MPRT Explained – Monitor Makers Lie To You

Monitor manufacturers lie to you. A lot, in fact I’d argue it’s one of the markets where their lies are most prevalent, and one of the biggest bugbears of mine is the “MPRT” or Moving Picture Response Time figure most often plastered everywhere from the box to the product page. They get away with this because monitors, or response times in particular, are an area that many reviewers don’t cover and most gamers aren’t well informed about. So it turns out I’m trying to fix that, both by developing my open source response time tool (OSRTT), and by making videos like this so let me explain.

The moving picture response time figure quoted by monitor manufacturers is, at best, a niche theoretical measurement, and at worst is a purposefully misleading marketing tool designed to trick you into thinking their product is better than it actually is – sometimes by a horrendous margin. It’s really important that you understand that MPRT is not a panel measurement. It’s not a measure of a physical property or characteristic, it’s a trick enabled by a mode that gives you eye strain and headaches. It is NOT the same as the Grey-to-grey response time, or pixel response time figures you see people like myself, Hardware Unboxed or TFT Central publish.

In short, moving picture response time is a measure of how long a frame is on screen for. All of the modern LCD and OLED style displays are what’s called “Sample and Hold” displays, meaning they take the frame, draw it on screen, then hold it there until the next frame is ready. But it’s that holding that is actually kind of a problem. Let’s say you are watching a video of a basketball being thrown in a parabolic arc from left to right. At 30 frames per second, you’ll see the basketball move every 33.3ms. In slow motion that would look like this, the ball skipping quite a long distance between frames. Compare that to the same clip at 60 FPS, which is now just 16.7ms per frame, it’s skipping, well, half as far.

Now here’s the trick. Your brain makes a whole load of assumptions based on what it sees, like where it expects an object to be next. In a simplified graph you can see how an object travelling diagonally upwards is going to keep moving “in real terms”, but that movement can’t be displayed at just 60 FPS. That means your eyes have to skip around to find the new position, meaning you perceive the object as more blurry than it is. Don’t believe me? Head to the Blur Busters UFO site and look for yourself. Head to the ghosting option, let it sync, then move your head in time with the UFO across the screen and look just how blurry it looks. Then change your refresh rate down to the lowest it will do and try again, and it will look much more blurry.

The moving picture response time is the exact same thing as persistence time, or frame time. At 60Hz that’s 16.7ms, at 144Hz that’s 6.7ms, and at 360Hz that’s 2.8ms. So how the hell are manufacturers quoting 1ms, or even sub-1ms MPRT times? Well that’s the trick. Instead of making the panel draw more frames, they are just turning the backlight off for all bar 1ms per frame. As a graph it would look like this – a lot like a PWM square wave, because in essence that’s all it is. The backlight is turned on at the start of the frame time, the panel tries its best to actually draw the frame, but just 1ms later the backlight is turned off and you see black. Rinse and repeat for the next frame, and so on.

There are a few problems with this trick though, primary of which for me is eye strain and headaches. Personally I can’t look at a monitor with that kind of flickering without getting a blinding headache and strained eyes. There is a reason why monitor manufacturers all had a massive push over the last decade to produce “flicker-free” monitors, ones without PWM controlled backlights, because it’s terrible for you.

To add to that, because you are turning the backlight off for a large majority of the frame time, our brains perceive that as a more dim display. Some monitors try to counteract that by cranking the backlight brightness up dramatically so the small amount of time the light is actually being emitted is bright enough to offset the complete lack of light the rest of the time, but in the vast majority of monitors you’ll just be getting a less-bright display.

The final problem is how the actual response time measurement, the pixel response time, how long the liquid crystals in the panel take to move to the right position, interacts with this strobing. I’m yet to test an LCD display where the pixel response time is consistently below 4 or 5 ms. That means if the monitor is only turning the backlight on for the first 1ms of a frame, what you’ll see is just the start of the transition. In graph-form that would look like this. You’d only see this much of a transition, or functionally nothing.

Luckily since you’ll never experience overshoot, the monitor can be fitted with an extra overdrive profile which if you were to keep the backlight on might look like this, but since what you see is cut short you’ll get closer to seeing the actual output instead. It’s still imperfect though.

To make it clear, that kind of backlight strobing / black frame insertion type effect does improve the visual clarity of a display. You will be able to perceive a more crisp image with smoother motion and that can translate into a better experience for gaming. It just has its drawbacks.

It’s also worth noting that thanks to the time the backlight needs to be off for, the vast majority of displays don’t support variable refresh rate – where your graphics card controls when the display refreshes (within the VRR limits) – at the same time as a black frame insertion mode like ULMB, Asus’ ELMB mode or AOC’s MBR mode.

So, next time you are shopping for a monitor and it lists a “1ms MPRT” response time you’ll now know that unless it’s a 1000Hz display, that’s only saying they have a black frame insertion mode you probably don’t want to use. The “actual” pixel response time is what matters for your visual experience when comparing two otherwise similar displays, say two 165Hz 1440p IPS monitors, which I’m happy to say you’ll be able to do more easily as more reviewers are now actually testing response times and displaying those results in their reviews!