LCD
Technology: Response Time
Enabling crisp full-motion video
With the recent widespread use of PC and online games, and PCs
equipped with DVD drives give users more opportunities to see
moving images such as those in 3D games or action movies on
screen. This increase in motion picture content means computer
monitors must be able to display not only still images, but
moving ones as well.
Manufacturers and IT
publications often cite a fast response time as an indication
that a display can play videos or games with little or no
blurring. Hence, we would like to share with you what response
time is, and how helpful it is in determining how well an
individual LCD display can portray moving images.
Response time:
Why is it
increasingly important for LCD applications?
If response time is slow, the
transition from one picture (or frame) to another can produce an
afterimage or blurring effect. This problem occurs not only when
looking at motion pictures, but also during scrolling. For this
reason, panels with faster response times are typically
recommended for displaying moving images. Listed below are
calculations for the liquid crystal response times that LCD
displays meet, with consistent reliability, for various
application standards. Response time is measured in milliseconds
(ms, 1/100 second). The shorter the time frame, the better the
display quality.
Crisp Low Response Rate at Left and High Response Rate at Right.
Notice the blurring that occurs (right image) with High Response
Rates.
|
LCD RESPONSE RATES |
|
30 ms:1/0.030 = 33 fps |
meets specs of NTSC (30
fps), PAL (25 fps) or movie (24 fps) standards |
|
16 ms:1/0.016 = 63 fps |
meets
the spec of HDTV (60 fps) standards |
|
12 ms:1/0.012 = 83 fps
|
meets VESA flicker-free
display with CRT of 72 fps and human-eye perception |
|
8 ms:1/0.008 = 125 fps |
3D PC games requirement |
|
4 ms:1/0.004 = 250 fps |
Professional 3D PC games
requirement |
|
fps = frame (picture) per
second |
What is response
time?
The transition time when LC
materials are rotating on each of the required white/black or
gray levels is called "rise time" and "fall time," respectively.
Normally, the transition time of 256 x 256 LC rotation levels
needs to be measured. However, some companies don't measure
degree levels due to limitations of equipment capability.
Liquid crystals are rarely
completely turned on or off. Instead, they cycle in between gray
states. The following are two common methods some manufacturers
use to measure response time:
On-Off response time Refers
to the change time for screen pixels to turn from white to black
(Tr) and from black to white (Tf) when the screen receives the
signal. However, it does not indicate the transit time between
gray levels.
Gray-to-Gray response time:
Since virtually all moving
images include gray levels, and the frequency of gray-to-gray
transitions is typically far greater than black-and-white
transitions, we use the Gray-to-Gray response time definition to
address the gray-to-gray transition time, allowing us to make an
accurate assessment of a displays' suitability to portray moving
images.
At present, there is no
accepted standard for the computation of Gray-to-Gray response
time. However, as a company that emphasizes product reliability,
most manufacturers insist on using the average to gauge
performance, delivering better value to the end user.
How
some manufacturers accelerate response times and guarantees
reliable products:
Lower rotational viscosity
liquid crystal materials and reduced cell gap thickness enhance
"On-Off Response Time" performance.
To rapidly improve liquid
crystal on-off response time, some manufacturers have developed
products with lower rotational viscosity liquid crystal
materials and reduced cell gap thickness during the first stage.
Many manufacturers overcome
technical challenges such as non-uniformity and side effects
caused by new LC materials in the LC-cell manufacturing process.
Furthermore, new products undergo strict testing before
launch.
Higher voltage with overdriving technology reduces the
moving image's "Gray-to-Gray response time."
These quick response times
modeled with new LC materials and a thick cell gap have earned
such products much praise in the market in terms of capability
and reliability, encouraging their makers to keep seeking new
technologies for product upgrades. Models with overdriving
technology have been integrated into many LCD displays, from
manufacturers such as Acer, accelerating response times,
especially for gray-to-gray.
Faster
gray-to-gray response time via overdrive (OD) technology
The key benefit of OD
technology is the clear improvement of the gray-to-gray level,
which is the most important factor in the moving-picture viewing
experience. Liquid crystal molecules respond faster to the high
voltage that's needed for black-white transitions than to the
low voltage that's needed for transitions between gray areas.
Therefore, even though going from one grayscale level to another
is less of a jump than going from black to white, the
gray-to-gray transition time can actually take longer. Two LCD
panels with the same black-white response times but with
different gray-to-gray response times will have different moving
picture playback capabilities.
As the figures below show,
using an overdriving algorithm, LCD displays can reduce the
deviation in the transition time and approach ideal performance.
This significant improvement allows LCDs to deliver high-quality
moving pictures for 3D games and videos.
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