LCD Response Time - Is faster always better?

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

Similarly, a plasma TV with a quoted contrast ratio of 2,000,000:1 is supposed to deliver a better picture than one with a contrast rating of 50,000:1.

These are just a few typical cases where some manufactures are taking customers for a ride by quoting unrealistic figures for technical specs resulting from unspecified testing methodologies; these aim more at inflating the quoted parameter values 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, the different specs used by different TV makers to quote LCD response times are often leading to a lot of confusion among consumers and vendors alike.

Lack of standards 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. This is due to LCD pixels that remain lit between frames - leading to a sort of 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 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 exactly on par with the excellent performance of top plasma displays when it comes to displaying fast moving content.

What about the latest super-fast LCD HDTVs?

Admittedly, the 'pixel response time' of the latest generation LCD display panels has improved significantly. 4msec LCD response times has become the standard while LCD displays with a rated 2msec response time are becoming relatively common, this when up to just over a year ago, 8msec was the norm. However...

Are these LCD displays fast enough in order to be able to display all video content without exhibiting even the slightest 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 perfectly adequate, however 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 sports events 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; this 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 leads to a much faster pixel response in plasma displays.

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But what is exactly '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 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 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.

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 response time figures 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 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.

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Motion Blur and 120Hz/240Hz LCD HDTVs

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 closing the performance gap with plasma and CRT displays to eliminate motion blur in LCD displays.

The advancements in LCD display technology have been such that it is extremely unlikely you will ever experience motion blur with any of the latest 60Hz LCD HDTVs. Most of the blurring during fast action scenes is generally inherent in the source. In other words, it would look practically the same when watching the same content over the same LCD TV irrespective of whether you operate that TV at 60Hz refresh rate, 120Hz, or 240Hz. In other words, the real issue is not the 60Hz refresh rate.

Unfortunately, there is a lot of consumer misinformation surrounding the issue of motion blur and 120Hz/240Hz LCDs. In particular, the general notion among consumers has so far been that the slower refresh rate of 60Hz HDTVs coupled with the slower LCD pixel response time in comparison to plasma TVs, leave 60Hz LCD HDTVs more susceptible to motion blur. There is a twofold reason for this:

Admittedly, this is where things start to get complicated. 120Hz, and now 240Hz are relatively new technologies that first turned out to be hot specs with LCD HDTVs in 2007. These high TV refresh rates work by adding an extra frame in the case of 120Hz LCDs and three extra frames in the case of 240Hz processing for every single video frame. These extra frames are either interpolated content as in the case of Samsung and Sony using the using the so called ME/MC (motion-estimation/motion-compensation) system, or simply added empty black frames.

Termed differently by different display manufactures - Sony's Motionflow/Motion Enhancer, Smooth 120Hz by Mitsubishi, and Auto Motion Plus 120Hz and 240Hz in the latest Samsung LCD HDTVs, this higher 120Hz and 240Hz technology should help smooth out the action by improving the image sequence the eye perceives.

This advanced video processing is also designed to increase the effective motion resolution for a virtually blur-free motion. However, while it is possible for the eye to detect the improved motion resolution between 60Hz and 120Hz processing, yet it is not the same with the resultant improvement with 240Hz over 120Hz. This explains the use of test patterns in commercials by TV makers to help push 240Hz LCDs.

But while the virtually blur-free motion is the result of the faster video processing (coupled with the faster pixel response time necessary to support these higher refresh rates), the smoothing action resulting from 120Hz and 240Hz processing applies mainly to film-based (24 frame-per-second) content.

The use of these higher refresh rates helps eliminate judder that would otherwise result from the 3:2 pulldown processing necessary to display 24p content over a 60Hz refresh rate HDTVs. [Note that we refer to 3:2 pulldown as it is often referred to as such; in reality, it should be referred to as 2:3 as according to SMPTE standards, the first frame in film-based content should be associated with the first and second fields of one video frame, and is therefore scanned twice, not three times.]

Judder is the resultant visual artifact leading to a jerky movement when film is transferred to video since you cannot really repeat every single frame in 24-frames-per-sec film-based content to appear an equal number of times on a 60Hz video frame rate. As a result, some frames are repeated twice while others three times during the 2:3 pulldown sequence, with the 'dirty' extra frame appearing every fifth frame. It is this uneven repetition of movie frames that causes judder. Judder is most noticeable in scenes that incorporate slow camera pans or in scenes shot with a handheld camera. 120Hz is the lowest refresh rate that simultaneously allows both 24Hz film-based and 30Hz video-based programs to be displayed without the need to kick in 3:2 pulldown. Unlike 3:2 pull-down processing, with 120Hz and 240Hz processing each frame in a 24p movie content is repeated an equal number of times

This means that what you get with a 120Hz/240Hz system - when done properly - is a smoother action by eliminating judder - while still preserving the natural cadence of film. We say done properly as some of the first LCD HDTVs using 120Hz did exhibit a number of video artifacts that were really annoying.

In other words, 120Hz/240Hz refresh rates alone have got nothing to do with motion blur and all to do with judder!

However, combined with additional video processing technologies, 120Hz and 240Hz processing may help increase motion resolution. This is the case with the latest LED TVs from Samsung as well as the latest 120Hz and 240Hz CCFL LCDs from both Samsung and Sony.

The 'Clear' setting for Sony's Motion Enhancer - which is Sony's dejudder processing - adds sequential backlight firing to improve motion resolution. Similarly, Samsung's Motion Plus 240Hz refresh rate dejudder mode combined with Samsung's LED Motion Plus on its B8500 LED TVs, adds similar sequential firing sequence to its LED backlighting for an improved motion resolution, reaching 1080 lines - much the same as the best plasmas from Panasonic. But there again, this improved motion resolution is a totally different beast to motion blur resulting from image lag due to a slower LCD response time.

It is therefore evident that the term 'motion blur' is often being used by both TV manufactures and retailers to describe a rather hazy concept without differentiating between say a 'jerky image' or judder due to a different frame rate content, 'motion resolution' due to limitations within the video processing in an HDTV, and image lag or 'blur' due to a slow LCD response time. This despite that judder, motion resolution, and image lag due to a slower pixel LCD response time - while all lead to blurring and lack of motion smoothness, are technically completely different issues.

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A Few Final Recommendations:

1] As things stand today, it is unlikely you will ever experience image lag due to slow LCD response times with the latest 60Hz HDTVs. 120Hz and 240Hz operation carry a number of benefits but these have nothing to do with image lag due to a slower pixel response time.

2] Should you decide to compare LCD response times between different TV makers, the best way is to judge with your own eyes. Product specs published by manufactures for LCD response time should only be taken as indicative; do not just rely on reported numbers especially 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 brand - Samsung, Sony, Sharp, etc. 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.

3] 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 the offered price-performance deal suits your needs.