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# Dynamic Voltage LCD

Dramatically reduce transition speed of LCD monitors
 (+3) [vote for, against]

A common problem of LCD-based monitors for computers is that the liquid crystal elements have a less than instant response to a change in color. This can lead to distracting and ugly "ghosting" effects around the edges of moving objects and is generally one of the prime reasons for keeping your CRT.

Part of the problem is that for a liquid crystal, making a small color change actually often takes longer than making a large color change. That is, going from pure white to pure black takes less time than going from white to 50% gray. This is because the intensity is controlled by the voltage applied to the LCD element--less voltage=less color. The problem with this is that a small change in color means a small change of voltage, and a small change of voltage has a smaller and slower effect on the liquid crystals (see attached article for more info about this).

My proposal is to counteract this by "spiking" the voltage when a color change is initially requested. By analyzing the response time of a particular LCD element, it is possible to determine how quickly it will change color when given a particular voltage. This information can be applied to the signal being given to the display. When a color change is requested, the voltage change will initially be much greater than necessary, to "shove" the crystals in the right direction. Once the crystals have moved most of the way to the correct intensity (probably around 15-20 ms) the voltage is returned to the level actual needed for the desired final color. The amount and duration by which the voltage is spiked will be proportional to the amount of chnage necessary, as related to the data on how quickly the LCD element reacts to a given voltage.

The end result: The LCD element adjusts its color much more quickly due to the increased voltage difference, but the difference is returned to normal levels just before the goal color is achieved, leaving you with only the desired final result. Response time for subtle changes will be improved to nearly match that of white-black transitions and the overall image will be much sharper.

 — 5th Earth, Nov 30 2004

X-bit’s Guide: Contemporary LCD Monitor Parameters and Characteristics http://www.xbitlabs...play/lcd-guide.html
How to judge an LCD monitor [5th Earth, Nov 30 2004]

 I believe some vendors already use such techniques. One difficulty is that because changes to intermediate values can happen faster than changes to extremely light or dark values, this can cause temporal distortion which can be even more distracting than ghosting.

This can be compensated for somewhat if the areas of the screen that can change quickly are delayed relative to those that will change more slowly. This is certainly doable but has the unfortunate side effect of causing the on-screen image to be delayed relative to the supplied signal. This can cause noticeable sound synchronozation problems unless the audio is delayed to match.
 — supercat, Nov 30 2004

 I know it's late, but does this make sense:

//making a small color change actually often takes longer than making a large color change. That is, going from pure white to pure black takes longer than going from white to 50% gray//

 Yes, it does make sense. The transition rate is not linear with voltage, so a sharp drop from full voltage to zero will produce a more rapid full transition than a drop to half voltage.

 I believe this is baked already.

 In addition to the use of better quality LCD panels, some of the more pricey newer LCD monitors have preprocessing circuitry that takes steps to reduce ghosting.

 1: The voltage profile is adjusted to account for the rate of change. Instead of a pure square wave transition, the voltage spikes (or dips) past the intended level, then returns.

 2: The actual image is modified with an inverse ghost of the previous frame to remove the artifacts that step 1 can't eliminate. Basically, the circuitry knows how much of a ghost will show up, and actually subtracts that from the current frame, effectively displaying exactly what is intended.

These hacks can't fully compensate for portions of the image that are on the extreme edges of the display's capabilities (for example, it can't completely remove the ghosting of a white object moving rapidly across a black background) But in general, it works fairly well.
 — Freefall, Nov 30 2004

[swamilad], never mind. I know what you mean.
 — Ling, Nov 30 2004

 Well, it seemed like an obvious solution, I'm not suprised to hear it's being done. Incidentally [Freefall], a white box on a black background is actually a better case scenario. What would be a worst case scenario would be a light gray box moving on a dark gray background (less contrast=more time, remember?)

Incidentally, [Swamilad] was right, I actually DID get it confused (read it carefully!). I've edited to correct this.
 — 5th Earth, Dec 02 2004

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