Info: Explanation of LCD Response Time - Overclock.net

stargate125645 · 06-03-06

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.

Terminology & Explanation:
Response time is simply the time it takes for the monitor (LCD specifically in this case) to completely redraw the screen with a new frame. The faster the response time, the faster the monitor redraws 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 afore mentioned 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 monitor 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 is used to speed up monitor frame generation. (A very detailed and technical explanation of how LCD monitors 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. Many users claim to see ghosting in games where they scroll fast with their mouse, which quickly changes the view of the 3D environment. If your response time is not fast enough, then parts of the previous frame will still exist while you are viewing pieces of the new frame, hence the ghosting effect. Some will argue that ghosting is more a product of the human eye mixing multiple successive images in conjunction with the LCD monitor, 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 TVs employ filters and faster refresh rates to "clean up" the images between frames in an attempt to eliminate artifacts. Since this is not possible with most LCD monitors, the easiest way to fix ghosting is framerate control. Ensuring that the LCD monitor has enough time to refresh the image completely will help mitigate any chances of you noticing artifacts since the monitor has more time to display 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) displayed by monitor. This gives each frame 1/30 = 0.033s (seconds) to display. 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 monitor is less than 33ms, there should not be ghosting, assuming the monitor always operated at its reported response time. But because the monitor does not necessarily operate at the manufacturer-specified response time continuously, one should also add a "buffer zone" of extra time for the monitor to change between frames as compensation.

So, the solution to ghosting, if viable, is simply to make sure your framerate is such that your monitor 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 monitors easily if we manage our framerates properly - we don't have to purchase super-fast "gaming" LCD monitors in hopes of preventing ghosting, which can mean sacrificing monitor 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 monitor's refresh rate. A typical refresh rate of 60Hz implies 60fps (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 monitors 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 monitor may be the only viable solution if you are experiencing ghosting (though on a PC you might have the option to turn up the graphics to reduce your framerate such that it is slightly slower than your monitor's response time, thereby eliminating any ghosting).

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 monitor received new frames to display faster than it is capable of displaying them. In other words, the refresh rate for the monitor is too slow to cope with the higher framerate the graphics card is telling it to display. This phenomena, sharing some of the same causes as ghosting, can also be fixed by controlling the framerate as discussed earlier. In my experience, tearing occurs during scenes where the environment is very simple to display rather than during fast-paced action. This further supports the idea that tearing occurs due to a monitor inability to display frames as fast as the graphics card outputs them, as simple-to-render scenes would naturally result in a much higher framerate.

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.

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 monitor's contribution to lag time would then be the time necessary for the received signal to be processed and sent to be displayed (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 monitor is very hard to come by, it is generally not something that is considered when purchasing a monitor, and is almost never mentioned in any reviews I have seen. Despite this, your best bet would be to read reviews of any monitor your are looking to buy (which you should do anyway) and look for any mention of a noticeable time delay between when the image is drawn and when the command was initially sent by the user.

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. 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.

In closing, I would just like to add that there are many factors that affect a monitor'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 monitor, there may be more factors that affect a monitor's performance to which I am not fully aware (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.

stargate125645 · 06-09-06

My first FAQ! Maybe it should go into my baby book...

**SpookedJunglist** · 06-09-06

You might want to add that the pixel response time that is quoted from the manufacturer is usually the time it takes to go from true black to true white to true black again .

This is a link I found on the subject. It is highly technical and might be of some help if you want to update your FAQ
XBIT labs on response time