V-Sync

In short: “Vertical Sync” — a graphics setting that ties the graphics card’s frame output to the monitor’s refresh rate, to avoid screen tearing.

In more detail: V-Sync caps the frame rate at the monitor’s refresh rate (e.g. 60 Hz) at most. The downside: if the frame rate drops below that, V-Sync can cause noticeable input lag — modern alternatives like G-Sync/FreeSync solve this problem more elegantly.

In Depth

How tearing occurs

Without V-Sync, the graphics card sends new frames independently of the monitor’s refresh cycle — the monitor builds up its image row by row from top to bottom (a relic from the CRT era that lives on in the driving logic to this day). If the graphics card switches frames right in the middle of this build-up cycle, the screen shows two half, different frames stacked on top of each other for a moment — visible as a horizontal offset (“tearing”), especially noticeable during fast camera movements in games, where vertical lines in the image suddenly get a visible kink.

How classic V-Sync solves the problem

V-Sync prevents this by forcing the graphics card to wait with its frame output until the monitor is ready for a completely new frame — technically, the finished frame is first written to a second frame buffer (back buffer) and only made visible at the monitor’s next refresh cycle (buffer swap). This means the monitor only ever sees complete, internally consistent frames.

The trade-off: stuttering and input lag

The downside: if the graphics card doesn’t finish a new frame in time before the next monitor refresh cycle, classic V-Sync has to fully repeat the old frame once more (“stuttering”, jerky behaviour) instead of immediately showing the new, delayed frame — this creates noticeable input lag, because more time passes between an input (e.g. a mouse movement) and its visible effect on screen than without V-Sync. At frame rates considerably below the monitor’s refresh rate, classic V-Sync can even “snap” the frame rate to clean fractions of the monitor’s rate (e.g. at 60 Hz down to 30 instead of, say, 45 FPS), which further worsens the stuttering.

Adaptive synchronisation as a modern solution

Adaptive technologies like NVIDIA G-Sync and AMD FreeSync (today usually grouped under the vendor-independent VESA Adaptive-Sync standard) solve this problem fundamentally differently: instead of tying the graphics card’s frame rate to a fixed monitor rate, they dynamically adjust the monitor’s own refresh rate to the frame rate the graphics card has actually finished — the monitor updates its image exactly when the graphics card has a new frame ready, instead of on a rigid time cycle. This avoids both tearing and the stuttering/input-lag problem of classic V-Sync at the same time, but needs correspondingly compatible hardware on both sides (graphics card and monitor).

See also: Sync (Display Sync)