AMD could counter Nvidia's G-Sync with simpler, free sync tech


— 11:50 PM on January 6, 2014

CES — During an impromptu meeting in a hotel ballroom this morning, I got an eye-opening demo orchestrated by a senior AMD engineering executive. He had a pair of relatively inexpensive laptops sitting side by side running a simple graphics demo showing a windmill with the blades in motion. One of the laptops was using traditional vsync, only refreshing the display at a fixed rate, and the quantization effect of the fixed refresh cycle introduced obvious roughness into the animation. On the other laptop, however, the motion was much smoother, with no apparent tearing or slowdowns—much like you'd see from Nvidia's G-Sync technology.

He explained that this particular laptop's display happened to support a feature that AMD has had in its graphics chips "for three generations": dynamic refresh rates. AMD built this capability into its GPUs primarily for power-saving reasons, since unnecessary vertical refresh cycles burn power to little benefit. There's even a proposed VESA specification for dynamic refresh, and the feature has been adopted by some panel makers, though not on a consistent or widespread basis. AMD's Catalyst drivers already support it where it's available, which is why an impromptu demo was possible.

Dynamic refresh works much like G-Sync, varying the length of the vertical blank period between display refreshes on a per-frame basis, so the screen can be drawn when the GPU has a finished frame ready to be displayed.

The lack of adoption is evidently due to a lack of momentum or demand for the feature, which was originally pitched as a power-saving measure. Adding support in a monitor should be essentially "free" and perhaps possible via a firmware update. The only challenge is that each display must know how long its panel can sustain the proper color intensity before it begins to fade. The vblank interval can't be extended beyond this limit without affecting color fidelity.

In AMD's assessment, it's possible to reduce some of the problems with traditional vsync that Nvidia described in its G-Sync presentations—particularly the quantization effect felt most painfully at frame rates between 60 and 30 FPS—through the use of triple-buffering. Triple-buffering is a time-honored technique that can be implemented by a game developer in software or even enabled via a software switch in a graphics driver control panel. AMD used to have an option to force the use of triple buffering in its driver control panel, in fact, and would be willing to consider bringing it back.

The quantization problem can only be completely resolved via dynamic refresh rates. However, the exec initially expressed puzzlement over why Nvidia chose to implement them in expensive, external hardware.

The exec's puzzlement over Nvidia's use of external hardware was resolved when I spoke with him again later in the day. His new theory is that the display controller in Nvidia's current GPUs simply can't support variable refresh intervals, hence the need for an external G-Sync unit. That would explain things. I haven't yet had time to confirm this detail with Nvidia or to quiz them about whether G-Sync essentially does triple-buffering in the module. Nvidia has so far been deliberately vague about certain specifics of how G-Sync works, so we'll need to pry a little in order to better understand the situation.

Regardless, the good news here is that AMD believes a very effective G-Sync-like variable refresh technology shouldn't add any cost at all to a display or system. The term "free sync" is already being spoken as shorthand for this technology at AMD.

That said, AMD is still in the early stages of cooking up a potential product or feature along these lines, and it has nothing official to announce just yet.

AMD believes the primary constraint in making this capability widespread is still monitor support. Although adding dynamic refresh to a monitor may cost next to nothing, monitor makers have shown they won't bother unless they believe there's some obvious demand for that feature. PC enthusiasts and gamers who want to see "free sync" happen should make dynamic refresh support a requirement for their next monitor purchase. If monitor makers get the message, then it seems likely AMD will do its part to make dynamic display synchronization a no-cost-added feature for Radeon owners everywhere.

Update: The original version of this story had some confused wording about AMD's position on whre triple-buffering could be useful. We've updated the text to correct the error.

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