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.
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
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
*A chicken crossing the road is poultry in motion.*
Dave Plowman email@example.com London SW
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
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 (fast forward to midway to skip the prologue).
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. :-(
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.
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.
Setting a good example for your children takes all the fun out of middle age.
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.
On Sunday, 31 December 2017 13:55:06 UTC, Dave Plowman (News) wrote:
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.
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?
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
It is said to take tens of milliamps to electrocute, and B & W TVs would
probably not produce this amount of EHT current.
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
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!
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