What is Response time?
Response time of a LCD TV is certainly an important specification to consider when you make a choice among various LCD TV models available in the television market. Generally, considered as a weakness of a LCD technology, the response time of LCD TV naturally needs lot more study and attention. Perhaps no other area related to the performance of LCD TV attracted such an amount of attention from the LCD manufacturers. LCD TV makers are continually bringing improvements in their high end LCD to overcome this weakness.
Response time means how fast the pixels are able to make transition between two states – inactive and active or vice versa. In other words, how fast they go from black to white or vice versa. LCD technology uses liquid crystals which allow and block light provided by backlight device. Response time is usually quoted in milliseconds or ms. TVs having faster shifts between active and inactive will have lower response time while higher response time means delay which can be vexing to the viewers.
Ghosting and flat-panel displays
The Plasma TVs or CRTs which use phosphors to produce light, are able to make the transitions much faster due to their technologies. For instance, the conventional CRT display gets the image on the screen refreshed when the electron beam travels over the screen. Plasma TVs also work in a more or less similar way. However, LCD takes much longer time to make these transitions resulting in smudges or blurs around the image. This is what is so called ‘ghosting’.
The negative aspect of displays with high response time is that they cannot produce satisfactory display when playing fast paced games or fast actions. This is also true when you use them for displaying CAD designs which require fast, precise operations.
Usually older LCD displays suffered ‘ghosting’ due to their higher response time although this is not true with latest LCD TVs as the manufacturers have come with numerous solutions.
Measuring Response time
Response time between 8 ms and 16 ms had been quite acceptable until recent time. The ISO standard usually considers the full black and white transitions as standard and the response time of display systems has to be quoted with TrTf (Time rising, Time falling). However the gray transitions which are mostly in practice are far behind the ISO standard thus misleading the customers with regard to response time. The recent Response Time Compensation or RTC technology has been adopted in many LCD TVs to make measurement of response time to fall in line with ISO standard. This is done by reducing the grey transitions in a LCD TV.
New LCD TVs come with improved response time
However, LCD manufacturers in the recent past found two ways to minimize the ghosting. One such way is to decrease the screen size as ghosting is not much visible smaller panels. Another way is to use a technique called overdriving which uses black-to-gray and gray-to-gray transitions.
Newer LCD TVs adopt two methods to reduce the response time to eliminate the ghosting.
According to first method, the backlight device is fired at a rapid rate which will be usually less than the screen refresh rate. However, this method is likely to result in flickering which can be perceptible by eyes.
The second method is to use ‘motion compensation’ which actually doubles the screen refresh rate and produces transitional frames. Most high end LCD TVs combine these two techniques in their LCD
