Operating below the minimum refresh rate?

Talk about AMD's FreeSync and VESA AdaptiveSync, which are variable refresh rate technologies. They also eliminate stutters, and eliminate tearing.

Operating below the minimum refresh rate?

Postby Glide » 24 Mar 2015, 21:12

Now that FreeSync/Adaptive-Sync is finally shipping, reviews have found some limitations when comparing it against G-Sync displays.

One of the main issues seems to be that once you get below the minimum refresh rate of the panel (40Hz TN/48Hz IPS) FreeSync is entirely disabled and you are left with the choice of a standard V-Sync on/off behavior operating at the minimum refresh rate.

I would not be happy if a game was running below 40 FPS, but a lot of people seem to be concerned about this.
Is this not something that AMD could fix in software if they simply repeat frames when operating below the minimum refresh rate of the panel?

E.g. To display 39 FPS on a 40-144Hz TN panel, output 78Hz or 117Hz instead and repeat the frame two or three times.
It would still be a problem for these first IPS panels which only have a range of 48-75Hz, but it seems entirely solvable in software for any display which has a maximum refresh rate of at least double its minimum.

As long as you have a flicker-free display, it seems like that should eliminate the problem, since LCDs are sample and hold.
I wonder if this might also be a partial solution for the overdriving issues that we are seeing, because you could keep the panel operating over a narrower range of frequencies. E.g. 73-144Hz rather than 40-144Hz.
Glide
 
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Re: Operating below the minimum refresh rate?

Postby Sparky » 24 Mar 2015, 21:38

Glide wrote:Now that FreeSync/Adaptive-Sync is finally shipping, reviews have found some limitations when comparing it against G-Sync displays.

One of the main issues seems to be that once you get below the minimum refresh rate of the panel (40Hz TN/48Hz IPS) FreeSync is entirely disabled and you are left with the choice of a standard V-Sync on/off behavior operating at the minimum refresh rate.

I would not be happy if a game was running below 40 FPS, but a lot of people seem to be concerned about this.
Is this not something that AMD could fix in software if they simply repeat frames when operating below the minimum refresh rate of the panel?

E.g. To display 39 FPS on a 40-144Hz TN panel, output 78Hz or 117Hz instead and repeat the frame two or three times.
It would still be a problem for these first IPS panels which only have a range of 48-75Hz, but it seems entirely solvable in software for any display which has a maximum refresh rate of at least double its minimum.

As long as you have a flicker-free display, it seems like that should eliminate the problem, since LCDs are sample and hold.
I wonder if this might also be a partial solution for the overdriving issues that we are seeing, because you could keep the panel operating over a narrower range of frequencies. E.g. 73-144Hz rather than 40-144Hz.

Presumably, AMD can fix this in drivers. I do think it's a significant problem, because a lot of games will hitch occasionally, and this will make that a lot more noticeable.

As for window shifting, Maximum 2x the minimum is only sufficient if your frame times are perfectly predictable. Say your range is 30 to 60hz. 1 refresh takes 16.7ms 2 takes 33.3ms If your frametime is fluctuating between 30 and 35ms, the display doesn't know if it has to force an early refresh or not. You need the window wide enough to contain your prediction error, which depends on a whole mess of things, including the game engine and what the user happens to be looking at.
Sparky
 
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Re: Operating below the minimum refresh rate?

Postby Glide » 24 Mar 2015, 22:15

Sparky wrote:As for window shifting, Maximum 2x the minimum is only sufficient if your frame times are perfectly predictable. Say your range is 30 to 60hz. 1 refresh takes 16.7ms 2 takes 33.3ms If your frametime is fluctuating between 30 and 35ms, the display doesn't know if it has to force an early refresh or not. You need the window wide enough to contain your prediction error, which depends on a whole mess of things, including the game engine and what the user happens to be looking at.
So you would be better off polling the GPU at 1ms then? :?
Glide
 
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Re: Operating below the minimum refresh rate?

Postby Sparky » 24 Mar 2015, 23:08

Glide wrote:
Sparky wrote:As for window shifting, Maximum 2x the minimum is only sufficient if your frame times are perfectly predictable. Say your range is 30 to 60hz. 1 refresh takes 16.7ms 2 takes 33.3ms If your frametime is fluctuating between 30 and 35ms, the display doesn't know if it has to force an early refresh or not. You need the window wide enough to contain your prediction error, which depends on a whole mess of things, including the game engine and what the user happens to be looking at.
So you would be better off polling the GPU at 1ms then? :?

You're better off with a wider window. Say you double the max refresh rate, you now can run a single refresh in 8.3ms, so if your frametime is 30ms ± 10ms, you can force a refresh at 11.5ms, and be pretty sure the next new frame can be displayed on time.
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