Question of TV technology, if anyone can answer two questions

snipped-for-privacy@aol.com wrote: > This has to do with why they do what they do. I consider myself fairly > knowedgable on that subject, but noone knows everytthing so here goes.

I have two questions.

Why would anyone give 2 hoots about CRTs? The optics are crap. There isn't enough light even from 3 mono CRTs. Convergence and geometry is mediocre at best. Stability is poor and as you mentioned, reliability is poor.

Question 2. Have you looked at the newer display types? Even the 'worst' one will blow CRTs out of the water.

We have a 4 year old Samsung DLP that has had one lamp fail and a color wheel in 9100 hrs operation. Total parts cost < $200. Total repair time < 2 hours. Its brighter and clearer than ANY CRT based RPTV set ever. The 'convergence' is flawless and the geometry error is unmeasurable. Plus it has a DVI input for 1:1 pixel mapping from a modest PC that doubles as a high def video recorder. And it weighs less. And it uses less power. And it has a smaller footprint. And it has no fluids to leak. And it fits into a minivan. And it has no high voltage to attract dust. And it makes less RFI.

OK, I didn't convince you. I, however, am done with CRTs and judging by what is in the stores, I'm not alone.

GG

Using electrostatic deflection requires a CRT with deflection plates. Such a tube would have (I believe) a thicker neck. Also, the output transistors would have to swing at least a couple hundred volts to deflect the beam.

My guess is that Sony, et al, stick with magnetically deflected tubes because they've been the standard for 60 years. That's the kind of tube they build, and the kind of deflection circuits they design.

However "correct" your theories might or might not be, they run up against industry practice. Electrostatic deflection is considered obsolete, at least with respect to video displays. And pretty soon CRTs will be obsolete with respect to video displays.

I have a Toshiba CZ-3299K HDTV that's about 12 years old. It has a 32" magnetically deflected CRT, and runs at four times the normal scanning rate (~ 63kHz) without problems.

I don't think so. Try changing the screen brightness of a 'scope's CRT. Does the deflection change?

Again, I don't think so. If this were true, the image on any magnetically deflected CRT would show severe geometric distortion that varied with image brightness. It doesn't.

Are you sure? How can you change beam current without changing brightness?

Electrostatic deflection can be much faster, but the focusing properties are much worse. Electron microscopes all use magnetic focusing and magnetic deflection for that reason--you can't get any sort of decent spot size with electrostatic. For analogue oscilloscopes, the spot size hardly matters--1mm is fine--but that's not true for TVs.

Cheers,

Phil Hobbs

And they use narrow deflection angles, infeasible in a TV. The problem would only be worse with wider angles.

Such a

Actually, it's not too bad. At a paltry 1800V acceleration, the tube in my Heathkit IO-103 gets pretty well 10V/cm, which is at least a 60V supply for its window. Focus works just fine at this voltage, as sharp as my Tek. It's beyond me why they designed the deflection circuits for 150 and 180V supplies.

Tim

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

But that's not what he asked.

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

--snip--

Permanent magnets do not provide the optimum field shape for pincushion correction, but a worse problem is that the pin correction in all three tubes must *match* for convergence. That requires electronic circuitry which can be adjusted.

That fact that the convergence circuit fails a lot is just a natural result of either incompetent design or overly aggressive cost-cutting. It does not need to be that way.

Electrostatic deflection causes uncorrectable astigmatism which defocuses the spot, especially in the corners. One solution (used in oscilloscopes) is a much longer tube and therefore much smaller deflection angles, but O-scopes also use a much smaller ultor voltage. The high accelerating voltage necessary for high brightness in TVs would necessitate ridiculously high deflection voltages on the plates, or else equally ridiculous tube length. High currents for deflection coils are far easier to achieve. A fundamental difference between electrostatic and magnetic deflection means that the latter causes essentially no spot astigmatism.

Isaac

--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.