Monitor overdrive: ghosting, overshoot and the right setting

Something follows moving objects on your screen. Turn quickly in a game or drag a window, and edges leave a faint smear, or a thin bright or dark outline that was never in the picture. Your monitor's menu has a setting for exactly this. It may be called overdrive, Trace Free, AMA or something else, and it offers a few levels with no explanation.

The right level depends on your panel and on the refresh rate you play at, and the strongest one is not automatically the best. A test pattern settles it.

Two different response times

A response time on the box can mean two things. GtG (grey-to-grey) is how long a pixel takes to change from one colour to another. MPRT (moving picture response time) is how long a pixel stays continuously visible, which on an ordinary monitor is limited by how long each refresh and each frame stays on screen (Blur Busters).

They cause different blur. Slow GtG leaves a trail of the old colour behind an edge. Long MPRT smears the whole moving object, because your eyes follow it while each frame holds still; Blur Busters calls this the sample-and-hold effect and notes that, without strobing, doubling the frame rate and refresh rate halves it. Overdrive works on GtG; Blur Busters adds that it can reduce motion blur only very slightly, and only up to the limit sample-and-hold sets.

That is also why "1 ms" on the box says little. Blur Busters' FAQ has a section titled "1ms Response" Is Useless Without Specifying GtG or MPRT. Manufacturers measure GtG between the 10% and 90% points of a transition, and some advertise only the fastest transition they tested. As the input lag no software can fix says, these figures are not comparable across brands.

What overdrive does

Overdrive drives each pixel with a higher voltage so it reaches the new colour sooner (Blur Busters). Too little, and you see ghosting: a trail in the old colour. Too much, and the pixel overshoots its target and bounces back, which shows as overshoot, also called inverse ghosting or coronas: a bright or dark fringe in a colour the picture never had. Blur Busters describes the trade-off plainly: low settings can cause ghosting, high settings can cause coronas.

RTINGS, which tests the overdrive settings of the monitors it reviews, agrees: "Often, the strongest setting on a monitor has the most overshoot, which results in distracting inverse ghosting. If that bothers you, try using a lower setting." (RTINGS) RTINGS also notes that VA panels are typically slower in dark transitions, which shows as black smearing, and that some VA monitors compensate with overdrive at the cost of overshoot in bright scenes.

Makers name the control differently: Blur Busters lists ASUS's Trace Free, BenQ's AMA and Acer's Overdrive. Names and step counts are each maker's choice, and Blur Busters notes that artifacts may show up at lower or higher settings on another monitor, so Medium on one monitor tells you nothing about Medium on another. Judge the picture, not the label.

Why the right level can change with refresh rate

Blur Busters notes that different monitors and refresh rates can have different overdrive strengths. RTINGS tests at the maximum refresh rate, 120 Hz and 60 Hz, and publishes its recommended overdrive setting at each, "even if the setting is different between refresh rates."

It also matters with G-SYNC or FreeSync, where the frame rate keeps changing and the panel runs across a range of refresh rates instead of one (G-SYNC, FreeSync and V-Sync). RTINGS runs a separate VRR test that plots each overdrive setting from 60 Hz up to the maximum and identifies the one that performs best across the whole range. It also notes that some monitors lock out the overdrive settings while VRR is enabled.

Some monitors adjust overdrive themselves. NVIDIA lists variable overdrive among the features of monitors with an NVIDIA G-SYNC processor (NVIDIA), and says the Adaptive Overdrive in its G-SYNC Pulsar monitors "modulates overdrive based on both screen location and refresh rate". G-SYNC Compatible monitors do not use NVIDIA processors, so for those the monitor maker's own description is the one to read.

Judge it yourself

TestUFO's ghosting test is built for this: in its own words it "visually reveals display motion blur, ghosting artifacts, overdrive artifacts", and Blur Busters used it for its overdrive comparison photos.

Let the monitor warm up. RTINGS says an LCD responds faster after a warm-up than straight after power-on, and it warms monitors for 30 minutes before testing.
Set the refresh rate you play at and make sure it is applied (monitor stuck at 60 Hz).
Open the test and, as TestUFO advises, close other apps and browser tabs.
Step through the overdrive levels in the monitor's menu. A trail in the old colour behind the moving object is ghosting; a bright or dark outline in a colour that was not there is overshoot.
Keep the level where the trail no longer bothers you and no halo has appeared yet. RTINGS' own advice is to choose whichever looks best to you.
If you play with G-SYNC or FreeSync, repeat the check at a lower refresh rate, as RTINGS does at 120 Hz and 60 Hz, and prefer a level that looks acceptable at both.

If a review measured your exact model, its recommended setting is a starting point, not a rule.

Backlight strobing: ULMB, DyAc and ELMB

Strobing goes after the other kind of blur. The backlight is turned off between refreshes, which shortens how long each frame is visible and hides pixel transitions in the dark periods; ULMB (NVIDIA), DyAc (ZOWIE) and ELMB (ASUS) are names for it. The costs: RTINGS notes that some people are bothered by the flicker, and because a backlight lights the whole screen at once while the pixels change line by line, part of the screen can show double images, which NVIDIA calls crosstalk. Strobing and variable refresh can also exclude each other: ASUS says ELMB cannot be enabled with Adaptive-Sync or FreeSync at the same time, and NVIDIA's ULMB 2 asks you to disable G-SYNC. Some models combine the two, such as ASUS's ELMB Sync and NVIDIA's G-SYNC Pulsar. ASUS adds that ELMB Sync's strobing "will greatly reduce the overall brightness of the screen" and that ELMB Sync cannot be used at the same time as HDR, so check the manual for yours.

Why OLED behaves differently

An OLED has no backlight and turns each pixel on and off itself, which is why RTINGS calls its response time near-instantaneous. There is no slow transition for overdrive to hurry along. What is left is persistence: Blur Busters points out that most 60 Hz displays, OLEDs included, keep each frame visible for at least 1/60 of a second, 16.7 ms, so an OLED still blurs in motion. Unless the monitor inserts black frames between refreshes, motion clarity on an OLED follows frame rate and refresh rate.

What ClearMR tells you

VESA's ClearMR grades motion blur by "the ratio of clear pixels to blurry pixels", in tiers from ClearMR 3000 to 21000. VESA tests in the factory default mode, at the maximum refresh rate and native resolution, with backlight strobing disabled (ClearMR FAQ). A tier therefore describes the monitor as it ships, at full refresh rate: useful when comparing monitors, not a verdict on the overdrive level you pick or on lower refresh rates under VRR.

What does not help

The strongest level because it sounds fastest. RTINGS finds that the strongest setting often has the most overshoot.
Comparing "1 ms" claims between brands. Without GtG or MPRT, and without the method, the number cannot be compared.
Looking for it in Windows or the graphics driver. Overdrive is set in the monitor's on-screen menu, and the panel side of the chain is out of reach for software on the PC (the input lag no software can fix).
Expecting overdrive to remove all motion blur. It shortens transitions; the blur of a frame held still for a whole refresh needs a higher frame rate and refresh rate, or strobing.

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