80-lines again

[Panrock]

Some years ago I planned an 80-line Nipkow display and Steve A kindly did the honours by designing a PWM LED driver.

Three of Steve A's drivers eventually ended up in my 30-line colour 'Grosvenor', where they are over-specified and do a great job. Steve A's aperture correction has been included too. Anyway I'll be bringing this to the Convention at the end of the week.

I was rummaging around today and found an old 80-line stainless-steel laser cut disc. This is of the 20-inch size originally intended to fit the Grosvenor with almost no modification. However as you would expect, its holes are pinprick-tiny.

I have been thinking about the Convention after next and how I might revisit this 80-line idea. I see Darryl's WC-01 converter can even produce an 80-line signal! I remember DrZarkov (Volker) kindly had an 80-line disc made for me in Germany but it turned out bigger than expected so I returned it to him.

Volker, have you still got this 80-line disc? Are you coming to this year's convention? If so (and you don't need it) could you very kindly bring it back? If you can, I'll buy it back off you. Your larger disc has the advantage of larger holes and greater light throughput. I'd build a completely new 80-line monitor around it.

Thanks!

Steve O

[DrZarkov]

Hello Steve,

I'm sorry, I don't have that disk any more. I've sent it to one of our australian club members here in the forum. Unfortunally (for us) that friend of mine who has made the disk is now working for another company, so he can't make any new disks...

[Panrock]

Not to worry Volker.

I see I still have the .dxf file, so could go back to my laser cutters and have a giant disc made!

What do people here think is the largest I could go, bearing in mind safety considerations? The rotational speed for the 1930s Don Lee 80-line standard is 15 Hz.

Steve O

[gary]

Hello Steve,

I'm sorry, I don't have that disk any more. I've sent it to one of our australian club members here in the forum.

Ooooh I wonder who that was and hasn't told us?...

[gary]

Not to worry Volker.

I see I still have the .dxf file, so could go back to my laser cutters and have a giant disc made!

What do people here think is the largest I could go, bearing in mind safety considerations? The rotational speed for the 1930s Don Lee 80-line standard is 15 Hz.

Steve O

I think it is reasonable to say that the limits are really the strength/size of the hub, drive, bearings etc. rather than the diameter of the disk (within reason).

There is probably an optimal size whereby it is more economical to go either the electronic magnification method (ref Peter Yanczer "Mechanics of Television") or the mechanical method (ref Holtzman here on this forum for instance).

In other words multiplexed multiple aperture spirals (2 or 3) .

[DrZarkov]

Ooooh I wonder who that was and hasn't told us?...

Actually, he has: viewtopic.php?f=9&t=1127

[Panrock]

Actually, he has: viewtopic.php?f=9&t=1127

Right that's it! Not to be outdone, I shall have to make my new disc bigger than 1 metre!

Maybe a 4-footer...? This could provide an 80-line picture of comparable size and brightness to my current 30-line effort.

The advantage of a home-growner(groaner) is that I can design it to fit the central boss I have to hand. I may cannibalise my mechanical camera for this. Or... (maybe better) have a more massive boss specially made.

Who was it that said: "The difference between Men and Boys is the size of their... Toys"?

Steve O

PS. I recognise there will be serious safety issues with demonstrating something of this size. But still, there were issues with the 44lb mirror screw - that ran at 25Hz - and these were overcome. Because of its size, it would have to break down into parts and be assembled 'in the field'.

[AncientBrit]

Steve,

As long as 'the parts' don't end up 'in the field' when its running!

Some disc that.
Think of the 'windage'
Wonder if there'll be a siren effect?

Good luck,

Graham

[Panrock]

A long post now.

Let's explore this 'big disc' idea and try pushing it as far as it will go. The following should be considered as a 'thought exercise' rather than a formal vow to go ahead with it!

The aim of such a project would be to demonstrate the 'best' picture -ie. finest detail, flickerless, reasonably large and bright - obtainable from a practical Nipkow disc. The higher definition mechanical standards, which are available from the WC-01 converter, are all horizontal scan. Therefore the picture would be seen at the top of the disc.

To give convenient viewing when standing, the bottom of the viewing port should say, be 5ft 6ins above ground. Making certain allowances, this means a 5ft (1.5m) disk with its bottom rim about 1 ft above the floor. This would give sufficient clearance for a stout base for the viewing shield.

The viewing shield itself would cover the disc, act as a light shield and incorporate the viewing port, with magnifier. It would have sides that 'wrap round' to prevent any possibility of touching the edge of the disc, which would be rotating at hundreds of mph.

So we have a 5-foot disc. Such a disc would require a lot of power to rotate fast against windage. It would also be heavy! The weight and (presumably) the windage could be reduced by making the disc of a vaned or 'spider' construction. But this would also reduce the strength.

The aim is to demonstrate the best Nipkow picture practicable. So how fast should it rotate to give reasonable freedom from flicker while keeping the speed to a minimum? For a bright colour display, 12½ Hz is really a bit flickery, as visitors will see on Saturday if they look at my exhibit. However I would want at least a similar picture size with this giant disc.

One advantage of using a very large disc with a 'high' definition mechanical standard would be that the characteristic 'Nipkow' keystone picture shape distortion would be very small. The picture would look, to all intents and purposes, rectangular.

The best match out of the WC-01 looks to be the 96-line standard (1932), with a 6:5 picture and a 20Hz frame rate.

Now we switch to metric units. The circumference of the disc at the centre of the picture would be 415cm. Therefore the picture would be 415/96 cm wide, or 4.3cm... somewhat less than 2 ins. This size is similar to what I have been working with - and magnifying - with the Grosvenor. However, given a similar aspect ratio, it now consists of 96/30 squared or 10 times as many pixels. Whether the picture would be ten times dimmer too, I don't know, since although the area of each pixel would be 10x less, there would also be 10x as many of them! Maybe someone wiser out there can comment? Anyway, the present luxeon display on the Grosvenor is plenty bright enough and can be viewed in subdued daylight. If we consider a display 10x - or more than 3 camera stops - lower in intensity, this should still be OK.

This brings us to laser cutting tolerances. These are generally reckoned to be ±0.1mm. For 96-lines, the square holes (actually now a rounded square) on this disc would be 0.45mm across. So there could be some blurring of the raster lines here... If this is random, it could be OK. However laser cutting errors tend to be periodic, so we could see some horizontal brightness banding on the picture.

What about the mechanical stresses on the disc? From http://www.centrifuge.jp/cgi-bin/calc-e.cgi we can see that, at 20Hz, there would be 1220 g's at the rim. This would mean several tons of stress distributed inside the disc. Hairy!

The edge of the disc would be travelling at 214 mph or 345 Km/h. Still well under the speed of sound though, so no shock waves!

I don't know what the insurance at Loughborough covers, but to date nobody has raised any concerns about safety, even with my mirror screw, which was a brutal chunk of jagged, spinning steel. That might have to change!

Steve O

[AncientBrit]

Hi Steve,

Sounds like you'll be competing with Castle Engineering who are machining the ali wheels for the Bloodhound land speed attempt!

Good luck,

Cheers,

Graham

[Steve Anderson]

...Such a disc would require a lot of power to rotate fast against windage. Steve O

I think Baird hit the same problem, his solution I believe was to run the disc in a vacuum - I don't know how true that may be, perhaps an urban legend. Even a partial vacuum would help against windage and acoustic noise.

Steve A.

[M3DVQ]

...Such a disc would require a lot of power to rotate fast against windage. Steve O

I think Baird hit the same problem, his solution I believe was to run the disc in a vacuum - I don't know how true that may be, perhaps an urban legend. Even a partial vacuum would help against windage and acoustic noise.

Steve A.

So now you add something liable to implode if it gets a shock... Does that cancel out the problem of the disc exploding since presumably one would happen very shortly after the other?

[Steve Anderson]

Does that cancel out the problem of the disc exploding since presumably one would happen very shortly after the other?

Nope, as you jokingly imply, the centrifugal and centripetal forces are still there. The vacuum helps or deals with the windage drag and acoustic noise issue, that's all. It does add another layer of risk. But whether Baird actually did it or not I don't know. Whatever, it's a tad impractical!

Steve A.

[gary]

The 180 line and 240 line intermediate system Baird used in the "bake off" with Marconi was indeed a disk in a vacuum (3,000 rpm+). The disk, however, was not a Nipkow disk, as such, as it did not have a spiral but a single line scanning aperture. The vertical displacement being provided by the film moving past the aperture. It was, in all intents and purposes, (almost) real time telecine. The film was still wet as it past the scanner.

Far from being impractical the principle was eventually used in the dead end, and now obsolete, CRT technology of the mid to late 20th century to prevent electrons running into air molecules...

[Panrock]

The Baird telecine, which used the same scanning method but lacked the chemical shenanigans, was acknowledged to be very good and better than that of Marconi-EMI. A shame Baird didn't (or wasn't permitted to) develop this further.

Steve O