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Frame time/ refresh rate

post #1 of 11
Thread Starter 
Hey Guys, I've got some questions about frame times and monitor refresh rates. I've been reading about this lately as many of you have but I think my understanding of how refresh rates work is flawed.

As I understand, say that with a 60hz monitor, 60 frames are shown a second, so each fame lasting 16.7ms.

If a single graphics card can run at 60 fps then you should get a unique frame every 16.7ms and lasting for 16.7ms.

If a single graphics card can only run at 30fps then you should see 2 of the same frame in 16.7 +16.7 then on to an new unique frame.

With this (how I think it works) how can "runt frames" happen if the frame has to last at least 16.7ms minimum?

So what do I have wrong here?

Thanks.
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post #2 of 11
Ill take a stab at this. In AMD cards there is not a governor except for vsync to make the frames last a minnium of 16.7ms. So in a situation with runt frames a card can push out a frame that is displayed 8ms. This runt frame is only displayed half the time it is supposed to. When this happens the frame is partially displayed, then most of the time the next frame is displayed for 16.7+the missing 8ms. This will give you the stutter or dip you can see.

AMD is working on the drivers to govern the frame times so they will display every 16.7ms in a crossfire setup.
post #3 of 11
Thread Starter 
So the image on the 16.7ms flash of the screen can change during 16.7ms of display time? So it can re-draw mid flash??
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post #4 of 11
Quote:
Originally Posted by drewfus View Post

So the image on the 16.7ms flash of the screen can change during 16.7ms of display time? So it can re-draw mid flash??
With vsync its always going to be 16.7@60hz. Without vsync this is when you can get frames lower and higher than 16.7ms even though you use a program like rivatuner to cap frames at 60fps.
post #5 of 11
Thread Starter 
I still don't understand how the image can change during the 16.7ms display time.
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post #6 of 11
Images dont change. i think you are confused about timing, refresh rate and frame metering. Ill do my best to try to explain it. My explination is based off AMD cards only.

60fps=16.7ms frame time. 120fps=8.3ms frame time.

So, lets say you have a monitor that is 1080p at 60hz, this monitor will display 60 images per sec, hence 60 fps. Anything above 60fps on this monitor will not display whole frames because the monitor will refresh and display a new image every 16.7ms.

So now we have the GPU, the GPU does not automatically sync frames with the monitor unless vsync is applied. When vsync is applied the monitor will spit out, for our scenario, an image every 16.7ms, 60fps. So when your monitor is refreshing frames every 16.7ms and you have a frame rate of 110 fps which is roughly 8.5ms the monitor will refresh and the GPU is still constantly putting out images at 8.5ms, 110fps There will be tearing on the monitor.

When a whole image is not displayed this is called tearing. Runt frames and tearing are similar but not always the same. Tearing comes from FPS higher than the monitor refresh rate which sends frames from the video card to the monitor at a faster pace than the monitor can process.

Runt frames are frames that are sporadically pushed out faster than the average frame time the GPU is sending them out. Example of this is a game you are averaging 60fps and you are getting these frames from the GPU every 16.7ms. Every once in a while sporadically the GPU will spit out a frames lower than 16.7ms when averaging of 16.7ms or 60fps. These runt frames only display a portion of the frame similar to tearing.

Now if we go back to my 1st quote maybe you will understand that between the monitor and GPU there is no frame meter to make sure the GPU sends out the frames uniformly. AMD is working on this with a driver set to release in june/july.
Quote:
Originally Posted by dir_d View Post

Ill take a stab at this. In AMD cards there is not a governor except for vsync to make the frames last a minnium of 16.7ms. So in a situation with runt frames a card can push out a frame that is displayed 8ms. This runt frame is only displayed half the time it is supposed to. When this happens the frame is partially displayed, then most of the time the next frame is displayed for 16.7+the missing 8ms. This will give you the stutter or dip you can see.

AMD is working on the drivers to govern the frame times so they will display every 16.7ms in a crossfire setup.

Im not the best when explaining things but im trying to get better i hope this helps.
post #7 of 11
Thread Starter 
Let me see if I understand you. In the event of a frame that last longer than 16.7 but less than 16.7+16.7 , in the 2nd 16.7ms frame It will for example display (on the top half) the old frame and (on the lower half) the start of the new frame, thus causing a tear. Is this correct?

So runt frames do last 16.7ms on the screen but will only fill a small portion of the frame?
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post #8 of 11
Thread Starter 
BTW thanks for your patients. thumb.gif
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post #9 of 11
Quote:
Originally Posted by drewfus View Post

Let me see if I understand you. In the event of a frame that last longer than 16.7 but less than 16.7+16.7 , in the 2nd 16.7ms frame It will for example display (on the top half) the old frame and (on the lower half) the start of the new frame, thus causing a tear. Is this correct?

So runt frames do last 16.7ms on the screen but will only fill a small portion of the frame?


This will be Frame Lag, unsmooth or stuttering, because one frame was slower than 16.7, lets say 23ms and when the monitor projects the next frame it is actually displaying the same frame. Then the next frame may be short, lets say 10ms for the GPU to catch up (16.7+16.7=33ms) (23+10=33ms) and since it was so quick the monitor displays half of the frame, a quarter or less, which is a runt frame.

So when you see stuttering its actually a combination similar to picture i painted, that's why it looks choppy.

If you are locked at 60hz which is 60fps then a runt frame does lat 16.7ms but since it was rendered so fast by the GPU (under 16.7ms) it displays the next frame along with the runt, if you look at some of the pcper videos, this action looks like screen tearing and stuttering.
post #10 of 11
Thread Starter 
Ok I see, Got it. Thanks.
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