One of the LG 34UM67’s stand-out features is obviously support for AMD FreeSync. Although we’ve talked about FreeSync a few times in the past, we’d like to recap some of the specifics here. To put it simply, like NVIDIA’s G-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. Instead of the monitor controlling the timing and refreshing at regular intervals, like 60Hz, 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 quickly as it can with the next frame, in lockstep with the GPU.
To fully appreciate how adaptive refresh rate technologies 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. 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. 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 essentially allowed 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 a whole new 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 so on. V-Sync would be an ideal solution if the frames were rendered and output at 60Hz as well, but that's not how today's games and GPUs work. It's common for today's games to exhibit significant variations in frame time and it's rare that the GPU and display are actually in sync for any 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 keep up. What results is that unfinished parts of adjacent frames are displayed on-screen, and since the positioning of the scene's components 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, which introduces lag. We should mention that this is another 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 image below illustrates how frames are output to a FreeSync capable screen like the LG 34UM67, when FreeSync is enabled in AMD’s drivers.
As you can see, 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. 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 call within the 30Hz to 144Hz range).
