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You are here: MacNN Forums > Hardware - Troubleshooting and Discussion > Mac Notebooks > How to check refresh rate?

How to check refresh rate?
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james_squared
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Sep 29, 2002, 12:04 PM
 
Hello,

As the subject header says, I want to know how I can check the refresh rate on my iBook running 10.2.1.

Thanks.

James
     
janitor
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Sep 29, 2002, 02:37 PM
 
lol. are you serious? lcd's don't have refresh rates, they're always on. This is why they help reduce eye strain...
     
james_squared  (op)
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Sep 29, 2002, 02:52 PM
 
Hello,

Oh, I guess the eye strain from all the years of using a CRT has wrecked my brain. Or, maybe it was all the radiation coming from the monitors in the computer lab.

James
     
maddjn
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Sep 29, 2002, 04:53 PM
 
LCDs do have refresh rates.
if they did not have a refresh rate, they would serve a single, static picture.

i don't know how to check it though.

maddjn
     
CheesePuff
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Sep 29, 2002, 05:23 PM
 
I think its 60 Hz. Thats what most LCDs are at. But it doesn't matter much. What does matter is the pixel response rate of a LCD. They range on the iBook from 50 ms to 100 ms. The PowerBooks LCD has a faster pixel response time, thats why they look much better.
     
tooki
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Oct 1, 2002, 02:12 AM
 
Originally posted by maddjn:
LCDs do have refresh rates.
if they did not have a refresh rate, they would serve a single, static picture.
Nope. Refresh rate is something that you find only on displays that rely on scanning to draw an image, like CRTs, plasma displays, passive matrix LCDs, etc. and other displays that rely on persistance of vision. This is because the image will fade away if it doesn't receive a signal in time.

There is a response time for a pixel, but in an active matrix LCD, each pixel has a transistor that remembers what state the pixel is supposed to be in. It only receives signals to change states. Since the pixel won't fade away, it only needs to be addressed for changes, so the refresh rate is "as needed." You only run into problems when you are trying to change a pixel faster than it can respond, which causes the image to go blurry or slurred.

Now, what does exist is the frame rate, which is a function of the OS and graphics card. OS X, I think, renders at 30fps maximum (less if CPU time is insufficient), although 3D in games renders as fast as it can.

The difference between refresh rate and frame rate is often overlooked, but think about it: the frame rate and the refresh rate can be the same, but they don't have to be. TVs in the U.S., and most TVs elsewhere, happen to have the refresh rate in sync with the frame rate. (In places where PAL TVs are used, you can get "double-scan" TVs that have twice the refresh rate of the frame rate.)

The difference between frame rate and refresh rate has long been exploited by movie projectors: they knew that a refresh rate of 24Hz was too low, but increasing the frame rate would mean increasing cost a LOT. So they decided to just show each frame twice, meaning that the refresh rate is 48Hz, with a frame rate of 24fps.

tooki
     
kybernaut
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Oct 1, 2002, 08:05 AM
 
tooky,

thanks for the useful explanations

kybernaut
     
stew
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Oct 1, 2002, 08:22 AM
 
Originally posted by tooki:
Since the pixel won't fade away, it only needs to be addressed for changes, so the refresh rate is "as needed." You only run into problems when you are trying to change a pixel faster than it can respond, which causes the image to go blurry or slurred.
Notebook screen pixels are not refreshed "as needed" - transfer from the graphics RAM to the display happens in full frames and therefore they do have something comparable to a refresh rate.


Stink different.
     
Commodus
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Oct 1, 2002, 09:12 AM
 
I think that LCDs technically have a refresh rate, but it's not something you can adjust and not something that means the same thing as it does in CRT-land. If you go to most LCD makers' websites and look at the specs, you'll notice that the refresh rate is almost always 60 Hz, and that there's no variation in refresh rate based on colour depth or resolution.

I'm not sure what it is exactly that makes LCDs free of flicker. It may be the fact that they can address pixels more specifically in each scan (rather than having to refresh absolutely everything on every line).
     
stew
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Oct 1, 2002, 11:46 AM
 
Originally posted by Commodus:
I'm not sure what it is exactly that makes LCDs free of flicker.
Having a backlight that is always on instead of a beam of electrons that moves wildly over a phosphorus layer at 80,000 lines per second.


Stink different.
     
   
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