How Variable Refresh Actually Synchronises
The frame isn't pushed to the panel — it's pulled, and the panel holds its breath until the source is ready.
The Protocol Layer
Every fixed-refresh display runs on a tight clock: the source sends a defined number of horizontal lines per second, the timing controller (TCON) latches each frame at the vertical synchronisation pulse, and the panel's gate driver scans those lines sequentially. Miss a pulse and you get a torn image. The whole chain is a metronome.
Variable refresh rate (VRR) breaks the metronome deliberately. Under DisplayPort Adaptive-Sync — the underlying protocol that both AMD FreeSync and NVIDIA G-Sync Compatible use — the monitor advertises its supported refresh range to the source's GPU in its EDID (Extended Display Identification Data). From that point, instead of generating vsync pulses at a fixed interval, the GPU holds the last line of blanking until a new frame is ready, then releases it. The panel sees a variable-length vertical blanking interval (VBI) and treats it as the gap between frames.
HDMI's equivalent, Forum VRR introduced in HDMI 2.1, uses the same logical concept but carries it over HDMI's fixed-clock physical layer, stretching the blanking region rather than relaxing the pixel clock. The on-screen effect is identical; the electrical implementation differs.
The TCON's Job
When the GPU signals end-of-frame, the TCON receives the vsync and immediately begins driving the gate lines. It scans the pixel array top-to-bottom, applying the new charge to each row in sequence — exactly as in fixed-refresh operation. The difference is what happens between frames.
In the extended blanking interval, the TCON keeps the panel in a low-power hold state. This is not zero activity: the liquid crystal in an LCD must be periodically refreshed even when the image is static, or charge leakage degrades pixel voltage and the image dims and shifts in colour. The minimum refresh rate in the VRR range — typically 48 Hz on consumer panels, sometimes lower — is set partly by how long a pixel can hold its charge before the error becomes visible. Below that floor, panels either clamp to the minimum or use low-framerate compensation (LFC), which simply duplicates frames to stay above it.
OLED panels have less leakage concern but face a different constraint: the organic emitters are driven by current, and the TFT backplane's ability to maintain stable current during a long hold period affects uniformity. Oxide TFT backplanes (IGZO) hold charge significantly better than amorphous silicon, which is one reason high-refresh OLED monitors use oxide or LTPS backplanes.
Latency and the Real Synchronisation Point
The genuine synchronisation event is the moment the GPU commits the completed frame to the display link. Everything before that — rendering, rasterisation, compositing — happens inside the GPU's pipeline, invisible to the panel. VRR does not reduce render time; it eliminates the mismatch between when rendering finishes and when the panel was going to scan regardless. A frame that completes 3 ms before the next fixed vsync would have waited; under VRR, it ships immediately.
This is why VRR's perceptible benefit concentrates in the range just below the panel's maximum refresh rate. At 60 fps on a 144 Hz panel, frame pacing is irregular and tearing is probable under fixed sync. VRR turns each completed frame into its own vsync event. At very low frame rates — below the VRR floor — the protocol falls back to fixed timing, and stutter returns.
The hardware is, at its core, a negotiated pause: the GPU says "I'm not ready yet," the TCON waits, and only when the source releases the blanking does the scan begin. Frame synchronisation reduced to a handshake.