6 Ingredients of a Perfect LCD TV: 120Hz (or faster)

On May 13, 2008, I began a series of articles called 6 Ingredients of a Perfect LCD TV. In the opening article, I suggested that there are 6 major ingredients that make up a perfect LCD TV. The 6 ingredients are:

  1. Technologies to reduce motion blur on LCD TVs: 120Hz, LED Backlight, Black Frame Insertion, Motion Interpolation, Fast Response Time, Over Driving Circuitry.
  2. Technologies to increase contrast ratio: LED Backlight and Local Dimming
  3. Full HD or 1080p: LCD panels with a pixel format of 1920 x 1080.
  4. Enhanced Color Gamut: LCD panels with >72% NTSC color gamut using RGB LED Backlight or Wide Color Gamut CCFL.
  5. Video Inputs: HDMI, DVI, VGA, Composite, Component and what all of these mean.
  6. Sexiness: Like a car, your gut instinct needs to really like how it looks turned on and off, within a budget, of course.

So here we go with the first very important ingredient: Motion Blur Reduction Technology Part I: 120Hz

The refresh rate became an important specification starting from when users of cathode ray tube-based displays demanded more performance. The refresh rate is more accurately the vertical refresh rate and refers to the number of times that the display hardware draws the data in one second. A common challenge for CRT displays was the problem of flicker. Flicker is the visible fading between picture frames and this occurs when the CRT display is driven at a low refresh rate. Flicker on a CRT display can cause eye strain. The low refresh rate allows the phosphors to lose their excitation before the electron gun can re-excite them. For example, if a CRT display has a vertical refresh rate of 60Hz, most will produce a visible flicker. In most cases refresh rates of 70Hz and above enable flicker-free viewing on CRTs displays. However, refresh rates beyond the display’s specification can cause damage. LCD displays have a similar challenge but is different in nature due to how the LCD operates.

LG Electronics LG70 LCD TV

Much of the discussion of refresh rate does not apply to LCDs because LCDs employ a backlight structure that illuminates the front of the display. The liquid crystals act as shutters while a color filter adds color to the different brightness levels of light coming through. Similar to refresh rates on a CRT displays, LCD displays have what is called a response time. Response time is the time it takes for liquid crystals to shift from one state to another. The measurement of response time varies and manufacturers often cite the time it takes for liquid crystals to change their states from “off” to “on”.

With the popularity of LCD TVs growing, the response time measurement changed as well. When the level of brightness is analyzed for most moving picture content, the brightness of the content is generally darker relative to the brightness users experience in front of a LCD monitor using software applications such as Microsoft Office. Although there are exceptions, most of the content in moving pictures do not have extended periods of purely white. LCD manufacturers have also realized these results and in turn tuned the liquid crystals to exhibit better response times shifting from one level of gray to another and is often referred to as gray-to-gray (GTG) response time.

Another metric to measure the response time performance of LCD TVs that is becoming more popular is the Motion Picture Response Time (MPRT) measurement. The MPRT measurement method takes into account three basic LCD panel characteristics: hold-type brightness variation, brightness level dependent transition times and liquid crystal response time. MPRT measurements are also impacted by how the backlight is controlled and the way video signals are processed.

Improvements in response times have enhanced front screen performance of LCD displays in general. In particular, blurring of object edges has been improved. In motion pictures where objects are moving quickly, blurring of edges can become a nuisance for the viewer. However, due to the nature of how LCDs work, even a response time of 0ms will not completely eliminated blurring.

LCD displays can be described as a hold-type display. This is different from CRT or plasma displays that use a method that excite phophors to generate brightness and color. On a LCD the liquid crystals hold their positions a certain amount of time to create different levels of brightness and color is generated via the color filter. This hold-type nature of LCDs make the reduction of blurring a difficult challenge compared to CRTs or plasmas. Other technologies are required to improve blur to a point that is undetectable by most users.

Motion interpolation is a form of video processing used to reduce blur in motion picture, which is called motion blur. The word interpolation is a method of constructing new data points within the range of a discrete set of known data points in the field of numerical analysis, a subfield of mathematics. Motion interpolation creates new frames based on two standard frames. The new frame is inserted between the two standard frames to smooth the picture and reduce motion blur. For motion interpolation to work, a typical display with a refresh rate of 60Hz will need to be doubled to 120Hz. Not all LCD TVs with 120Hz refresh rates use motion interpolation to reduce motion blur but a refresh rate of 120Hz allows for the implementation of motion interpolation that greatly improves the reduction of motion blur. An advanced LCD TV that incorporates both 120Hz and motion interpolation can yield MPRT figures of around 11ms to 12ms. If additional technologies such as a scanning backlight are incorporated MPRT can be reduced to about 8ms.

Major TV manufacturers have implemented 120Hz refresh rates on their LCD TVs: Hitachi’s Reel120, Philips’ HD Digital Natural Motion, Samsung’s Auto Motion Plus, Sony’s MotionFlow, and Toshiba’s ClearFrame, among others. Recently, Samsung has announced its Blue Phase LCD prototype that has a frequency that quadruples the original to 240Hz.

Stay tuned for the rest of the ingredients in the next weeks…

[tags]LCD TV, 120Hz, 240Hz, LED Backlight, Black Frame Insertion, Motion Interpolation, Fast Response Time, Over Driving Circuitry, Local Dimming, Full HD, 1080p, 1920 x 1080, RGB LED Backlight, Wide Color Gamut CCFL, WCG-CCFL, HDMI, DVI[/tags]

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