Baird replica with phonic wheel http://www.earlytelevision.org/baird_kit_copy.html Last edited by old_tv_nut; 12-10-2011 at 08:18 PM. Reason: add info |
Audiokarma |
#6
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Baird's description of how phonic coils operated sounds like a sync pulse to me. There was a black band between frames that produced a 375 hz tone. If it weren't for the presence of the black band (the one constant component) there wouldn't be much point in applying a video signal to the coils. It follows that, since the sync tone was generated by masking the picture, if there was nothing but black being transmitted that would indeed result in little to no 375 hz signal being applied to the coils, likely resulting in loss of hold between scenes.
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#7
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