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LCD HDTV – Response Time

December 24th, 2010 Leave a comment Go to comments

Key facts
– A lower response time is better for faster moving video
– A typical value of 4 ms was found in most LCD HDTV in 2010
– Plasma HDTVs have much better response time
– Response time is generally defined as the time it takes for the pixel to change the state from black to white and then again to black.
– The Liquid Crystal Displays, the twisting of molecules take finite amount of time when changing states, therefore, we have definite response time.

Response Time is yet another parameter that is quoted in the manufacturer’s specification that you will come across when comparing HDTVs. Response Time is expressed in milliseconds. The lower the value, the better is the performance. Lower value of response time connected with higher screen refresh rate implies excellent performance (for example a response time of 8 milliseconds is better than 25 milliseconds). In simple words the time needed to change from complete black to full white (i.e. from inactive to the active state) for LCD displays is called response time. An LCD HDTV with faster response time will take lesser time to change the state and will therefore be preferable.

Response time is of great importance when you’re dealing with a video content full of speed (like watching a motor racing event or a baseball game). LCD screen with long response time will show a tail like smearing artifact while viewing objects in fast movement. LCD display is more susceptible to this problem. In the example of a racing event, consider two cars moving fast close to each other, a viewer can’t perfectly distinguish which car is leading.

Different manufacturers use different testing strategies to calculate and report the response time. Usually they rely on methods giving low response time, which armors the marketing division. Some others completely exempt the value from the catalogue to keep the customer attention far away from the actual problem.

We have seen the number 5 ms and 4 ms reported as the response time for the TVs in 2010. Some 240 Hz LCD HDTV also come with response time as low as 2 ms !!

Importance of the response time is this – Assume three alternate frames of a moving sequence and focus on one particular pixel that changes its state from white to black and then again to white. If the response of the molecule in LCD crystal is slow the change from white to black or from black to white may not complete, leading to incomplete contrast sequence and poorer image quality in case of fast moving image. LCDs work of twisting action of the crystals and if this twisting is not fast enough we have the response time issue. Plasma TVs, on the other hand work on a different technology and do not have the response time issue. In Plasma HDTVs the phosphor is driven instantaneously by the driving signal and they do not suffer from the slower twisting crystal issues as in LCDs. The response time in Plasma TVs can be as fast as 0.001 ms as compared to 4 second or so in case of the LCD TVs. If you really watch a lot of car racing, base ball and want to catch the really fast moving action details, plasma TV could be the way to go.

There is yet another subtle difference between the LCD HDTV and the plasma TV as far as the representation of the intermediate intensity level is concerned. In the LCD HDTV, the intermediate intensity level changes depend upon the drive strength of the driving signal. The incremental change in the intermediate level is slower than the full on ( at white) and full off response time, further slowing the real life response time.

Plasma TVs on the other hand works differently. A particular pixel is either off or on for a particular duration of time. The intensity is controlled by the amount of the time for which the pixel stays on. This further makes smaller response time in case of plasma TVs.
Response Time – The definition and Measurement

A casual definition for LCD response time, as expressed earlier is the time it takes for a pixel from one state to other. To complete this we need to define what exactly we mean by change of state.

One way of change of state is called rise and fall change of state and this measures the rise and fall response time. When the driving signal rises, it changes the state from black to white. A falling driving signal, changes the state from white to black. Listed as TrTf time ( Tr stands for Time rising and Tf for time falling), this time is a standard way of reporting the response time. Notice that the time is the sum of the two times, not any one of the two.

Manufacturer may also choose to represent rise time and fall time individually or may report the total time by adding them together. The fall time is usually slower than the rise time. Sometimes a manufaturer may just report the rise time which will show a smaller ( and therefore better) figure hiding the real TrTf (total) response time.

Gray to Gray (GtG) is another method some manufaturer use to report the response time. The Gray to Gray (GtG) is the time it takes for the pixel to change state from onde intensity level to other intensity level. Switching between the grey level is a slower phenomenon.

In case of the “Full off” on “Full On” pixel state change, you can “overdrive” the pixel to get a the crystal to act faster and get smaller response time. In the GtG you can not do the same thing – as can not over drive – you need to keep the precision in control as you change state from one precision gray level to another precision gray level. This means that the Gray to Gray response time is large in reported value.

Gray to Gray level potentially reflects that more real life situayion. However, it leads to another difficulty in uniformity in the way the response time could be measured and reported between different manufacturers. If a manufacturer does not specify anything, you should generally assume that it is the TrTf response time

However, there is no need to go to the extremities in case of response time. A low value of 2 milliseconds is available with most modern technology. But, our eyes can identify the improvement only up to 10 milliseconds. A value of 8 milliseconds is much better. This value is enough to view even the peak high quality blu-ray DVD video or a sports event. This can give high level viewer experience combined with 120Hz or more refresh rate.

Popular TV brands use different strategies to overcome the problems in using LCD screen for television purposes. These technologies are known with different names in the case of various manufacturers. But you can’t close your eyes by simply believing these shortcuts. While viewing images under scarce light conditions the problem may revert in the form of motion blur and vague image areas.

The artificial way of improving the viewer experience is achieved using image processors which add duplicated image sequences at certain regions. The input signal is electronically manipulated before reaching the screen. This is done using complex algorithms capable of doing mathematical calculations depending on the nature of input. Modifying the backlight interface is another method. The direct way is to improve the Liquid crystals in such way to give sudden changes from one color to another extreme color value.

Reading customer reviews and look for product ratings is a good way to make a decision before changing your TV set. If you’re having a good presence of mind and patience to examine all the required facts, outstanding performance is near your doorsteps. Keep away from usual steps. Perform the usual steps in an unusual way. This will make the real difference.

Some other facts you may be interested to know when planning to buy an LCD HDTV

– The truth behind the HDTV Contrast Ratio.

1080p Vs 720p resolution

LCD HDTV Refresh Rate

– Historical fact – LCD TV Vs older CRT Monitor.

We are also planning to bring the HDTV buying guide, a LCD Vs Plasma TV comparison and LED TV Vs LCD TV ( LED TV is basically LCD TV with backlight driven by LED). Stay tuned.

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  1. March 7th, 2011 at 19:23 | #1

    Great article. I really enjoy share for my friends and post on my blog.