Title: Method and apparatus for sparkle reduction by reactive and anticipatory slew rate limiting
Abstract: A circuit (10) for reducing errors due to adjacent pixel interdependence in a liquid crystal display includes a decomposer (12) for dividing an input signal into a plurality of signals having at least a high brightness signal and at least one low brightness signal, a delay match circuit (14) for the high brightness signal to provide a delay high brightness signal, one or more transient conditioner circuits (16 and/or 18) for processing the at least one low brightness signal to provide at least one transient conditioned low brightness signal, and a combiner (20) for combining the delay matched high brightness signal with the at least one transient conditioned low brightness signal to provide an output signal having reduced sparkle artifacts.
Patent Number: 6,961,039 Issued on 11/01/2005 to Willis
| Inventors:
|
Willis; Donald Henry (Indianapolis, IN)
|
| Assignee:
|
Thomson Licensing S.A. (Boulogne-Billancourt, FR)
|
| Appl. No.:
|
078904 |
| Filed:
|
February 19, 2002 |
| Current U.S. Class: |
345/88; 345/89; 345/204; 345/690 |
| Intern'l Class: |
G09G 003/36 |
| Field of Search: |
345/87-103,204,690
|
References Cited [Referenced By]
U.S. Patent Documents
| 5170152 | Dec., 1992 | Taylor.
| |
| 5526060 | Jun., 1996 | Raby.
| |
| 5786866 | Jul., 1998 | Sani et al.
| |
| 6344857 | Feb., 2002 | Matono et al.
| |
| 6359663 | Mar., 2002 | Gadeyne et al.
| |
| 2002/0126079 | Sep., 2002 | Willis et al.
| |
| 2002/0126080 | Sep., 2002 | Willis et al.
| |
| 2002/0126134 | Sep., 2002 | Willis et al.
| |
| 2003/0156091 | Aug., 2003 | Willis.
| |
| Foreign Patent Documents |
| 0457497 | Nov., 1991 | EP.
| |
| 08-088770 | Apr., 1996 | JP.
| |
Primary Examiner: Lao; Lun-yi
Attorney, Agent or Firm: Tripoli; Joseph S., Fried; Harvey D., Johnson; Christine
Claims
1. A circuit for reducing declination errors in a liquid crystal display, comprising:
a decomposer for dividing an input signal into a plurality of signals having
at least high, medium and low brightness signals;
at least one transient conditioner circuit including an anticipatory portion
and a reactive portion for limiting signal transients between brightness levels
in at least one of said medium and low brightness signals;
a delay match circuit for said high brightness signal; and,
means for combining the delayed high brightness signal with said at least one
signal transient processed brightness signal to provide an output signal, wherein
said output signal has reduced sparkle artifacts.
2. The circuit of claim 1, wherein the decomposer divides the input signal into
the high brightness signal, a medium brightness signal, and a low brightness signal
and the at least one transient conditioner circuit further comprises a second transient
conditioner circuit for processing the medium brightness signal to provide a processed
medium brightness signal.
3. The circuit of claim 2, wherein the combiner combines the processed high brightness
signal, the processed low brightness signal and the processed medium brightness signal.
4. The circuit of claim 1, wherein the decomposer further comprises at least
a threshold signal, wherein if the input signal is below the threshold signal,
then the processed high brightness signal is zero and the at least one processed
low brightness signal is the input signal and wherein if the input signal is above
the threshold signal, then the processed high brightness signal is the input signal
minus the threshold signal and the at least one processed low brightness signal
is the threshold signal.
5. The circuit of claim 2, wherein the decomposer further comprises a lower threshold
and an upper threshold, wherein if the input signal is greater than the upper threshold,
then the high brightness signal equals the input signal minus the upper threshold,
the medium brightness signal equals the upper threshold minus the lower threshold,
and the at least one low brightness signal equals the low threshold, and wherein
if the input signal is less than the upper threshold but greater than the lower
threshold, then the high brightness signal equals zero, the medium brightness signal
equals the input signal minus the lower threshold, and the at least one low brightness
signal equals the lower threshold, and wherein if the input signal is less than
the lower threshold, then the high brightness signal equals zero, the medium brightness
signal equals zero, and the at least one low brightness signal equals the input signal.
6. The circuit of claim 1, wherein the liquid crystal display is a liquid crystal
on silicon (LCOS) display.
7. The circuit of claim 1, wherein the at least one transient conditioner comprises
at least one recursive slew rate limiter.
8. The circuit of claim 1, wherein the at least one transient conditioner comprises
at least one finite response pre-conditioner for limiting bright going transients.
9. The circuit of claim 1, wherein the at least one transient conditioner comprises
at least one recursive slew rate limiter and at least one finite response pre-conditioner.
10. The circuit of claim 1, wherein the delay match circuit comprises a sample
delay circuit.
11. A method for reducing declination errors in a liquid crystal display, comprising
the steps of:
dividing an input signal into at least high, medium and low brightness signals
limiting, including an anticipatory limiting step and a reactive limiting step,
signal transients between brightness levels of at least one of said medium and
low brightness signals;
delay matching the high brightness signal; and,
combining said at least one limited brightness signal and said delayed high brightness
signal to form an output signal having reduced sparkle artifacts.
12. The method of claim 11, wherein the step of limiting includes a step of slew
rate limiting dark going transients of said at least one of said medium and low
brightness signals and finite response filtering bright going transients of said
at least one of said medium and low brightness signals.
13. The method of claim 12, wherein the step of slew rate limiting is carried
out asymmetrically.
14. The method of claim 12, wherein the step of limiting is carried out on said
medium brightness signal and further comprising the step of:
combining said slew rate limited and finite response filtered signal with said
high and low brightness signals.
15. The method of claim 14, comprising the steps of:
slew rate limiting and finite response filtering said medium brightness signal;
and,
applying different slew rates and different finite filter responses to said medium
and low brightness signals.
16. The method of claim 14, comprising the steps of:
slew rate limiting and finite response filtering said medium brightness signal;
and,
applying different slew rates and different finite filter responses to said medium
and low brightness signals.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
(not applicable)
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of video systems utilizing a liquid crystal
display (LCD), and in particular, to video systems utilizing normally white liquid
crystal on silicon imagers.
2. Description of Related Art
Liquid Crystal on Silicon (LCOS) can be thought of as one large liquid crystal
placed over a silicon wafer. The silicon wafer is divided into an incremental array
of tiny plates. A tiny incremental region of the liquid crystal is influenced by
the electric field generated by each tiny plate and a common plate. Each such tiny
plate and corresponding liquid crystal region are together referred to as a cell
of the imager. Each cell corresponds to an individually controllable pixel. Each
tiny plate is also a mirror for reflecting back a cell's light. A common plate
electrode is disposed on the other side of the liquid crystal.
The drive voltages are supplied to plate electrodes on each side of the LCOS
array. In the presently preferred LCOS system to which the inventive arrangements
pertain, the common plate is always at a potential of 8 volts. Each of the other
plates in the array of tiny plates is operated in two voltage ranges. For positive
pictures, the voltage varies between 0 volts and 8 volts. For negative pictures
the voltage varies between 8 volts and 16 volts.
The light supplied to the imager, and therefore supplied to each cell of the
imager, is field polarized. Incoming light is incident upon the common electrode
which is transparent. Each liquid crystal cell rotates the polarization of the
input light responsive to the RMS value of the electric field applied to the cell
by the plate electrodes. Generally speaking, the cells are not responsive to the
polarity (positive or negative) of the applied electric field. Rather, the brightness
of each pixel's cell is generally only a function of the rotation of the polarization
of the light incident on the cell. Furthermore, polarization rotation for each
cell is a non-linear function of the electric field. Polarization rotation for
a given cell occurs as the light passes through the liquid crystal both before
and after reflection from the cell plate. It is the rotation of the polarization
that is capable of being controlled. Light leaving the imager is approximately
the same intensity, but a different polarization. This may depend on the intensity
that is ultimately desired. It should be noted that it is undesirable to have the
imager absorbing light because it can get too hot. The imager will get hot due
to some spurious amount of absorption.
If adjacent pixels produce different brightness, then there must be a different
potential on the 2 cell plates corresponding to the adjacent pixels. When potentials
on adjacent cell plates are unequal, there is an electric field between them which
is known as a fringing field. The fringing field has some components, which are
orthogonal to the desired field. These orthogonal components are not a problem
in the space between adjacent mirrors. But, the orthogonal components of the electric
field, which is over the mirror, will have the effect of distorting the polarization
rotation. This distortion results in a substantial local increase in brightness.
This is a particular problem when the pixel is supposed to be dark, but is usually
an insignificant problem when the pixels are intended to be bright since the pixels
are not very different in voltage so the fringing field is not that great. Also,
for dark pixels, the additional brightness is much more noticeable. Contrast ratio
is also very important in making a high quality display. It is very important to
achieve sufficient black level. A proportionately larger drive voltage is needed
to create a slightly darker image in a normally white display. Often, a large difference
in voltage between adjacent pixels is needed. This results in a major fringing
field that produces a visible artifact denoted sparkle. Due to the rotational effects
of the fringing fields, this phenomenon is also referred to as a declination error
in the imager. Sparkle artifacts can be red, blue and/or green, but green is usually
the most prominent color.
Because of the particular manufacturing process used for many imagers, horizontally
adjacent pixels suffer more from the fringing field problem. Thus, a need exists
for overcoming the sparkle problem described above.
SUMMARY
A circuit for reducing declination errors in a liquid crystal display, in accordance
with the inventive arrangements, comprises: a decomposer for dividing an input
signal into a plurality of signals having at least a high brightness signal and
at least one low brightness signal; at least one transient conditioner circuit
for reducing declination errors by limiting signal transients between brightness
levels in said at least one low brightness signal; a delay match circuit for said
high brightness signal; and, means for combining the delayed high brightness signal
with said at least one signal transient processed low brightness signal to provide
an output signal, wherein said output signal has reduced sparkle artifacts. The
at least one transient conditioner can comprise at least one slew rate limiter
and at least one finite response filter.
A method for reducing adjacent pixel interdependence in a liquid crystal display,
in accordance with the inventive arrangements, comprises the steps of: dividing
an input signal into at least a high brightness signal and at least one low brightness
signal; slew rate limiting and finite response filtering the at least one low brightness
signal to reduce adjacent pixel interdependence by limiting signal transients between
brightness levels; delay matching the high brightness signal; and, combining the
at least one slew rate limited and finite response filtered low brightness signal
and the delayed high brightness signal to form an output signal having reduced
sparkle artifacts.
The method can further comprise the step of slew rate limiting dark going transients
of the at least one low brightness level signal and finite response filtering bright
going transients of the at least one low brightness level signal. The slew rate
limiting can be asymmetric.
The method can further comprise the steps of: further dividing the input signal
into a medium brightness signal having brightness levels between the high and low
brightness level signals; limiting signal transients between brightness levels
of the medium brightness signal to further reduce adjacent pixel interdependence;
and, combining the slew rate limited and finite response filtered signal with the
high and low brightness signals. The medium brightness signal can be slew rate
limited and finite response filtered. Different slew rates and different finite
filter responses can be applied to the medium and low brightness signals.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a decomposer, transient conditioners, and
a delay match circuit in accordance with the present invention.
FIG. 2 is a more detailed block diagram showing the transient conditioner in
accordance with the present invention.
FIG. 3 is a graph illustrating the operation of a system in accordance with
the present invention.
FIG. 4 is a flow chart illustrating a method in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reducing the difference in brightness between adjacent pixels when they
are dark, but not when they are bright can resolve the sparkle problem previously
described. A device called a decomposer
12 on the input divides the input
signal into at least two signals on a circuit
10 used to reduce adjacent
pixel interdependence in liquid crystal displays as shown in FIG.
1. It
should be understood that Sparkle or declination errors can also be considered
a subset of a broader phenomenon known as adjacent pixel interdependence. It should
be noted that the present invention is particularly useful for liquid crystal on
silicon (LCOS) displays but is not necessarily limited thereto. The decomposer
12 serves as an amplitude discriminator for the input signal which is preferably
an eight (8) bit video signal that preferably carries the desired brightness of
one color component (Red, Green, or Blue).
The input signal is decomposed in a manner that enables obtaining the original
signal when the decomposed or divided signals are added or combined back together.
The method in accordance with the present invention would further process the low
brightness portion (L and optionally M) and delay match the high brightness portion
(H). Then, the signals are recombined and sent to the imager. The same technique
can also be applied to the luminance signal (only need one of these decomposed).
Accordingly, the improved approach relies upon one decomposer for each color (Red,
Green, & Blue). It should be understood that the decomposer can divide the input
signal into two or more component signals within contemplation of the present invention.
The decomposer must have at least two inputs, a threshold input and a brightness
input signal. If a single threshold is used and the brightness input signal is
below the threshold, then the high output is zero (0) and the low output is the
brightness input signal itself. If the brightness input signal is above the threshold,
then the high output is the brightness input signal minus the threshold input and
the low output is the threshold input signal.
Referring once again to FIG. 1, the circuit
10 comprises the decomposer
12 for dividing an input signal into a plurality of signals having at least
a high brightness signal (H) and at least one low brightness signal (L). As shown
in the embodiment of FIG. 1, the input signal is optionally divided into three
(3) signals including a high brightness signal (H), a medium brightness signal
(M), and a low brightness signal (L). When the input signal is divided into three
signals, two threshold signals (Tu and Tl) are preferably used by the decomposer
12. The circuit
10 further comprises a delay match circuit
14
for processing the high brightness signal to provide a match delayed high brightness
signal and at least one transient conditioner (
18) circuit for processing
the at least one low brightness signal to provide at least one transient conditioned
low brightness signal. When the input signal is divided into more than two signals,
the "lower" brightness signals (M and L in FIG. 1) are preferably processed using
additional transient circuits as needed. In the embodiment shown in FIG. 1, the
low brightness signal (L) is processed using the transient conditioner
18
to provide a transient conditioned low brightness signal and a second transient
conditioner circuit
16 processes the medium brightness signal to provide
a transient conditioned medium brightness signal. The transient conditioners preferably
comprise an anticipatory portion and a reactive portion. The transient conditioner
circuits preferably comprise at least one recursive slew rate limiter for limiting
dark going transients and at least one finite response conditioner or pre-conditioner
for limiting bright going transients as will become apparent with reference to
FIG.
2. The recursive slew rate limiter is the reactive portion and the
finite response pre-conditioner is the anticipatory portion. The circuit
10
also comprises a combiner
20 for combining the processed high brightness
signal with the at least one transient conditioned low brightness signal to provide
an output signal, wherein the output signal has reduced sparkle artifacts. The
combiner
20 of circuit
10 in particular combines the match delayed
high brightness signal, the transient conditioned low brightness signal and the
transient conditioned medium brightness signal. In this instance, the "at least
one transient conditioned low brightness signal" includes both the transient conditioned
low brightness signal and the transient conditioned medium brightness signal.
When the decomposer
12 utilizes only a single threshold signal (Tl for
example) and the input signal is below the threshold signal, then the high brightness
signal is zero and the low brightness signal is the input signal and if the input
signal is above the threshold signal, then the high brightness signal is the input
signal minus the threshold signal and the low brightness signal is the threshold signal.
If the decomposer
12 includes a lower threshold and an upper threshold,
wherein if the input signal is greater than the upper threshold, then the high
brightness signal equals the input signal minus the upper threshold, the medium
brightness signal equals the upper threshold minus the lower threshold, and the
low brightness signal equals the low threshold, and wherein if the input signal
is less than the upper threshold but greater than the lower threshold, then the
high brightness signal equals zero, the medium brightness signal equals the input
signal minus the lower threshold, and the low brightness signal equals the lower
threshold, and wherein if the input signal is less than the lower threshold, then
the high brightness signal equals zero, the medium brightness signal equals zero,
and the low brightness signal equals the input signal. The scenario above where
the decomposer
12 divides the input signal into three signals, a high (H),
a medium (M), and a low (L) signal, can be summarized as follows:
Tl represents a lower threshold.
Tu represents an upper threshold.
Referring to FIG. 2, a transient conditioner
50 is shown in accordance
with the present invention. The transient conditioner preferably comprises an anticipatory
part and a reactive part. The anticipatory part consists of a sample delay line
implemented with latches (
52,
54,
56,
58,
60,
and
62), and with additional processing at each tap (
51,
53,
55,
57,
59, and
61). A slew rate value S, is utilized
to perform a subtraction at each tap except the output tap. The magnitude of the
constant subtracted is proportional to the time delay between that tap and the
output. Thus, the second to last tap (
61) has S subtracted from its value,
the third to last tap (
59) has 2S subtracted, the fourth to last tap (
57)
has 3S subtracted and so on. Then, the maximum (
64) of all the resulting
values is chosen and passed on. If a high positive value is coming, the output
signal is prepared for this by starting its increase early, but at no time is the
increase in the output from one sample period to the next sample period greater
than S. By the time the high value arrives at the output of sample delay
62,
it can be safely used unaltered if it is the maximum.
The reactive part of the transient conditioner
50 consists of a simple
slew rate limiter, which limits the slew rate of only negative going transients
to -S. This consists of a subtracter (
66), a MAX circuit
68, an adder
70, and a one-sample-delay latch
72. If the positive input to the
subtractor is lower than the output by more than S, then the new output equals
the previous output minus S.
The two transient conditioners can advantageously be set to different slew rates.
Different positive and negative slew rate limits can also be advantageously selected,
although such a selection is not shown in FIG.
2. The threshold or thresholds
can also advantageously be independently selected.
Referring to FIG. 3, an example of the operation of a system in accordance
with the present invention is shown in the graph. For this example, the upper threshold
is
60, the lower threshold is
15, the middle slew limit is
20,
and the lower slew limit is
3. In many instances, these values are chosen
by trial and error. Too much sparkle correction can cause an undesirably noticeable
reduction in resolution, so that the threshold values and the slew rates represent
a compromise between reducing declination errors and maintaining picture sharpness.
Referring to FIG. 4, a flow chart illustrating a method
400 for
reducing adjacent pixel interdependence in a liquid crystal display is shown. The
method preferably comprises the steps of dividing (
402) an input signal
based on the amplitude of the brightness of the input signal into at least a high
brightness signal and at least one low brightness signal, processing (
404)
the at least one low brightness signal preferably with at least one recursive slew
rate limiter to limit dark going transients and at least one finite response pre-conditioner
to limit bright going transients to provide at least one transient conditioned
low brightness signal, and processing (
406) the high brightness signal with
a sample delay to provide a match delayed high brightness signal. The method further
comprises a step
408 of combining the match delayed high brightness signal
with the at least one transient conditioned low brightness signal to provide an
output signal, wherein the output signal has reduced sparkle artifacts, declination
errors or adjacent pixel interdependence. The step of dividing optionally divides
the input signal into a high brightness signal, a medium brightness signal, and
a low brightness signal wherein the step of processing the high brightness signal
comprises delay matching the high brightness signal and wherein the step of processing
the at least one low brightness signal comprises transient conditioning the low
brightness signal and the medium brightness signal preferably using at least one
transient conditioner circuit. The method thus asymmetrically limits the slew rate
of predominantly low brightness transients.
Although the present invention has been described in conjunction with the
embodiments disclosed herein, it should be understood that the foregoing description
is intended to illustrate and not limit the scope of the invention as defined by
the claims.
*
