In the first part of this article, we explained what exactly is LCD response time and why is it such an important display parameter. We have also discussed how different TV manufactures use different ways of reporting this important display specification in what is often an attempt to hide the hard facts; this also explains why faster does not necessarily imply a better TV!
In this second part, we discuss to what extent LCD response time is related to the display panel refresh rate, and how higher refresh rates impact motion blur and judder performance.
Unfortunately, here things start to get really complicated, mainly as a result of the misconceptions delivered by both TV makers and retailers that their 120Hz, 240Hz and even more so the latest 480Hz LED LCD TVs, are cable of practically zero motion blur. Are these higher refresh rate HDTVs really better at displaying fast action content?
Motion Blur, LCD Panel Refresh Rates, and Pixel Response Times
LCD and LED TV specs seem to imply that the capability of a display panel to reduce motion blur during the presentation of fast action content is not just a function of the pixel ability to change state in a short time or pixel response time, but also of the display panel refresh rate. The implication here is that a higher panel refresh rate should yield less motion blur.
The whole issue is: To what extent a faster display panel refresh rate reduces motion blur, and is LCD response time related to the panel refresh rate?
Let’s make one point clear: The advancements in LCD technology are such that it is unlikely you will experience motion blur with the latest 60Hz TVs. Most blurring during fast action scenes is generally inherent in the source. Fast action would look practically the same when watched over the same LED/LCD TV, irrespective of whether the TV operates at a panel refresh rate of 60Hz, 120Hz, 240Hz or higher. In other words, the real issue is not the refresh rate.
Editor’s Note: 60Hz/120Hz/240Hz/480Hz refresh rates are applicable only to countries using the NTSC broadcasting standard such as the US. The equivalent refresh rates in countries using the PAL format are 50Hz/100Hz/200Hz/400Hz.
Unfortunately, there is a lot of consumer misinformation surrounding motion blur and 120Hz/240Hz LCD/LED TVs. 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 TVs more susceptible to motion blur.
There is a twofold reason for this:
1. Retail stores are pushing 120Hz and 240Hz LCD TVs as the ‘fastest TVs’ not because sales reps understand 120Hz and 240Hz technologies; most simply don’t. Their real reason is that higher refresh rate LCD HDTVs are more expensive and often carry higher profit margins than 60Hz versions!
2. LCD manufactures are pushing 240Hz and even 480Hz refresh rate display panels as the hottest TV specs, claiming these eliminate motion blur completely.
The reality however is that faster pixel response times are necessary to support these higher refresh rate; 2-msec is typical with 240Hz while 4-msec is the norm with 120Hz. These faster response times are often bundled by TV makers with 120Hz and 240Hz advanced video processing technologies to help smooth out the action, eliminate judder, and improve motion resolution. The ultimate aim is to combat the perception that LCD HDTVs are slower than plasma TVs.
To complicate matters, during these last years, plasma TV makers have come up with the so called 600Hz sub-field drive technology in an attempt to re-affirm that plasma are the fastest HDTVs you can buy!
But this 600Hz in plasma has nothing to do with actual video frame rate. Instead, 600Hz sub-field motion is a process whereby each pixel in the plasma display is flashed 10 times every frame, leading to 600Hz flash rate with 60Hz frame rate content. According to TV makers, this leads to the best realization of fast action content. Nevertheless, experts would tell you to ignore this 600Hz technology altogether as the eye would not perceive the resultant improvement in plasma TVs.
Unlike LCDs, plasma displays are generally not susceptible to motion blur; this extra hertz technology in plasma TVs is mainly there to help combat the 240Hz sales game by LCD TV makers. In fact, flagship 600Hz plasma TVs often make use of 96Hz or 72Hz frame rate processing.
These frame rates come into play only with 24p content to eliminating judder. In simple terms, this means that each movie frame is repeated four times in the case of 96Hz systems and 3 times in the case of 72Hz processing, leading to a judder free processing. Despite the lower panel refresh rate, these plasma HDTVs still support the full 1080 lines of motion resolution even though they lack the faster display panel refresh rates found on the latest LCD and LED TVs.
This proves our statement that higher panel refresh rates alone do not minimize LCD motion blur, and therefore do not lead to improved motion performance.
This is where things start to get really complicated. 120Hz and 240Hz refresh rate technologies first turned out to be hot specs with LCD HDTVs in 2007.
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, Sony and some of the latest LG HDTVs – using the so called ME/MC (motion-estimation motion-compensation) system, or simply added empty black frames.
The process of adding either an interpolated or empty black frame to achieve higher display refresh rate applies to all 120Hz HDTVs. In the case of 240Hz HDTVs, the additional extra frames necessary to generate the 240Hz rate are generated using either one of two ways:
1. ME/MC technology—in which case the three extra frames necessary to generate the 240Hz panel rate from the 60Hz video rate are all made up of interpolated content, or
2. Use ME/MC or a blank black frame to generate the extra frame for a 120Hz refresh rate and then apply a scanning backlight to get the 240Hz ‘panel refresh rate effect’.
This scanning backlight process is also used with some of the latest 480Hz systems where ME/MC technology generates the 240Hz signal and then a scanning backlight is used to effectively double this refresh rate to 480Hz.
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 virtually zero motion blur. However, while it is possible for the eye to detect the improved motion resolution between 60Hz and 120Hz processing systems, it is not as easy to perceive the resultant improvement brought by 240Hz refresh rate systems over 120Hz refresh rate technology.
And with 480Hz systems, the perceived improvement over 240Hz systems is practically negligible. Manufactures have reached the point of diminishing returns where the added expense associated with the more complex image processing necessary to drive these higher refresh rate HDTVs results in practically nil visual benefit for the viewer. This also explains the use of special test patterns in 240Hz and 480Hz HDTVs promotional material.
Judder and Motion Blur
It should be noted here that while this virtually blur-free motion is the result of the faster video processing coupled with the faster pixel response times necessary to support these higher refresh rates, the smoothing action resulting from 120Hz and 240Hz processing applies to film-based (24 fps) 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. [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; the reason is that you cannot 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; the so called ‘dirty’ extra frame would appear 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 with film-based content by eliminating judder while still preserving the natural cadence of film. We say ‘done properly’ as some LCD/LED HDTVs do exhibit a number of video artifacts that can be annoying.
In other words… 120Hz/240Hz refresh rates alone have nothing to do with motion blur and all to do with judder!
However, combined with additional video processing, 120Hz and 240Hz processing may help increase motion resolution. This is the case with the latest 120Hz and 240Hz CCFL and LED LCD TVs from all major brands. 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 Auto Motion Plus 240Hz refresh rate dejudder mode combined with Samsung’s LED Motion Plus employed on some of Samsung’s premium LED TVs, adds similar sequential firing to its LED backlighting for an improved motion resolution that exceeds 1080 lines; this is very much the same as the best plasma TVs.
But there again, this improved motion resolution is a totally different beast to the motion blur resulting from image lag due to a slower LCD response time.
It is 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 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.
While all three can lead to motion blur during fast action content, judder, motion resolution, and image lag are technically completely different in nature.
Conclusion: Putting everything in the right perspective
1. As things stand today, it is unlikely you will experience image lag due to slow LCD response times on the latest 60Hz TVs. 120Hz and 240Hz operation carries a number of benefits but these have nothing to do with image lag due to slower pixel response times.
2. Product specs published by manufacturers for LCD response time should only be taken as indicative; it is not possible to compare LCD TV response times between different brands due to the non-standard ways TV makers use to report pixel response for their display panels.
3. While an insufficient LCD response time may ruin an otherwise excellent picture and therefore your viewing experience, do not just focus on one display spec only. More important than just pixel response is the overall TV motion performance, or rather the TV’s ability to minimize motion blur as well as the correct handling of judder processing.
4. If you are in the market for an HDTV, look at the overall picture performance and see if the offered price-performance deal suits your needs.