Introduction:
Prior to writing this FAQ, I was in search of a new monitor (as of the date of this FAQ I have purchased a 20.1" widescreen LCD monitor, 8ms response time). The main thing that puzzled me when reading people's advice was response time and the differing beliefs on exactly what response time is acceptable for gaming, etc. I decided to do some "logical thinking" on the subject in an attempt to figure out, in my own mind, what exactly the response time constitutes, and what affects it. In doing my research, I also came across a few other items that are pertinent to monitors and I have included them in this FAQ as well. With that said, I do not claim to be an expert on this subject so if I have mistakes in my train of thought, feel free to give suggestions and/or corrections - feel free to give them even if you don't think I have a mistake.
Edit: Since the original writing of this FAQ, I have expanded it to include much more than just the LCD response time. I hope you can now view this FAQ as a good place to find a brief but thorough overview of the most important LCD terms and things to consider when shopping for a new LCD monitor or TV.
Terminology & Explanation:
Response time is simply the time it takes for a pixel in the display (LCD specifically in this case) to change color for a new frame. The faster the response time, the faster the pixel changes and the display can redraw the entire frame. The response time reported by the manufacturer is typically the fastest achievable value, which was traditionally recorded as the black-to-white-to-black response time - something very impractical for the consumer as such a transition is rarely experienced by the user. More recently, manufacturers have been reporting their response times in terms of grey-to-grey, which is more realistic. As X-bit lab explains, black is the perceived color by the user with minimal brightness, white is with maximum brightness, and grey is any color in between (i.e. grey is not the traditional grey color we generally refer to, but any mixture of red, blue, and green that isn't the aforementioned black or white). Thus, the grey-to-grey response time is more descriptive of what users would actually encounter; however, it should be noted that the reported value by the manufacturer is probably still the fastest possible response time the display is capable of unless otherwise stated (e.g. some manufacturers will specify that the value given is an average), and not necessarily the response time that you or I would see in practice. In order to achieve today's ultra low 2ms response time, "overdrive" technology may be used to speed up display frame generation. A very detailed and technical explanation of how LCDs actually work can be found in this review of 7 monitors by X-bit labs.
Ghosting occurs when action is fast enough that the response time cannot keep up, so a drawn frame on the monitor may have artifacts left over from a previous render as the pixels were not able to respond fast enough before the next frame is to be rendered. (Note that CRT monitors have negligible response time so the human eye cannot notice the delay for pixel color changes like with some LCDs). These out of place pixels create the visual artifacts that users refer to as ghosting. An excellent explanation and comparison between CRTs and LCDs (particularly TFT panel LCDs), complete with an example of ghosting, can be found here in an article by Digital Silence. Some will argue that ghosting is more a product of the human eye mixing multiple successive images in conjunction with the LCD refresh rate, which is why ghosting is perhaps more noticeable when the scenery changes faster, such as in open environments (versus say a hallway), because any artifacts will be significantly different than the current scenery and thus more easily noticed.
Many LCD TVs employ filters and faster refresh rates (see the "Myths & Stigma" section) to "clean up" the images between frames in an attempt to eliminate artifacts. Since this is not present with most LCD monitors, the easiest way to fix ghosting is framerate control (obviously, this would also work on LCD TVs). Ensuring that the display has enough time to refresh the image completely will help mitigate any chances of you noticing artifacts since the display has more time to present each image and your eyes have more time to adjust to the changing image. For example, suppose you have a framerate of 30fps (frames per second). This means that every second, 30 frames are generated by your graphics card and (are attempted to be) presented by the display. This gives each frame 1/30 = 0.033s (seconds) to be fully shown. This can also be written as (0.033s)*(1000ms/s) = 33ms (milliseconds). Thus, each frame is displayed for 33ms. So, ideally speaking, if the response time of a display is less than 33ms, there should not be ghosting, assuming the display always operated at its reported response time. But because the display does not necessarily operate at the manufacturer-specified response time continuously, one should also add a "buffer zone" of extra time for the display to change between frames as compensation.
So, a simple solution to ghosting is to make sure the framerate fed to the display is such that it has enough time to generate each frame (including the buffer zone time) before the next one is sent from the graphics card. This means setting a maximum FPS for the game to generate. I am in no way saying that we can all go out and buy 25ms monitors and just make sure we have low framerates. What I am saying is that we can get by with slower-than-2ms displays easily if we manage our framerates properly - we don't have to purchase super fast "gaming" LCDs in hopes of preventing ghosting, which can mean sacrificing display quality in terms of settling for lower contrast ratios, brightness values, etc. Lower response times today are the norm, so ghosting is a much less talked-about issue; nevertheless, it is always good to know a possible fix should the problem arise!
Note that when you are doing something such as surfing the net, your framerate is essentially your display's refresh rate as the graphics card isn't going to be working very had and will have an easy time sending as many frames to the display as it will accept. A typical refresh rate of 60Hz implies 60fps (more on this later) assuming your graphics card can keep up, or 16.7ms per frame. Thus, one could expect to see ghosting when moving the mouse, etc., on displays with response times near that or higher. In addition, it is important to realize that you probably are not going to be able to cap the FPS for a console game, and this may also be an issue for some PC games. In cases such as these, a low response time LCD may be the only viable solution if you are experiencing ghosting (though on a PC you might have the option to improve the graphics to reduce your framerate).
Tearing is the appearance of a line or streak in the middle of the displayed image, giving the appearance that the image has been "ripped" into two or more pieces. The viewer is actually seeing two frames being rendered on the screen simultaneously: The previous frame and the current frame. This occurs when the frame buffer of the display is informed to begin drawing a new frame when it has not completed rendering the previous frame. This phenomena can also be fixed by controlling the framerate as discussed earlier to the point where the frame buffer is no longer overloaded. In my experience, tearing occurs during scenes where the environment is very simple to display rather than during fast-paced action, as in these situations the framerate is much higher and the display frame buffer is therefore more likely to be overloaded.
Another solution to tearing, and in some instances to ghosting (depending upon how lenient your eyes are to noticing artifacts), is to enable "v-sync" in the game. V-sync, or vertical synchronization, works by ensuring that no new frame is displayed until the previous frame has been entirely displayed (i.e. the frame buffer of the TV or monitor is clear and able to accept information about a new frame from the graphics card without the risk of mixing the new frame with the previous frame).
Lag time defines the amount of time that elapses between when a command is issued by the user and when the monitor starts to display the image. The display's contribution to lag time would then be the time necessary for the received signal to be processed and sent to the panel (versus the time to draw the image which is the response time). A higher lag time could lead to a noticeable delay between when the command is issued and when the result is viewed on-screen. Things such as image processing can contribute to the lag time, so disabling image processing should be one of the first trouble-shooting options explored when significant lag time is experienced. Very few manufacturers report this value, instead relying on the response time to define their product's value. Because the lag time value of a display is very hard to come by, it is generally not something that is considered when purchasing one, and is almost never mentioned in any reviews I have seen. Despite this, your best bet would be to read reviews of any display you are looking to buy (which you should do anyway) and look for any mention of input lag. An excellent, in-depth article covering input lag by Anandtech can be found here.
Example:
As a personal example, my (now old) Dell 15" monitor with a 25ms response time is essentially slow, but I can use it without ghosting because I set my FPS properly: I have games set to cap my FPS at 30. This gives each frame 33ms on-screen before it is overwritten, so my 25ms monitor has 8ms extra to slack off, regardless of how fast I move my mouse in-game, helping to compensate for the fact that my monitor will not always operate at its reported 25ms. My new monitor has an 8ms response time, and performing the above calculations in reverse yields (1frame)/(8ms)*(1000ms/s) = 125fps as my ideal, maximum framerate without ghosting (disregarding the refresh rate as being the true maximum framerate my monitor is capable of displaying; see below). If I set my maximum FPS lower than that, with a generous buffer zone, I am good to go! A benefit of capping the framerate is also that your graphics card does not work as hard, which not only reduces the operating temperature but can help reduce the overall load on your computer and improve temperatures around the board.
Myths & Stigma:
There are many myths associated with LCDs, and many ideas that people have that are simply false. Unfortunately, this misinformation gets passed along all too often so I will attempt to put as much of it to rest here as possible.
Perhaps the biggest piece of misinformation is the common belief that a monitor or TV can display more frames per second than its refresh rate allows. In actuality, a display can truly only present as many frames per second as its refresh rate (e.g. a 60Hz display can only show 60fps, and a true 120Hz display can only show 120fps), so providing a higher framerate than that is virtually worthless. In fact, exceeding the display's refresh rate can often lead to the tearing phenomena mentioned earlier, and is the reason vertical sync is an important tool.
It is important to realize that although a TV or monitor may be able to display 120 frames per second (or more), if it is not a "3D" display or a true 120Hz display it cannot accept more than a 60Hz signal. A 60Hz display simply receives a signal at x frames per second (but no more than 60), and either displays the same frame multiple times or interpolates to make the remaining frames up to its refresh rate. When implemented properly, this can lead to a perceived reduction in motion blur and studder and is indeed a useful technology; however, such a display still only accepts a 60fps signal at most. Displays that are 3D-capable may accept a 120Hz signal (i.e. some 3D displays can, and some cannot), but even then will often still interpolate or repeat frames to achieve refresh rates higher than 120Hz. This means that different 3D display models will interpret 3D signals differently depending upon what type of signal it accepts and how the 3D image is dispalyed to the viewer. Be absolutely sure you know the pertinent 3D specifications and capabilities of the display if you are buying it for 3D viewing to ensure that it meets your requirements and preferences, and to know what accessories, if any, you will need to buy in addition. Also make sure that if you desire a display truly capable of more than 60fps that you purchase a true 120Hz (or more) display (i.e. a display that will actually show all 120 frames and not interpolate or repeat frames). It should also be noted that a TV or monitor's processing engine (if one is present) will also play a big role in how an image appears. For example, high-end TV processing engines (such as Sony's Bravia XBR engine) will almost invariably lead to a better picture than the engines in low-end TVs (such as a Vizio). In other words: Features are great, but if the power to use those features well do not exist in the TV/monitor, then those same features are useless - and may even damage the picture quality. More information on 3D display technology, including potential hardware changes and requirements that accompany the use of a 3D display, can be found here in an FAQ by CNET.
In closing, I would just like to add that there are many factors that affect a display's performance, and many terms any consumer should know. Although I have explored and discussed several of the major factors and terms that will help you in understanding what you need to know when looking for a display, there are more factors that affect a display's performance (another reason why a generous buffer zone is recommended), so you should always do your own research on top of what you read here. Happy hunting!
Special Thanks:
I would like to thank SpookedJunglist for his suggestions and the review link.
Edited by stargate125645 - 2/7/11 at 6:48am
Prior to writing this FAQ, I was in search of a new monitor (as of the date of this FAQ I have purchased a 20.1" widescreen LCD monitor, 8ms response time). The main thing that puzzled me when reading people's advice was response time and the differing beliefs on exactly what response time is acceptable for gaming, etc. I decided to do some "logical thinking" on the subject in an attempt to figure out, in my own mind, what exactly the response time constitutes, and what affects it. In doing my research, I also came across a few other items that are pertinent to monitors and I have included them in this FAQ as well. With that said, I do not claim to be an expert on this subject so if I have mistakes in my train of thought, feel free to give suggestions and/or corrections - feel free to give them even if you don't think I have a mistake.
Edit: Since the original writing of this FAQ, I have expanded it to include much more than just the LCD response time. I hope you can now view this FAQ as a good place to find a brief but thorough overview of the most important LCD terms and things to consider when shopping for a new LCD monitor or TV.
Terminology & Explanation:
Response time is simply the time it takes for a pixel in the display (LCD specifically in this case) to change color for a new frame. The faster the response time, the faster the pixel changes and the display can redraw the entire frame. The response time reported by the manufacturer is typically the fastest achievable value, which was traditionally recorded as the black-to-white-to-black response time - something very impractical for the consumer as such a transition is rarely experienced by the user. More recently, manufacturers have been reporting their response times in terms of grey-to-grey, which is more realistic. As X-bit lab explains, black is the perceived color by the user with minimal brightness, white is with maximum brightness, and grey is any color in between (i.e. grey is not the traditional grey color we generally refer to, but any mixture of red, blue, and green that isn't the aforementioned black or white). Thus, the grey-to-grey response time is more descriptive of what users would actually encounter; however, it should be noted that the reported value by the manufacturer is probably still the fastest possible response time the display is capable of unless otherwise stated (e.g. some manufacturers will specify that the value given is an average), and not necessarily the response time that you or I would see in practice. In order to achieve today's ultra low 2ms response time, "overdrive" technology may be used to speed up display frame generation. A very detailed and technical explanation of how LCDs actually work can be found in this review of 7 monitors by X-bit labs.
Ghosting occurs when action is fast enough that the response time cannot keep up, so a drawn frame on the monitor may have artifacts left over from a previous render as the pixels were not able to respond fast enough before the next frame is to be rendered. (Note that CRT monitors have negligible response time so the human eye cannot notice the delay for pixel color changes like with some LCDs). These out of place pixels create the visual artifacts that users refer to as ghosting. An excellent explanation and comparison between CRTs and LCDs (particularly TFT panel LCDs), complete with an example of ghosting, can be found here in an article by Digital Silence. Some will argue that ghosting is more a product of the human eye mixing multiple successive images in conjunction with the LCD refresh rate, which is why ghosting is perhaps more noticeable when the scenery changes faster, such as in open environments (versus say a hallway), because any artifacts will be significantly different than the current scenery and thus more easily noticed.
Many LCD TVs employ filters and faster refresh rates (see the "Myths & Stigma" section) to "clean up" the images between frames in an attempt to eliminate artifacts. Since this is not present with most LCD monitors, the easiest way to fix ghosting is framerate control (obviously, this would also work on LCD TVs). Ensuring that the display has enough time to refresh the image completely will help mitigate any chances of you noticing artifacts since the display has more time to present each image and your eyes have more time to adjust to the changing image. For example, suppose you have a framerate of 30fps (frames per second). This means that every second, 30 frames are generated by your graphics card and (are attempted to be) presented by the display. This gives each frame 1/30 = 0.033s (seconds) to be fully shown. This can also be written as (0.033s)*(1000ms/s) = 33ms (milliseconds). Thus, each frame is displayed for 33ms. So, ideally speaking, if the response time of a display is less than 33ms, there should not be ghosting, assuming the display always operated at its reported response time. But because the display does not necessarily operate at the manufacturer-specified response time continuously, one should also add a "buffer zone" of extra time for the display to change between frames as compensation.
So, a simple solution to ghosting is to make sure the framerate fed to the display is such that it has enough time to generate each frame (including the buffer zone time) before the next one is sent from the graphics card. This means setting a maximum FPS for the game to generate. I am in no way saying that we can all go out and buy 25ms monitors and just make sure we have low framerates. What I am saying is that we can get by with slower-than-2ms displays easily if we manage our framerates properly - we don't have to purchase super fast "gaming" LCDs in hopes of preventing ghosting, which can mean sacrificing display quality in terms of settling for lower contrast ratios, brightness values, etc. Lower response times today are the norm, so ghosting is a much less talked-about issue; nevertheless, it is always good to know a possible fix should the problem arise!
Note that when you are doing something such as surfing the net, your framerate is essentially your display's refresh rate as the graphics card isn't going to be working very had and will have an easy time sending as many frames to the display as it will accept. A typical refresh rate of 60Hz implies 60fps (more on this later) assuming your graphics card can keep up, or 16.7ms per frame. Thus, one could expect to see ghosting when moving the mouse, etc., on displays with response times near that or higher. In addition, it is important to realize that you probably are not going to be able to cap the FPS for a console game, and this may also be an issue for some PC games. In cases such as these, a low response time LCD may be the only viable solution if you are experiencing ghosting (though on a PC you might have the option to improve the graphics to reduce your framerate).
Tearing is the appearance of a line or streak in the middle of the displayed image, giving the appearance that the image has been "ripped" into two or more pieces. The viewer is actually seeing two frames being rendered on the screen simultaneously: The previous frame and the current frame. This occurs when the frame buffer of the display is informed to begin drawing a new frame when it has not completed rendering the previous frame. This phenomena can also be fixed by controlling the framerate as discussed earlier to the point where the frame buffer is no longer overloaded. In my experience, tearing occurs during scenes where the environment is very simple to display rather than during fast-paced action, as in these situations the framerate is much higher and the display frame buffer is therefore more likely to be overloaded.
Another solution to tearing, and in some instances to ghosting (depending upon how lenient your eyes are to noticing artifacts), is to enable "v-sync" in the game. V-sync, or vertical synchronization, works by ensuring that no new frame is displayed until the previous frame has been entirely displayed (i.e. the frame buffer of the TV or monitor is clear and able to accept information about a new frame from the graphics card without the risk of mixing the new frame with the previous frame).
Lag time defines the amount of time that elapses between when a command is issued by the user and when the monitor starts to display the image. The display's contribution to lag time would then be the time necessary for the received signal to be processed and sent to the panel (versus the time to draw the image which is the response time). A higher lag time could lead to a noticeable delay between when the command is issued and when the result is viewed on-screen. Things such as image processing can contribute to the lag time, so disabling image processing should be one of the first trouble-shooting options explored when significant lag time is experienced. Very few manufacturers report this value, instead relying on the response time to define their product's value. Because the lag time value of a display is very hard to come by, it is generally not something that is considered when purchasing one, and is almost never mentioned in any reviews I have seen. Despite this, your best bet would be to read reviews of any display you are looking to buy (which you should do anyway) and look for any mention of input lag. An excellent, in-depth article covering input lag by Anandtech can be found here.
Example:
As a personal example, my (now old) Dell 15" monitor with a 25ms response time is essentially slow, but I can use it without ghosting because I set my FPS properly: I have games set to cap my FPS at 30. This gives each frame 33ms on-screen before it is overwritten, so my 25ms monitor has 8ms extra to slack off, regardless of how fast I move my mouse in-game, helping to compensate for the fact that my monitor will not always operate at its reported 25ms. My new monitor has an 8ms response time, and performing the above calculations in reverse yields (1frame)/(8ms)*(1000ms/s) = 125fps as my ideal, maximum framerate without ghosting (disregarding the refresh rate as being the true maximum framerate my monitor is capable of displaying; see below). If I set my maximum FPS lower than that, with a generous buffer zone, I am good to go! A benefit of capping the framerate is also that your graphics card does not work as hard, which not only reduces the operating temperature but can help reduce the overall load on your computer and improve temperatures around the board.
Myths & Stigma:
There are many myths associated with LCDs, and many ideas that people have that are simply false. Unfortunately, this misinformation gets passed along all too often so I will attempt to put as much of it to rest here as possible.
Perhaps the biggest piece of misinformation is the common belief that a monitor or TV can display more frames per second than its refresh rate allows. In actuality, a display can truly only present as many frames per second as its refresh rate (e.g. a 60Hz display can only show 60fps, and a true 120Hz display can only show 120fps), so providing a higher framerate than that is virtually worthless. In fact, exceeding the display's refresh rate can often lead to the tearing phenomena mentioned earlier, and is the reason vertical sync is an important tool.
It is important to realize that although a TV or monitor may be able to display 120 frames per second (or more), if it is not a "3D" display or a true 120Hz display it cannot accept more than a 60Hz signal. A 60Hz display simply receives a signal at x frames per second (but no more than 60), and either displays the same frame multiple times or interpolates to make the remaining frames up to its refresh rate. When implemented properly, this can lead to a perceived reduction in motion blur and studder and is indeed a useful technology; however, such a display still only accepts a 60fps signal at most. Displays that are 3D-capable may accept a 120Hz signal (i.e. some 3D displays can, and some cannot), but even then will often still interpolate or repeat frames to achieve refresh rates higher than 120Hz. This means that different 3D display models will interpret 3D signals differently depending upon what type of signal it accepts and how the 3D image is dispalyed to the viewer. Be absolutely sure you know the pertinent 3D specifications and capabilities of the display if you are buying it for 3D viewing to ensure that it meets your requirements and preferences, and to know what accessories, if any, you will need to buy in addition. Also make sure that if you desire a display truly capable of more than 60fps that you purchase a true 120Hz (or more) display (i.e. a display that will actually show all 120 frames and not interpolate or repeat frames). It should also be noted that a TV or monitor's processing engine (if one is present) will also play a big role in how an image appears. For example, high-end TV processing engines (such as Sony's Bravia XBR engine) will almost invariably lead to a better picture than the engines in low-end TVs (such as a Vizio). In other words: Features are great, but if the power to use those features well do not exist in the TV/monitor, then those same features are useless - and may even damage the picture quality. More information on 3D display technology, including potential hardware changes and requirements that accompany the use of a 3D display, can be found here in an FAQ by CNET.
In closing, I would just like to add that there are many factors that affect a display's performance, and many terms any consumer should know. Although I have explored and discussed several of the major factors and terms that will help you in understanding what you need to know when looking for a display, there are more factors that affect a display's performance (another reason why a generous buffer zone is recommended), so you should always do your own research on top of what you read here. Happy hunting!
Special Thanks:
I would like to thank SpookedJunglist for his suggestions and the review link.
Edited by stargate125645 - 2/7/11 at 6:48am
BladeRunner v2.2
(13 items) |
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BladeRunner v2.2
(13 items) |
