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March 13, 2008

LCD Display Response Time

For clear action images without motion blurring an LCD display needs a fast response time (minimal latency). Response time is more critical for motion-intensive applications such as gaming. The faster the response time, the faster the screen can be refreshed and the less chance there is of the motion blurring or ghosting that LCD displays are suspect to. The blurring is a result of a display not being able to keep up with the output from the video processor.

Unlike LCD displays, plasma and CRT displays have a virtually instantaneous response time and are not prone to motion blurring.

The two most common measurement methods are “rise-and-fall” and “gray-to-gray”. A technical specification for rise-and-fall is available from VESA (the Video Electronics Standards Association). Rise-and-fall is measured as the time taken for a pixel to change from black to white (rise) and back to black again (fall). Gray-to-gray is the time to change from one shade of gray to another. Transitions between the fine graduations of gray-to-gray can be 3 to 4 times slower than those of rise-and-fall because of the lower driving signal for the transition.

Consumers can easily become misled if they are not aware response times quoted may be for different measurement techniques. As next to all video images displayed contain grayscale images, the gray-to-gray response rate gives a better idea of the real world response rate of a display.

It should be noted that LCD manufacturers are making excellent headway in closing the response time performance gap with plasma and CRT displays with LCD response times being some three times faster than a couple of years ago. An 8 ms or less response time is not hard to find. Faster response times have also given manufacturers the option of utilizing another LCD blurring reduction techniques including a faster frame rate through interpolated frames or inserted black frames.

A response time lowering technology also worth mentioning is Viewsonic’s Amplified Impulse Technology which delivers LCD displays with a 4ms response time. The technique used is the application of a short, higher drive signal to jump-start LCD pixels in their transition.