DuckieHo

CRT Refresh Rate
Cathode ray tube (CRT) monitors use a phosphor-coated screen. A cathode ray fires from electron gun(s) and hits the different color pixels in the screen. This narrow beam makes the pixels glow for a very brief period before the pixel goes back to black. Therefore, the pixels must be repeatedly hit by a beam to glow consistently. The electron gun(s) quickly draws the image on the display and must repeatedly do so for the image to remain. Hence, the number of times the display is redrawn is the "refresh rate". If the refresh rate was 1Hz, each pixel would be hit by the electron beam only once per second. The pixels glow for much less than a second. So at 1Hz, you would see the screen flash for something like .01s and be black for .99s. As you increase the refresh rate, more displays are flashed. Once you get over a certain frequency of frames per second (around 30Hz), the image would appear constant to the human eye. Again, CRTs displays fade to black if the screen is not updated. Viewing a CRT at 30Hz, you actually are seeing the display, black screen, display, ect. This is why running a CRT at 30Hz or even 60Hz seems like it is flashing... because it is. CRTs need to be ran over 70-85Hz (subjective to user) to reduce the amount of black screen.

LCD Refresh Rate
Liquid Crystal Displays are based on a different technology completely. They are comprised of a glass sheet with embed electrodes array. Each of these electrodes generate a color and can individually be turned on or off. A backlight behind the glass sheet illuminates the electrode colors and hence you get a display. A good metaphor would be holding a flashlight behind a film negative. The negative has colors but it is too dark to see by itself. The light source behind it allows you to see the image. Pixels can be individually controlled and can stay on. If a portion of an LCD display needs to change colors, only those required to change actually update to the new color. If you are looking at a static image, an LCD screen does not need to update or refresh at all. There is no momentary black screen as in CRTs. As mentioned before, the human brain interpretes smooth motion at around 30FPS or 30Hz. This is why a 60Hz LCD is ALWAYS smoother than a 60Hz CRT. Therefore, "refresh rate" for a LCD takes on slightly different meaning. A 60Hz refresh rate would mean how often the LCD checks for updates to the current displayed image. The human brain cannot easily tell the difference above 60Hz.

FPS vs Refresh Rate
Frames per second discribes how fast a video processor can output full screen updates. Refresh Rate describes how often a monitor updates the display. That is an important distinction. Refresh rate is a set period of time. At 60Hz, a monitor pulls out whatever image is in the video processor's frame buffer and displays it every 1/60s. FPS is how fast the GPU can write out a full image to the frame buffer. This rate varies and is NOT sychronized with the frame rate. Therefore, the monitor may pull from the frame buffer but the video card has not completed updating the frame buffer. In this case, a portion of the old display maybe still present in the image you see on the screen. This is called "tearing" since the buffer is written top to bottom.... so you would see a horizontal line across screen. This often happens but is not noticable if does not occur to many times in a row. You would not notice tearing if only 3-5 frames out of 60 in a second. If it does become an issue, you may use something called V-sync. This forces the video card not to release the new updated image until is completely done. The downside to this is if the video card is too slow to update the new image. The monitor would continue to display an "old" image.

Response Time
Another major difference between a LCD and CRT is response time. This value is how fast an individual pixel can update. CRTs always update their pixels and each screen is actually a pixel refresh. Therefore, response time does not really matter for CRTs. However, LCD pixels are made of physical electrodes which take time to turn off and on. If an electrode takes to long to switch, "ghosting" will occur. This is when the previous image is still apparent when the new image is appearing. The lower the response time rating, the faster the pixels can switch colors. 12-16ms is probably the minimum for a home users. Gamers are recommended displays with less than 12ms response time.

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AMDLUVR

Good faq but I believe the light source for an lcd is at the edge(s) and not behind the panel. Anyways Rep+.

adam144

Great guide, now I understand refresh rate a lot more? Rep for you

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DuckieHo

Yeah... but keeping it simple. The lighting is behind the screen in all cases. There are newer lighting technology like LED instead of cathodes and wasn't sure about LED placement.

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Unknownm

you should this: (stargate125645 did this before but i'm Passing it on)

1\8msx1000 = 125

125 frames the LCD can display before it ghost, If higher than 125 Frames..



1\5msx1000 = 200

200 frames the LCD can display before it ghost, If higher than 200 Frames..

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stargate125645

This is very good information, though I covered the last two items in my FAQ on response time:
Info: Explanation of LCD Response Time.

I believe Unknownm is quoting me on the math there, so I would just like to add that those are for ideal situations only. You will need a buffer zone, since the image is not displayed instantaneously.

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DuckieHo

Elaborate? Those equations don't make sense...

Most LCDs have a refresh rate of 60Hz or 75Hz. Frame rate has nothing to do with ghosting. A gaming running at 999FPS will ghost on a LCD with a response time of 20s.

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stargate125645

We posted at the same time. See my previous post.

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Unknownm

Read's stargate125645 post. I just quoted he's math

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DuckieHo

ok, yeah... those equations are only valid in theoretically ideal situations (which is not possible with current technology). Nothing wrong with multiple FAQs. This post was mainly to clear up the confusion between CRTs and LCDs... especially why CRTs need a 60Hz+ resfresh rate.

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