Output lag

Output lag is a phenomenon associated with some types of LCD displays, and nearly all types of HDTVs, that refers to latency, or lag measured by the difference between the time a signal is output into a display and the time it is shown by the display. This lag time has been measured as high as 68ms [cite web |title=Face to Face cameras, printers, ... - DigitalVersus |date= |url=http://www.digitalversus.com/duels.php?ty=6&ma1=36&mo1=332&p1=3107&ma2=36&mo2=290&p2=2810&ph=12 |accessdate=2008-03-07 ] , or the equivalent of 3-4 frames on a 60 Hz 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 output lag than P-MVA panels, while IPS, S-IPS and AS-IPS panels are not or only minimally affected.Fact|date=January 2008

Causes of output lag

While the pixel response time of the display is usually listed in the monitor's specifications, no manufacturers advertise the output lag of their displays, likely because the trend has been to increase output lag as manufacturers find more ways to process output at the display level before it is shown. Possible culprits are the processing overhead of HDCP, DRM, and also DSP techniques employed to reduce the effects of ghosting - and the cause may vary depending on the model of display. Investigations have been performed by several technology related websites; some of which are listed at the bottom of this article.

LCD and plasma displays, unlike CRTs, have a native resolution. That is, they have a fixed grid of pixels on the screen that show the image sharpest when running at the native resolution (so nothing has to be scaled full-size which blurs the image). One common source of output lag that is separate from others is the introduction of latency due to internal scaling for non-native resolutions. As an example, a display that has a native resolution of 1600x1200 being provided a signal of 640x480 must scale width and height by 2.5x in order to display the image provided by the computer on the native pixels. In order to do this, advanced signal processing is required, which can be a source of introduced latency. Anecdotally, output lag is significantly less when displays operate in native resolutions for a given LCD screen. External devices have also been shown to reduce overall latency by providing faster image-space resizing algorithms than those present in the LCD screen.

However for many LCDs they use a technology called "overdrive" which "reads" what is being sent to the display several frames ahead and processes the image to reduce blurring and streaks left by ghosting. The effect is that anything displayed on the screen is several frames behind which makes anything, even mouse movements, feel delayed as explained in the introduction of the article.

Testing for output lag

Output lag is easier "felt" than seen. As such, proving the existence of this phenomenon requires these things: a test display (the display being measured), a control display (usually a CRT) that would ideally have no output 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 output lag between two displays and cannot determine the absolute output lag of a single display. CRTs are preferable to use as a control display because their output 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.

Effects of output lag on users

Depending on the monitor, output 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. Sometimes, fighting games such as Super Smash Bros. and Street Fighter contain extremely tight windows (sometimes 1-3 frames) for output of certain moves, making them considerably more difficult to play at a high level than on a CRT display.

Output lag contributes to the overall latency in the interface chain of the user's outputs (mouse, keyboard, etc...) to the graphics card to the monitor. Most sensitive users can tolerate latency under 20ms.Fact|date=November 2007

"Game mode"

Many televisions, scalers and other consumer display devices now offer what is often called a "game mode", in which the extensive preprocessing responsible for additional lag is specifically sacrificed in order to decrease, but not eliminate, latency. While typically intended for videogame consoles, this feature is also useful for other interactive applications. Similar options have long been available on home audio hardware and modems for the same reason.

Output lag versus response time

LCD screens with a high response time value often do not give satisfactory experience when viewing fast moving images (They often leave streaks or blur; called Ghosting). But an LCD screen with high response time AND significant output lag is unsuitable for playing fast paced computer games or performing fast high accuracy operations on the screen (e.g. CAD design) due to the mouse lagging behind. Manufacturers only state the response time of their displays and do not inform customers of the output lag value.

Mislabeling the issue as input lag

There is confusion as to the proper terminology of output lag. Some consider this effect as input lag because of the delay between user input versus on screen response to the input. However, input is received and processed by the system at the correct times. The output device can not keep the display in sync with the internal state of the program.

For example–if a game has a meter, which you must hit at an exact moment, the screen may display the cursor at the position where you must press a button. The player will press the button, and will be off by a couple tenths of a second. This is mistakenly assumed it was the button press which was not processed on time (Input Lag). The issue was the state on the screen, where the player pressed the button has already been passed. On screen is it displayed it is the correct time to press the button, but in actuality, it was a couple of tenths of a second before. The screen is just too slow to keep up with the correct frames (Output Lag).

References

External links

* [http://www.behardware.com/articles/632-1/lcds-images-delayed-compared-to-crts-yes.html BeHardware test]

* [http://www.behardware.com/articles/647-4/which-22-inch-to-choose-six-monitors-tested-with-reaction-times-from-2-to-5-ms.html BeHardware tests 22inch TFT monitors for output lag]

* [http://www.tftcentral.co.uk/articles/content/dell_2xx7wfp_2.htm Article at TFT Central demonstrating 2405FPW output lag]

* [http://www.avsforum.com/avs-vb/showthread.php?t=558125 Avs Forum posting on output lag]

* [http://www.youtube.com/watch?v=pi2OE6hSh00 Video demonstrating output lag in Quake]

* [http://www.youtube.com/watch?v=hVXTeGTp5tw&NR Video demonstrating output lag in UT2003]

* [http://www.tftcentral.co.uk/features.htm Output Lag and TFT Features]

* [http://fuchur.dyndns.org/nik/leaveasitis/TFTvsCRT.wmv Video demonstrating output lag in Windows UI]