Author Topic: What determines the maximum horizontal and vertical resolution of a CRT tube?  (Read 7975 times)

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Offline Fusion916

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I'm trying to learn more and more about CRT tech (both theory, design, repair, etc) and one thing I want to understand is what determines the maximum resolution of the tube itself, meaning with the entire chassis removed to remove any concern about video bandwidth, horizontal drive, etc. Just the tube, yoke, and the electron guns.

So with that said, in the tube design itself. What about the physical construction determines the resolution? Lets take standard NTSC tvs for example. I know that the spec is 525 scanlines, but only 480 of them are viewable. Does that mean the physical construction of the tube has 480 lines of resolution? Also, what is the determining factor of the horizontal resolution? And how do TV lines relate to this.

One more thing is displaying as interlaced vs progressive. Again, removing the chassis part of it, is there anything inherint in the CRT tube itself that prevents progressive display vs interlace display? All SD CRTs have the 480 total scanlines, so what in the tube itself prevents progressive display? Is the design of the yoke too slow to scan 480 lines every 1/60th of a second? If that is the case, in theory shouldn't it be able to do 480 lines at 30fps?
 

Offline helius

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In general, a CRT does not have a "resolution" per se. It's an analog device with variable acceleration voltage and beam current, which affect the size of the bright spot. Since you can control the X-Y deflection in any way you want, to draw curves of any shape, there is no such thing as a "resolution" in the tube itself.
Most color tubes work on a parallel tricolor system with three electron guns, and a shadow mask or aperture grille that lets electrons from each gun strike spots of their respective color phosphor. The spots have a definite pitch (measured horizontally, or at an angle), and those are a lower limit on the number of distinguishable pixels. But the CRT can certainly be scanned at a higher resolution; it will act as a low-pass filter.
On monochrome tubes (or tubes with sequential or penetration-based color), there are no spots, as the phosphor is a continuous coating. But the tube's electron beam must be focused to stay in a small spot, and it can't be both infinitely sharp and bright. More beam current makes the spot "bloom" and so increasing brightness leads to decreased acutance.
 

Offline T3sl4co1l

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In general, a CRT does not have a "resolution" per se. It's an analog device with variable acceleration voltage and beam current, which affect the size of the bright spot. Since you can control the X-Y deflection in any way you want, to draw curves of any shape, there is no such thing as a "resolution" in the tube itself.
Most color tubes work on a parallel tricolor system with three electron guns, and a shadow mask or aperture grille that lets electrons from each gun strike spots of their respective color phosphor. The spots have a definite pitch (measured horizontally, or at an angle), and those are a lower limit on the number of distinguishable pixels. But the CRT can certainly be scanned at a higher resolution; it will act as a low-pass filter.

To refine the point:
1. Without any grille, a monochrome display is just phosphors, in which case the mentioned limits apply: beam focus, scattering, and the beam's focal plane.  These, in turn, are largely set by the CRT design, and by physical law.  2nd anode voltage is the most important figure in beam sharpness; but you can't simply crank up the voltage on a poorly made tube (which will probably flash over before you get very high!).  Sharp focus is best achieved by simply finding a well-made tube.

2. The grille doesn't just act as a low-pass filter, though: it's a sampling aperture.  That means any high frequency information is downconverted -- giving aliasing, better known as Moire patterns!

If you display a checkerboard pattern on a Trinitron CRT, you see a Moire pattern, which gives the mismatch between electrical raster and the physical aperture grille.  By counting the Moire fringes across the screen, and where they fall, you can see how many pixels the image is off by, and how that varies over the screen (the edges and corners are the worst, of course).  You also see a rainbow of subtle color, because much as interference colors arise from an oily film, the rasters of different colors aren't quite identical.

So it's quite important to have lowpass antialias filtering on a CRT display, to avoid drawing very sharp graphics that fall apart under these errors.

LCDs completely fix the electrical-to-spacial mapping, by physically addressing each pixel (yay!).  But, their very square pixels, and the high contrast ratio between neighboring pixels, can make visual problems of their own, again making antialiasing very important to the viewer.  (I wonder if anyone ever thought about making a hexagonal* over-sampled and antialiased LCD.  Hmm...)

*A hex grid can resolve circular features properly (like the round blur of antialiasing), where a square grid cannot.  Also, a hexagon is easily divided by thirds, which is nice for coloring, I guess?

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Offline T3sl4co1l

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I think I read at one stage HD CRT TV's  were developed in Japan that had 1200 approx lines. 

I still use a 1600x1200x85Hz Trinitron.  I prefer the color fidelity, and haven't met a single LCD that can compare (though they're finally starting to get comparable).

The highest I've seen is 2048 x 1536, which is probably the practical limit of compensating an analog raster scan system in production.  The maze of correction components in one of these displays is astounding: magnetally biased saturable reactors for correcting nonlinearity, switched capacitors for correcting sweep rate and ringing; etc.  And at horizontal sweep rates over 100kHz, the poor yoke is being pumped with ~1kVA of reactive energy, just to push around a nearly massless electron beam!

Tim
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Offline helius

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Lets take standard NTSC tvs for example. I know that the spec is 525 scanlines, but only 480 of them are viewable. Does that mean the physical construction of the tube has 480 lines of resolution? Also, what is the determining factor of the horizontal resolution? And how do TV lines relate to this.
The physical construction of the tube is that it should have an aspect ratio that is close to 4:3 for broadcast signals that are in that aspect ratio, or else the image will appear distorted. The physical tube has almost nothing to do with the 483 "visible lines", in fact on most old televisions not all those lines were visible due to overscan. The determining factor of horizontal resolution is the bandwidth and signal fidelity of the tuner, filters, and amplifiers; it was not generally constrained by the response of the electron gun, although that can play a role in displays with higher video bandwidths. "TV lines" is a measurement of the horizontal resolution that is normalized to the screen height, not to its width; this cancels out the effects of overscan or underscan, as long as the picture tube is 4:3. The "lines" are not actually pixels, but a number of alternating white and black signals that are displayed with a minimum level of contrast, similar to line pairs/mm to measure film resolution.
The NTSC signal contains a specific number of lines vertically (525, of which 483 or 480 visible), but the data horizontally is a continuous signal. It might make sense to use 640 pixels to represent each line, since 640:480 = 4:3, but there is a problem because the color signal does not have an even multiple of 640 cycles per line. So in broadcast video it's standard to use 720 pixels per line, even though the pixels are not square (not the same distance horizontally as vertically).
 

Offline Paul Moir

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Regarding the overscan (525 vs 480 thing) that was done so that the picture would still fill the screen even given variances in line voltage, temperature, tolerances, etc.  You have to remember early TVs had not much by the way of regulation or anything like that going on, and were optimized for the minimum number of tubes.
If you scan a normal picture tube at 480/30fps, then by the time the last line on the bottom of the screen is being swept the top line is dimming.  That's why they went with interleaved in the first place:  to prevent noticeable flicker.
Tube shape has a lot to do with vertical resolution.  Even before LCD everyone hated a deep TV, so the electron gun was squashed up close to the screen.  As a consequence, at the top and bottom of the vertical scan the electron beam hits the phosphor at a steep angle.  This causes the lines near the top and bottom to be larger.  Think sloped tank armour.  You'll notice high resolution CRTs such as in computer monitors were much deeper to prevent this.
 

Offline helius

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The highest I've seen is 2048 x 1536, which is probably the practical limit of compensating an analog raster scan system in production.
There were higher resolution displays used for specific applications. The Sony DDM-2802C had 2Kx2K used for air traffic control. 4Kx4K might have been sold, but I was unable to find documentation. There was a government grant in 1992 to fund development but nothing seems to have come of it.
 

Offline helius

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Tube shape has a lot to do with vertical resolution.  Even before LCD everyone hated a deep TV, so the electron gun was squashed up close to the screen.  As a consequence, at the top and bottom of the vertical scan the electron beam hits the phosphor at a steep angle.  This causes the lines near the top and bottom to be larger.  Think sloped tank armour.  You'll notice high resolution CRTs such as in computer monitors were much deeper to prevent this.
That's a good point; a lot of televisions did have very shallow tubes. Although few were as thin as the Sinclair TV80!
On the other side, oscilloscopes had really nice, wonderfully long tubes.
 

Offline james_s

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I think this has been mostly covered, but the primary limiting factors of the tube itself are the spot size and the pitch of the shadow mask or aperture grill. For monochrome tubes the only real limit is the spot size; the phosphor screen on those tiny 0.5" viewfinder tubes is essentially the same as on a 25" B&W TV tube but the viewfinder tube can produce a far smaller spot, at the expense of much lower total energy.

For the whole system, the vertical resolution is determined by the number of scan lines which is determined by the ratio between the vertical and horizontal sweeps. The horizontal resolution is determined by the bandwidth of the video amplifier and that of the image source.

CRTs are a fascinating technology and it's sad to see them disappearing so rapidly. I'm of the opinion that for viewing analog content nothing else comes close to a CRT, and the picture looks much better than the specs on paper would suggest. There are a few other tricks they can do too, the light guns used in some classic video games rely on the raster scanning of the CRT to identify where the gun is pointed. An even more impressive use is the CRT vector monitor, if you've never played Atari's 1979 arcade game Asteroids on an original CRT vector monitor you've never really played it. They used velocity modulation to create eye-searing bright photon bullets that streak across the screen. If you play it emulated on a raster monitor it's nowhere near the same.
 

Offline David Hess

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So with that said, in the tube design itself. What about the physical construction determines the resolution? Lets take standard NTSC tvs for example. I know that the spec is 525 scanlines, but only 480 of them are viewable. Does that mean the physical construction of the tube has 480 lines of resolution? Also, what is the determining factor of the horizontal resolution? And how do TV lines relate to this.

Ignoring linearity and sticking with monochrome CRTs, the spot size determines the number of lines of resolution which is an easy enough test to make using a raster.

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One more thing is displaying as interlaced vs progressive. Again, removing the chassis part of it, is there anything inherent in the CRT tube itself that prevents progressive display vs interlace display?

Except for constraints on how quickly the beam can be scanned and controlled, the CRT design has no influence on progressive versus interlaced display formats.

It would have to either be the ability to focus the beam accurately corner to corner. Which would be more difficult the more the screen deviates from spherical. Which I suppose is one reason an oscilloscope screen tends to be long and narrow.

Oscilloscope CRTs are long (for their target area) because deflection sensitivity which limits bandwidth is proportional to length.

A major limitation of bandwidth in oscilloscope and scan converter CRTs is charging and discharging the deflection plates to relatively high voltages.  The best scan converter CRTs limited to a length suitable for rack mounted equipment were about 5 GHz.  During the cold war, the Soviets had a much easier solution; they made scan converter CRTs which were 6 meters long to achieve a bandwidth of 13 GHz.

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Also there is the phosphor itself. as the beam strikes the phosphor there must be a small amount of scatter to surrounding phoshor which would decrease the dot sharpness. Photographic film had a certain amount of light scattering within the emulsion that limited resolution. The phosphor has a certain thickness that may cause a similar phenomonon.

There is also the minimum dot size that permits sufficiently bright images. The smaller the beam the smaller and dimmer the image and the longer it will take to refresh the screen if it requires scanning more lines.

There are a bunch of things which affect the spot size.  Lower acceleration voltages mean that the electrons have more time to mutually repel each other. (1) Different types of electron lenses suffer from various amounts of aberration.  If scan expansion is used, it also expands the spot size. (2) The phosphor target itself diffuses the spot. (3)

(1) So higher acceleration voltages yield both a smaller and brighter spot.
(2) This explains why the last generation of pre-scan expansion mesh CRTs (50 MHz Tektronix 547) have a smaller spot size than later CRTs.  On the other hand, scan expansion meshes yielded higher bandwidth because of higher deflection sensitivity.  Some later CRTs replaced scan expansion meshes which quadrupole lenses but I think they bend up the beam a lot because they have little or no increase in sharpness.  Or maybe my one oscilloscope like this is just old.
(3) The bright disc around the spot seen at low sweep speeds is from secondary emission; the electrons blasted free from the spot get pulled back by the high PDA (post deflection acceleration) voltage.  The ghost seen at low sweep speeds which catches up to and then precedes the spot is produced by (secondary emission from?) the electron beam hitting the scan expansion mesh.

2nd anode voltage is the most important figure in beam sharpness; but you can't simply crank up the voltage on a poorly made tube (which will probably flash over before you get very high!).  Sharp focus is best achieved by simply finding a well-made tube.

If you ever have the pleasure of working on an oscilloscope which supports reduced scan, then you can see the effect of the acceleration voltages on the sharpness and brightness.  Increasing the cathode voltage (more negative) makes the focus proportionally sharper, incredibly so, but also lowers the deflection sensitivity proportionally.

I do not know why someone could not in theory have made a lower bandwidth higher deflection sensitivity CRTs with a higher cathode voltage and tiny spot size across a standard 10x8cm or larger graticule but I do not know of any.  I assume diffusion through the phosphor target would have limited the improvement.

Lowering the PDA lowers the brightness and increases the spot size of course but counter intuitively also changes the deflection sensitivity because it acts as a final lens.  If a scan expansion mesh is used, lowering the PDA counter intuitively *lowers* the deflection sensitivity.  The Tektronix Circuit Concepts book on CRTs discusses this.

I did a bunch of tests on my 7904 and 7603 last year to see what effects altering the PDA has so we would have a better idea of what to look for on the TekScopes@yahoogroups.com list when someone suspects a missing PDA or bad high voltage multiplier.

I think I read at one stage HD CRT TV's  were developed in Japan that had 1200 approx lines.

I still use a 1600x1200x85Hz Trinitron.  I prefer the color fidelity, and haven't met a single LCD that can compare (though they're finally starting to get comparable).

The highest I've seen is 2048 x 1536, which is probably the practical limit of compensating an analog raster scan system in production.  The maze of correction components in one of these displays is astounding: magnetally biased saturable reactors for correcting nonlinearity, switched capacitors for correcting sweep rate and ringing; etc.  And at horizontal sweep rates over 100kHz, the poor yoke is being pumped with ~1kVA of reactive energy, just to push around a nearly massless electron beam!

I have a 21" shadow mask computer monitor which does 2048 x 1536 but I usually ran it at 1600x1200 to match my 19".  Its horizontal output circuit keeps failing.
 

Offline helius

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(3) The bright disc around the spot seen at low sweep speeds is from secondary emission; the electrons blasted free from the spot get pulled back by the high PDA (post deflection acceleration) voltage.  The ghost seen at low sweep speeds which catches up to and then precedes the spot is produced by (secondary emission from?) the electron beam hitting the scan expansion mesh.
Something similar happens in storage CRTs, right? The second electron gun is focused to spread out evenly over the whole face, and somehow kicks electrons from a dielectric layer out to strike the phosphor. There were also some regular tubes that used flood electrons to light the graticule!

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I have a 21" shadow mask computer monitor which does 2048 x 1536 but I usually ran it at 1600x1200 to match my 19".  Its horizontal output circuit keeps failing.
I have two monitors with issues that I need to diagnose. One has visible green lines in the retrace: I hope it's just a bad blanking circuit, but I'm afraid there could be something wrong with the guns.
The other has helpful MCU error messages, but I have no idea what "Malfunction: ELT down" means. Extra low voltage??

There are a few other tricks they can do too, the light guns used in some classic video games rely on the raster scanning of the CRT to identify where the gun is pointed.
Light pens should also work on vector monitors, if the display list can be synchronized with the pen input. You can only "click on" active drawn lines or points; raster scan has the advantage that you can detect clicks anywhere if the raster is turned up enough for the pen to detect.
 

Offline james_s

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I suppose a light gun could work on a vector monitor, I never saw it done but it's certainly possible from a technical standpoint. I suppose I should have just said that only a CRT display will work.

I doubt the retrace problem with your monitor is a problem with the guns. Usually it's either a problem with the blanking or excessive sub brightness.
 

Offline helius

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I suppose a light gun could work on a vector monitor, I never saw it done but it's certainly possible from a technical standpoint. I suppose I should have just said that only a CRT display will work.
I seem to remember that SAGE (on the AN/FSQ-7) and Sketchpad (on the TX-2) both had light pens. They were vector display based.


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I doubt the retrace problem with your monitor is a problem with the guns. Usually it's either a problem with the blanking or excessive sub brightness.
Right, but if the AGC is compensating for a blown gun it could overpower the blanking.
 

Offline Siwastaja

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I doubt the retrace problem with your monitor is a problem with the guns. Usually it's either a problem with the blanking or excessive sub brightness.

At least several Sony computer monitors tend to have an issue with a failing G2 HV regulator (around 1 kV IIRC), producing increasing G2 voltage, causing overall brightness increase that cannot be compensated with the brightness controls - eventually even the retrace appears. I tried to properly fix my Sony, and the schematics for the HV regulator, IMO, were showing overly complex design for such a simple linear regulator. I ended up kludging some resistors somewhere in the circuit to drop the G2 back to acceptable levels before I ditched the monitor.


What defines the actual resolution of the CRT, is the focusing of the electron beam so that it can produce a small enough dot in every part of the screen. It can be fairly complex combination of electrostatic focusing (focus voltage applied to the tube), static astigmation magnets that can be adjusted (typically small gray plastic levers in the neck of the tube), and electromagnetic focus coils dynamically driven and adjusted for different parts of screen, controlled digitally. In a typical computer CRT, most of these things are hidden from the user or nonexistent - but getting a high-end CRT projector allows you to become very familiar with all the gazillion of adjustments and settings (mechanical, analog and digital) and how they affect the image. With CRT projectors, 1600x1200 is often considered "mid range" resolution (such as my Barcographics 808s); some go considerably higher, but "normal" CRT stuff tends to stop before 4K. But I'm rather sure there are hyperexpensive specialty units produced only in hundreds that can do 4Kish with crisp image. For real-world examples of mass produced CRT projectors, see http://www.curtpalme.com/Projector_Rankings.shtm
 

Offline CJay

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Oscilloscope CRTs are long (for their target area) because deflection sensitivity which limits bandwidth is proportional to length.


I was under the impression that another reason the tubes are long is because a narrow deflection angle is easier to keep linear across the scan, shorter tubes necessitate wider deflection angles and are proportionally more difficult/expensive to keep linear, also electrostatic deflection is less 'powerful' than magnetic deflection (but much easier, again, to keep linear)?
 

Offline nctnico

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I'm trying to learn more and more about CRT tech (both theory, design, repair, etc) and one thing I want to understand is what determines the maximum resolution of the tube itself, meaning with the entire chassis removed to remove any concern about video bandwidth, horizontal drive, etc. Just the tube, yoke, and the electron guns.
From an electronics perspective the resolution is limited by the speed of the horizontal sweep. A higher resolution needs a faster sweep to draw more lines in the same time for a given framerate. Another key component is the amplifier which modulates the output of the electron guns. These need high voltages and achieving high frequencies and good linearity makes these amplifiers expensive.

Also getting a sharp picture on the entire tube is difficult. Eizo had a system in their higher end monitors which controlled the focus dynamically during sweeping so the picture is sharp right into the corners. Someone else already mentioned Trinitron but that always gave me a headache. The best CRT monitors I've seen are the LG Flatron types. Even after years of use the picture is still sharp.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline David Hess

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(3) The bright disc around the spot seen at low sweep speeds is from secondary emission; the electrons blasted free from the spot get pulled back by the high PDA (post deflection acceleration) voltage.  The ghost seen at low sweep speeds which catches up to and then precedes the spot is produced by (secondary emission from?) the electron beam hitting the scan expansion mesh.

Something similar happens in storage CRTs, right? The second electron gun is focused to spread out evenly over the whole face, and somehow kicks electrons from a dielectric layer out to strike the phosphor. There were also some regular tubes that used flood electrons to light the graticule!

Yes, secondary emission from the flood gun plays a critical role in storage CRTs.  I just thought this was interesting because it is an example where one can literally see the results of secondary emission.  Unlike the spot from the electron beam, the disc produced by secondary emission has uniform brightness and a sharp boundary.

Oscilloscope CRTs are long (for their target area) because deflection sensitivity which limits bandwidth is proportional to length.

I was under the impression that another reason the tubes are long is because a narrow deflection angle is easier to keep linear across the scan, shorter tubes necessitate wider deflection angles and are proportionally more difficult/expensive to keep linear, also electrostatic deflection is less 'powerful' than magnetic deflection (but much easier, again, to keep linear)?

As far as I know, that was never an important consideration.  There are various things which can be done with the electron optics to improve linearity, reduce aberration, and increase deflection sensitivity but deflection sensitivity and effectively bandwidth are proportional to tube length and bandwidth was always a problem.

Image CRTs are a completely different matter since they are not limited by deflection sensitivity do to their low bandwidth requirements and there is a high premium on having a large area while being short.  As nctnico points out, their horizontal resolution is limited by their z-axis bandwidth which is completely different from an oscilloscope CRT although they are not slouches for z-axis bandwidth either.
 

Online Moshly

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This Tek video discuses some of the issues involved in making CRT's

 

Online rstofer

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Fascinating!  No wonder Tek analog scopes were so expensive.
 

Offline _Wim_

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This Tek video discuses some of the issues involved in making CRT's



Thanks for posting this one! Makes you appreciate gear like this even more!
 

Offline Audioguru

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Years ago my Sony 32" Trinitron flat screen TV failed. I tried to fix it but it failed again. I thought it was pretty good so my son bought me a new one that was also pretty good.
I was given an LCD high definition TV for the recent Christmas and it is much[/b clearer that the Sony CRT TVs that had standard definition.

I wrongly assumed that my cable TV company would charge more for high definition but no, it is standard now. Next year I might receive a 4k or 8k TV for Christmas. :)
 

Offline james_s

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You can get HD over the air for free, have been able to for years.

For standard definition content a good CRT still looks superior, but there's no shortage of HD content anymore.
 

Offline BurningTantalum

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Siwastaja:
For the Sony CRT monitor issue-
From memory one needs to download Sony DAS (digital alignment software) and use a TTL to RS232 converter from the DB9 port hidden round the side of the monitor. The G2 voltage and many other adjustments can then be made.
I have done this on a couple of G1 chassis over the years.
BT
 

Offline tooki

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I think I read at one stage HD CRT TV's  were developed in Japan that had 1200 approx lines. 
I know Barco made 3-CRT projectors that could exceed 1080p. (Never saw one in operation though.)

Speaking of weirdo projectors that are technically CRTs, though not in any form resembling a "normal" CRT: who remembers the Eidophor? Now that was cool technology if ever there was one!
 

Offline Siwastaja

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I know Barco made 3-CRT projectors that could exceed 1080p. (Never saw one in operation though.)

Nothing special exceeding 1080p on a CRT projector! These over-full-hd units are surprisingly commonplace, from many manufacturers, often considered mid-range units. My Barco exceeds 1080p, and scans to 1200 lines at 100Hz, although I use it at 1080p since going further doesn't give much extra real resolution with this mid-range, slightly worn-out 8" unit; at least it would require very careful and experienced setup, so I'm happy with the standard full-HD input.

Exceeding about 1400-1500 scanlines with a crisp image, getting near to the 4K territory (but not quite) would bring you up to the high-end CRT projectors, which are expensive, special, and somewhat rare, but still not "legendary" magical things; still several manufacturers and models to choose from!

And a good, high-end CRT projector still exceeds the visual image quality of even some of the new, expensive DLP projectors when displaying natural (sampled) images where square-pixel mispresentation of the sampled analog image is not desired. OTOH, when computer graphics and text are to be presented, often in a brightly lit conference room, CRT projector is totally wrong tool for the job, which is why mid-to-high-end CRT projectors from the late 90's and early 2000's are being decommissioned from such environments, often in useable shape.
« Last Edit: February 12, 2017, 01:39:29 pm by Siwastaja »
 
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