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Why weren't electrostatic deflection CRT's more popular for TVs and monitors?

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Berni:

--- Quote from: bdunham7 on August 26, 2023, 09:31:46 pm ---
--- Quote from: ELS122 on August 26, 2023, 09:20:19 pm ---Out of curiosity, what typically is the frequency limit for magnetic deflection? I suppose you could get slightly higher bandwidth by lowering the screen size.

--- End quote ---

A 1600 x 1200 CRT monitor at 120Hz would be 144kHz and that would be a pretty ambitious specification.  I suppose you could go quite a bit faster with a smaller tube if you didn't care about efficiency or precise geometry.

--- End quote ---

Yep my last CRT was running at 1600x1200 at 80Hz.It was one of the better CRT monitors both in terms of specs and image quality.

It was pretty common for computer CRTs to support resolutions this high and fast as long as you didn't get the cheap ones.

The limit is simply how hard you can drive the deflection coils. More drive voltage = more sweep speed. Tho i suppose at some point the windings might arc over, but it would have a pretty monsterus drive amplifier at that point. It was more that we didn't really need more resolution back then.

ELS122:

--- Quote from: Berni on August 27, 2023, 06:01:20 am ---
--- Quote from: bdunham7 on August 26, 2023, 09:31:46 pm ---
--- Quote from: ELS122 on August 26, 2023, 09:20:19 pm ---Out of curiosity, what typically is the frequency limit for magnetic deflection? I suppose you could get slightly higher bandwidth by lowering the screen size.

--- End quote ---

A 1600 x 1200 CRT monitor at 120Hz would be 144kHz and that would be a pretty ambitious specification.  I suppose you could go quite a bit faster with a smaller tube if you didn't care about efficiency or precise geometry.

--- End quote ---

Yep my last CRT was running at 1600x1200 at 80Hz.It was one of the better CRT monitors both in terms of specs and image quality.

It was pretty common for computer CRTs to support resolutions this high and fast as long as you didn't get the cheap ones.

The limit is simply how hard you can drive the deflection coils. More drive voltage = more sweep speed. Tho i suppose at some point the windings might arc over, but it would have a pretty monsterus drive amplifier at that point. It was more that we didn't really need more resolution back then.

--- End quote ---


It would be interesting to see how high you can drive the resolution for a B&W tube, since there's no shadow mask you can theoretically get as high a resolution as you can drive it. Focus would be a problem, would have to search for a tube with better geometry for getting super sharp focus. 4k gaming on a black and white roundie  :popcorn:

David Hess:

--- Quote from: ELS122 on August 27, 2023, 07:03:00 am ---It would be interesting to see how high you can drive the resolution for a B&W tube, since there's no shadow mask you can theoretically get as high a resolution as you can drive it. Focus would be a problem, would have to search for a tube with better geometry for getting super sharp focus. 4k gaming on a black and white roundie  :popcorn:
--- End quote ---

Color CRTs always seemed to be higher resolution than B&W CRTs.

It might have had to do with acceleration voltage and secondary emission.  Without a shadow mask, the high acceleration needed for a small enough spot size on a B&W CRT will produce secondary emission from the phosphor resulting in a circular "spot" around the point where the electron beam hits.  This will not be visible as such, but it will severely reduce the contrast like it does with a bright oscilloscope CRT.

I think a shadow mask captures the secondary emission so the contrast is higher.

ELS122:

--- Quote from: David Hess on August 27, 2023, 08:31:17 am ---
--- Quote from: ELS122 on August 27, 2023, 07:03:00 am ---It would be interesting to see how high you can drive the resolution for a B&W tube, since there's no shadow mask you can theoretically get as high a resolution as you can drive it. Focus would be a problem, would have to search for a tube with better geometry for getting super sharp focus. 4k gaming on a black and white roundie  :popcorn:
--- End quote ---

Color CRTs always seemed to be higher resolution than B&W CRTs.

It might have had to do with acceleration voltage and secondary emission.  Without a shadow mask, the high acceleration needed for a small enough spot size on a B&W CRT will produce secondary emission from the phosphor resulting in a circular "spot" around the point where the electron beam hits.  This will not be visible as such, but it will severely reduce the contrast like it does with a bright oscilloscope CRT.

I think a shadow mask captures the secondary emission so the contrast is higher.

--- End quote ---

With a shadow mask you have a fixed resolution

secondary emission hits the HV supply not the phosphor, and it does that by design. The electrons hit the phosphor and bounce back into the HV supply.

Berni:
Shadow mask spot size =/= resolution.

We figured out how to make really fine pitch shadow masks, so for those the spot size of the electron beam would tend to be bigger than the pitch of the shadow mask. But TVs did indeed have pretty huge pitch shadow masks because you don't need anything finer when displaying such a low PAL/NTSC resolution on a huge screen. The big masks help with brightness, but they are always a fair bit finer than the CRTs designed resolution.

Some cheap PC monitors also skimp on the shadow mask pitch, but the good one have one so fine you can't see it with your bare eyes even when pushing your face right up to it. More of a problem is that CRT computer monitors didn't tend to come in sizes big enough to be useful at 4K resolution (remember that UI scaling was not a thing in the year 2000). But if you are to find a large enough CRT tube with the nice fine shadow mask, it could certainly do 4K. Tho i wouldn't expect the geometry to be within anything better than a dozzen pixels across the whole area without employing some fancy calibration methods.

Don't think you could even get that much resolution from any normal consumer graphics card back then or even had a standard video connector that can handle it. The early digital video standards didn't have the bitrate to handle 4K while the analog standards would need a pretty huge amount of bandwith in the video DAC to pump out so many pixels. Heck even reasonably modern standards struggled with 4K when it arrived in the consumer world, HDMI could only do it at 30Hz

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