Input lag
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Input lag is a phenomenon associated with some types of LCD displays that refers to latency, or lag measured by the difference between the time a signal is inputted into a display and the time it is shown by the display. This lag time has been measured as high as 65ms, or the equivalent of 3-4 frames on a 60Hz display. Currently, the only TFT panels known to have this phenomenon are so-called overdrive panels. These include S-PVA, S-MVA, and Overdrive-TN panels. S-PVA have been observed to suffer from greater input lag than P-MVA panels, while IPS, S-IPS and AS-IPS panels are not or only minimally affected.
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[edit] Causes of input lag
While the pixel response time of the display is usually listed in the monitor's specifications, no manufacturers advertise the input lag of their displays, likely because the trend has been to increase input lag as manufacturers find more ways to process input at the display level before it is shown. Possible culprits are the processing overhead of HDCP, DRM, and DSP techniques employed to reduce the effects of ghosting, however no conclusive investigations have been performed, and the cause may vary depending on the model of display.
[edit] Testing for input lag
Input lag is easier "felt" than seen. As such, proving the existence of this phenomenon requires an elaborate set-up (at least for a home computer system). Tests for this phenomenon usually involve a test display (the display being measured), a control display (usually a CRT) that would ideally have no input lag, a computer capable of mirroring output to two displays, stopwatch software, and a high-speed camera pointed at the two displays running the stopwatch program. The lag time is measured by taking a photograph of the displays running the stopwatch software, then subtracting the two times on the displays in the photograph. This method only measures the difference in input lag between two displays and cannot determine the absolute input lag of a single display. CRTs are preferable to use as a control display because their input lag is typically negligible. Also, video mirroring does not guarantee that the same image will be sent to each display at the same point in time. However, since the results of this test are easily reproducible, even when the displays are plugged into different ports and different cards, suggests that the effect is attributable to the display and not the computer system.
[edit] Effects of input lag on users
Depending on the monitor, input lag times between 10ms and 65ms have been measured. However, the effects of the delay on the user depend on the user's own sensitivity to it. In the application of gaming, the type of game being played is another factor that may increase one's perception of lag. For instance, in a relatively slow MMORPG such as World of Warcraft, slight delays are far more tolerable than in medium paced tactical shooters like Counter-strike or especially in fast-action shooters such as Unreal Tournament.
Input lag contributes to the overall latency in the interface chain of the user's inputs (mouse, keyboard, etc...) to the graphics card to the monitor. Most sensitive users can tolerate latency under 20ms.
[edit] Input lag versus response time
LCD screens with a high response time value often do not give satisfactory experience when playing fast paced computer games. But an LCD screen with high response time AND significant input lag is unsuitable for playing fast paced computer games or performing fast high accuracy operations on the screen (e.g. CAD design). Manufacturers only state the response time of their displays and do not inform customers of the input lag value.
