This is not a complete article and is therefore posted here in HD Library until I can reach a final conclusion and version regarding the ins and outs of motion related artifacts. Most of this content is based on observed experience living with an LCD for over 60 days. At this time the main purpose is to provide a reference point for LCD reviews and consumers seeking relevant information. A great starting point is an article from Rodolfo, LCD Specs Playing with Your Eyes.
Motion blur is being used as a generic term to describe a multitude of different and independent motion artifacts that any display technology can suffer from or that any capturing system creates before ever getting to a display. The only reason it has come to the forefront is due to the unique sample and hold properties of direct view LCD display imaging, the very high contrast ratios direct view LCD provides along with detail motion blur, an artifact all displays can suffer from yet is far more degrading with direct view LCD and therefore considered a unique artifact of direct view LCD.
Motion Blur, Your Eyes and Frame Rate
The human visual system is sensitive to frame rate and based on testing peaks out at 60 frames per second. This is the magic number where most folks simply can’t tell a difference and higher frame rates have little to no benefit. Hard core PC gamers will likely disagree with that statement which is understandable yet their application is very different from ours of simply enjoying an image. The first step in understanding is that still images are not affected by frame rate or our eyes but objects in motion are.
Film is our reference point yet that uses 24 frames per second and with television and the old analog NTSC system we are limited to a maximum of 30 frames per second. On the surface this would seem to present a huge problem with content that is in motion but there is a catch; both of these formats introduce their own form of motion blur that is beneficial to the way we perceive images. In most cases images are captured as an open shutter directly related to the frame rate of the capturing device which means that each frame of an image is captured over time. An object in motion will leave a trail of motion blur following it directly related to the speed of the motion. While this may be an imaging artifact, in reality, with these lower frame rates, this is beneficial as it creates the illusion of smooth motion to our eyes. This blurring can be overcome by using a high speed shutter in front of the film or imaging device of a camera which opens and closes far faster than the frame rate being captured making it possible to create perfectly clear images of an object in motion frame by frame. Unfortunately using a high speed shutter creates another artifact with low frame rates called strobing and that is how your eye perceives it; a choppy, stuttering or strobing effect of the object in motion.
There is only one answer to this dilemma and that is a higher frame rate which allows us to capture the movement that would occur between frames. With HDTV this is finally possible for mass distribution and broadcasting by using the 720p format which allows 60 frames per second. With a high speed shutter engaged on the capturing device your eyes will not only perceive smooth motion but also better definition and clarity of the object in motion!
Real World Direct View LCD Testing
I just recently completed an intimate 60 days with a 46” LCD flat panel from Mitsubishi and the following information was derived from that experience.
Mitsubishi LT-46148 LCD HDTV - Review Essentials
Mitsubishi LT-46148 LCD HDTV - On the Test Bench
LCD Pixel Timing
Let’s get this particular LCD artifact out of the way first! This artifact creates tails on moving balls or multiple ghosts of moving objects. You can save this concern for inexpensive bottom of the line LCD displays or even some mid line; buyer beware. It is not an artifact with current state of the art LCD technology. While a 120hz processing feature is not a guarantee of ample LCD pixel speed in most cases the panel is fast enough for the manufacturer to justify the additional 120hz processing expense.
24 or 30 frame Content
This frame rate is simply not fast enough for any imaging creating a strobing effect at your local film cinemaplex or with any display. This frame rate represents the majority of content. The higher the contrast ratio the more pronounced it becomes making this a natural problem for top of the line LCD displays. Another artifact that comes with low frame rates is detail blur when capturing the image. 120hz processing can improve the response for both aspects removing those artifacts. 720p distribution of 30 and 24 frame content does not provide a solution since those sources are merely converted to 720p retaining their 30 and 24 native frame rates.
60 Frame Content
Only native 720p from capturing to distribution can provide this frame rate and is notoriously reserved for sporting events. With 120hz turned off such sources showed no obvious problems and there was nothing obvious to be gained from using 120hz processing. ESPN sports HD content never showed a problem but 1080i HD sports programming could due to the 30hz frame rate.
Detail Motion Blur
This is a very difficult artifact to catch! While testing with Digital Video Essentials Blu-ray or HQV Benchmark Blu-ray all I could find were rare and very quick errors that you had to be looking for. You will have to be intimately familiar with the material on a display that does not have the problem to catch it!
120hz Processing Artifacts
For high fidelity, matching the original, the best use of this processing is to add a black frame after each image frame. For 60 frame sources all you would so is add a black frame. For 30 and 24 frame sources you simply repeat the original frame and add a black frame. As noted in Rodolfo’s article the problem is this 120hz application reduces light output. In a world where the sales mode of a display is the first and foremost concern of the design process adding a black frame is not the most likely application to spend time on or provide lest your panel is viewed as dimmer when compared to the competition.
What manufacturers appear to have concentrated on is keeping all 120 frames active by deriving new frames from the originals that go in between the original frames or dumping the original frames altogether in favor of processed original frames that match up better with the additional new processed frames. A part of this process also attempts to remove motion blur from the original image. Ultimately this can only lead to the delivery of artificial imaging with the ultimate goal of improving the original. In a world of Videophiles and high fidelity this kind of 120hz LCD processing surely is counter intuitive! The golden rule is anytime you try to manipulate a signal the process to do so creates distortion; it is a pro / con equation. It is critical that the improvement far out weighs the artifacts created by the process
With the Mitsubishi LCD and their Smooth 120hz processing LCD motion artifacts were removed but the cost was an image that was not high fidelity. Smooth 120hz processing made a 30 year old film appear as if it had been just recently captured using a state of the art HD camera. The process made the natural motion of people and objects appear artificial. With images that had lots of motion the process would lose frame cadence lock creating the very motion artifact it was trying to remove.
At this time most reviewers agree that current 120hz processing leaves much to be desired and prefer not using the feature.
It’s all in the Cinematography!
How an image is directed and captured plays the biggest role in your ability to perceive motion artifacts from direct view LCD flat panel technology. A central focal point of interest in the image and the speed of objects in motion play the biggest role. The vast majority of such images for television and film use a central focal point, typically one or more characters in the story or an object in the image. The following examples are based not using 120hz processing, the feature is turned off, yet this feature did correct these errors.
As an example lets say two people are walking down a city street having a discussion and our camera is capturing this from the side following them. Our focus will naturally be on the characters and their dialogue yet in the background the buildings, sign posts and other people are horizontally panned. There won’t be motion blur of our characters but if your eye strays to the background you will likely see motion blur; due to the nature of this scene that is unlikely. For the most part this represents the vast majority of images we experience via television programming. There is a focal point or grounding for the image keeping our focus at that point on the screen. With common television programming I rarely noticed a problem!
Motion pictures represent the most likely content to provide motion blur. Far more attention is paid to the art of capturing images and evoking emotion from the visual experience. Complete horizontal and vertical pans of a room or outdoor scene can be common in which case there is rarely a focal point in the image along with our eyes being far more likely to wander to any place on the screen. Speed of the pan is important; low speeds rarely invoke the artifact but any high speed pan will. These kinds of images flesh out motion blur of frame rate or detail quite easily. The vast majority of motion blur I detected came from motion pictures via my DVD player!
Add to that your Viewing Distance and Contrast Ratio
Using the HD critical viewing distance of 3 screen heights the artifacts from 24 and 30 frame content was very apparent if not irritating. In the upstairs system though normal viewing distance for the 46” LCD was 5 screen heights with another viewing position at 7 screen heights. Both greatly reduced the impact of 24 and 30 frame strobing. Viewing distance will play a huge role in your perception of any LCD motion artifacts and for the general public 5-10 screen heights is quite normal. Very few customers or clients use 3-4 screen heights in their applications.
LCD displays though are unique due to the sample hold method of reproduction along with very high contrast ratios for both on/off and intra-field and the combination makes strobing with 24 and 30 frame content far more apparent! Turning down the bias lighting as provided by the Mitsubishi did improve on this aspect but do you really want to forsake all that dynamic range? I didn’t. Again, viewing distance helps a lot.
Conclusion
Motion blur is not as self evident as one might expect. If it were it would have been an issue for display imaging well before LCD flat panels. LCD displays though are unique due to the sample hold method of reproduction along with very high contrast ratios for both on/off and intra-field and the combination makes strobing with 24 and 30 frame content far more apparent! Even though LCD flat panels suffer from motion blur regularly the real question is whether or not you will notice in your application and even if you do the question becomes deeper based on how often you notice the problem with the sources and content you regularly view. Based on my own experience and other reviewers the current application of 120hz processing to remove these artifacts may work but so far it also introduces new artifacts due to the process.
Motion blur is being used as a generic term to describe a multitude of different and independent motion artifacts that any display technology can suffer from or that any capturing system creates before ever getting to a display. The only reason it has come to the forefront is due to the unique sample and hold properties of direct view LCD display imaging, the very high contrast ratios direct view LCD provides along with detail motion blur, an artifact all displays can suffer from yet is far more degrading with direct view LCD and therefore considered a unique artifact of direct view LCD.
Motion Blur, Your Eyes and Frame Rate
The human visual system is sensitive to frame rate and based on testing peaks out at 60 frames per second. This is the magic number where most folks simply can’t tell a difference and higher frame rates have little to no benefit. Hard core PC gamers will likely disagree with that statement which is understandable yet their application is very different from ours of simply enjoying an image. The first step in understanding is that still images are not affected by frame rate or our eyes but objects in motion are.
Film is our reference point yet that uses 24 frames per second and with television and the old analog NTSC system we are limited to a maximum of 30 frames per second. On the surface this would seem to present a huge problem with content that is in motion but there is a catch; both of these formats introduce their own form of motion blur that is beneficial to the way we perceive images. In most cases images are captured as an open shutter directly related to the frame rate of the capturing device which means that each frame of an image is captured over time. An object in motion will leave a trail of motion blur following it directly related to the speed of the motion. While this may be an imaging artifact, in reality, with these lower frame rates, this is beneficial as it creates the illusion of smooth motion to our eyes. This blurring can be overcome by using a high speed shutter in front of the film or imaging device of a camera which opens and closes far faster than the frame rate being captured making it possible to create perfectly clear images of an object in motion frame by frame. Unfortunately using a high speed shutter creates another artifact with low frame rates called strobing and that is how your eye perceives it; a choppy, stuttering or strobing effect of the object in motion.
There is only one answer to this dilemma and that is a higher frame rate which allows us to capture the movement that would occur between frames. With HDTV this is finally possible for mass distribution and broadcasting by using the 720p format which allows 60 frames per second. With a high speed shutter engaged on the capturing device your eyes will not only perceive smooth motion but also better definition and clarity of the object in motion!
Real World Direct View LCD Testing
I just recently completed an intimate 60 days with a 46” LCD flat panel from Mitsubishi and the following information was derived from that experience.
Mitsubishi LT-46148 LCD HDTV - Review Essentials
Mitsubishi LT-46148 LCD HDTV - On the Test Bench
LCD Pixel Timing
Let’s get this particular LCD artifact out of the way first! This artifact creates tails on moving balls or multiple ghosts of moving objects. You can save this concern for inexpensive bottom of the line LCD displays or even some mid line; buyer beware. It is not an artifact with current state of the art LCD technology. While a 120hz processing feature is not a guarantee of ample LCD pixel speed in most cases the panel is fast enough for the manufacturer to justify the additional 120hz processing expense.
24 or 30 frame Content
This frame rate is simply not fast enough for any imaging creating a strobing effect at your local film cinemaplex or with any display. This frame rate represents the majority of content. The higher the contrast ratio the more pronounced it becomes making this a natural problem for top of the line LCD displays. Another artifact that comes with low frame rates is detail blur when capturing the image. 120hz processing can improve the response for both aspects removing those artifacts. 720p distribution of 30 and 24 frame content does not provide a solution since those sources are merely converted to 720p retaining their 30 and 24 native frame rates.
60 Frame Content
Only native 720p from capturing to distribution can provide this frame rate and is notoriously reserved for sporting events. With 120hz turned off such sources showed no obvious problems and there was nothing obvious to be gained from using 120hz processing. ESPN sports HD content never showed a problem but 1080i HD sports programming could due to the 30hz frame rate.
Detail Motion Blur
This is a very difficult artifact to catch! While testing with Digital Video Essentials Blu-ray or HQV Benchmark Blu-ray all I could find were rare and very quick errors that you had to be looking for. You will have to be intimately familiar with the material on a display that does not have the problem to catch it!
120hz Processing Artifacts
For high fidelity, matching the original, the best use of this processing is to add a black frame after each image frame. For 60 frame sources all you would so is add a black frame. For 30 and 24 frame sources you simply repeat the original frame and add a black frame. As noted in Rodolfo’s article the problem is this 120hz application reduces light output. In a world where the sales mode of a display is the first and foremost concern of the design process adding a black frame is not the most likely application to spend time on or provide lest your panel is viewed as dimmer when compared to the competition.
What manufacturers appear to have concentrated on is keeping all 120 frames active by deriving new frames from the originals that go in between the original frames or dumping the original frames altogether in favor of processed original frames that match up better with the additional new processed frames. A part of this process also attempts to remove motion blur from the original image. Ultimately this can only lead to the delivery of artificial imaging with the ultimate goal of improving the original. In a world of Videophiles and high fidelity this kind of 120hz LCD processing surely is counter intuitive! The golden rule is anytime you try to manipulate a signal the process to do so creates distortion; it is a pro / con equation. It is critical that the improvement far out weighs the artifacts created by the process
With the Mitsubishi LCD and their Smooth 120hz processing LCD motion artifacts were removed but the cost was an image that was not high fidelity. Smooth 120hz processing made a 30 year old film appear as if it had been just recently captured using a state of the art HD camera. The process made the natural motion of people and objects appear artificial. With images that had lots of motion the process would lose frame cadence lock creating the very motion artifact it was trying to remove.
At this time most reviewers agree that current 120hz processing leaves much to be desired and prefer not using the feature.
It’s all in the Cinematography!
How an image is directed and captured plays the biggest role in your ability to perceive motion artifacts from direct view LCD flat panel technology. A central focal point of interest in the image and the speed of objects in motion play the biggest role. The vast majority of such images for television and film use a central focal point, typically one or more characters in the story or an object in the image. The following examples are based not using 120hz processing, the feature is turned off, yet this feature did correct these errors.
As an example lets say two people are walking down a city street having a discussion and our camera is capturing this from the side following them. Our focus will naturally be on the characters and their dialogue yet in the background the buildings, sign posts and other people are horizontally panned. There won’t be motion blur of our characters but if your eye strays to the background you will likely see motion blur; due to the nature of this scene that is unlikely. For the most part this represents the vast majority of images we experience via television programming. There is a focal point or grounding for the image keeping our focus at that point on the screen. With common television programming I rarely noticed a problem!
Motion pictures represent the most likely content to provide motion blur. Far more attention is paid to the art of capturing images and evoking emotion from the visual experience. Complete horizontal and vertical pans of a room or outdoor scene can be common in which case there is rarely a focal point in the image along with our eyes being far more likely to wander to any place on the screen. Speed of the pan is important; low speeds rarely invoke the artifact but any high speed pan will. These kinds of images flesh out motion blur of frame rate or detail quite easily. The vast majority of motion blur I detected came from motion pictures via my DVD player!
Add to that your Viewing Distance and Contrast Ratio
Using the HD critical viewing distance of 3 screen heights the artifacts from 24 and 30 frame content was very apparent if not irritating. In the upstairs system though normal viewing distance for the 46” LCD was 5 screen heights with another viewing position at 7 screen heights. Both greatly reduced the impact of 24 and 30 frame strobing. Viewing distance will play a huge role in your perception of any LCD motion artifacts and for the general public 5-10 screen heights is quite normal. Very few customers or clients use 3-4 screen heights in their applications.
LCD displays though are unique due to the sample hold method of reproduction along with very high contrast ratios for both on/off and intra-field and the combination makes strobing with 24 and 30 frame content far more apparent! Turning down the bias lighting as provided by the Mitsubishi did improve on this aspect but do you really want to forsake all that dynamic range? I didn’t. Again, viewing distance helps a lot.
Conclusion
Motion blur is not as self evident as one might expect. If it were it would have been an issue for display imaging well before LCD flat panels. LCD displays though are unique due to the sample hold method of reproduction along with very high contrast ratios for both on/off and intra-field and the combination makes strobing with 24 and 30 frame content far more apparent! Even though LCD flat panels suffer from motion blur regularly the real question is whether or not you will notice in your application and even if you do the question becomes deeper based on how often you notice the problem with the sources and content you regularly view. Based on my own experience and other reviewers the current application of 120hz processing to remove these artifacts may work but so far it also introduces new artifacts due to the process.
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