LCD TVs: LCD Response Time, Image Lag and 120Hz Operation

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 should render better fast actions during movies, sport events, and gaming, but...

In the case of LCD HDTV sets, response time is surely among the latest battlegrounds between display manufactures, with some quoting response figures as low as 6, 4, and even 2msec! The most common LCD response time being quoted for 2008 models is 4 msec; this is also the case with the Samsung LN52A650 featured above.

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 50,000:1 is supposed to deliver a better picture than one with a contrast rating of 20,000:1.

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 have become rather common while LCD displays with a rated 2msec response time have also started to emerge, this when up to a year ago, 8msec was the norm. However...

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 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 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.

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, 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 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.

Motion Blur and 120Hz 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. Mind you, we do see a lot of blurring during fast action scenes but this is always inherent in the source. In other words, it would look the same irrespective of whether you are watching over a 60Hz LCD, a 120Hz LCD HDTV, or a plasma TV.

Unfortunately, there is a lot of consumer misinformation surrounding the issue of motion blur and 120Hz 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 times in comparison to plasma TVs, leave 60Hz LCD HDTVs more susceptible to motion blur. There is a twofold reason for this:

Retail stores in particular are pushing '120Hz LCD HDTVs' as the fastest TVs you can buy.' This is not because sales reps understand 120Hz technology; most simply do not grasp 120Hz basics at all. Instead, their real reason for proposing 120Hz LCD HDTVs is that these are significantly more expensive, and therefore carry higher profit margins!

LCD manufactures are pushing 120Hz as the hot spec of the moment. Faster response times and 120Hz technology are often being bundled together by manufactures to combat the perception that LCD HDTVs are slower. In fact, 120Hz is often being coupled with other advanced video processing technologies designed to smooth out the action, eliminate judder, and help improve motion resolution.

The issue is - To what extent today's advanced video processing technology reduce motion blur?

Admittedly, this is where things start to get complicated. 120Hz is a relatively new technology that first turned out to be a hot spec with LCD HDTVs in 2007. It doubles the standard 60Hz refresh rate in HDTVs to 120Hz by adding interpolated, repeated, or black frames.

Termed differently by different display manufactures - Sony's Motionflow/Motion Enhancer, Smooth 120Hz by Mitsubishi, and Auto Motion Plus in the latest Samsung LED Series A950 LCD HDTVs, 120Hz technology should help smooth out the action by improving the image sequence the eye perceives.

But the resultant 120Hz smoothing action applies mainly to film-based (24 frame-per-second) content as it helps eliminate judder due to the use of 3:2 pulldown processing otherwise necessary with 60Hz 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 60Hz video. As a result, some frames are repeated twice while others three times during the 2:3 pulldown process, with the 'dirty' extra frame appearing every fifth frame. 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.

This means that what you get with 120Hz - when done properly - is a smoother action by eliminating judder. We say done properly as some of the first LCD HDTVs using 120Hz did exhibit a number of video artifacts that could be annoying.

In other words, 120Hz technology alone has got nothing to do with motion blur and all to do with judder!

However, combined with additional video processing technology, a 120Hz frame refresh rate may help increase motion resolution. This is the case with the latest LED LCD HDTVs from both Sony and Samsung. 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 120Hz refresh rate dejudder mode combined with Samsung's LED Smart Motion, adds similar sequential firing sequence to its LED backlighting for an improved motion resolution of over 1000 lines - much the same as the best plasmas from Pioneer. 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 clear 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 when displaying fast action content, are technically, completely different issues.

A Few Final Recommendations:

1] As things stand today, it is extremely unlikely that you will every experience image lag due to slow LCD response times with the latest 60Hz LCD HDTVs. 120Hz operation carries 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.

For the latest LCD HDTV sets available on the market, we advise to visit the amazon storefront; the vast choice of products and the various buying options available from the different retailers featured on their site are among the best online.

However, prior to any decision, we recommend to do some research of your own. In particular, look at what other customers had to say about their LCD HDTV following their purchase. This will give you extensive insight about the product of interest. A good starting point in this respect is the customer feedback posted on the amazon site.

You can search the amazon storefront without leaving our site by using the amazon search box below; your search results will appear here under.

LCD Response Time - Is faster always better?

LCD Televisions, Image Lag and 120Hz

LCD TV Response Times

...playing with numbers!