Browse Prior Art Database

Liquid Crystal Display Gray Scale Method using a Pseudo Linear Algorithm

IP.com Disclosure Number: IPCOM000110551D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2005-Mar-25

Publishing Venue

IBM

Related People

Stein, F: AUTHOR

Abstract

This invention relates generally to the display of information on a Liquid Crystal Display (LCD). More particularly, it relates to a method of driving the LCD to produce shades of gray on the display, also called a gray scale.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 14% of the total text.

Liquid Crystal Display Gray Scale Method using a Pseudo Linear Algorithm

       This invention relates generally to the display of
information on a Liquid Crystal Display (LCD).  More particularly, it
relates to a method of driving the LCD to produce shades of gray on
the display, also called a gray scale.

      Several methods for producing gray scales are disclosed in the
prior art.  These include individually controlling an analog voltage
applied to each picture element or pixel of the LCD; controlling the
pulse width (but not amplitude) of the voltage applied to each pixel;
and controlling the frequency of the pulses (but not the amplitude or
width) applied to each of the pixels.  Of the three, the latter
method is the most common.  In this method, the full brightness is
obtained by turning a pixel on each frame, and a 50 percent
brightness is obtained by turning the pixel on every other frame.

      The pulse frequency method causes several visual problems.
These include a flickering of some pixels, and difficulty generating
linear looking gray scale values.  The flicker is caused by turning
the pixels on and off at a low enough frequency to be visible to the
human eye.  As the number of shades of gray is increased, it becomes
necessary to use a wider range of duty cycles, which results in lower
and more visible flicker frequencies.  For example, 8 shades of gray
may utilize 0/8, 1/8, 2/8, 3/8, 5/8, 6/8, 7/8, 8/8.  The 1/8 means
that the pixel is turned on only once every 8 frames.  If the frame
frequency is 60 Hz, then this pixel would be on once every 133
mseconds, which is slow enough to be plainly visible to the human
eye.  The 7/8 case, where the pixel is turned off once every 8
cycles, also has this slow repetition frequency and is similarly
visible.

      The more pixels which are turning on and off at the same time,
the more visible this flicker is.  One prior-art attempt to reduce
flicker by partitioning the pixels within the LCD into multiple
display groups, each composed of a predetermined number of adjacent
pixels.  If the pixels in one display group are equal in intensity
level, these pixels are turned on with the same duty cycle, but with
different phase.  This helps somewhat to reduce the flicker but does
not eliminate it.

      The objectives of this invention are:
       -  To minimize the flicker in the generation of gray scale.
       -  Produce a substantially linear scale of gray values.
CONCEPT

      The light intensity of a shade is approximately proportional to
the duty ratio of the driving waveform.  Sixteen shades can be
produced by using different duty ratios; one for each shade.  It is
common to use duty ratios that have sixteen as a divisor (i.e., 1/16,
8/16, 11/16, etc.) because they are easily produced with digital
circuits.  However, a low frame rate can cause some of these duty
ratios to produce useless shades due to flicker or swimming effects....