What is Motion sharpness on televisions? The sharpness of the image you see on television is not only determined by the resolution of the screen. In fact, with fast moving images, a complex interaction between your eyes and the screen technology plays the most important role. What about terms such as Black Frame Insertion and Motion Interpolation, and how do they impact motion sharpness?
Motion sharpness and blur explained
To show you how complex the interaction between our eyes and the screen is, all you need to do is check out a few simple demos here . Focusing on a fixed point or focusing on a moving object makes a huge difference. When an object moves quickly across the screen, in many cases we see a vague outline around it, and a lot of detail is lost. That concept is called ‘motion blur’ (not to be confused with the motion blur that results from taking a photo with a slow shutter speed, so that moving objects leave a trail against a still background).
How does this effect arise? When an object moves in real life, it does so smoothly, and our eyes follow it in a smooth motion. However, the screen only shows a limited number of frames per second, for example 50 fps. That image is displayed for 20 ms and the transition from one frame to the next is not smooth, the object jumps one step (the faster it moves, the more it jumps). We call such a technique sample and hold. However, our brain expects movement, and therefore our eyes keep moving smoothly to follow it. Result: the object is then 1/50 e of a second fixed in one place, and as we move our eyes slide a little out of our central view point. We perceive that as a blur.
A second factor that contributes to motion sharpness is the response speed of the pixel. After all, a pixel cannot switch infinitely fast from one state to another. OLED screens switch very quickly in practice (0.1ms), which is fast enough to no longer be relevant. But LCD screens can still be affected by the response time. How did that happen?
Suppose a white plane slides over a black background. At the front of the white plane, the pixels should switch from black to white, and at the back of the plane they should switch from white to black. If the next frame is displayed, and the transition of pixels takes too long, the previous image is still slightly visible while the next image is already appearing. Obviously, that causes even more blur.
In some extreme cases, it can also cause colored borders around the moving object. After all, a pixel is made up of a red, green and blue sub-pixel. If the response times differ significantly, you will get a colored border. For example, if you see a blue line behind a moving object, this means that the blue reacts more slowly and therefore remains visible for longer.
All very difficult problems. How are these solved in practice?
Sharper images due to Black Frame Insertion
One of the most obvious solutions to motion blur can be found by looking at the cause. The blur occurs because our eyes glide over an object that is actually stationary. When we display that object for a shorter time, we see less blur. A possible solution is, for example, to temporarily black out the screen (more specifically the backlight). We call this ‘Black Frame Insertion’ or BFI. This technique is very effective, but it has a few drawbacks. First, the picture gets significantly darker, because we show a black screen for part of the time. And second, if the frequency with which we introduce black images is not high enough, we will see flicker in the image. We insert more black images, for example twice as many as we have frames,
Another term related to BFI is Backlight Scanning (BLS). Instead of dimming the entire screen, it dims it to pieces. This is possible because an LCD screen does not put each frame on the screen at once, but typically builds up from top to bottom. By dividing the backlight into, for example, 10 rows and synchronizing it with showing a new frame, you get even sharper images. Conceptually you can say that a backlight that is divided into 10 sections is the equivalent of 9 Black Frame insertions since every part of the screen is dark 90% of the time.
BFI and BLS are due to the high Hz ratings that you find on televisions. A manufacturer can use a 50Hz panel and combine it with BFI to then claim a 100Hz frame rate. Obviously that is not the same as a real 100Hz panel (which does not use BFI). However, that 100Hz panel can also be combined with BFI, and we are talking about a 200Hz frame rate. It gets even more complicated when BLS is used. A 100Hz panel combined with, for example, a 10-part BLS, is then referred to as a 1000Hz framerate. How manufacturers calculate these numbers is internal, and not always clear. It is therefore best never to compare Hz ratings or ‘motion ratings’ from manufacturers. Within the range of a manufacturer, that rating is indicative of better motion sharpness.
BFI or BLS are good gaming techniques. They do not cause any delay, but you have to include any flickering of the image and some darker images.
Another way to combat ‘motion blur’ is to show more images per second. Gamers undoubtedly know that it is best to get as many fps (frames per second) as possible to ensure a smooth gaming experience. That is why there are specific 120Hz monitors for gaming. And there are also 100Hz panels for television. Good examples of the effect of higher frame rates can be found here. They indicate what happens if you step from 30 to 60 fps, but the concept remains the same if you step from 60 to 120 or from 50 to 100. The more images you can display, the shorter the frame is visible, and the shorter your eyes move across a stationary image. The blur is then increasingly limited.
But a typical video contains only 50 frames per second (50 fps), and film even only 24 fps. Repeating the same image will not help you (eg 2x to go from 50 to 100 Hz) unless you do some form of Black Frame Insertion. So you have to get intermediate images. Since they are not in the video file, the television has to calculate it itself. But that does not always work perfectly. Simple, slow-moving images are rarely a problem. But complex things (fast moving things in front of a complex background, or explosions) can cause image artifacts. This is due to the fact that our videos are compressed and therefore not every frame is perfectly available. And to convert a film from 24 fps to 100 fps, the TV even has to calculate three intermediate images. Even despite powerful image processors, these image artifacts are rarely avoidable. They usually look like a strange block or seemingly transparent halo around the moving object.
Motion interpolation is avoidable for gamers as it causes delay.
The Soap Opera Effect
A lot of people hate motion interpolation for another reason. Yes, the images look much smoother and more detailed, but that’s exactly what bothers them. Movies are shot at 24 fps, which gives a very specific movie look. Motion interpolation converts the film to 50 or 100 fps and that removes the magic of film for some. The name ‘soap opera effect’ refers to cheap TV series (soaps) that were not recorded on expensive film, but on cheap video cameras, often at 60 fps (interlaced). Whether you use motion interpolation on your television is a matter of taste. We personally like it, but if you really can not watch it, you will always find a setting in the menus of the TV with which you can switch off the effect or make it less strong.
Settings in the menu
Each manufacturer uses its own terminology (unfortunately) to indicate settings that improve motion sharpness. At LG, this is TruMotion, Sony uses MotionFlow, Samsung Auto Motion Plus and Led Clear Motion, Philips has Perfect Natural Motion, and Panasonic calls it Intelligent Frame Control. In many cases you will find two subdivisions there, one to prevent motion blur (look for something like deblur, or clear motion) and one to prevent stuttering of the image (judder) (dejudder). The latter usually refers to forms of motion interpolation. We generally recommend using a mild combination of both, but as mentioned, personal taste should be the deciding factor here.
Motion sharpness on televisions: Conclusion
Motion sharpness is a complex matter. The combination of what our brain expects, our eye does and what the screen shows, results in blurred edges. These problems can be tackled with techniques such as Black Frame Insertion, Backlight Scanning and Motion Interpolation, but each solution has specific advantages and disadvantages.