Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

I have never really paid any attention to the refresh rate setting on computers with LCD screens. I thought with LCD displays the pixels are either on or off, they don't flicker like in old CRT monitors.

Seems trivial to me, but my own graphics card allows me to select 59Hz or 60Hz - which leads me onto my question:

Does the refresh rate affect LCD screens? Technical info to back up any answers would be great.

share|improve this question
add comment

3 Answers

up vote 5 down vote accepted

Yes, but it is worthwhile clarifying.

LCD pixels get turned on by current and turned off when the current stops. On the other hand, CRT pixels get excited when the electron beam hits them and immediately start to fade.

The implications of this are:

  • With a too low refresh rate, a CRT appears to flicker as the brain has time to notice that it went dark. This depends on the individual and which part of the eye sees it since our peripheral vision is more sensitive to light.
  • An LCD does not appear to flicker because pixels do not go dark between refreshes. They simply transition from one state to the next.
  • The refresh rate of both CRTs and LCDs affect how they render motion. Higher refresh rates give smoother motion.
  • LCDs have an upper limit on the refresh rate due to the time it takes for a pixel to change state. This is known as response time. If the refresh rate is too fast, then some pixels do not have enough time to change; this results in ghosting.
  • Is it possible on a CRT to have ghosting as well, but only at very high refresh rates, since phosphors fade quite fast.
  • LCDs do not have lower limits to their refresh rates. I once tested a display with 3830×2400 resolution and the first version had a 24 Hz rate. No flicker was noticeable. It was then upgraded to 48 Hz, and then, sadly, discontinued. I guess not enough people paid the 27K price tag!
share|improve this answer
1  
An excellent summary of the points that I lacked time to supply. I will say though that LCDs do have a lower limit to their refresh rate. Fortunately it is down around 0.5Hz or so. –  Ignacio Vazquez-Abrams May 22 '11 at 0:24
1  
hmm interesting stuff, especially about ghosting and the effects on motion. Thanks for that! –  Kez May 24 '11 at 7:07
1  
Thanks! I spent a few years evaluating LCDs for a company doing film and video processing when the CRTs stopped being produced. LCDs had a really rough start :) –  Itai May 24 '11 at 13:12
1  
I believe that you may be confusing refresh rate with frame rate. If a video source (e.g., an MPEG file) has 30 frames per second, and the screen has a refresh rate of 60 Hz, then each frame will be displayed twice. Increasing the refresh rate to 90 Hz will just result in each frame being displayed three times; I don’t see how that would give smoother motion. In fact, motion might look less smooth at 70 Hz because you no longer have an n to 1 ratio. –  Scott Jan 14 at 1:07
add comment

Yes. But since it takes longer for a LCD cell to depolarize than for a CRT phosphor to extinguish, it doesn't affect them very much.

share|improve this answer
add comment

A significant point is that the phosphors on a CRT screen have their "persistence" designed to support a particular fairly narrow range of refresh rates. The phosphors could be made to have really long persistence (seconds), so there would be no serious flicker down to even maybe a 5 second refresh interval, but then, since the phosphors can only be "turned on" and not "turned off", you wouldn't be able to see motion much faster than that. (Some early CRT terminals used long-persistence phosphors, with the characters "drawn" on the screen instead of scanned. This didn't provide very fast "refresh", but it only had to be as good as a 10 CPS Teletype.)

LCDs have the property that they can be turned on or off, at some relatively high rate, and once set one way or the other they have a relatively long persistence, on the order of a second or so. For this reason they can support a wide range of refresh rates.

The upper limit on refreshing an LCD is a function of capacitance and of the fact that the L in LCD stands for "liquid" --

LCDs are "scanned" via an X-Y matrix of wires, with a pixel at each point where two wires cross. Only one pixel can be manipulated at a time. The voltage on a pixel must be maintained long enough to "charge" the pixel, so that it will hold the charge until refreshed, and all pixels must be visited on each refresh cycle.

And, in addition to the charge time, the liquid inside needs time to mechanically reorient its crystal structure (though, at a physics level, this reorientation is tied at least partially to the "charge" time). Both of these factors place an upper limit on refresh rate.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.