LCD Technology: Response Time
Enabling crisp full-motion videoWith 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, 1/1000 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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