Display Motion Blur - Cause and Solution

The afterimage on a display means happening distortion of image. The reason of this afterimage used to be an optical illusion or problems of a movie or a display. LCD often has a problem of afterimage and it is called 'motion blur'. It is phenomenon to be shown a blur on the edge of a video when frames are changed. The motion blur can be divided two. First is caused by that a subject is moved before a camera focuses it and it is called 'captured motion blur'. The other is 'displayed motion blur' and it is generated by driving of a display. In this article, you can get useful information why 'displayed motion blur' is happened and how pioneers can overcome this problem.

Left : Original Image,  Right : with Motion Blur

Why is motion blur happened on LCD?

The motion blur is a phenomenon to be shown a blur on the edge because of overlaping images between privios one and present one. It means than display can not divide continous images correctly. We often listen an information of motion blur that PDP is better than LCD and it is the reason that PDP is better when watching an action movie. Then why is the phenomenon of motion blur specially discussed on LCD?

1) Motion blur by responsive time of liquid crystal

LCD uses polaraizer film and liquid crystal to adjust amount of light. When a voltage is generated on liquid crystal, it moves to change direction of light and polalozers make to pass a light as an its angle. At this time, liquid crystal needs time to move where you want. For example, it is supposed to be needed as 20ms to move it's position when 5V is supplied. And there is 120Hz display needing 8ms to express a frame. Then can this display show correctly a frame? 8ms looks shorter than 20ms liquid crystal needs, isn't it? In this case, there is a problem can't show a image fully when frame is changed to other frame.

2) Motion blur by an afterimage of eyes

Almost people used to think that motion blur is mainly happened because of slow responsive time of liquid crystal, but the afterimage of eyes remebering privious an image also is playing an important role to make the motion blur. When frames are changed, eyes remember privios a frame and it is seen as motion blur to be overlaped with current frame. That's why LCD's analog driving method keeping a same image during a frame time is more fragile than PDP's digital driving showing impulsive light.

How can LCD overcome these obstacles

Aforementioned, Although LCD has a weak structure about motion blur, this display has tried to improve the problem. Thanks to these efforts, motion blur was less recognized to viewers and it positively influenced LCD which could become the strongest display on TV market. Then, look into techniques to improve morion blur.

1) Improve problem by slow responsive time of liquid crystal 

- overdriving

What is there method to improve problem by responsive time of liquid crystal?

It can come to mind firstly that making new liquid crystal with fastly responsive time. There are many tries to be related that, it is hard to improve easily when considering cost / yield / reliability. When some problem need to be improved, nomally the most easy method is which change circuit area with remaining panel area. Through this point of view changing circuit area to supply different voltage, 'Over Driving' technique is generated.

The Graph shows overdriving is how to be realized

Usally responsive time means a change time from 10% to 90%. The thing which responsive time of liquid crystal would be slow means its change time is over a frame time. So, for revising responsive time, change time of liquid crystal to change 90% makes to be in a frame. For realizing this, an idea was derivated, which generates a higer voltge to the changing frame. When high voltage is generated to the liquid crystal, it has a feature to be changed steeply. Like above fiqure, supplying high voltage is at the first frame it changes and the voltage inteneded originally is supplied when frames shows same scenes. Although slow responsive time of liquid crystal, you can reach over its 90% change in a frame time by using this technique. Actually, it looks easy but there is a problem to be implemented. Because responsive time is diffrent among grays and it means different additioanl voltage is necessary every gray value. Anyway the algorithm to be looked easily plays really important role to revise motion blur on LCD.

2) Improved problem of motion blur by afterimage of eyes - Black data insertion and frame rate doubling

Like aformentioned, there is not only liquid crystal's slow responsibe time to be problemed by motion blur. By a feature of eyes remembering static image, motion blur used to be happened. How can this problem revise? LCD approached to two different method and you can get information through the below movie.



Did you catch diffternt method to imporve the motion blur? In the movie, first, compare a movie with motion blur than original one. And there is shown techniques 'black data insertion' and 'frame rate doubleling' controbuting to improve motion blur problem.

- Black Data Insertion / Scanning Backlight

The first method is impulse driving like CRT or PDP. It is not maintain a light for a frame time. it means showing a light during shorter time than a frame time. For the realization, black frame is inserted to between scenes. 'Scanning Backlight' is representative method to insert black frame on LCD and it is possible by turning on / off its backlight.

Black Data Insertion's basic concept

There is, however, demerit to need more light for showing same brightness during short time. (Link -> What's difference between LCD and PDP)

Like aforementioned, the technique 'Scanning Backlight' shows black image by turning off its backlight in middle of a frame time. It helps to refresh remembering image using black like impulse driving. This method is very effective and easy to implementation, but, it has a demerit to be lower power efficiency because it has to show more light as much as reducing a frame time.


- Frame Rate Doubling / MEMC

Frame Rate Doubling's basic concept. Let's compare between 60Hz and 120Hz

The other method is derived from an idea 'Raise frame refresh time to don't aware motion blur'. In other words, a number of frames are increased during same time and it means some images should insert to between existed frame. Then, input frequency is increased from 60Hz to higher frequency and it is decided by a number of insert images like 120Hz or 240Hz. For realization, there is a technique such as frame rate doubling and it is one of method MEMC. This method extracts the vector from images inputting 60Hz and estimates change of between vectors. From estimated result, a new frame is made by interpolating motion between two frames. This method needs complicated algorithom extracting the vector from images changing so fast and needs many memory ICs for buffering images. Also fast frame rate by this technique makes many demerits to raise cost and power consumption, to weak noise and to require changing many components like T-Con and Panel IC. But it has merit to get high power efficiency of backlight unit because it can maintain same brightnees for a frame time.
About competition to refresh frequency

The problem by slow responsive time of liquid crystal is almost overcomed. But motion blur by afterimage of eyes is still issue and many company are trying many technologies to jump off the obstacles. It is true the high frame rate helps to improve problems of motion blur on LCD because time previous image is shown is reduced and it means the time eyes remember image is also reduced. High frame rate of 240Hz makes considerable improvement as people don't catch this problem.

At this time, we need to detailed check about frame rate, in other words, frame refresh time. PDP uses sub filed driving method and it means 8~10EA subframe is shown through 600 times of sustain. Then they advertise it uses 600Hz driving. But, is it true to show 600EA different images?

TV set receives a frame image every 60Hz and it is also not real 60Hz. The original movie format is almost 24fps and multimedia player likes DVD player makes to change this format to 60Hz using 3:2 full down method it is just repeat image according to ratio. In other words, 60Hz TV receives is just 24frame image. Although PDP sustains 600times for a frame time, it is to shows just 24 different frames.

Then, we can get a question 240Hz makes really different 240 frames for displaying? Nobody say motion blur to PDP despite of its 60Hz driving. Then high speed driving is necessary? If it is true, what speed is suitable?

These questiones is connected to several problem from standard movie format to driving method. More detalied contents relaed above question will be shared soon. Good luck.