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The world of CRT



Ladies & Gentlemen of the TIG:
I've read the paper submitted by Jerry Rodgers, and I have come to the
conclusion that there are several difficulties with this document. One of
the opening statements is that the best image scanning source available
could be determined by mathematics and physics. Agreed, so I was looking
forward to both of these techniques being applied to the problem.

1. The equation for "space charge current". Why is this introduced, and
then just as quickly dropped? I have to say that I had no idea that this
could be written as 14.6x10E-6.(L/ra).(Eb/b^2)^3/2. Having said that, I
don't care! What's more, neither does the author! Apart from a fleeting
mention that "active tube length is an interesting parameter", the result
of this statement is not used anywhere else in the document.

2. We have some sophomore maths/physics to show that the deflection ANGLE
is proportional to the applied transverse magnetic field and inversely
proportional to the sq.root of accelerating voltage. Again, so what? This
information does not really figure in the following  non-rigorous treatment
of resolution. The elementary conclusion gained from all this math  is that
the smaller the deflection angle, all else being equal, the better the H
and V linearity. I should have thought this qualitative conclusion could be
derived from one simple drawing.

3. The above conclusion is used to demonstrate the intuitively obvious:
that the spot size will grow as a result of a non perpendicular beam
landing. Absolutely true! why did we need irrelevant equations for this:
again, a simple drawing would have sufficed. This is where the maths starts
becoming a bit suspect. See #4.

4. The author states that theta for an active tube length of 254mm (where
did this figure come from?) and a deflection of 83.8mm is equal to
sin(83.8/254) = 19.27degrees. Not in my math class!
The correct answer is ARCTAN (83.8/254) which is about 18 degrees.
This equates to growing the spot by ((1/cos(arctan(83.8/254)))-1)*100
percent or ABOUT 5.3 PERCENT.

5. Even leaving the gross mathematical errors aside, I cannot agree with
the apparently arbitrary values to active tube length and deflection. My
experience with Cintel tubes is that active tube length is roughly of the
order of 16" or 400mm.
The patch is about 4" square, giving a deflection of about 2" or 50mm.
Now lets do the (correct) math again:
Spot percentage increase = ((1/cos(arctan(50/400)))-1)*100 or about 0.78
percent.
I believe this figure represents the real world far more: around a 1% or so
fall-off in resolution at the picture edge.

6. Real world viewing of the Cintel C-Reality tends to support the math:
Has anyone tried looking at the PICTURES? I have. A group of us from POP
had the chance to go to Valencia to "C the Reality" last Monday. The
pictures I saw on the Hi-Def machine were clean, resolute and seemed to be
free of artifacts. Unfortunately, the machine kept crashing on me (sigh). I
anticipate that, given the software cleanup required, Cintel will have a
very worthy competitor to the Spirit, (which I also like a geat deal),
around NAB time if not sooner, notwithstanding any half-assed mathematical
"evidence" to the contrary.

7. Xenon is spelled with an "X"

Finally, may I say it is customary for "scientific" papers about to be
published to be subject to peer review. This avoids any errors in such
documents being pomulgated, and may help prevent embarrasment to their
authors. It also stops people like me taking the piss. The paper here
presented cries out for review. I'd like to recommend that about a
half-dozon TIGrs be available for a peer review process, nominated by
people on the TIG. I'll start off by nominating Dave Corbitt and Martin
Euridjian for this job.

TTFN

Mike (mine's a pint) Orton



---
Thanks to Howard Lukk for support in 1998.
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