General > General Technical Chat
Why do Digital Displays have a Screen saver. When CRT's Don't?
David Hess:
--- Quote from: jklasdf on June 07, 2020, 04:23:48 am ---I think if high-end analog oscilloscopes had continued to be developed into the late 90s, screensavers for the CRT might have become a thing, but for just a plain old analog oscilloscope it seems like a lot of work to implement. The only ones I can think of (as already mentioned) are the Teks with MCP.
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MCP oscilloscopes are a special case because the MCP has a limited operating life.
--- Quote from: SiliconWizard on June 07, 2020, 03:45:37 pm ---As said above, just blanking the CRT display would have been an easy way to avoid screen burns.
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You have to be careful about blanking a CRT. Extended operating time with the heater active but no cathode current can damage the cathode. (1) This becomes a problem in applications where the instrument is powered for long duration but the CRT intensity is turned off to preserve the CRT. Turn the intensity down, but not off, and defocus it if you must.
Vacuum tubes have the same problem when operated in cutoff. Special cathode materials were used to avoid this issue in applications where operation in cutoff would be routine like digital logic.
The above suggests that analog oscilloscope CRTs were constructed to operate in cutoff, at least after the problem was recognized, but *raster* CRTs were not.
(1) Robert B. Tomer calls this "interface resistance" in his book but does not describe the mechanism because I do not think anybody at the time knew how it happened. It results in a severe loss of transconductance. My guess is that ion poisoning is responsible when the ion trap is not operating.
--- Quote ---On old analog scopes which basically had no "fixed" area of the display, the problem would be minor unless the user left the exact same signal on display for a very long time. Probably not that much of a problem in practice.
The problem started to appear when scopes started to have on-screen display of extra information. I guess automatically blanking the screen after a defined amount of time would have solved the issue, but OTOH that could have been pretty annoying for many use cases.
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The common aluminized phosphors used in oscilloscope CRTs are not susceptible to screen burn except at very high beam currents which explains why I have never seen ones of these CRTs with screen burn from the readout. Contrast them with storage and MCP CRTs where screen burn in the readout area is common.
Cyberdragon:
OLED displays can also suffer burn-in (it's a problem with TVs).
David Hess:
Low voltage phosphors are also more delicate which becomes a problem in plasma and field emission displays. This might also apply to oscilloscope CRTs which lack post deflection acceleration but there are so few of them that I am not sure. I think CRTs with MCP image amplification also have to use low voltage phosphors but in that case, the MCP itself wears out with use so it may not matter. I am not sure where storage displays fit here although they operate with lower acceleration voltages.
Krotow:
--- Quote from: Labrat101 on June 06, 2020, 02:26:46 pm --- To stop that nasty screen burn. which plagues them All. Also the Plasma & Phosphor screens suffer from screen burn . :'(
I have never seen screen burn on a Digital screen . ie LCD, LED, But all have a screen saver .. Why.. ??
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Actually I seen burn-in lines on at least two laptop LCD screens. My old workplace in nineties had two laptops with Intel 386 and 486 CPUs and LCD screens, made circa 1992 and 1994 by Compaq and probably Toshiba. We used them for inventory accounting and customs declaration preparing. Both programs used forms with distinct white line frames. Also laptops worked without any power save attempts (who need power save when computer is (mostly) always is plugged in wall). During 6 years white frames burned in screens so well that became visible when laptops was turned off. Go figure, why it happened.
HwAoRrDk:
--- Quote from: Cyberdragon on June 08, 2020, 09:25:30 pm ---OLED displays can also suffer burn-in (it's a problem with TVs).
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Yes. My previous smartphone had an OLED screen and had noticeable burn-in after 5 years. The screen had a slight yellow tint to it, with the exception of the Android status bar area at the top of the screen; that area had a black background, so did not suffer from the burn-in. The yellow tint was only noticeable when displaying a white background full-screen - there would be a brighter, whiter strip at the top of the screen - so I wasn't bothered by it, but was disappointed that the OLED display had degraded after a relatively short time.
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