Standards Group Adds Adaptive-Sync To DisplayPort

MojoKid (1002251) writes "Over the past nine months, we've seen the beginnings of a revolution in how video games are displayed. First, Nvidia demoed G-Sync, its proprietary technology for ensuring smooth frame delivery. Then AMD demoed its own free standard, dubbed FreeSync, that showed a similar technology. Now, VESA (Video Electronics Standard Association) has announced support for "Adaptive Sync," as an addition to DisplayPort. The new capability will debut with DisplayPort 1.2a. The goal of these technologies is to synchronize output from the GPU and the display to ensure smooth output. When this doesn't happen, the display will either stutter due to a mismatch of frames (if V-Sync is enabled) or may visibly tear if V-Sync is disabled. Adaptive Sync is the capability that will allow a DisplayPort 1.2a-compatible monitor and video card to perform FreeSync without needing the expensive ASIC that characterizes G-Sync. You'll still need a DP1.2a cable, monitor, and video card (DP1.2a monitors are expected to ship year end). Unlike G-Sync, a DP1.2a monitor shouldn't cost any additional money, however. The updated ASICs being developed by various vendors will bake the capability in by default."

Comments

It's a great idea

[Bryan Ischo]

I have to wonder why the idea of adaptive vsync wasn't thought of earlier or implemented into display standards earlier. It just seems like such an obvious idea once you've heard of it. Surely someone else in the graphics/display industry must have had the idea before NVidia?

I can't think of any downsides to having this technology; it's pure upside as far as I can tell. Although, I guess I could imagine that there could be some technical downsides, depending upon how displays are typically implemented. For an LCD, I can imagine that knowing the frequency ahead of time allows the LCD panel to perhaps "pipeline" some of its operation, allowing faster grey-to-grey transitions. For example, if the display knows that the next frame is going to come at exactly X milliseconds in the future, then perhaps it could start transitioning all pixels to grey at time X - N, where N is the average time it takes for pixels to transition to grey, and then when the frame is received, it could then transition all pixels from grey to the next frame pixel colors faster. With adaptive vsync, the display would not be able to do this; it would have to start the transition from frame M pixel values to frame M + 1 pixel values only as soon as frame M + 1 becomes available.

Not being able to play grey-to-grey optimization games is I guess a possible downside of adaptive vsync; but I suspect it's a pretty small downside. Aside from gamers who want to see "the next frame" with the smallest latency possible, I don't know that anyone is really going to care much about that potential downside.

Re:It's a great idea

[wonkey_monkey]

For example, if the display knows that the next frame is going to come at exactly X milliseconds in the future, then perhaps it could start transitioning all pixels to grey at time X - N, where N is the average time it takes for pixels to transition to grey, and then when the frame is received, it could then transition all pixels from grey to the next frame pixel colors faster.

What's the reason for transitioning to grey? Is it to minimise the likely "distance" (time) to the new colour?

Won't most pixels, most of the time, remain a similar colour in the next frame? I don't understand the ins and outs, but wouldn't you lose as much, if not more, as you'd gain?

Re:It's a great idea

[michelcolman]

I have to wonder why the idea of adaptive vsync wasn't thought of earlier or implemented into display standards earlier. It just seems like such an obvious idea once you've heard of it. Surely someone else in the graphics/display industry must have had the idea before NVidia?

It's just a vicious compatibility circle.

CRTs have a fixed frame rate for technical reasons.
Therefore graphics cards have a fixed frame rate to support CRTs
Therefore LCD displays have a fixed frame rate to support graphics cards
Therefore graphics cards continue to have a fixed frame rate
etc...

New stuff has to remain compatible with old stuff, so nobody even thinks of breaking the circle. Until now, fortunately.

Re:It's a great idea

[neilo_1701D]

The Amiga did this sort of stuff when it first came out. You could create a Copper (the display coprocessor) list that was synced to the vscan quite easily; "beam-synced blitting" I think was the name. Basically, you built your copper list so the screen writes were always just behind the video beam so you could have flicker-free drawing.

Re:It's a great idea

[chihowa]

There's no reason to go through grey between every frame. As you say, most pixels will remain the same color, and making every pixel grey at the V-sync frequency will just make the whole display strobe and look washed out.

Grey-to-grey is just an easy thing to test for benchmarking displays. You don't actually do that in normal operation.

Re:It's a great idea

[Immerman]

That's clever in that they presumably accomplished that without a back-buffer back when RAM was expensive, but basically you're describing the vsynced based rendering which has been the standard for decades: Wait until the screen starts updating (the vsync), then start working on the next frame to maximize rendering time. It's nothing like G-sync/free-sync/adaptive sync though - you still have the issue that if your screen updates at 60FPS you have exactly 1/60 of a second (~16.67ms) to render each frame.

Adaptive sync means that if you finish rendering an easy frame in only 14ms the screen can display it immediately instead of waiting an extra 2.67ms for the next scheduled refresh. Even more importantly if a complex frame takes 20ms to render you don't miss the refresh and have to have to wait an extra 13.3ms for the next scheduled refresh, wasting almost an entire frame - instead the screen can hold off on refreshing until the rendering is complete.

TLDR: Adaptive sync means that if you enter a graphically intensive area that you can only render at 50fps, then your monitor will automatically refresh at 50fps, instead of continuing to try to refresh at 60fps and having to spend every other frame waiting for the rendering to finish, for an effective framerate of only 30fps. (or possibly a jittery 40fps with double-buffering: update,update, wait,u,u,w,...)

Re:It's a great idea

[pla]

I have to wonder why the idea of adaptive vsync wasn't thought of earlier or implemented into display standards earlier.

I have to wonder why we still use the concept of sync and porch and blanking interval and even frames, etc at all, when we all now run pixel-addressable digital displays rather than a magnetically confined analog electron beam physically sweeping over a surface.

"Tearing" results from the display updating halfway through a complete refresh. Why the hell do displays still do complete refreshes? No need whatsoever to update anything but the small subset of pixels that have changed. And no need whatsoever to do that in some blessed-from-on-high linear scan pattern from left-to-right top-to-bottom manner, either.

How about if the next gen of video hardware stops pretending it still needs to support CRTs, and we can all move on from caring about metrics like "refresh rate" that haven't meant a damned thing in over a decade?

Re:I didn't realise they didn't already did that.

[gigaherz]

The protocol used for digital signaling is internally surprisingly similar in concept to the analog equivalent. The idea of "adaptive" sync is that instead of starting a new frame after a fixed exact period, it can be "or later". There's no other technology involved other than allowing a frame to come late.

About bloody time...

[danknight48]

This tech has been a long time overdue, nothing else to say.

Re:I didn't realise they didn't already did that.

[50000BTU_barbecue]

What's "purely digital" about a LCD? You can have analog VGA inputs, which are digitized in the monitor, then sent over some ridiculously fast serial interface to column driver ICs on the glass... to be converted back to the analog voltages needed to control the LCD shutters.

Guess what? Your LCD monitor has thousands of D/A converters in it!

So for example, a relatively cheap monitor (like mine) 1680x1050, requires 1680x3=5040 columns to be driven in the actual glass. Each pixel has RGB, right? Well, those voltages have to come from somewhere!

www.intechopen.com/download/pdf/11273

Column drivers are the most amazing things I've seen in a while. They are bare dies about 2 x 11 mm with hundreds of pins, attached directly to the flex PCB that drives the glass. Each IC contains hundreds of digital-to-analog converters and opamps! It's crazy! There's usually 10 per panel, so each IC drives about 500 lines. You should see the flex PCBs, the traces are so fine you need a magnifying glass to resolve the traces.

http://oi59.tinypic.com/whmc74...

This is as close as I can get this morning. Yes, those traces are so fine they just look like a green patch.

I'd say that means a LCD monitor is more analog than digital, but that's just me.

So what's so strange about a serial device needing synchronization signals anyways?

Re:I didn't realise they didn't already did that.

[50000BTU_barbecue]

It's tragic to hear the kind of nonsense people tell themselves. It's like a cyclist buying a car and saying "that's silly, why would a car have a speed?"

It's the same thing, dingus!

A monitor is just a high-speed serial device. Stuff comes in at some rate. The only reason CRTs had such tight timing requirements was because of the humongous amount of reactive power flowing in the deflection coils. You can just short them out but then all that reactive power becomes real (waste) heat. Lots of it. So people didn't do that.

Remember how old Multisync monitors used to click relays as they shifted to different horizontal frequencies? That was the monitor swapping in different capacitors to create the LC tank with the deflection coils. So they could swap the power around between the coil and the cap instead of dissipating it.

But that meant you better be ready to send me those pixels when I'm ready! I can't wait!

There is no such large power being bounced around inside an LCD, it's really just thousands of analog voltages being sent to a glass panel. It can wait a bit, the picture won't fade that quickly. Eventually the capacitor that is formed by the LCD shutter will leak, but that takes time.

Re:I didn't realise they didn't already did that.

[chihowa]

The pixel addressing of a modern graphics system (GPU to LCD) is purely digital, which is what he meant by "purely digital". Of course there are analog components in the displays, but the signal path is digital.

It seems very inefficient to dump whole frames to the panel at a fixed (or even variable) interval. Why not just change individual pixels only when they are damaged?

Re:I didn't realise they didn't already did that.

[50000BTU_barbecue]

Why does it matter if you dump whole frames to the LCD? It's not like the cable is miles long or that transmitting a signal takes so much power compared to the backlight.

You'd just be adding a lot of complexity to arbitrarily refresh a bunch of pixels.

Oh and suddenly programmers are worried about *efficiency*? I doubt it! You'd just be adding complexity to the monitor. Right now a monitor is a 2 dimensional serial to parallel converter. It does the job just fine.

And I'd argue your assertion that pixels are addressed on a LCD. If you're not using it at its native resolution, what are you addressing? That's purely a concept on the computer side. It's actually the TCON in the monitor that does any "addressing", and it doesn't do anything more fancy than a shift register. It's not like you can go back on the line and say "oops, I wanted that pixel to be purple, not yellow, so please address it". By that time, it's too late. Next line!

Re: grey

[kimvette]

There goes the great contrast ratio of monitors. Just as we're mourning the loss of vertical resolution thanks to the economics of reusing 16:9 television panels, we'll be mourning the good old days of nice dark blacks and well-saturated colors if they were to completely grey out the screen between each frame. Thanks but no thanks.