A televisual curiosity

[Panrock]

Hi David,

Not quite clear about the details of this arrangement but if a blank raster on the Decca was used as the source of the 'flying spot', then phosphor lag would have been a problem... more a 'flying streak'...

Steve

[ppppenguin]

A standard TV tube has a relatively long persistance phosphor. Not long enough to blur motion but more than enough to foul up a flying spot scanner. Even the special CRTs designed for FSS had a finite persistence which had to be corrected in the electronics. Effectively an adjustable HF boost to the video.

Phosphor persistence isn't a simple exponential decay. There are at least 2 different time constants. ISTR that the Rank Cintel afterglow correctors had 5 knobs, each with a different time constant.

[Panrock]

Not the Mirror Screw, but the 'brown lump' seen to the left - a 30-line colour monitor - uses a nipkow disc with almost-square scanning holes and an almost-square picture. Here, unless I'm mistakn, a rather similar technique is used - aperture correction - to extract the maximum definition from the signal.

Steve

[ppppenguin]

I'm not 100% sure about this, but if you can scan with a spot that's exactly 1 pixel by 1 line and has uniform light distribution (or sensitivity distribution if it's a camera) then that's an ideal aperture. There's still the zero order roll-off which would need to be corrected with an aperture corrector. This provides HF boost but with linear phase, unlike simple filters. (A practical aperture corrector is a transversal filter with symmetric taps. Can be analogue or digital.)

If you had a very small scanning aperture then there would be no such roll-off but sensitivity would be poor.

Practical cameras and FSS need greater aperture correction since the spot doesn't have an ideal rectangular profile. A vertical aperture corrector requires a pair of 1 line delays. They used to be an expensive add-on in the old days.

A lot of this was understood in 1934, in Mertz & Gray's seminal paper: https://archive.org/details/bstj13-3-464
Stiil worth reading but the maths is quite tricky. There once was a time when I understood all of this properly. Now it's just an overall feel for what's happening.

[djrm]

Hi David,

Not quite clear about the details of this arrangement but if a blank raster on the Decca was used as the source of the 'flying spot', then phosphor lag would have been a problem... more a 'flying streak'...

Steve

I think that just about sums up the problem, I seem to remember that a coloured filter was meant to be used to limit this problem - I dont remember if I actually had one fitted though. David.

[Panrock]

There are three side-by-side R,G and B vertical lines of light reflected off the (intersecting) horizontal edges of the  mirror screw - resulting in scanning spots for the R,G and B signals. There is no need for colour mixing with the resultant wasted light, since the R, G, and B channels are differently delayed to compensate for their lateral displacement by Karen Orton's rather ingenious timing corrector box.

Steve

[ppppenguin]

Hi David,

Not quite clear about the details of this arrangement but if a blank raster on the Decca was used as the source of the 'flying spot', then phosphor lag would have been a problem... more a 'flying streak'...

Steve

I think that just about sums up the problem, I seem to remember that a coloured filter was meant to be used to limit this problem - I dont remember if I actually had one fitted though. David.

Don't think a coloured filter helps. If the different colour components that make up a white phosphor had significantly different time constants you'd see coloured smears on moving pictures.

Perhaps you were thinking of the dual colour radar CRTs (P7 phosphor?) that had a short persistance blue and long yellow. This gave very good radar displays but was hopeless for TV. I think some who built TVs using these CRTs used a blue filter to get rid of the yellow smear on moving pictures.

[djrm]

That will be it, I had one of those blue/yellow tubes back then as well, it's all a bit dim in the past now.

[Panrock]

The tube in question was the CV2810 and there were indeed greenish smears on fast picture changes. Though the two phosphors together could give a passable 'black-and-white', as on the Argus shown here.

Steve

[Ed_Dinning]

Hi folks, I think I saw the article on the flying spot scanner in a 50's copy of PTV and it may also have been included in one of their derivative books

Ed