LCD TVs - LCD Response Time Explained
Date: January 26, 2012

LCD Response Time and Motion Blur
Is Faster Always Better?

Pixel Response, Image Lag and 120Hz/240Hz/480Hz Operation

LCD Response Time is surely among the most important specification you should look at when buying a CCFL or LED LCD TV. Generally, the faster the better, but is it always so? And how fast is good enough? How does LCDs and the latest LED TVs compare with plasma HDTVs? What about 120Hz, 240Hz and 480Hz LED LCD HDTVs? Are these really better at eliminating motion blur?

Equally important, is it possible to compare LCD TV response times between different TV manufactures when the different ways of reporting this important display specification seem simply designed by some TV makers to disguise the hard facts?

We discuss these issues with the hope that the information contained in this LCD TV guide would help you find your way through this incomprehensible mess!

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LCD TV Response Times:
Are TV makers playing the number game?

Normally, bigger numbers sell better but in the case of pixel response time, it is the smaller numbers that are better. Manufactures know this number game very well. A faster LCD TV response time should render fast action better but...

Response times have long been among the battlegrounds between TV makers, with some quoting response times as low as 1msec. More common LCD response times range from 4msec for 120Hz TVs to 2msec for 240Hz LCD HDTVs.The 1msec response time was first introduced during 2010 on LG's flagship 480Hz LED TV. As with most 480Hz LCD TVs, this 480Hz video rate was not the result of a 480Hz panel but the combined effect of a 240Hz panel refresh rate and a scanning LED backlight.

During the last year, TV makers managed to complicate things further! What used to be relatively easy for end-customers to understand has now become rather confusing terminology. It is no longer milliseconds. Instead, we have some fancy wording—like Samsung's Clear Motion Rate and Sony Motionflow XR.

But what does this new terminology really mean to end-customers? Nothingexcept that many would simply associate 960 as being better than 480, or 480 as being better than 240; in fact, HDTVs using Samsung 960 CMR and Sony's MotionFlow XR960 still make use of 240Hz refresh rate display panels.

The truth is that this is more of a number game than anything else, or rather a number battle between TV makers. The use of fancy wording in defining new technology means nothing unless backed by defined standards and a proper scientific definition.

We have seen a similar number battle between TV makers with HDTV contrast ratio ratings. Many would think that a plasma or LED TV with a quoted contrast ratio of 5,000,000:1 is capable of delivering a better picture than one with a contrast rating of 100,000:1. The reality however is that numbers in themselves mean nothing unless you know what exactly one is talking about.

Unfortunately, this obscure way of reporting certain technical specifications is repeating itself also in the way some manufactures are quoting LCD response times. The lack of standard specifications for measuring LCD response time means manufactures can choose to quote or leave out whatever suits them best.

This has a twofold effect:
  1. The different specs used by different TV makers to quote LCD response times are leading to a lot of confusion among consumers and vendors alike.

  2. Lack of standards mean a quoted fast LCD response time does not necessarily imply an LCD or LED TV is capable of handling fast moving images better than one with a slower quoted pixel response time. In other words, response times quoted by different manufactures are becoming almost of no use to buyers when it comes to comparison-shopping.

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Why is LCD Response Time Important?

Some LCD panelsespecially older generation LCDshave a tendency to blur images during fast moving scenes in movies, sports, and gaming events. This is due to LCD pixels that remain lit between frames, leading to a sample-and-hold effect as a result of the relatively long time required by the liquid crystals to align themselves i.e. twist or untwist, to represent a new pixel state. This twisting action of liquid crystals acts as a light valve that allows or blocks light emitted by the backlight from reaching the surface of the display panel in response to the driving video signal.

Even some older generation plasma displays have a similar tendency to blur images. In the case of plasma televisions however, the reason is 'phosphor-lag'; this would normally manifest itself mostly when a bright object is moving fast against a dark background. However, phosphor-lag on the latest generation plasma TVs has practically become a non-issue.

Recent developments in LCD response times mean that handling of fast moving subjects is becoming less and less of a concern. Still, LCD TVs are not exactly on par with the excellent performance of the latest plasma displays when displaying fast moving content.

How fast is fast enough? How do the latest super-fast LED TVs compare with Plasma TVs?

The latest LCD panels (both CCFL and LED) come with significantly improved LCD response times. 4-msec LCD response times has become the standard with LCD displays having a panel refresh rate of up to 120Hz, while a rated 2-msec response time is standard with all 240Hz displays. These figures represent a significant improvement over the 8-msec quoted by LCD TV makers up to a few years ago

However... Are these LCD display panels fast enough to be able to display all video content without exhibiting visible image lag?

As we will further explain in this article, it all depends on what exactly is being measured by the respective LCD panel manufacturer when quoting LCD TV response times.

In general... For many, the latest 4-msec and 2-msec LCD displays should be more than adequate. However the discerning eye may still be able to detect a slight 'trail' or blurring effect where individual pixels in the LCD display appear just out of step with the image on the screen during very action content say during sports events.

How does this compare with plasma TVs?

In contrast to LCD displays, plasma televisions and CRT TVs have a virtually instantaneous pixel response time that is at least a thousand times faster than that of the fastest LCDs. This is mainly limited by the speed of the video processing engine rather than by the time it takes to fire the display phosphor; this is in the order of a few hundred nano-seconds as against the milliseconds required for the liquid crystals to change state in LCD panels.

In this respect, LCD panels still have a long way to go. The latest Samsung and LG plasma TVs have a rated pixel response time of just 0.001msec.

Yet, there is a further technical difference between the two technologies that renders plasma superior when it comes to pixel response time. Control of pixel intensity in a plasma display does not rely on the same drive process used in LCD panels. In the latter, a minor adjustment in pixel intensity is brought about by altering the drive signal in very small steps to adjust the twisting action of the liquid crystals to control the amount of light reaching the surface. This method of adjusting the drive voltage in an LCD panel in small steps to represent an intermediate level, leads to a relatively slow response, much slower than full 'on' or 'off' states.

Instead, plasma displays reply on the pulsing of currentusing time division multiplexing techniques (TDM). In other words, the pixel drive signal in a plasma display is either fully 'on' or fully 'off', and you simply adjust its duration using TDM to control the pixel intensity. This leads to a much faster pixel response in plasma display panels.

Defining Pixel Response Time: What exactly is 'LCD Response Time'?

By definition, pixel response time is the time it takes a pixel to change state. However, this definition in itself is not complete as one needs to specify the change of state itself to which the response time is referring.

If it is a rise-and-fall response, then the response time is a measure of the time it takes a pixel to change state from black-to-white-to-black again. More specifically, it represents the pixel ability to change from 10% 'on' to 90% 'on' and then back from 10% 'off' to 90% 'off' again. Originally, this was the standard way of reporting response times of LCD TVs and computer monitors, and was normally listed as a TrTf (Time rising, Time falling) measurement.

This spec is reported with either the rise and fall response times separate or just the total response by adding both figures. It should be noted however that if reported as a total measurement, a good portion of the figure represents the fall response time since this is generally substantially longer.

A TrTf LCD response time measurement is shorter than any other measurement as it is relatively easy to over-drive the pixel to change state fast when operating between extreme levels i.e. 'on' or 'off' states.

Things got complicated when some manufactures started using a gray-to-gray (GtG) measurement for LCD response times.  Gray-to-gray LCD response time is a measure of the time it takes a pixel to change state from one level of gray to the next. Switching between gray states is much slower, typically 3 to 4 times slower than rise-and-fall; the reason is that it is more difficult to overdrive a pixel to reach the next gray state while maintaining accurate gray-scale levels.

There is a whole debate surrounding the gray-to-gray LCD response time. While some argue that this better reflects the display capability of responding to changing subtle picture detail in a real world TV application, others argue that measuring gray-to-gray LCD response time is pointless, since manufacturers rarely tell where in the cycle they start and end their measurements.

LCD Response Time: Lack of Standards

The Video Electronics Standards Association (VESA) has a defined standard for rise-and-fall response time, but there is no such defined standard for companies to adhere to and that defines the exact parameters to be adopted for gray-to-gray response time reporting. This is rendering quoted pixel response times by manufactures for their LCD HDTVs rather meaningless.

In the absence of defined standards, some manufactures are quoting the rise-and-fall LCD response time giving the rise and fall times separate or in total, others quote the gray-to-gray, while others may quote both, total, or just the response time without saying what is exactly being measured.

To complicate matters, manufactures tend to quote the fastest LCD response time instead of the average or typical response time that a display is capable of achieving overall under normal usage conditions.

In other words, in the absence of precise defined standards, manufacturers' specifications tend to rely on best-case scenarios rather than on the slower, yet real-world performance.

However, irrespective of the way LCD response times are being reported by TV makers, it remains a fact that LCD display manufactures have moved a great way towards eliminating motion blur in LCD displays while closing the performance gap between LCD and plasma displays in this respect.

LCD response time is extremely important, yet this is not the only parameter that comes into play when judging a TV's display ability to handle fast action content correctly. In particular, TV makers are coming up with higher refresh rate HDTVs to deliver what they claim to be practically 'zero' motion blur.

In the second part of this article, we discuss to what extent LCD response time is related to the panel refresh rate, and how does higher refresh rates impact motion blur; what about judder performance when displaying movie content? Unfortunately, here things start to get really complicated - mainly as a result of the misconceptions delivered by TV makers and retailers about their higher refresh rate HDTVs.

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 Article Content

Issues discussed in this article

Blue bullet  LCD Response Times: Are TV makers playing the number game?

Blue bullet  Why is Pixel Response Important? How fast is fast enough; how does LCD response times compare with plasma TVs?

Blue bullet  Defining Pixel Response Time: What is exactly LCD response time?

Blue bullet  Motion Blur, Panel Refresh Rate, and Pixel Response: How are these interrelated? Do higher refresh rate technologies help eliminate motion blur?

Blue bullet  Conclusion: Putting things in the right perspective

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