Re: Question of TV technology, if anyone can answer two questions

Haha William, nice try, I know there is more to this but for now ; the main reason that CRT necks got smaller was to get the deflection yoke coils closer to the beam, to enhance deflection sensitivity.

strat, I have a problem with any lightbulb based system. It does not generate light, it only controls it. That light is being produced at the maximum at all times, in fact bulb based projectors usually run hotter when you turn down the brighness, such as for night viewing.

If the unit is showing you less light, you are using the same or more energy.

Actualy I know I am not going to talk anyone out of it, but just what kind of DLP is it ? I know they are more reliable than an LCD, slightly. but I have seen some pretty new ones broken.

Also, they go to great lengths to compensate for beam current and actual HV level. I mean there are sets that do not even use an active HV regulator at all, but use a scheme by which they modify the deflection drive and geometry circuits to maintain proper geometery. Look up the ,,,,, it's either a TA8859 or a TDA8859, one or the other. This chip can give an RPTV, CRT based, VERY stable geometry. More stable than you think.

Granted, CRT based RPTVs have their own set of problems, believe me, but the thing is, with plasmas and other types of units, very few are repairable out of warranty.

In fact a buddy of mine calls me the other day. Computer geekish, not completely but close. Bought a Samsung DLP in May of this year. Went out around Thanksgiving, and parts are not available. The ASC has been out there twice.

So it boils down to if you get one of these things, when it breaks, throw it away.

Phil, you might be in error abiout that. Magnetic focusing is very rarely done on modern CRT based RPTVs. Mitsubishi did it, but some still had an eletrostatic electrode.

Plus most already have dynamic focus. All that circuitry added up together, it gets to be nuts. The 8859 chip works on the standard Phillips bus, and has lots of functions, integrates and monitors HV and beam current, modifies not only the drive to the pincushion circuit but the vertical drive as well.

So it is my belief that the level of complexity is already there.

Now, to address the question of electrostatic deflection, I realize it is a pain in the ass. Instead of four parameters, two more which could almost be called axes night be focus and astigmatism.

And I do not understand how it could be true that the same small spot size cannot be achieved elecrostatically. That is bacause it is already in almost any CRT based set. While deflection is magnetic, focus is usually electrostatic.

The only reason I can thing of for this is that with current technology, there is a problem with designing the deflection plates, which do operate as an element of the tube. The only reason that makes sense is that they need to have a certain surface area, as such, they must be longer longitudinally, therefore there is some variant in the focus of the particles. This is only a theory. If it were the most important point of this whole thing I might go find out. I might, but not tonight. It is after midnight.

JURB

--snippety-snip--

If not for the requirement to deflect the beam, it could be.

With electrostatic (but not magnetic) deflection, there is an increase in the momentum of the electrons -- they are accelerated sideways while traversing the deflection plates.

Recall that inside the tube, the "beam" is not a constant small diameter "rod" the size of the spot; it's really two cones, base-to-base, with the largest diameter located at the focus electrode. The spot on the screen is really an image of the cathode.

Because of the momentum change in the electrostatic case, the position of an electron within the beam as it transits the deflection plates determines how much it is deflected (the electrons are not "running parallel", they are converging), and so the spot becomes more oval the more it is deflected (and that is why electrostatic tubes must be so long). But it's even worse than that, because the ellipticity of the beam caused by the first set of plates makes the distortion caused by the second set very much worse (which is why things are so bad in the corners).

The distortion of the beam (astigmatism) caused by deflection is not correctable, even in theory. There is another source of astigmatism in CRTs, caused by cathode asymmetry, which *is* correctable, by a properly asymmetric lens.

Isaac

I'll show my ignorance: why doesn't magnetic deflection cause an increase in momentum? Seems to me the electrons would be accelerated sideways while traversing the magnetic field.

while

Del cross B ;-)

The force is perpendicular to the beam, so it just arcs around.

Tim

-- Deep Fryer: A very philosophical monk. Website @

Fundamentally, magnetic fields are conservative; you can't get work out of them. If an electron came out of the field moving faster than when it went in, work must have been done on it. That's why those geometry-correcting magnets stuck all over CRTs don't "run down".

Think of it this way:

The effect of magnetic deflection is a function of the electron's velocity -- a stationary electron will not be deflected (accelerated) at all, and high velocity ones are deflected faster than low velocity ones (because a moving electron acts like a current-carrying conductor). Assuming a uniform field (and in a well-designed yoke, it's pretty close), the math works out such that while a slower electron spends more time in the field than a fast one, they both wind up ultimately being deflected by the same angle.

With electrostatic deflection, the lateral force an electron feels is unrelated to its forward velocity -- a stationary one will still be accelerated towards the positive plate. The result is that slower electrons spend more time under the influence of the plates, and so are deflected more than faster ones.

Even if all the electrons in the beam left the cathode with precisely the same velocity (and despite best effort, they don't), they'd still be moving in different directions -- remember that cone? It's the "forward vector", not the absolute velocity, that determines how long an electron is influenced by the deflection field, so all electrons not on the axis of the cone spend more time being deflected.

Isaac

Don't forget to mention the relativistic effects.