Here's why G-SYNC monitors flicker

Talk about NVIDIA G-SYNC, a variable refresh rate (VRR) technology. G-SYNC eliminates stutters, tearing, and reduces input lag. List of G-SYNC Monitors.
Alexious
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Here's why G-SYNC monitors flicker

Post by Alexious » 03 Dec 2014, 17:13

Completely stopping the panel refresh would result in all TN pixels bleeding towards white, so G-Sync has a built-in failsafe to prevent this by forcing a redraw every ~33 msec. What you are seeing are the pixels intermittently bleeding towards white and periodically being pulled back down to the appropriate brightness by a scan.
http://techreport.com/news/27449/g-sync ... d-here-why

blargg
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Re: Here's why G-SYNC monitors flicker

Post by blargg » 03 Dec 2014, 19:21

So it has a 33ms timeout that forces an internal refresh. If this were lessened to reduce the variation in brightness/increase flicker rate so it's less-perceptible, it might add latency. While it's doing this internal refresh, if the PC decides to do its own refresh, the monitor will have to wait until this internal refresh is done to refresh again with the new frame from the PC. So more often internal refreshes (say every 16ms) could cause a game that is say in a period of updating every 20ms (50 FPS) to keep having these delayed until the timeout-triggered internal refreshes are done.

flood
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Re: Here's why G-SYNC monitors flicker

Post by flood » 03 Dec 2014, 21:47

hm well that explains why gamma changes when refresh rate increases

but are you saying that when a game is running at 0fps so that the screen updates every 33ms (=30fps), you can see 30fps flicker?

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Re: Here's why G-SYNC monitors flicker

Post by Chief Blur Buster » 03 Dec 2014, 21:56

This is an interesting discussion.

I see this flicker behavior in my GSYNC monitors from time to time, but it isn't major enough to bother me -- as it is very faint.

One idea is that NVIDIA drivers could do a predictive "pre-refresh" if it detects that the current frame is currently being rendered slowly and won't be finished in the next ~1/144sec.

In that case, it could predictively automatically do an early re-refresh, and keep re-refreshing more often until the GPU was very certain that the currently-rendering frame is finished during the next ~1/144sec (the time of a refresh scanout).

That could decrease the flicker a lot.
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flood
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Re: Here's why G-SYNC monitors flicker

Post by flood » 03 Dec 2014, 22:06

(or nvidia could just invest more $ into oled)

IwantGSYNC
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Re: Here's why G-SYNC monitors flicker

Post by IwantGSYNC » 04 Dec 2014, 00:56

Completely stopping the panel refresh would result in all TN pixels bleeding towards white
Do other panel types (MVA, PVA, IPS, PLS, etc) suffer from the same problem in 24+ hz range? It looks like it is impossible to create a non-flickering monitor with dynamic 24-60hz refresh rate.

Also, Ryan posted an interesting comment on the 2nd page: http://www.pcper.com/reviews/Editorial/ ... 1#comments
Will this problem occur if I take any non-GSync monitor on the market and set its refresh rate to 24 hz in settings?
No, this won't happen because the refresh rate / redraw rate stays the same throughout content.
How is that possible? If pixels start bleeding towards white after less than 33 msec (30hz) without refresh, they should start bleeding towards white after 41,6 msec (24hz) as well.

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Re: Here's why G-SYNC monitors flicker

Post by Chief Blur Buster » 04 Dec 2014, 14:58

flood wrote:(or nvidia could just invest more $ into oled)
OLED suffers from the same fade-to-equilibrium problem. (In this case, equilibrium on OLED is black), and GSYNC OLEDs will have a minor udulation/flicker during refreash rate variances. How visible the flicker will be, relative to LCD, is the question.

That said, I also noticed a large proportion of flicker is caused by the LCD inversion algorithm, rather than TN fade-to-white. The checkerboard pattern (http://www.testufo.com/inversion) caused by alternating DC polarities every opposite refrehs, on a checkerboard pattern, actually causes more of the flicker, than the TN nature of fading to white. This is what I have noticed. Especially at framerates just below or above 30fps. You end up getting stationary checkerboard pattern effects that briefly flicker to opposite polarity and then back. So you got asymmetric time between polarities; leading to the flicker problem. This is the bigger cause of GSYNC flicker than TN fade-to-white! I should let TechReport know (or you can point them to this thread).

A modified inversion pattern may solve this, but I think predictive pre-refresing would be a better thing to do (continually refresh until rendertime for the rest of current refresh is predicted to finish in less than 1/144sec)
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Re: Here's why G-SYNC monitors flicker

Post by Sparky » 04 Dec 2014, 18:05

Chief Blur Buster wrote:
flood wrote:(or nvidia could just invest more $ into oled)
OLED suffers from the same fade-to-equilibrium problem. (In this case, equilibrium on OLED is black), and GSYNC OLEDs will have a minor udulation/flicker during refreash rate variances. How visible the flicker will be, relative to LCD, is the question.

That said, I also noticed a large proportion of flicker is caused by the LCD inversion algorithm, rather than TN fade-to-white. The checkerboard pattern (http://www.testufo.com/inversion) caused by alternating DC polarities every opposite refrehs, on a checkerboard pattern, actually causes more of the flicker, than the TN nature of fading to white. This is what I have noticed. Especially at framerates just below or above 30fps. You end up getting stationary checkerboard pattern effects that briefly flicker to opposite polarity and then back. So you got asymmetric time between polarities; leading to the flicker problem. This is the bigger cause of GSYNC flicker than TN fade-to-white! I should let TechReport know (or you can point them to this thread).

A modified inversion pattern may solve this, but I think predictive pre-refresing would be a better thing to do (continually refresh until rendertime for the rest of current refresh is predicted to finish in less than 1/144sec)
While I do think the forced refresh should be dynamically pulled forward, I think the best implementation is a bit more complicated than that.

Scenario time:
It takes ~6ms to update the display
you can go 20 ms between refreshes without flicker (assume for the moment you prioritize flicker over judder and latency, and a 20ms maximum refresh interval is sufficient to eliminate flicker)
your frame time is varying between 25ms and 45ms

Essentially, you're trying to squeeze 20ms of frametime variability into a 14ms window, so you need to decide which frames to risk colliding. You want to minimize the risk of a late frame being even later(as that's very noticeable), but you don't want to delay your window too much or you add latency to the frames that finish more quickly. So you weigh the penalty against the probability of it happening, and put a forced refresh around 23ms, putting your window at 29ms to 43ms. This means you'd collide with short frames much more frequently than long frames, which is okay because the visual penalty for colliding with a long frame is much higher.

(for comparison) G-sync currently has a 26ms window, where a frame gets presented exactly when it's ready, but it pushes every frame between 33 and 39ms up to 39ms. It doesn't shift that window to center around the predicted frame time. In this scenario that' would hit you pretty hard, because your average frametime is right around 35ms.

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Re: Here's why G-SYNC monitors flicker

Post by Chief Blur Buster » 04 Dec 2014, 20:08

That's an idea; shifting around the window dynamically based on current frametime trends. What I describe is sort of the same thing, but interpreted in a different point of view. There will be compromises no matter what happens, but I think we can do better than status quo.
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Re: Here's why G-SYNC monitors flicker

Post by tygeezy » 08 Jul 2020, 13:56

Chief Blur Buster wrote:
04 Dec 2014, 14:58
flood wrote:(or nvidia could just invest more $ into oled)
OLED suffers from the same fade-to-equilibrium problem. (In this case, equilibrium on OLED is black), and GSYNC OLEDs will have a minor udulation/flicker during refreash rate variances. How visible the flicker will be, relative to LCD, is the question.

I come from the future to let you know that frametime spikes do indeed cause noticeable flicker in games using an oled with "Gsync compatibility." It seems like even smaller spikes are more noticeable on OLED due to the instant pixel response.

phpBB [video]


no graph here but I bet it's due to spikes in frametimes.
phpBB [video]

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