Seeing colour in a B&W film..

I have been watching an occasional old film on 81 (Talking Pictures).
This morning there was a short BFI B&W kids film on (The Christmas Tree), from 1966, with kids involved in journey taking a Christmas tree to London. Rather oddly and despite it being a B&W film, the tree's foliage showed up quite faintly as green. None of the grass, nor the other trees showed as even slightly green. All rather puzzling, but I remember a TV experiment from way back, where they tested B&W TV's to hint at showing some colour. Anyone remember it?
I like watching some of these older low budget films, for the quiet roads and vehicles from the early days of my motoring career.
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A B&W TV can't show colour - other than that of its phosphor. Assuming you are talking old CRT sets - not sure I've ever seen a B&W LDC, other than tiny ones.
It might be possible to confuse the eye into thinking a part of the picture is in colour by using some sort of pattern. But that isn't the set producing colour.
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*A chicken crossing the road is poultry in motion.*

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) pretended :

The BBC conducted some experiments in the 1960's, I think Tomorrow's World might have been involved. Then of course all TV's were CRT, but I did see a slight hint of colour, on a B&W CRT. Some saw nothing as I remember.
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Yes indeed. It used different mark/space ratios of flashing to make the effect. As a boy I made a spinning disc optical illusion which has four circular tracks with different ratios. There is a youtube demo of it here:
https://youtu.be/hf3KTsRRPLs

https://youtu.be/hf3KTsRRPLs
(fast forward to midway to skip the prologue).
--
Dave W



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I can't seen any hint of colour in the four different circles. They are different shades of grey, dependent on the differing mark:space ratios, and there is a quarter-circle sector that rotates (beating between rotational rate and video scanning rate), but no colour whatsoever.
Evidently I'm one of the people who can't see the effect. :-(
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On Sun, 31 Dec 2017 12:35:38 GMT, Harry Bloomfield

I think I remember it, I couldn't see any effect but there always some like my mum who reckoned they could. Did it involve in strobing the image at certain rate? There was a craze for looking at hidden images amongst a load of squiggles printed on paper about 20-30 years ago, I could never see those either but others found them quickly, OTOH as the printed pattern looked a bit like the ones sometimes printed on paper surrounding sensitive information such as a salary slip and we convinced a colleague that the within the pattern the company trade mark could be seen and he then announced he could see it I've been a bit skeptical as to how many were genuine.
BBC 4 returned to the subject in more recent times.
https://www.youtube.com/watch?v=3P8q_dCU3RI

G.Harman
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Those were called surds or stereograms. No idea how you were supposed to see them, I just got vague instructions like "don't focus on it". I never saw anything but dots. I also never found any reason as to why an image should have appeared from random dots.

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Setting a good example for your children takes all the fun out of middle age.

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I don't remember seeing it when it was originally broadcast. but I've seen a Youtube video of that item on the programme (it showed a drinks can with different "colours" on different parts of the can) and I saw a pattern, but no hint of colour whatsoever. It's possible that modern technology was hiding the effect: a) LCD screen rather than CRT, b) progressive scan rather than interlaced, c) it may have been a film recording of the CRT screen, rather than videotape, which would have destroyed the interlaced scan and altered the gamma of the image.
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Was this the famous OXO cartoon? If it was then all most saw was an ever changing patterning effect as if something was interfereing with some parts of the pictures. Brian
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On Sunday, 31 December 2017 12:20:07 UTC, Dave Plowman (News) wrote:

Early CRT TVs were known to produce a pink picture when the EHT partially shorted, causing the current limiting red lamp to light behind the CRT.
NT
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How early are you talking about? I've never heard of that one.
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*Heart attacks... God's revenge for eating his animal friends

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Sun, 31 Dec 2017 13:46:52 +0000 (GMT), "Dave Plowman (News)"
viable and meaningful comprehension...

Not too sure myself, but on balance he is probably referring to a so-called "Barretter" https://en.wikipedia.org/wiki/Iron-hydrogen_resistor
--

Graham.
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On Sunday, 31 December 2017 16:38:30 UTC, Graham. wrote:

no connection at all
NT
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On Sunday, 31 December 2017 13:55:06 UTC, Dave Plowman (News) wrote:

g
d
First generation CRT sets used a mains transformer to provide EHT for the C RT. A red pygmy lamp was fitted in series with the mains side for short pro tection. There's an old story out there about a woman that phoned Ally Pall y to congratulate them on the lovely shades of red & pink in the picture on ly to be told she should switch it off before it caught fire.
The EHT was deadly on these sets.
NT
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I presume this was a transformer that had mains on the primary and EHT on the secondary, needing only a rectifier and smoothing circuitry to give the EHT for the tube, whereas modern CRTs used a relatively low voltage (maybe mains), that was rectified and then multiplied by a ladder rectifier/capacitor stack so the full EHT was achieved after several mains cycles - one to charge each capacitor in the ladder.

Isn't EHT *always* deadly? If a current-limiting resistor was fitted to reduce the severity of an electric shock from the several thousand volts of EHT, wouldn't it prevent enough current flowing to drive the CRT?
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I am sure the mains ones had a transformer with a single digit kV output and used a voltage multiplier (the names of Cockcroft and Walton spring to mind). The difference was that the later ones used a transformer (driven by a power valve) at line frequency, about 10kHz originally (well, 10kc/s originally). This more easily limited the maximum power avaliable from the transformer. I would guess that the mains transformer secondary was more lethal than the higher voltage output of the voltage multiplier as more current would be available. The EHT is more likely to cauterise the skin than to electrocute, especially at the higher frequency. That was certainly the result the one time I put a finger too near the anode of an HF output valve operating at about 1.5kV.

It is said to take tens of milliamps to electrocute, and B & W TVs would probably not produce this amount of EHT current.
--

Roger Hayter

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snipped-for-privacy@hayter.org says...

I'm fairly certain that it was simply a very high voltage secondary winding - no need for a multiplier.

The EHT was derived from an overwind on the Line Output Transformer which coupled the output stage to the scan coils.
The same principle was used in early colour sets (where the EHT was 25kV) but later sets had a lower voltage overwind and a tripler.

Certainly - if my experiences are anything to go by! But I wouldn't take any chances with a colour set.
EHT on a monochrome set only needs to supply a few tens of microamps but colour tubes - particularly the early ones - used the shadowmark principle whereby the three angled electron beams converged on the shadowmask mpounted behing the face plate. Small holes permitted the electrons to pass though the mask where they started to diverge again so that each beam only hit its corresponding phosphor to produce the three primary colours. A lot of the energy was absorbed by the steel shadow mask so a cuurent of up to 1.5mA was required and that, at 25kV, is lethal!
--

Terry

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On Monday, 1 January 2018 14:27:57 UTC, Terry Casey wrote:

it wouldn't make any sense to implement a valve multiplier chain.
NT
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EY51, as I recall, in Pye monchrome studio monitors
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from KT24 in Surrey, England

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snipped-for-privacy@candehope.me.uk says...

Yes, three of them!
They would each, of course, have needed an individual insulated heater winding!
--

Terry

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