G-SYNC reduces input lag for fixed refresh rates too (emulators, etc.)

Posted Oct 24, 2013 by

It is observed NVIDIA G-SYNC is also useful for low-latency fixed refresh rate applications as well: It accelerates frame delivery from GPU to monitor: Frames are delivered from GPU to monitor in 1/144sec instead of 1/60sec when running at 60Hz!

Low-lag fixed-refresh-rate G-SYNC has uses for emulators and other applications that requires a fixed refresh rate (e.g. 60Hz) with an ultra-low input lag.

About Mark Rejhon

Also known as Chief Blur Buster. Founder of Blur Busters. Inventor of TestUFO. Read more about him on the About Mark page.

5 Comments For “G-SYNC reduces input lag for fixed refresh rates too (emulators, etc.)”

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Ewok
Member
Ewok

That’s the first time I hear DisplayPort introduces lag (especially a huge 16.67ms). Do you have a source?
What about DVI, HDMI and VGA?

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10 years 11 months ago
Mark Rejhon
Author
Mark Rejhon

It has nothing to do with the cable. It’s not cable specific, it’s the finite speed of scanning from top-to-bottom. One needs to how display refreshes traditionally workeds:

See the high speed videos:

High speed video of CRT:
http://www.youtube.com/watch?v=zVS6QewZsi4

High speed video of LCD:
http://www.youtube.com/watch?v=nCHgmCxGEzY

Observe how the display is refreshed from top-to bottom. And VGA, CRT, analog, DisplayPort, HDMI, DVI, all have finite-speed transmission time and scan-out time. Sometimes some displays (e.g. plasma) frame buffers the incoming frame delivery instead, and then flash the whole frame all at once (e.g. subfield refreshes). But you always have a latency to deliver a whole frame buffer from top-to-bottom, during VSYNC ON.

As you have seen in the high speed video, no displays in the last 75+ years have ever had instantaneous entire-frame-at-once frame delivery from source (e.g. computer) to the destination (e.g. human eyeballs). The displays always have required a finite amount of time to scan-out the refresh.

No matter what cable (in the last 75+ years), a 60Hz refresh always took 1/60sec to deliver over any cable in the past, even if it’s scanned-out to the screen in real time. That adds input latency for the bottom edge of the screen. No longer; during G-SYNC, the computer can now deliver a 60Hz refreshes faster to the display (e.g. 1/144sec) so that a single refresh gets displayed all at once faster, no matter what the refresh rate is (e.g. 30Hz, 60Hz). Vertical blanking intervals can now be much bigger than the refresh itself.

The Leo Bodnar Input Lag tester has three flashing squares (top, center, bottom), and the bottom square always has more input lag than the top edge of the screen. Check this displaylag.com article about the explanation that the top edge of the screen has less input lag than the bottom edge of the screen. Many other sites (e.g. HDTVtest.co.uk and others) have confirmed this. This is just the sad reality of slow finite-speed delivery of individual refreshes from a video source (e.g. computer) to the display (e.g. seen by human eyes). Although a CRT has zero input lag, that’s only for the top edge of the screen.

Now, yes, you can reduce lag using VSYNC OFF, and continually splice new frame fragments into the current scanout (creating tearing):


(credit: AnandTech liveblog of G-SYNC launch)

A variable refresh rate display such as G-SYNC (operating at maximum bandwidth) revolutionizes this by accelerating frame delivery times, independently of the refresh rate, and allows faster whole-frame-at-once delivery times without needing splices (tearing). Frame delivery times and scanout times can be made faster, even at lower refresh rates! Permitting beautiful stutter-free and tearing-free G-SYNC (a greatly improved VSYNC ON) with much less input lag.

For the first time in history, frame delivery time become truly independent of refresh rate.

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10 years 11 months ago
Ewok
Member
Ewok

Thanks for the explanation. I now understand that the top of the screen has almost no lag, and the bottom of the screen has about 16.67 ms lag, on a traditional (non G-Sync) 60 Hz LCD monitor.

I’m a bit surprised that the human eye cannot detect this top-to-bottom refresh. (Our eyes suck ^^)

Couldn’t the top-to-bottom refresh be even faster than 1/144 sec? For example, using DisplayPort 1.2, the available bandwith is 17.28 Gb/s, so a single 1920x1080x24 frame would take only 2.88ms (1/347 sec).

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10 years 11 months ago
Mark Rejhon
Author
Mark Rejhon

Oh, by the way, http://www.testufo.com/blurtrail configured to full screen mode, also tilts on an iPad (in landscape mode) too. Very interesting to visually witness the finite speed of the LCD scanout on the iPad!

Also works on a 120Hz monitor in non-LightBoost mode, especially during 60Hz refresh rate. The tilting of the line still occurs during 120Hz/144Hz non-LightBoost, but only very slightly (due to the faster scanout).

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10 years 11 months ago
Mark Rejhon
Author
Mark Rejhon

> I’m a bit surprised that the human eye cannot detect this top-to-bottom refresh. (Our eyes suck ^^)

Actually, we can see it:
1. Go to a 60Hz CRT or a 60Hz LCD (not plasma, DLP or LightBoost)
2. View http://www.testufo.com/blurtrail, height configured to “Full Screen”.

Notice how the fast-moving line tilts?
Exactly: The tilting is the distortion caused by the top-to-bottom scanout.

>Couldn’t the top-to-bottom refresh be even faster than 1/144 sec?

Correct, it’s only limited by the bandwidth of the cable, and how fast the electronics can receive it. We just have to use the maximum dotclock possible, and pad the largest possible blanking interval right after each refresh (blanking intervals can be bigger than the actual image if needed, for refresh rates less than half the cable’s maximum bandwidth). The upcoming DisplayPort 2.0 can transmit 1920x1080p frames in 1/354th of a second, and 3840x2160p frames in 1/88th of a second. Whether monitor manufacturers take advantage of this, remains to be seen. G-SYNC 2.0 of the future, maybe?

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10 years 11 months ago
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