Narrow-Bandwidth Television Association

Having previously given my first impressions on the Nipkow disk I bought from Peter Yanczer, I thought it fair to give further impressions now that I've had it in operation...

Originally I wrote...

Andrew Davie wrote:My Nipkow disc from Peter Yanczer arrived today! Here are my candid first impressions... this looks to me like a second-hand disc, which is not what I expected. Not that I'm unhappy with it, but it's not the shiny thing I had anticipated. The center label is a bit torn/scuffed, but I would probably have removed it anyway, and the hub looks a bit beat about -- a few dings, etc. Having said that, I must point out that Peter kindly added a couple of suitable cassette motors for me, which I much appreciate.

That was a fair enough call, I think -- but let's have a look at the actual results of using the disk (with reference to the attached image)...

1. The bottom of the image is a near-perfect straight line. This indicates to me that the angular separation of the holes is spot-on. Not just very good, but excellent. I couldn't ask for any better.

2. The spacing (horizontally, or radially, if you wish) is very good. There is only one 'gap' in the image, and it is a very minor one (indicated in the picture attached).

3. The hole size/consistency. Perfect. There are no apparent brightness changes caused by differing hole sizes.

4. Balance. This disk is superbly balanced, insofar as when it is rotated on the hub it appears to be spinning perfectly around a centerpoint (ie: not offset). I'm quite impressed at how accurate this part of the manufacture is.

5. Flex. Average. The disc tends to want to flex into one of two positions (in other words, it would not lie perfectly flat on the table. This was one of my main concerns originally -- but in actual usage it makes no detectable difference. There is some slight 'wobble' if you look at the disk edge-on when it is spinning, but I can see no artifact of this when viewing the image. So, although this flex/buckling nature is not something I particularly like, it makes no difference whatsoever to the image.

6. Finish. I still think the disk has a secondhand look, as noted before. I won't dwell on this point, as I'll probably paint the disc with a matt black anyway.

So there we go. Mechanically this is a superb Nipkow disc. Definitely far better than anything I could make by hand. Cosmetically there are some minor flaws -- but with the benefit of hindsight, I wouldn't hesitate to purchase the same from Peter again.

Andrew, disc wobbling is because of the spokes. They turn very fast and then split the air. This causes turbulence, which demonstrates itself by some wobbling of the disc.
Denis Asseman made quite some Baird-like discs, 50 cm Ø and with spokes, just like the original Baird-discs have. You will have seen them on the photographs on the NBTV-pages. For me he made a similar disc, on purpuse without spokes. This disc has absolutely NO wobble, while his discs wobble exactly like the original Baird discs. The disc without spokes runs more quiet and looks to stand still.
Some years ago we had here problems with a brand new bridge in Rotterdam, where the bridge road was hanging at steel lines. When we had heavy wind the lines started to oscillate and the road became unstable. That was the reason that I asked Denis to make my disc without spokes........
My 30 cm aluminium disc has large holes in the center part. It wobbles too and is lying absolutely flat on the table.

Andrew,

I agree, the image quality of Peter's discs are really outstanding.
As for wobble, Here is what I did for my disc ( also made by Peter ) to minimize wobble once it was firmly mounted to the hub and cabinet. Leave the belt off so the disc will rotate freely. Standing directly overhead and viewing the disc directly edge on, give it a slow to medium rotation by hand and study any irregularity with one eye so to eliminate binocular vison. It also helps to make a reference slot with paper or just reference the slot in the top of your cabinet. Try to stop the disc right at the farthest point of the wobble and VERY CARFULLY and GENTLY apply pressure to the disc in the opposite direction. It may take numerous times to slowy work out any subtle irregularities.
I did this with mine and it now spins almost perfectly true and wobble free. Best regards,

Allan

Here is an editied close-up of a small section of a recent image. This can be used to very closely judge the relative spacing of each hole, the overlap, and gaps. In reality the image looks much better than this section would indicate. The only real problem hole is #11 (numbering from right) which gives the image a black vertical band. I will soon manually file this hole to make it slightly larger. The differing brightnesses *within* the area overlaid by this hole suggests that there's some dirt/obstruction in the hole -- I will report back once I've worked on the disc.

I have hand-corrected the hole sizes and spacing by using a very small drill bit to enlarge/clear holes as required. The attached image shows the final result -- here can be seen the vast improvement in the spacing.

Wow, what an improvement!

Andrew,

I wonder why white line overlaps are more acceptable to the eye than black "underlap" lines?


Regards,


Graham

Yes, that is a funny remark. I think it is the same effect as why gamma is a good idea. Our eyes look at contrast in light as a proportional effect:

[b]Twice the amount of light gives the same impression as half the amount of light.[/b]

In a 20% light area black lines give a contrast of 20 : 0, this is infinity
In the same 20% light area overlapped lines, say 100%, i.e 40% light, give a contrast of 40 : 20, this is a factor of 2.
In practice with circular holes the overlap is never 100%, but say 30%, this gives a contrast of 26 : 20, a factor of 1,3. Hardly visible.

Klaas,

Perhaps then instead of circular holes we should be using elliptical (or rectangular, if cut by laser) holes ?

Any small mechanical errors would then be more acceptable to the human eye.

Regards,

Graham

No, do you still remember the cats eye holes of late O Hopkins? I think that is optimal. But I fear lasers can't cut them. Laser spots are rather large compared to the hole size on not too large discs. So holes could be cut, but a precise shape is impossible.

I think circular is not that bad as a compromise, as long as they have some overlap. Andrew tried some time ago to calculate the optimum. But now we know that some oversize is better than some undersize.

Klaas,

A modest overlap it is then,

Regards,

Graham