One of the ASUS MG279Q’s stand-out features is obviously support for Adaptive-Sync / AMD FreeSync. Although we’ve talked about FreeSync a few times in the past, we’d like to recap some of the specifics here. Like NVIDIA’s G-SYNC, Adaptive-Sync / AMD FreeSync keeps a display and the output from a compatible GPU in sync, regardless of frame rates or whether or not V-Sync is enabled, within certain limits. For the ASUS MG279Q, the range when FreeSync is enabled is 35-90Hz.
Instead of the monitor controlling the timing and refreshing at regular intervals, like say 60Hz for example, the timing control is transferred to the GPU. The GPU scans a frame out to the monitor where it's drawn on-screen. And the monitor doesn't update until a frame is done drawing. As soon as a frame is done, the monitor will update again as needed, in lockstep with the GPU.
To fully appreciate how adaptive refresh rate technologies like FreeSync or G-SYNC work, it’s best to experience them in person. We do have a video produced by AMD that does a good job demonstrating how FreeSync works, but it's not a replacement for actually experiencing a compatible display first-hand. We need to point out that the demo of the technology doesn’t really do it justice because the simulations and camera don’t perfectly portray all of the potential issues and fixes. To see it live is to truly appreciate FreeSync, but the tech is explained in plain English in the video, which some of you may find helpful.
As it stands today, gamers can typically choose to play their games with V-Sync (vertical sync) enabled or disabled. V-Sync is an ancient technology that allows the output from a video source to synchronize properly with a display at a given frequency--the most common of which is 60Hz. That may sound well and good, but if the graphics output is coming at a rate above or below the vertical refresh rate of the screen, a number of issues are introduced. Disabling V-Sync may seem like the simple answer, but that causes another set of problems.
The diagram above illustrates what happens between a GPU and a display when V-Sync is enabled. In the image, the panel is refreshing at a fixed interval, but the GPU is rendering frames at different intervals. Frame 1 renders and is displayed, but Frame 2 takes a little longer, so that frame is shown on-screen twice during two refresh cycles, which causes stutter during the animation and input lag. And the process continues. V-Sync would be an ideal solution if the frames were rendered and output at exactly 60Hz as well, but that's not how today's games and GPUs work. It's common for games to exhibit significant variations in frame time and it's rare that the GPU and display are actually in sync for any meaningful length of time.
Disabling V-Sync does away with the input lag, but introduces tearing on-screen. When V-Sync is disabled, the GPU essentially pumps out frames as fast as it can, regardless of whether or not the display can actually show them. What results is that unfinished parts of adjacent frames are displayed on-screen, and since the positioning of the scene's elements are usually in different positions, tear lines are introduced.
The two graphs above show how frame rates are affected when enabling / disabling V-Sync. With V-Sync enabled (red line) on a display that has a refresh rate of 60Hz, and the games configured for high image quality settings to target the 40-60 FPS range, it is not uncommon to see frame rates bounce between 60 and 30 FPS for a time (half the monitor's refresh rate), which means many frames are duplicated, and that introduces lag. We should mention that this is an area where FreeSync has an advantage over G-SYNC. With FreeSync, if V-Sync is disabled, frame rates are not limited by the max refresh rate of the connected display.
Now that we’ve explained what happens when V-Sync is enabled or disabled, understanding what AMD FreeSync technology does should be simple. The slide above illustrates how frames are output to a FreeSync capable screen like the ASUS MG279Q, when a compatible GPU is used and FreeSync is enabled in AMD’s drivers.
When a frame is complete, it is scanned out to the screen. Frame 1 finishes in X amount of time, and it’s sent to the display. Frame 2 takes a bit longer, but when it is done, it is sent out to the screen, and so on. FreeSync removes the fixed refresh rate limitation of most of today’s desktop displays and transfers the timing to the GPU. The screen’s refresh rate is dynamically adjusted to stay in-sync with the GPU, regardless of the frame rate, within certain limits. In doing so, FreeSync eliminates the screen tearing associated with disabling V-Sync, and eliminates the lag and stutter that come when V-Sync is enabled. We should mention that FreeSync is limited to refresh rates between 9 and 240Hz, but those limitations are beyond what any current monitor is reasonably capable of (most LCDs fall within the 30Hz to 144Hz range). As we've mention, this particular ASUS display supports FreeSync frequencies between 35 and 90Hz.
We should note that FreeSync is not universally compatible with all AMD GPUs. Though it may seem like only newer, GCN 1.1 GPUs are supported, it’s not actually the core type that determines whether or not adaptive refresh rates are supported. It is the display controller in the GPU that determines compatibility, and it’s only the display controllers in newer AMD GPUs (Hawaii, Tongo, and newer) and APUs that are fully compatible. Older GPUs, like those used in the Radeon HD 7000 series are partially compatible for video playback, but not with 3D applications, i.e. games. Since FreeSync is a riff on the open Adaptive-Sync spec, Intel has announced support for the technology as well, but it's still a work in progress.
