Published: August 30, 2010 1:45 PM
By Pierre-Jean Alzieu
Translated by: Sam McGeever
We often get asked about how we measure the input lag of video on a TV compared to a traditional CRT monitor.

Before we explain that, though, we should make sure we get the difference between response time and input lag clear:
  • Response time is a measure of the monitor's ability to display one frame without any of the previous frames being visible on screen.  If a display isn't fast enough, then one or more 'ghost' frames may hang around, leading to blurry movements and a noticeable fall in the level of detail.  Manufacturers quote a response time in milliseconds, which, theoretically, is the amount of time it takes for a pixel to change colour.
  • Input lag is a measure of how long the gap is between a monitor showing one frame and the same one being visible on a standard CRT monitor.  It's a stat that's only of interest to gamers, and everybody else can ignore it.  For instance, a screen with an input lag of 67 ms will show every frame 67 ms later than our reference CRT.  If the screen is refreshing at 60 Hz, that means that it will always be around four frames behind.  That penalises whoever is using the LCD screen, as they see all of the action a fraction of a second after everybody else.  In the worst cases, you can see the start of actions which have actually already finished, with a typical case being seeing your enemy opening fire before realising that he's shot you already.

How our tests work



The input lag is the time it takes for a TV or monitor to receive a signal from a video source, handle it and display it onscreen.  If you have a look at our monitor face-offs, you'll probably see that they often do much better at this than TVs.  That's because the latter usually have image correction chips not found in the former which take a while to work through each frame.

To test input lag, we connect a TV using HDMI, and put into 'Gaming' mode, if that's available, which usually turns off all of the extra filters that could be applied by the chip.  If that isn't an option, we turn them off manually ourselves.

In practice, as long as the input lag is under four frames, or 67 ms (maximum, not on average), it will be hard to spot and won't have a negative impact on your gaming.  Beyond that, though, it will start to be problematic and can be a real disadvantage.  At the moment, very few televisions manage this feat, and are often above the four frame threshold. 
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