HD NBTV?

Forum for discussion of narrow-bandwidth mechanical television

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Postby Viewmaster » Mon Apr 14, 2008 6:34 pm

Nice image at 48 lines, and isn't 4:3 aspect just the very best ever?
Not like that 'orrible 16:9 that everyone seems to be going potty over.

Your piccy reminds me of the grand ol' 405 raster. :)
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A linear CRT?

Postby Steve Anderson » Thu Apr 17, 2008 1:47 pm

One thing I have completely omitted to mention is the matter of Gamma. It was the main reason for building the PWM display in the first place.

Below is a side-by-side comparison. The picture on the left shows conventional linear grid modulation, the one on the right the PWM method.

This is the very same tube using basically the same signal source with the exception of number of lines and scan directions which shouldn't affect the outcome.

Sadly the lighting conditions in the left picture were not ideal but it can readily be seen that the right-hand photo shows a much more accurate rendition of the greyscale. Without an accurate means of measuring the light output on various parts of the screen I can't assess how linear it now is.

Steve A.

A thought, I've got some small photodiodes, placed in direct contact with the screen it should be able to give me a clue.
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Postby Panrock » Mon Apr 21, 2008 10:03 pm

Well done Steve A. 8)

Peter Yanczer was over from the States at the convention, and he showed me one of his mirror screws. He said he can supply 30 and 60-line versions as standard.

Maybe I'm going way off beam for NBTV now but I wonder if this is the way 120-lines could be practicably achieved? It would presumably have to be quite a large m/screw and certainly cost, but it could be an idea for the future, if 80-lines doesn't prove possible with my existing rig. I would no longer use a mechanical picture source but electronic means for this. At this sort of definition, with a converter, I could even use it as my main household TV !

Vic's demonstration of 60 lines colour at the convention was impressive. He chose to display some quite subtle and difficult picture subjects and the small picture was entertainment quality I would say.

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Postby AncientBrit » Mon Apr 21, 2008 10:37 pm

>Steve A.

Very interesting circuits for your PWM CRT equipment. (I tend to collect stuff like this, might be of use in the future!)

BTW the presentation of your projects with text and embedded circuits is excellent. You must spend quite a bit of time on this.

Well done,

Regards,

Graham
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Postby Steve Anderson » Mon Apr 21, 2008 11:34 pm

AncientBrit wrote:...the presentation of your projects with text and embedded circuits is excellent. You must spend quite a bit of time on this. Regards, Graham


Thanks Graham for your comments, it doesn't really take much time as long as you do it as you progress. I made a comment somewhere else about documention and the benefits it brings in later review. It's a pain, it's an overhead, but there's nothing much worse that picking up a board sometime later and asking yourself, "What in heck did I do this for?", and "What in goodness is an XYP27C chip?"

Accurate documentation is a skill aquired out of not doing so. Fag-packet sketches are fine, as long as you know where to find them when needed. The computer has really given us no excuse to be lax about this now. It's the same as backing-up. Again, through the agony of having had a crash I now back-up as if it were some kind of fetish.

Anyway, thanks again Graham. I like to present things if I can in a magazine format when the subject has gelled and I feel that for the moment that's the end of it. Which for me, 48 lines has come and gone.

Steve A.

P.S. It might well appear in the next issue of CQTV.
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Postby Steve Anderson » Tue Apr 22, 2008 12:54 am

Panrock wrote:Well done Steve A. 8)
Maybe I'm going way off beam for NBTV now but I wonder if this is the way 120-lines could be practicably achieved? Steve O


Steve, as ever you've beaten me to it. It's my ambition to do 128 line colour in time, I'm just working my way up to it. At which point I think I'll retire.

Steve A.
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Postby Stephen » Tue Apr 22, 2008 1:47 am

Panrock wrote:Peter Yanczer was over from the States at the convention, and he showed me one of his mirror screws. He said he can supply 30 and 60-line versions as standard.

Maybe I'm going way off beam for NBTV now but I wonder if this is the way 120-lines could be practicably achieved?
The mirror screw concept is an elegant way to achieve a high line count. TeKaDe had 180 line mirror screw sets in the mid 1930s.

I have added patents by Delamere B. Gardner and Franz Von Okolicsanyi relating to mirror screws in the Patents and Articles section for reference.
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Postby chris_vk3aml » Thu Apr 24, 2008 9:09 am

Actually the 240 line Baird system WAS transmitted for the consumption of the general public. In fact, it was the system used for the very first official public program from the Alexandra Palace on 2 November 1936. The usage of the Baird 240 line system went ahead on a back-to-back public comparison against the Marconi-EMI system until 30 January 1937.

Doug Pitt told me years ago that he saw one or two of the transmissions, the general consensus being that the Baird system was much the superior for telecine, but that Marconi-EMI had the edge on practically every other account - camera mobility, multi-camera switching, flexibility of production, and lack of breakdowns. Rather more obviously, the lack of interlaced scanning on the Baird 240 line system gave it a flicker level that was obtrusive on the few sets capable of producing really good overall picture brightness at that time. (Perceived flicker reduces when the screen illumination is dim, which it then was on many sets having no screen aluminisation and EHT voltages of 6KV or lower).

The scanning discs used for telecine did not have 240 holes, as one might expect. The disc, with a relatively limited number of holes, spun at very high speed in an evacuated chamber. Its holes were not in the spiral pattern, but were concentric, and the light from a high intensity arc was focussed through them onto a constantly-moving film in a gate having no intermittent advance mechanism. A gearing-down from the rotational speed of the scanning disc to the steady movement of the film set the 240 line per frame scan. Effectively, the disc only provided the horizontal scan while the steady film advance set the frame (low speed) scan. Baird could have produced interlaced scanning by setting the holes in the disc in a spaced spiral pattern, but his rejection of interlace may have had something to do with EMI patent rights - Baird, however, had specified interlace in patents of the 1920s but not in this instance.

The optical inefficiency of Nipkow discs at this very high scanning rate meant that Baird was always battling for more light - see my notes on Nipkow disc efficiency in another recent posting here. The 240 line disc system could never be used for 'live' pickup without recourse to the flying spot pickup system, so all link announcements were made in total darkness by either Elizabeth Cowell or Jasmine Bligh who had to memorise their lines - no teleprompt or reading from scripts was possible in total darkness!

For "live" shows the Baird 240 line system used intermediate film - and this system permitted of only using one fixed camera position in the studio. Lens changes were made during transmission and were clearly visible to the viewing audience in necessary but obtrusive brief blackouts.

This was mechanical scanning pushed to its uttermost limit - some would even say "beyond".

The problem with the Marconi-EMI system on telecine involved the Emitron camera tubes producing appalling shading and smear artifacts which were, of course, absent in Baird's. And the Emitron gave a rather soft image which went nowhere near exploiting the 405 line definition of which the scanning system was eventually capable. The difference in perceived definition between Baird's 240 and EMI's 405 was actually almost impossible to choose at the time, according to contemporary reports and the memories of today's few survivors who saw the transmissions.

The Baird pioneers were brave, but Marconi-EMI - some would say unfortunately - had access to a patent pool involving Blumlein, Brown, RCA and Zworykin which became a formidable joint force.

Hope this sets the record straight,

All the best,

Chris Long VK3AML
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A snapshot.

Postby Steve Anderson » Sat May 31, 2008 2:50 pm

After a couple of requests here's a picture of the CRT display. I'm a bit shy about posting it as my construction standards fall well short of others, but for what it's worth here it is.

The extra holes are a result of previous versions, it has been re-built five times, and I'm sure this will not be the last!

Also a picture taken some years ago when it had valves/tubes for the deflection.

Steve A.
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Postby DrZarkov » Sat May 31, 2008 3:55 pm

What's wrong with the design? It's looking safe and stable, and very clearly laid out. Nothing more or less I would expect from any "commercial" made chassis. The only thing missing would be a nice housing for the chassis, for my taste of course made of wood. But that is another kind of art.
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Postby Steve Anderson » Sat May 31, 2008 4:10 pm

DrZarkov wrote:What's wrong with the design? It's looking safe and stable, and very clearly laid out.


Well thank you for the kind comments Volker, but you have to admit it's nowhere near the standards that Klaas and others produce. As for the case/housing, I'm no carpenter or cabinet-maker, if I were to do that I would hand the task over to someone like Steve Ostler who has the expertise to turn out some remarkable works of art.

This particular chassis is an 'industrial' test-bed for not only NBTV but also SSTV. Hence it's current fifth incarnation. For 'classic' SSTV I use a DP7-5 CRT that has a very long persistence phosphor, fits in the same chassis and uses (almost) the same voltages so I don't have to rebuild the power supply each time.

Once again, thanks.

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Re: A snapshot.

Postby Viewmaster » Sat May 31, 2008 6:17 pm

Steve Anderson wrote:After a couple of requests here's a picture of the CRT display. I'm a bit shy about posting it as my construction standards fall well short of others, but for what it's worth here it is.


No need to be shy about that as it looks very neat and nicely put together.

If you really wish to see construction whose standards are falling short of other builds then I'm the kiddy! Call me Mr. Bodger! :)
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Re: A snapshot.

Postby Steve Anderson » Sat May 31, 2008 7:11 pm

Viewmaster wrote:No need to be shy about that as it looks very neat and nicely put together. Albert.


Albert, again, thanks for the kind comments. It's what I can do with my level of skills and the facilities at home which are not that extensive. It was a proof-of-concept project and based on the results I would like to think successful.

Anyway, there it is in all its resplendent ugliness.

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Postby Panrock » Sat May 31, 2008 7:54 pm

Steve Anderson wrote: As for the case/housing, I'm no carpenter or cabinet-maker, if I were to do that I would hand the task over to someone like Steve Ostler who has the expertise to turn out some remarkable works of art.


Don't be too sure about that.... My cabinets can conceal some absolute horrors within. For example the steel frame of my colour camera turned out to be warped after it had been bolted together, necessitating various odd spacer blocks between it and the cabinet panels covering it.

But this is the name of the game when we build things up from scratch and having to make changes as we go. Your work looks perfectly fine to me. I raise my hat to you! :)

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Postby chris_vk3aml » Sun Jun 01, 2008 4:46 am

I think you've managed to build a creditably neat and small CRT monitor, Steve. No need for embarrassment at all. That wiring and soldering standard leaves my chassis-bashing for dead!

I am amazed that you've managed to get a reasonably good grey scale out of a P1 phosphor working at relatively low EHT, though. With simple cathode modulation, I found that 2 KV EHT - AT LEAST - was necessary for a relatively bright phosphor display with plenty of greyscale. The P2 phosphor was slightly better than P1 in that respect, but had an inconveniently long persistence phosphor, producing slight smear on fast-moving subjects. My current electronic CRT receiver uses a 7 inch diameter tube, a P4 (white) phosphor, 4 KV of acceleration and wholly electrostatic deflection. But by the looks of the pictures, 6 KV would have been a better choice for brightness and enhanced grey scale.

A beautiful job, Steve!

Chris Long VK3AML.
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