LCD Response Time - Is faster always better?

LCD Televisions and Image Lag

LCD Response Time is surely one of most important specs you should look at when buying an LCD TV. Normally, the faster the better, but how low is good enough, and how does LCDs compare with plasma TVs? Is it possible to compare LCD TV response times between different manufactures when the different ways of reporting this important spec seems simply designed by some to disguise the hard facts?

We hope that the info contained in this short LCD TV guide would help you find your way through this incomprehensible mess.

Samsung LNS4051D 40" Wide LCD HDTV

LCD TV Response Times: Playing with Numbers!

Normally, bigger numbers sell better; in the case of response time, it is the smaller number that is better. Manufactures know this number game very well. A faster LCD TV response time is supposed to render better, fast actions during movies, sport events, and gaming, but...

Response time of LCD HDTV sets is surely among the latest battlegrounds between display manufactures, with some quoting response figures as low as 8, 6 and even 3msec! A case in point is the ViewSonic VX924 19-inch LCD Monitor, which comes with a quoted 3msec response time.

We have seen number battles in the scanner and digital camera world when some manufactures started quoting interpolated instead of true optical image resolution figures. A scanner with 19200DPI image resolution would seem better than one with 2400DPI ...unless you know what you are really talking about.

Similarly, a plasma TV with a quoted contrast ratio of 10,000:1 is supposed to deliver a better picture than one with a contrast rating of 4,000:1.

These are just a few typical cases where some manufactures are taking customers for a ride by quoting unrealistic high or low figures, for technical specs resulting from unspecified testing methodologies that aim more at inflating the end figure to cut on competition, than to deliver a better product to the end customer.

Unfortunately, this obscure way of reporting hard facts 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 that manufactures can choose to quote, or in that case even leave out, whatever suits them best.

This has a twofold effect:

• First and foremost, it is leading to a lot of confusion among customers and vendors alike about quoted LCD response time terms.

• It also means that a quoted fast LCD response time does not imply that an LCD TV is capable of handling moving images better. In other words, response times quoted by different manufactures are turning out to be almost useless to buyers when it comes to comparison-shopping.

Why is LCD Response Time important?

Some LCD panels – especially from 2nd tier manufacturers - and older generation models, have a tendency to blur images particularly during fast moving scenes in movies, sports, and gaming events; the main reason being the relatively long time required for the liquid crystals to align themselves - more precisely, to twist or untwist - when activating and de-activating pixels.

This twisting action of liquid crystals acts as sort of a light valve that allows or blocks light from the backlight source, from reaching the glass 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 on par with the excellent performance of plasma displays when it comes to displaying fast moving subjects.

What about the latest super-fast LCD HDTVs?

Admittedly, the 'pixel response time' of the latest generation LCD displays have greatly improved, with some manufactures quoting 8msec to  6msec LCD response times; a few LCD displays with a 3msec response time have also started to emerge. However...

Are these LCD displays fast enough in order to be able to display all video content without exhibiting a slight 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.

Our experience show that even with some of the latest 8msec LCD display panels, the discerning eye may still be able to detect a slight 'trailer' or blurring effect, where the individual pixels on the LCD display appear to be just out of step with the image on the screen during very fast moving sports and action movie scenes. 

How does this compare with plasma TVs?

In contrast to LCD displays, plasma televisions and CRT TVs have a virtually instantaneous response time.

This is mainly limited by the speed of their video processing engine rather than by the time, it takes to fire the display phosphor - which is in the order of 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.

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 an LCD panel. In an LCD display, a minor adjustment in pixel intensity level is brought about by altering the drive signal in very small steps to adjust the twisting action of the liquid crystals, and which in turn controls the amount of light reaching the surface.

As we will soon explain in this article, this method of adjusting the drive voltage in an LCD panel in small increments to represent an intermediate intensity level, leads to a relatively slow response that is substantially slower than the full 'on' or 'off' states.

Instead, plasma replies on the pulsing of current - using 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 renders for a much faster pixel response in a plasma display.

But what is exactly 'LCD Response Time'?

By definition, pixel response time is the time it takes a pixel to change state. If it is a rise-and-fall response, then it 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 as it is more difficult to overdrive the 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, yet 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.

Lack of Standards:

The Video Electronics Standards Association (VESA) has a defined standard for rise-and-fall response time, but not for gray-to-gray. Without an approved standard from VESA for all companies to adhere by, this measurement of LCD response time is effectively being rendered 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 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. 

We know that VESA is in the process of introducing a spec standardizing response time measurements; it is hoped that this would arrive sooner rather than later in order to cut on this whole mess of confusion among vendors and buyer alike.

A final recommendation: As things stand today, the best way when looking for an LCD HDTV or monitor, is to judge with your own eyes. Manufacturers specs for LCD response time should only be taken as indicative, but do not just rely on reported numbers.

This is especially important if you are opting for some cheap model from a 2nd or 3rd tier manufacturer. If you are buying online, try to go for a 1st tier manufacturer and check the vendor return polices; the cheaper prices online should normally justify this approach; alternatively, check a model performance at a brick-and-mortar store and if satisfied, buy online to enjoy the cheaper pricing options.

Finally, remember that while an insufficient LCD response time may ruin an otherwise excellent picture and therefore, your viewing experience, yet do not just focus on one spec., instead look at the overall picture performance and see if this suits your needs.

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