LCD - Vertical refresh vs scan frequency

M

Michael Daly

[H]ard|Gawd
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I'm so confused.

LCD's don't refresh, right? Then why do they quote vertical refresh/scan frequencies? Is it just because video cards still try to send refresh signals and the monitor has to deal with it?
 
LCD's still have to refresh. How else are they going to know when the video card has change a pixel's color value? They just don't have the pixel fade problem that CRTs do, so you don't see refresh flicker. Although it's been said the smooth fading and reigniting of the phosphers in a CRT is less visually jarring to some people compared to the LCD's near instant changing.
 
Michael Daly said:
But if they can update at any frequency, why not just one instead of several?
Click to expand...

One update?
Is that per mS, per second or per year?
Because when it updates, it does just that, one update.

How many frames of video would you like to see when watching a movie or playing a game?
1 update isnt going to be much fun.
 
High refresh rates havent always been available, with CRT they started from around 30fps on TV type displays and reached hundreds of Hz on higher definition CRTs.
As products developed, the resolution improved, allowing for a higher bandwidth signal to be used. At lower res, this leaves room for higher refresh rates, this will be explained below.

The higher the refresh rate, the higher the max number of frames that can be displayed as long as the graphics system can match the speed.
Twitch shooters and many other discerning games players like to have high fps so when they perform a quick moving action, the screen doesnt miss displaying something crucial to the game. Too low a framerate and/or refresh rate, frames will be missed.

Those gamers that need to use VSync to prevent screen tearing may choose the refresh rate carefully as there have been performance issues running at framerates not close to 1/4, 1/2 etc the refresh rate.
This can be impossible though as every game plays differently and the framerate changes throughout a game.
Triple buffering sorts this out to some degree but also adds 1 frame lag for each buffer used.
Many gamers that need Vsync enabled dont use triple buffering so they dont get the lag, but may then be prone to the performance issues.

30fps on a TV video system looks fine because the phosphor glows longer and video is recorded with motion blur.
60fps on computer monitor is for many too slow and flickering is perceived due to the quicker response phosphors, this can result in headaches, eye problems and even epileptic siezures.
The remedy is to use a higher refresh rate so the picture doesnt get a chance to fade before it is refreshed. Normally 70 to 75Hz is enough but some people need higher.
I use 85Hz.

It is also worth noting that a monitors maximum refresh rate at max resolution determines the highest bandwidth of the signal needed to send the video signal from the graphics card.
On VGA cabled systems this involves a RAMDAC on the grfx card to convert the digital signal to Analogue VGA and the maximum speed of the ramdac determines the max resolution/refresh rate.
Because a VGA cards max resolution and refresh rate is partially determined by the speed of the ramdac, not all refresh rates are available in all resolutions.
If you reduce the resolution, you can usually get access to higher refresh rates and conversely, if you lower the refresh rate, higher resolutions may become available.

A side effect of higher refresh rates on CRTs is that some of them blur significantly so on some CRTs the higher refresh rates might not be usable.

So there are many reasons to use different refresh rates.



LCDs dont have phosphor so they retain the image without any degrading over time.
This means the whole screen does not need refreshing in order to remain the same.
However it does take time for an LCD to display a change of image and this speed determines how many changes it can display per second and hence determines the response of an LCD.
As this response is pretty low/slow, it isnt the determining factor for the refresh rates an LCD uses.

Many LCDs come with 60Hz, 70Hz, 72Hz and 75Hz refresh rate options.
Sadly this is only a gimmick on the vast majority of LCDs as they actually cut frames to display a 60fps image.
This can result in a loss of smoothness on video or while gaming.
There are 100Hz and 120Hz LCDs on the market but they will only accept a 50Hz/60Hz refresh rate signal. The monitor fills in the other frames with either a blank frame or an interpolation of the previous and the next image.
There are no true 100/120Hz LCDs on general sale.
So for LCDs unless you have a special monitor, you should use 60Hz.

That explains some of it.
 
Nenu said:
snip
Click to expand...

That doesn't really answer his question. A 8ms response time would allow a 120Hz refresh rate, then 5ms = 200Hz.


Refresh rate is a misnomer. It's really the vertical sync rate.. When the VSync signal goes high it tells the monitor to go back to the top of the screen. Also V-sync is timed just right so that when it goes high, H-Sync also goes back high moving the beam (in a CRT) or active pixel (in a LCD) to the left side. The reason we call it refresh rate is because of the nature of CRT phosphers losing their glow within a short time after the beam hits it. It just so happens that vertical sync rate is also the rate at which phosphors are refreshed by the CRT gun.

Maybe in 20 years when CRTs are a thing of the past and LCD are starting to lose their foothold, we'll call it "update rate" since the pixels in an LCD are updated in a simlar way phosphors are refreshed.

Note: Vsync setting in game is not really the same thing as the Vsync I refer to. It's also a misnomer of sorts. The Vsync setting in a game locks the video card to only output to the montior in the period just before the Vsync signal to goes back high (that period is called Vertical blanking)
 
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