Pushed Too Hard: Overdrive Artefacts Explained
LCD overdrive speeds up pixel transitions — push the voltage too far and you create a ghost in front of the object instead of behind it.
The Overdrive Bargain and Where It Breaks
Liquid crystals rotate slowly. Left to their own physics, the time a crystal takes to reach a new alignment — and therefore a new grey level — can stretch past 20 ms, long enough to smear a moving object visibly across multiple frames. Overdrive is the fix: the panel's timing controller applies a voltage higher than the target grey level demands, forcing the crystal to rotate faster, then relies on it arriving at the correct alignment as the frame is displayed. Done well, the pixel hits its target in time. Done aggressively, it overshoots.
Overshoot is the entire problem. If the controller pushes too hard, the crystal rotates past the target orientation. During that frame the pixel is briefly brighter than intended — and because this happens on the leading edge of a moving object, the result is a bright halo that precedes the object rather than trailing it. Calibrators call this inverse ghosting or a corona. It reads to the eye as an unnatural rim of light, often white or complementary-coloured, and it is immediately jarring on dark-background content with fast lateral motion.
The magnitude of overshoot depends on two things: how far the pixel has to travel (the source grey level), and how aggressively the lookup table pairs that transition with an overdrive voltage. Manufacturers build a two-dimensional table — source grey by target grey — and the values in it represent years of tuning. An entry miscalibrated by even a few grey steps produces visible coronas on specific transitions, which is why overdrive artefacts can appear only with certain content and disappear on others.
Variable refresh rate complicates the picture further. Overdrive tables are built for a fixed refresh cadence; when the frame rate drops, the crystal has more time to settle, and the same overdrive voltage pushes it further past its target. Panels with variable refresh must either run multiple tables or accept that overdrive will worsen at low frame rates — a trade-off few spec sheets acknowledge.
The underlying lesson is simple: overdrive is open-loop. The controller applies a calculated push and hopes the physics cooperates. It cannot read where the crystal actually is mid-frame and correct in real time. That fundamental limitation is why inverse ghosting has no clean engineering solution, only careful table calibration and a reluctant compromise between response speed and overshoot.