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NVIDIA's G-Sync Is VSync Designed For LCDs (not CRTs)
Phopojijo writes "A monitor redraws itself top to bottom because of how the electron guns in CRT monitors used to operate. VSync was created to align the completed frames, computed by a videocard, to the start of each monitor draw; without it, midway through a monitor's draw process, a break (horizontal tear) would be visible on screen between the two time-slices of animation. Pixels on LCD monitors do not need to wait for above lines of pixels to be drawn, but they do. G-Sync is a technology from NVIDIA to make monitor refresh rates variable. The monitor will time its draws to whenever the GPU is finished rendering. A scene which requires 40ms to draw will have a smooth 'framerate' of 25FPS instead of trying to fit in some fraction of 60 FPS."
NVIDIA also announced support for three 4k displays at the same time. That resolution would be 11520×2160.
For starters it reduces memory contention because the display device doesn't have to read and send the display data over the wire 60 times a second, while rendering the next frame. Theoretically, if there's nothing happening on the screen, such as an idle desktop, the display device won't consume x*100 Mbyte of bandwidth just to show a still image on the screen.
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Vsync still tend to add noticeable input lag even in games today. And some games still have issues with tearing even on lcd screens. So I'm guessing this is what they are trying to fix.
MMMMkay.... even if LCDs don't have an explicit refresh rate, interface standards DO.
On the wire, be it VGA, DVI, HDMI, they all, without exception, must conform to timing standards. This includes a pixel clock and for digital transport and hsync/vsync for VGA and always fixed sizes for blanking and active regions. This maps to a fixed frame rate which the input hardware of the monitor will synchronize to. You can't just arbitrarily decide to send frames at variable rates - it doesn't work that way.
NVidia would have to change the whole industry for this - it could work but we're talking about new interface standards here... I expect to see an NVidia-branded LCD which supports this, which of course will cost 4 times as much as a standard LCD.
No marketspeak here, but if you're not familiar with the technical details you might be a bit lost. First of all, in order to understand the solution, you need to identify the problem.
The problem is that, currently, refresh rates are STATIC. For example, if I set mine to 60Hz, the screen redraws at 60fps. If I keep vsync disabled to allow my gfx card to push out frames as fast as it can, my screen still only draws 60fps, and screen "tearing" can result as the screen redraws in the middle of the gfx card pushing out a new frame (so I see half-and-half of two frames).
As described, let's say my gfx card is pushing out 25fps. Currently the optimal strategy is to keep vsync off, even though this can result in screen "tearing", because with low fps bigger problem emerges even though screen "tearing" is fixed, with vsync on.
Every time my gfx card pushes out a frame, since vsync is on, it waits to ensure it will not be drawing to the screen buffer while the screen is updating. Since it waits, the screen only draws complete frames. So at 60fps the screen updates in 1/60 second intervals, and the gfx card render at 1/25 second intervals. So, at the beginning of a frame render, the gfx card renders... and the screen redraws twice, and then the gfx card has to wait for the third opportunity to draw before syncing up again. Since it is waiting instead of rendering, I am now rendering at 20fps (since I lose 2/3 refresh opportunities) instead of the optimal 25fps. If I disable vsync, I get tearing, but now 25fps.
This "G-sync" claims to solve that issue by making refresh rates DYNAMIC. So if my gfx card renderas at 25fps, the screen will refresh at that rate. It will be synchronized. No tearing or gfx card waiting to draw.
LCD monitors absolutely do not ignore VSync. Now let's not forget that the primary function of a VSync signal is to tell the monitor (CRT or LCD) where the start of the picture begins. There's also HSync to break the picture into scanlines. VSync always takes a certain amount of time which the monitor will "take a breath" (CRT will also move the gun back to top). At this time, it is the perfect moment for the GPU to quickly swap its framebuffers in the video memory. The "scratch" draw buffer will be moved as the final output image and then the GPU can begin drawing the next one in the background. At the same time the completed image is sent to the monitor in the normal picture signal when the monitor gets back to work to draw a frame. If the buffers are swapped in the process of the monitor drawing the frame, the halves of two frames will get shown together which leads to the video artifact called "tearing".
If we are a good citizen and swap buffers only during the VSync period we can get a nice tear-free (typically 60fps) image. However if instead it takes more than the time of one picture (which about 16ms) to draw the next one, we have to wait a long time for the next VSync and that means that we also slide all the way down to a 30fps frame rate. Now if the game runs fast at some moments but slower at some others, the bouncing back between 60fps and 30fps (or even 15fps) makes this annoying jerky effect. NVIDIA's G-Sync tries to solve this problem by making the frame time dynamic.
Actually, it's the reverse. Instead of forcing the GPU to wait for the screen's refresh rate (as is the case with V-sync), potentially causing some pretty bad frame drops, G-sync makes the monitor wait for the GPU's frames. Whenever the GPU outputs a frame, the monitor refreshes with that frame. If a frame takes longer, the screen keeps the old frame shown in the meantime.
Remember, V-sync forces the GPU to wait for the full frame's duration, regardless of how long it's taken to render the frame. If the GPU renders the frame in 3ms but V-sync is at 10ms per frame, the GPU waits around for 7ms. Flip side, if the GPU takes 11ms, it's "missed" a frame (lag) and still has to wait 9ms until it can start drawing the next frame. G-sync is supposed to make it so as soon as the GPU's done rendering a frame, it pushes it to the monitor, and as soon as the monitor can refresh the display to show that new image, it will.
In theory, this could effectively give the visual quality of V-sync (no screen tearing) with a speed similar to straight rendering without V-sync.