Motion interpolation is an optional feature that TVs use to increase the frame rate of content by adding intermediate frames between each actual frame sent by the source. The goal is to help with the appearance of motion and smooth out fast-moving objects, but if it doesn't work properly, there are a few downsides. High levels of motion interpolation also cause what's known as the soap opera effect, which not everyone may like. No motion interpolation feature is perfect, meaning that they can create some artifacts in more difficult scenes. When it works well, it's especially useful for reducing stutter in low frame rate content like movies and many shows.
We don't test motion interpolation directly anymore, but this article explains how we used to test for this subjectively and the pros/cons of using this feature on your TV.
Test Methodology Coverage
Our Motion Interpolation test was first added with our 0.9 test methodology. As part of our 2.2 test bench update, it was replaced with objective measurements of stutter using a high-speed camera, both with and without motion interpolation enabled instead. This objective approach provides a better way of comparing the effectiveness of the motion interpolation feature between different TVs. Learn how our test benches and scoring system work.
| Test | 0.9-2.1 | 2.2 |
|---|---|---|
| Motion Interpolation (30 fps) | ✅ | ❌ |
| Motion Interpolation (60 fps) | ✅ | ❌ |
| Sample And Hold | ❌ | ✅ |
| Sample And Hold (Interpolation) | ❌ | ✅ |
When It Matters
Motion interpolation is a feature found on just about every TV on the market. It's disabled by default in the most accurate picture mode on most TVs, but you can enable it with low frame rate content, like movies, TV shows, or sports.
Sensitivity to motion interpolation varies between people. Some don't even notice the difference until it's pointed out, but others see it right away. Overall, this feature isn't essential (most people will get by just fine without motion interpolation), but it could be a nice bonus for those who like it.
Why TVs Have Motion Interpolation
Before we explain how we used to test for motion interpolation, it's important to know what exactly it is and how it works. Most media is recorded at 24, 30, or 60 frames per second (fps). TVs, on the other hand, usually refresh their image 60 or 120 times per second. This is the refresh rate, and it's measured in Hertz (Hz). When there's a mismatch between a TV's refresh rate and the content frame rate, the TV needs to display the incoming frames multiple times. So if you're watching 30 fps content on a TV with a 120Hz refresh rate, the TV needs to display each incoming frame four times. This is a bit of an oversimplification, as most TVs can actually adjust their refresh rate to be closer to the incoming signal, but you get the idea.
As the TV holds each frame for a long time, this creates an effect known as stutter. Moving objects appear to jump as they move across the screen, and that motion isn't smooth.
Motion interpolation offers an alternative use for the difference between the video frame rate and the TV refresh rate. Instead of playing each frame multiple times, the TV's processor will enhance the frame rate of the video by creating transitional frames and inserting them between the originals. This makes the video match the TV's refresh rate and makes it so that the movement of the video looks smoother. The image above offers a basic idea of what this looks like. Essentially, the TV tries to guess what the frame would look like in between each actual frame (highlighted in red in the illustration above). The black frames are the ones that the TV generates itself; it tries to estimate the location of each object, so it's basically creating them out of nothing.
Limitations of our Testing
Our motion interpolation testing was very simplistic. It couldn't objectively measure how good the motion interpolation feature is because the effectiveness depends on the scene, so any judgment was entirely subjective. You can judge partially by the photos we publish, but it doesn't tell the whole story because it's a simple video with one object moving left to right.
Our Tests
We stopped directly testing motion interpolation as part of our 2.2 test bench update. The information in this section shows how we used to test motion interpolation and is provided for historical reference only, as a significant portion of the TVs we've tested over the years still have this section.
Motion Interpolation (30 fps)
To pass our 30 fps test, a TV had to be able to interpolate a 30 fps video by increasing the frame rate up to at least 60 fps. The results of this test reflected whether a TV could add smoothing to 30 fps video sources.
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To test for this capability, we played a 30 fps video on the TV and enabled the TV's motion interpolation feature. We took a pursuit photo and looked at the resulting image to determine if the TV passed or failed. Above left, you can see a photo of unsmoothed 30 fps video, and on the right, a picture of the same video after it has been interpolated to 60 fps on a 60Hz TV.
Motion Interpolation (60 fps)
Similar to our 30 fps interpolation test, this test instead looked for a TV's ability to smooth 60 fps videos. To pass, a TV needed to be able to increase a 60 fps video’s frame rate to 120 fps.
The results of this test reflected whether a TV can add smoothing to 60 fps video, like sports, some video games, and some streaming videos. Because 60 fps video is relatively rare, this test wasn't as important as our 30 fps interpolation test. It's worth noting that a TV that passed this test should be able to smooth 24 and 30 fps videos even more than a TV that only passes the 30 fps interpolation test. Only 120Hz TVs can interpolate 60 fps video because 60 fps content on a 60Hz TV matches the refresh rate without interpolation needed.
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To test for this, we played a 60 fps video on the TV and then saw if there was a motion interpolation setting that would make the video look smoother. Above left, you can see a photo of the unsmoothed 60 fps video, and on the right, a photo of the same video after it has been interpolated to 120 fps.
Additional Information
Motion Interpolation and Motion Blur
One of the main reasons people enable motion interpolation is to help with the appearance of motion. There are a few different factors that affect the appearance of motion. Response time is the time it takes for pixels to change from one color to the next, and a slow response time generally results in blur trails appearing behind fast-moving objects. However, enabling the motion interpolation feature doesn't change the response time at all. What the feature does is increase the frame rate so that each frame doesn't stay on the screen for a long time, which is the cause of stutter. Motion interpolation certainly helps with the appearance of motion when it works well, but it's not the end-all solution.
Motion Interpolation vs. Other Motion Settings
Judder-Free
With 24 fps content, there's an effect called judder, which is different from the stutter caused by 30 fps content. This is because 60 isn't a multiple of 24, so you can't simply interpolate 24 fps content up to 60 fps. TVs can remove 24p judder by multiplying some frames three times and others two times. However, removing 24p judder on 120Hz TVs is easier because the 24 frames are repeated five times each for 120 fps. It's different from motion interpolation because the TV isn't trying to guess what frames are in between, but they're repeating frames that they already have.
Black Frame Insertion
The black frame insertion feature on OLEDs, or backlight strobing on LEDs, aims to improve the appearance of motion blur by inserting black frames in between each frame to try to reduce the amount of motion blur. It's different from motion interpolation because it's not trying to create additional frames, but it's just adding black frames.
Motion Interpolation's Downsides
No feature on a TV is perfect, and there are always downsides. Motion interpolation is no exception. The main downsides are usually the reason why people don't use the feature.
Soap Opera Effect
Using motion interpolation causes an effect known as the soap opera effect. It's called this because it takes away from the traditional cinematic experience and makes content look more like soap operas on TV. It makes content almost too realistic-looking, and that's not how some people want to watch movies. Some don't like this, while others prefer it, so it's entirely up to you whether or not you use it.
Artifacts
It's especially noticeable when the motion interpolation features don't work because there are many artifacts. It's often noticeable in 'busy' scenes, meaning there's more action going on, and the TV struggles to keep up. How it performs depends on the scene; one TV might perform better with one show than another. Because the TV is trying to guess what's happening between frames, and if two subsequent frames are very different from each other, it will be hard for the TV to come up with frames in between. You may see artifacts like haloing. If the scene is really busy, the motion interpolation may stop working altogether or drop frames.
On the plus side, you'll rarely see these artifacts in slow scenes, like panning shots or talking scenes. Our test pattern was a straightforward video with our moving RTINGS logo, so it's easy for the TV to guess what's happening next. This is why we also check real content to see how the motion interpolation works with that.
Increases Input Lag
Most TVs don't offer motion interpolation in Game Mode, so if you want to use motion interpolation, you'll need to disable Game Mode, which dramatically increases input lag. Samsung is the only brand that offers motion interpolation in Game Mode, so input lag is low with the feature enabled.
How To Get The Best Results
Simply put, there's no way to improve the effectiveness and performance of the motion interpolation feature. Generally speaking, 120Hz TVs do a better job of interpolating lower-frame-rate content than 60Hz TVs, and usually, higher-end models perform better than entry-level ones. Since most brands have sliders for their motion interpolation settings, you can easily adjust to your preference. For our testing, we always set the sliders to their max to see how the TV performs. If you don't like the intensity that the motion interpolation is working at, you can set it lower. You should adjust it one step at a time and watch a bit of the content to feel how it works and see which setting you prefer the most.
Conclusion
Most 4k TVs have a motion interpolation feature to increase the frame rate of lower-frame-rate content. It helps with the appearance of motion because it displays content at the same frame rate as the TV. When it works well, it smooths out motion. However, motion interpolation may not be for everyone because it causes the soap opera effect, and it may cause some artifacts when it doesn't work well. We no longer test for motion interpolation directly, as we look at its impacts on motion handling overall through the stutter reduction metric. Motion interpolation features on TVs are optional, so whether you want to use it or not is entirely up to you.