LCD Refresh Rates - The Final Thread

[Jimb0]

After a lot of googling I’ve read many contradicting view points on this issue (even on this forum), so I thought I’d create a hardocp account and ask this question one last time, sorry.

There seems to be two different prevailing opinions:

Opinion 1) - Say we have an LCD with a response time of 8ms, yielding a theoretical maximum frame rate of 125. If I were to play a game with VSYNC turned off and the max FPS capped at 125 within the games configuration file, then I could assume (providing my videocard is good enough) that the LCD is literally displaying 125 full frames per second, regardless of what the refresh rate is set to within Windows.

Opinion 2) - Now, the other school of thought says that even though the LCD is theoretically capable of displaying 125 frames per second with VSYNC turned off, it is still limited by the refresh rate/sampling rate/vertical frequency/some_strange_thing of LCD technology, which basically means the LCD is not communicating fast enough with the videocard in order to produce the full 125 frames.

So to put it simply, with LCD’s “response time” = the speed at which a given pixel can physically change while the “refresh rate” = how fast new pixel/update information is actually being sent to the screen.

The most reputable website I could find with Opinion 2 (aside from various forum posts) was tweakguides.com on this page: http://www.tweakguides.com/Graphics_8.html

“Let's look at an LCD's theoretical refresh rate, based on its response time rating. Consider the example of an LCD monitor nominally rated at an 8ms response time. Given 8 milliseconds is 8/1000ths of a second, in one full second it can refresh all the pixels on the screen (if necessary) 1000/8 = 125 times, which makes it equivalent to a 125Hz refresh rate. Yet no 8ms LCD monitor allows you to set a refresh rate even remotely close to this in Windows, nor do even 4ms LCD monitors.”

Interestingly that article also says the reason the DVI standard was built with such a limitation was because LCD’s must accommodate the current Windows/Videocard architecture, which works on a full frame by frame basis, rather than a per-pixel one:

“Well it appears that LCD monitors need to emulate a refresh rate in Windows primarily for compatibility purposes with games and hardware. Games, Windows and your graphics card are all still designed around composing individual frames in the frame buffer, and sending these whole frames to your monitor one by one, with the timing for buffer flipping typically based on Vertical Blank Intervals - all things which were originally required for CRT monitors. Therefore LCD panels have to try to operate on the same basis, despite the fact that they don't have the same physical limitations of a CRT.”

I still don’t know who is right

 

[UrielDagda]

It's basically a legacy thing.. If they want to change it, they will have to make that change in the circuitry, it would probably require a new connection, new logic on the video card and new drivers. It's probably just too complex to do right now, and even if they did, the monitors that used it would probably cost five times as much.

 

[ToastyX]

#1 is completely wrong. Response times have nothing to do with refresh rates or frame rates.

Refresh rates work the same way on both LCD and CRT monitors. The only difference is the way the display physically updates. That's it. LCD monitors don't flicker because they don't fade between refreshes. They still only update 60 times per second at 60 Hz, which means they can only display 60 unique frames per second.

Also, many LCD monitors only output 60 frames per second regardless of the refresh rate, so at 75 Hz, every 5th frame is skipped. Not all LCD monitors do that, but it's very obvious to me on the ones that do because motion becomes jittery.

 

[Jimb0]

Thanks for clearing that up ToastyX, but can anyone tell me why not many LCD manufacturers are currently producing displays with higher supported refresh rates to match their advertised response times?

Since they are a digital device, could it be some sort of internal memory buffer issue?

If it is, then they probably think most consumers don’t care (and they would be right) - so it’s not worth the expense to improve the technology.

 

[Tamlin_WSGF]

Refresh rate is used in several chips internally on screen (a/d converter, scalers etc), but is also significant to bandwidth.

(resolution x refresh rate) x (1 + blanking interval) = pixel clock rate

A single link dvi have a limit of 165 mhz bandwidth and higher refresh rates combined with higher resolutions will exceed this.

Analoge signals like VGA needs to be converted via A/D converter before processed (all LCD's are digital). Refresh rate limitations due to chips are also there.

Since screens have had problems showing each frame below 16,67 ms due to response time, they have decreased response time for even 60 hz (and changed how they measure response time too, to get better results). First they need to make LCD's fast enough for 60 hz before they try on 75 hz.

Not to mention the extra costs, since chips are mostly made for 60 hz.
As you say, for current techology, its not worth it to produce higher refresh rate LCD's.