Title: Conversion of a video signal for driving a liquid crystal display
Abstract: Method and apparatus for the conversion or generation of a video signal intended to be displayed on an image display with different luminance response times for rise and fall. The conversion or generation of the video signal is so that motion artefacts which are caused by the difference in luminance response times for rise and fall such as large area luminance jumps, large-area flicker and faulty temporary large-area luminance are fundamentally cancelled in the displayed image.
Patent Number: 6,909,472 Issued on 06/21/2005 to Gadeyne, et al.
| Inventors:
|
Gadeyne; Koen (Anzegem, BE);
Vandenberghe; Patrick (Hareelbeke, BE)
|
| Assignee:
|
Barco N.V. (Poperinge, BE)
|
| Appl. No.:
|
061185 |
| Filed:
|
February 4, 2002 |
Foreign Application Priority Data
| Current U.S. Class: |
348/750 |
| Intern'l Class: |
H04N 003/14 |
| Field of Search: |
348/790,791,792,793,674,675,613,616,625
345/63,87,89,904,95
|
References Cited [Referenced By]
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| 3643011 | Feb., 1972 | Engel et al.
| |
| 4855831 | Aug., 1989 | Miyamoto et al.
| |
| 4888529 | Dec., 1989 | Madsen et al.
| |
| 4910598 | Mar., 1990 | Itakura et al.
| |
| 5396157 | Mar., 1995 | Hackett et al.
| |
| 5416599 | May., 1995 | Ubukata et al.
| |
| 5438342 | Aug., 1995 | Yamaguchi.
| |
| 5526129 | Jun., 1996 | Ko.
| |
| 5592190 | Jan., 1997 | Okada et al.
| |
| 5619224 | Apr., 1997 | Hoshino et al.
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| 5619349 | Apr., 1997 | Ueda et al.
| |
| 5627555 | May., 1997 | den Hollander.
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| 5936621 | Aug., 1999 | Medin et al.
| |
| Foreign Patent Documents |
| 0 300 754 | Jan., 1989 | EP.
| |
| 0 487 140 | Nov., 1990 | EP.
| |
| 0 553 865 | Aug., 1993 | EP.
| |
| 0 603 713 | Jun., 1994 | EP.
| |
| 0 608 056 | Jul., 1994 | EP.
| |
| 2 191 667 | Dec., 1987 | GB.
| |
| 04-288589 | Oct., 1992 | JP.
| |
| WO 94/0947/5 | Apr., 1994 | WO.
| |
| WO 94/2353/2 | Oct., 1994 | WO.
| |
| WO 97/1235/5 | Apr., 1997 | WO.
| |
| WO 97/3327/1 | Sep., 1997 | WO.
| |
Other References
Okumura et al., "32.3: A New Low-Image-Lag Drive Method for Large-Size LCTVs,"
SID 92 Digest, Toshiba R & D Center (Kawasaki, Japan), p. 601-604, (1992).
McCartney et al., "The Primary Flight Instruments for the Boeing 777 Airplane,"
98/SPIE vol. 2219, Honeywell, Inc. (Phoenix, Arizona), p. 98-107, (1994).
Haim et al., "A2.2: Full-Color Gray-Scale LCD with Wide Viewing Angle for Avionics
Applications.," SID 94 Applications Digest, Honeywell, Inc. (Phoenix, Arizona),
p. 23-26, (1994).
McCartney et al., "Performance Testing of the Primary Flight Instruments for
the Boeing 777 Airplane," 86/SPIE vol. 2734, Honeywell, Inc. (Phoenix, Arizona),
p. 86-93, (1996).
Schönfelder, H., "Digitale Filter in der Videotechnik", pp. (besides front
3) 10, 79-82, 113-115, 125-128, 204-206, 208-209, 212 and 213. Published by Drei-R-Verlag,
Berlin, Germany (1988). See IDS filed Feb. 10, 2003.
|
Primary Examiner: Miller; John
Assistant Examiner: Tran; Trang U.
Attorney, Agent or Firm: Pillsbury Winthrop LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of application Ser. No. 09/459,853 filed
Dec. 14, 1999 which was a continuation of International Application No. PCT/EP99/02050
filed Mar. 25, 1999, both of which claim priority from European Application No.
98870086.0 filed Apr. 17, 1998, the contents of all three applications being incorporated
hereinto by this reference thereto.
Claims
1. A method for video signal conversion, said method comprising:
receiving a first video signal; and
producing a second video signal for display on a display device with different
luminance rise and decay times, the display device comprising a display screen,
and operating at a frame period,
wherein said producing a second video signal includes changing at least one of
a level and a duration of at least a portion of the first video signal such that
the luminance response of a picture element of the display device to a change of
the first video signal from a first amplitude value to a second amplitude value
is substantially equal in shape, time, and amplitude but reversed in slope compared
to the luminance response of a picture element of the display device to a change
of the first video signal from the said second amplitude value to the said first
amplitude value.
2. A method according to claim 1, whereby the said luminance responses are substantially
equal in amplitude and not slower than the luminance response of the same or another
picture element of the display device caused by the first video signal displayed
without conversion.
3. A method according to claim 1, whereby the said luminance responses are substantially
equal to predefined luminance responses.
4. A method according to claim 1, whereby the conversion of the first video signal
into the second video signal is so that the second video signal is built up in
consecutive steps during corresponding consecutive correction periods.
5. A method according to claim 4, whereby at the start of a correction period,
for the determination of the next step, one or more of the following parameters
are taken into account:
the present luminance of the picture element as predicted at the instant of the
previous correction period,
the present amplitude of the first video signal,
the physical location of the picture element on the display screen,
the present temperature at the location of the picture element.
6. A method according to claim 4 wherein a correction period is equal to a multiple
of the frame period of the first video signal.
7. A method according to claim 1, wherein the frame rate of the second video
signal is different from the frame rate of the first video signal.
8. A method according to claim 1, wherein the conversion of the first video signal
into the second video signal is so that the luminance response of a picture element
of the display screen to a change of the first video signal is slowed down in order
to match the luminance response in time and amplitude to the known slower luminance
response of the same or another picture element of the display device for the opposite
change of the first video signal.
9. A method according to claim 1, wherein the conversion of the first video signal
to the second video signal is so that the luminance response of a picture element
of the display screen to a change of the first video signal is accelerated in order
to match the luminance response in time and amplitude to the known faster luminance
response of the same or another picture element of the display device for the opposite
change of the first video signal.
10. A method according to claim 2, Whereby the conversion of the first video
signal into the second video signal is so that the second video signal is built
up in consecutive steps during corresponding consecutive correction periods.
11. A method according to claim 3, whereby the conversion of the first video
signal into the second video signal is so that the second video signal is built
up in consecutive steps during corresponding consecutive correction periods.
12. Apparatus for converting a first video signal into a second video signal,
the second video signal being for display on a display device comprising picture
elements with difference luminance rise and decay times, the display device further
comprising a display screen and operating at a frame period, comprising:
a device to convert the first video signal to the second video signal such that
the second video signal causes the luminance response of a picture element of the
display device to a change of the first video signal from a first amplitude value
to a second amplitude value to have substantially the same amplitude/time characteristic
but inverse in slope compared to the luminance response of the same or another
picture element of the display device to a change of the first video signal from
the said second amplitude value to the said first amplitude value.
13. Apparatus as in claim 12, further including a device which modifies the luminance
response time of the picture element by stepwise formation of the second video signal.
14. The apparatus according to claim 12, whereby the said luminance responses
are substantially equal to predefined luminance responses.
15. The apparatus according to claim 12, wherein the device is configured to
build up the second video signal in consecutive steps during corresponding consecutive
correction periods, and
wherein at the start of a correction period, for the determination of the next
step, one or more of the following parameters are taken into account:
the present luminance of the picture element as predicted at the instant of the
previous correction period,
the present amplitude of the first video signal,
the physical location of the picture element on the display screen,
the present temperature at the location of the picture element.
16. The apparatus according to claim 12, wherein the device is configured to
build up the second video signal in consecutive steps during corresponding consecutive
correction periods, each correction period being substantially equal to i times
the frame period of the first video signal, where i is a positive nonzero integer.
17. The apparatus according to claim 12, wherein the frame rate of the second
video signal is different from the frame rate of the first video signal.
18. The apparatus according to claim 12, wherein the device is configured to
convert the first video signal into the second video signal so that the luminance
response of a picture element of the display screen to a change of the first video
signal is slowed down in order to match the luminance response in time and amplitude
to the known slower luminance response of the same or another picture element of
the display device for the opposite change of the first video signal.
19. The apparatus according to claim 12, wherein the device is configured to
convert the first video signal to the second video signal so that the luminance
response of a picture element of the display screen to a change of the first video
signal is accelerated in order to match the luminance response in time and amplitude
to the known faster luminance response of the same or another picture element of
the display device for the opposite change of the first video signal.
20. The apparatus according to claim 12, wherein said device modifies the luminance
response time of the picture element by stepwise formation of the second video signal.
Description
FIELD OF THE INVENTION
The present invention relates to the display of images on image displays with
different luminance rise and fall response times, such as liquid crystal displays,
in particular to the display of TV pictures and/or data information on a video
display system equipped with a liquid crystal display device.
DESCRIPTION OF RELATED ART
The display of video images on display devices such as a Cathode Ray Tube (CRT)
or a Liquid Crystal Display (LCD) is a known art. Image displays equipped with
such CRT or LCD display devices are capable of displaying on a display screen images
consisting of a number of picture elements (or pixels) which are refreshed at a
refresh rate generally above 25 Hz. These images may be monochromatic, multicolor
or full-color. Common standards are in use to display the images as a succession
of frames.
The light of the successive frames which are displayed on the display screen
of such a CRT or LCD display device is integrated by the human eye. If the number
of displayed frames per second-further called the frame rate-is sufficiently high,
an illusion of the images being displayed in a continuous way, and therefore an
illusion of motion, can be created.
The way images are formed on the display screen of a CRT display device is fundamentally
different from the way images are formed on the display screen of an LCD display device.
On a CRT display device, the luminance of a picture element is produced by an
area of a phosphor layer in the display screen when the area is hit by a writing
electron beam.
On an LCD display device, the luminance of a picture element is determined by
the light transmittance state of one or more liquid crystal elements in the display
screen of the LCD display device at the location of the picture element, whereby
the light itself originates from ambient light or a light source.
For a faithful reproduction of moving images or moving parts of an image, the
luminance response of the display device being used is of utmost importance.
The luminance responses and the luminance response times of display screens are
known to be very different for CRT and LCD display devices. The luminance response
time, being the time needed to reach the correct luminance on the display screen
in response to an immediate change in a corresponding drive signal is shorter than
a frame period for a CRT display device but up to several frame periods for a typical
LCD display device according to the state of the art.
For LCD display devices, the luminance responses and luminance response times
are also known to be different for a darker-to-brighter luminance transition as
compared to the responses and response times for a similar brighter-to-darker luminance
transition. Furthermore, the luminance responses and luminance response times are
temperature dependent, drive voltage range dependent and, due to production tolerances,
unequal over the LCD screen area (location dependent).
Various solutions are known for changing luminance response times with LCD
display devices. They however have the aim to shorten the overall luminance response
times, not to make the luminance rise and fall times equal. EP 0 487 140 discloses
a method for speeding up display response times by doubling the display frame rate.
The luminance rise and fall times remain different. EP 0 553 865 describes luminance
flicker phenomena related to luminance response, but these phenomena are not due
to the difference between luminance rise and fall times, but rather to how image
lines are written.
There exist a number of images, further referred to as specific images, which
when moved over a display screen with different luminance rise and fall times,
give rise to visible and measurable artefacts in the displayed image, even when
the luminance responses are shortened.
It is characteristic of such specific images that they contain a number of isolated
or clustered picture points, which are in high contrast to their surroundings in
the image.
The artefacts are due to the difference between luminance rise and fall times,
which is typical for an LCD display device. This causes the luminance fall (or
rise) of a white spot at a first location to be different from the simultaneous
luminance rise (or fall) of a white spot at a second location, when the white spot
is moved from the first to the second location. The total luminance integrated
over the screen area immediately before, during and after the movement of the white
point is not constant. The integrated luminance shows a "luminance jump".
In practice, the artefacts will only be visible when more picture elements change
luminance at the same time within the observation field of the viewer.
In practice, various different artefacts may appear dependent on various parameters
such as the difference between luminance rise and fall times, the frame rate of
the displayed image, the video signal levels, the speed with which the image is
moved over the screen, the image content.
The visible artefacts cause the quality of the displayed image to range from
being inferior to unacceptable. The known solutions of increasing the frame rate
do not fundamentally solve the problems but only make them in the best case less perceptible.
SUMMARY OF THE INVENTION
It is the aim of this present invention to remove luminance jumps and visible
artefacts resulting from luminance jumps in a displayed image during and immediately
after the movement of the image, the luminance jumps and the artefacts being caused
by a difference in luminance rise and fall times of the display screen on which
the image is displayed.
This is obtained by a method for converting a first video signal into a second
video signal, the second video signal being intended for being displayed on a display
device with different luminance rise and fall times, which comprises a display
screen, and which operates at a frame period. The conversion is so that the second
video signal causes the luminance time response of a picture element of the image
to a change of the first video signal from a first amplitude value to a second
amplitude value to be substantially equal in shape and amplitude but reversed (i.e.,
inverted) in slope compared to the luminance time response of the same or another
picture element of the image to a change of the first video signal from the second
amplitude value to the first amplitude value. The luminance time responses can
be made substantially equal to predefined luminance time responses.
The luminance time responses can be made substantially equal in amplitude and
not slower than the luminance response of the same or another picture element which
would be caused by the first video signal if this were displayed without conversion.
The choice of the same or another picture element can be the same picture element
itself, a reference picture element from a selected group of picture elements (e.g.
a window) to which the same picture element belongs, any picture element which
can be displayed on the display screen of the display device. The chosen same or
another picture element can be that picture element of all picture elements which
are aimed to be displayed of which the luminance response is the slowest. The conversion
permits the compensation of the unevenness of the luminance rise and fall times
over the surface of the display screen, as well as the compensation of the temperature
dependency of the luminance rise and fall times.
According to a preferred embodiment, the conversion is such that the second
video signal is built up in real time in consecutive steps during corresponding
consecutive correction periods.
For the determination of a next step, one or more of the following parameters
may be taken into account at the start of a correction period:
- the present luminance of the picture element as predicted at the instant
of the previous correction period,
- the present amplitude of the first video signal,
- the physical location of the picture element on the display screen,
- the present temperature at the location of the picture element.
Preferably, a correction period is equal to a multiple of the frame period
of the second video signal.
Preferably, the frame rate of the second video signal is a multiple of
the frame rate of the first video signal.
According to an embodiment of the present invention, the conversion of
the first video signal into the second video signal is so that the faster luminance
response of a picture element to a change of the first video signal is slowed down
in order to match the luminance response in time and amplitude to the known slower
luminance response of the same or another picture element for the opposite change
of the first video signal.
According to another embodiment of the present invention, the conversion
of the first video signal to the second video signal is so that the slower luminance
response of a picture element to a change of the first video signal is accelerated
in order to match the luminance response in time and amplitude to the known faster
luminance response of the same or another picture element for the opposite change
of the first video signal.
According to another embodiment of the present invention, the conversion
of the first video signal to the second video signal is so that the second video
signal causes the luminance time response of a picture element to a change of the
first video signal from a first amplitude value to a second amplitude value to
be substantially equal in shape and amplitude but inverted in slope compared to
the luminance time response of the same or another picture element for a change
of the first video signal from the second amplitude value to the said first amplitude
level, the luminance responses being equal to predefined luminance responses.
Furthermore, an apparatus is disclosed and claimed for carrying out
a method as described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1
a, 1
b and 1
c illustrate the display
of a specific video signal and its scrolling down over the display screen;
FIG. 2 illustrates the display of a specific "text window" video signal and
its movement over the display screen;
FIGS. 3
a, 3
b, and 3
c illustrate the movement
of a white point between a first location and a second location on a display screen;
FIG. 4 shows luminance responses on a display screen of which the luminance
rise time is shorter than the luminance fall time, when a white point moves from
a first to a second location (prior art);
FIG. 5 shows luminance responses on a display screen of which the luminance
rise time is longer than the luminance fall time, when a white point moves from
a first to a second location (prior art);
FIGS. 6
a, 6
b, and 6
c illustrate a horizontal
movement of two white points on a display screen;
FIGS. 7
a, 7
b, 7
c illustrate a horizontal
movement of three white points on a display screen;
FIGS. 8
a, 8
b, and 8
c illustrate a vertical
movement of two white points on a display screen;
FIGS. 9
a, 9
b, and 9
c illustrate a movement
of a cluster of white points on a display screen;
FIG. 10 illustrates a movement in three steps of a white point on a display screen;
FIG. 11 shows a luminance response on a display screen of which the luminance
rise time is longer than the luminance fall time, when a white point moves on the
display screen during three consecutive frame periods (prior art);
FIG. 12 shows a prior art connection of a video generator to an image display;
FIG. 13 is a block diagram of an embodiment of the present invention;
FIG. 14
a shows a waveform of a first video signal corresponding to an
image point which changes first from black to white and later from white to black;
FIG. 14
b shows a waveform of a prior art RMS drive voltage to an individual
liquid crystal cell in an LCD display screen to let it change luminance first from
black to white and later from white to black;
FIG. 15
a shows the luminance response of a picture element on an LCD
display screen of which the luminance rise time is shorter than the luminance fall
time, according to the present invention and compared to prior art;
FIG. 15
b shows a waveform according to the present invention of a RMS
drive voltage to an individual crystal cell in an LCD display screen to let it
change luminance first from black to white and later from white to black;
FIG. 15
c shows a waveform according to the invention of a second video
signal corresponding to a picture element which changes first from black to white
and later from white to black;
FIG. 16 shows how a luminance response is controlled according to the invention;
FIG. 17
a shows the luminance response of a picture element on an LCD
display screen of which the luminance rise time is longer than the luminance fall
time, according to the present invention and compared to prior art;
FIG. 17
b shows a waveform according to the present invention of an RMS
drive voltage to an individual crystal cell in an LCD display screen to let it
change luminance first from black to white and later from white to black;
FIG. 17
c shows a waveform according to the present invention of a second
video signal corresponding to a picture element which changes first from black
to white and later from white to black;
FIG. 18 shows a stand-alone apparatus according to the present invention;
FIG. 19 shows an apparatus according to the present invention, connected between
a video generator and an image display;
FIG. 20 shows a video generator with a built-in apparatus according to the present
invention, which is connected to an image display; and
FIG. 21 shows a video generator which is connected to an image display which
contains an apparatus according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
A first example of a specific image is illustrated in FIG. 1
a, FIG. 1
b
and FIG. 1
c. An image display
1 has on its display screen
2
a specific image
3 characterized by a high noise content, the image scrolling
down at such a slow speed that the scrolling steps are individually perceptible.
FIG. 1
b shows an enlarged part
4 of the specific image
3,
its location referred to the image being shown in FIG. 1
a. FIG. 1
b and
FIG. 1
c illustrate a downward scrolling step equal to the difference between
the distance
5 of a bright image point
6 to the top border of the
image before a scrolling step and the distance
7 after the scrolling step.
A second example of a specific image is illustrated in FIG.
2 and shows
a window
8 with text inside, which is moved over a display screen
2
from a location
9 to a location
10. Depending on the luminance rise
and fall responses of the display device and on the scrolling or movement speed,
artefacts are seen as a large-area luminance flash, large-area luminance flicker,
or a temporary faulty large-area luminance.
FIGS. 3
a,
3b and
3c illustrate the movement
on a display screen of a white point
11 with the dimension of a picture
element from a first position
12 (FIG. 3
a) to a second position
13
(FIG. 3
c). Only a small part
14 of the display screen enclosing the
two locations
12 and
13 is shown in an enlarged way.
When the white point changes instantly from the first position
12 to
the said second position
13, the following happens.
On a display screen of which the luminance response is immediate, the white point
will at the same instant fully disappear at the first location
12 and fully
reappear at the second location
13; the luminance integrated over the screen
area
14 at time instances before, during and after the move of the white
point will be equal to the luminance corresponding to one white point.
On a display screen with a luminance rise time different compared to the luminance
fall time as it is typical for an LCD display device, the luminance fall (or rise)
of the white spot at the first location
12 will be different from the simultaneous
luminance rise (or fall) of the white spot at the same instant at the second location
13; the total luminance integrated over the screen area
14 is not
equal immediately before, during and after the movement of the white point.
FIG. 4 shows the luminance before, during and after the movement of the white
point
11 at a time instant T
0 from a first location
12 to
a second location
13 on a display screen of which the luminance rise time
is shorter than the luminance fall time. The horizontal axis
15 is a time
scale and the vertical axis
16 is a luminance scale. Graph
17 shows
the luminance of the picture element at the first location
12, graph
18
shows the luminance of the screen picture element at the second location
13,
and graph
19 shows the integrated luminance over the screen area
14.
FIG. 5 shows the luminance before, during and after the move of the white point
11 at a time instant T
0 from a first position
12 to a second
position
13 on a display screen of which the luminance rise time is longer
than the luminance fall time. Graph
20 shows the luminance of the picture
element at the first location
12, graph
21 shows luminance of the
picture element at the second location
13, and graph
22 shows the
integrated luminance over the screen area
14.
FIGS. 4 and 5 show that when the white point
11 moves from the first
position
12 to the second position
13, there is a short luminance
jump, upwards or downwards depending on how the rise and fall times of the display
screen relate to each other. Within the same time period, the luminance of the
picture element at the second location
13 is changed differently compared
to the luminance of the picture element at the first location
12, the difference
determining the amplitude of the luminance jump. This luminance jump is at the
origin of the artefacts mentioned above and further explained below.
If more white points are moved at the same instant and within the same small
area
of the display screen, a luminance jump will occur as well but its amplitude depends
on how the white points are distributed within the same area.
FIGS. 6
a,
6b and
6c illustrate how two white
points laying side-by-side move in the same horizontal direction over a distance
of one picture element. FIG. 6
b shows that one picture element does not
change luminance, while two other picture elements change luminance. Compared to
the move of one white point as described above, the amplitude of the luminance
jump within the area
14 is equal; however the relative luminance jump being
the absolute luminance jump related to the total luminance of the moving points
is smaller.
FIG. 7
a, FIG. 7
b and FIG. 7
c illustrate how three white
points laying side-by-side move in the same horizontal direction over a distance
of one picture element. FIG. 7
b shows that two picture elements do not change
luminance, while two other picture elements change luminance. Compared to the move
of one white point as described above, the amplitude of the luminance jump within
the area
14 is equal. Compared to the move of two white points as described
above, the amplitude of the luminance jump within the area
14 is equal;
however the relative luminance jump being the absolute luminance jump related to
the total luminance of the moving points is smaller.
FIGS. 8
a,
8b and
8c illustrate how two white
points above each other move in the same vertical direction over a distance of
one picture element. FIG. 8 shows that four picture elements do change luminance
at the same time. Compared to the move of one white point, the luminance jump is
doubled, but the relative luminance jump is the same.
Different combinations of white points moving at the same time in the same
direction from one first location to a second location within an area of the image
screen will give different absolute and relative luminance jumps within that area.
FIGS. 9
a,
9b and
9c illustrate a movement of
a larger combination or cluster of white points from one location to a more right-down location.
FIG. 10 illustrates a white point
11 moving during a time interval T
0-T
3
of three frame periods from location
23 to location
26 over locations
24 and
25, within a screen area
14. FIG. 11 shows the luminance
graph
27 in function of time, integrated over the area
14. A temporary
lower luminance
28 occurs during the move of the white point. The luminance
is temporarily faulty. This artefact is related to the image jump and further mentioned
as a "temporary faulty luminance".
The "luminance jump" and "temporary faulty luminance" artefacts were explained
hereinabove for simple moving images composed of one or more white points. These
artefacts however occur more or less visible and/or measurable with any image moved
on a display screen of an image device with different luminance rise and fall times.
When an above mentioned specific image, for example the image illustrated by means
of FIG. 1
a, is moved over the screen whereby its content remains unchanged,
depending on the speed of the movement, artefacts ranging from a luminance jump
(or a brighter or darker luminance flash), over a large-area flicker to a large-area
faulty luminance may occur. The artefacts occur only in the images or in parts
of the image which are moved.
FIG. 12 shows a prior art connection
31 of a video generator
29
to an image display
1 which has a screen
30.
An embodiment of the present invention is explained by means of block diagram
FIG.
13 and figures of waveforms. It is an apparatus in which a first video
signal is converted into a second video signal.
FIG. 13 shows a block diagram of apparatus
32 (specifically, a video
signal converter) according to the present invention. The input is a first video
signal
33, and the output is a second video signal
34 which is a
conversion of the first video signal
33. The apparatus
32 contains
several functional blocks including an optional inverse gamma corrector
35,
a subtractor
36, a first adder
37, a second adder
38, a processing
block
39, a one-frame memory FM, and an optional gamma corrector
40.
The functional blocks are interconnected through several interconnections for the
interchange of values between the functional blocks. These values may correspond
to luminances, or to gamma corrected video signals, or to video signals without
gamma correction, or to a combination of one or more of these, depending on where
the apparatus
32 is located in a video chain between a video generator and
a display device. For the description of the apparatus
32, it is assumed
that the values are linearly related to luminances on the display screen and that
the first and second video signals are not gamma corrected. It will however be
easy to extend the apparatus for gamma corrected video signals by the addition
of an inverse gamma-correction
35 at the input side, and a gamma corrector
40 at the output side, or by integrating gamma awareness into the apparatus
32.
The processing block
39 has an optional input for values TL, these values
being related to the present status of a picture element of the display screen
such as temperature, location of the picture element being processed, differences
in display behaviour between production batches, ageing of the display, intended
to be used for compensations in the conversion of the first video signal into the
second video signal. These values may come from a sensor in the display device,
or be user-configurable through an on-screen display or an external data entry device.
For explaining the operation of the apparatus
32 of FIG. 13, reference
is made to FIG. 14
a which shows a chosen first video signal IN
1.
This chosen first video signal corresponds to a white picture element on a black
background, the white picture element appearing at time T
0 and disappearing
at time T
10. In FIG. 14
a, the horizontal axis is a linear time scale
with divisions TF
1 corresponding to frame periods of the first video signal,
and the vertical axis is a linear voltage scale. The first video signal amplitude
changes at T
0 from I
0 to I
1, and at T
10 from I
1
to I
0.
FIG. 14
b shows the waveform of the RMS drive voltage applied inside a
typical LCD display device to the one or more liquid crystal image cells of the
display screen of LCD display device which are driven to display the white point
of the first video signal IN
1, this being according to prior art.
FIG. 15
a shows a number of luminance time responses of a picture element
on a display screen of an LCD display device of which the luminance rise time is
shorter than the luminance fall time. The horizontal axis is a linear time scale,
and the vertical axis
41 is a linear luminance scale. The luminance responses
in FIG. 15
a correspond to one unique LCD display device; the response is
dependent on the display device, the location of the picture element on the display
screen, and on the temperature.
Graph
42 on FIG. 15
a shows the prior art luminance response to
the first video signal IN
1 at the location of the displayed picture element.
As shown, the luminance rises from time instant T
0 for a duration of several
frame periods from L
0 to L
1, and falls from time instant T
10
for a duration of several frame periods. The luminance rise time is shorter than
the luminance fall time.
Graph
43 shows the prior art luminance response of the same picture
element to a first video signal which is reversed in amplitude compared to video
signal IN
1 and which is further called -;IN
1. Luminance rise and
fall times are as with Graph
42.
Video signals IN
1 and -;IN
1 do not occur at the same instant
for driving the same picture element, but may both be present at the input within
a time interval shorter than an input frame period when e.g. a white picture element
moves from one location to another within the image.
According to the present invention, the luminance rise and fall times are
made equal, obtained by slowing down the faster response to match with the slower
response, or accelerate the slower response to match with the faster response,
or make the faster and the slower response equal to a predefined luminance response,
the three methods being possible with the here described embodiment. Accelerating
the slower response will however not always be useable in practice because higher
drive voltages will be needed and saturation may occur in the image display.
The solution is only fully explained for making the faster response slower. Making
the slower response faster, or making the faster response and the slower response
equal to predefined responses, can easily be implemented by the skilled person.
In accordance with the present invention, graph
42 in FIG. 15
a is
slowed down to graph
44 during the time interval of rising luminance and
matches as close as possible to graph
45 being the inverse of the falling
graph
43. During the interval of falling luminance (from T
10 on),
the response should not be modified but should remain as in graph
42.
FIG. 16 is an enlarged version of a part of FIG. 15
a, namely between
time instances T
0 and T
3. To the first vertical axis
41 is
added a second vertical axis
46 in order to show the relation between the
second video signal and the luminance of the image on the display screen.
The method for converting or modifying the first video signal to develop the
second video signal is further explained referring to the block diagram in FIG.
13.
The conversion is such that the second video signal is built up in real time
in consecutive steps during corresponding consecutive correction periods TC. A
correction period (TC) is by preference equal to the frame period of the displayed
image. A correction period may be different from the frame period (TF
1)
of the first video signal.
From the present value of the first video signal
33 is subtracted in
the subtractor
36 a value FR which corresponds to the present luminance
as it was predicted one correction period before. The result is a value Δ.
The value Δ determines how the luminance will have to change during the next
correction period. Luminance should increase or rise when Δ is positive,
decrease or fall when Δ is negative, and remain equal when Δ is zero.
The value Δ is applied to a first input of the processing block
39.
At a second input is applied the predicted present luminance FR. With input values
Δ and FR and if present the input of one or more temperature values TL related
to the connected display screen, two output values, ΔC and ΔR are determined.
How these values ΔC and ΔR can be determined is explained further.
ΔC is a correction value to be added to the predicted present luminance FR
in order to reach a chosen luminance (to match to a chosen response) at the end
of the next correction period. ΔR is the value with which the luminance will
have changed after the next correction period when ΔC is added to the predicted
present luminance FR taking into account the parameters of the display screen (of
which some are screen-location, voltage and temperature dependent).
The value ΔC is added in the first adder block
37 to the predicted
present luminance FR. The predicted present luminance FR was predicted at the beginning
of the previous correction period and has been delayed over one correction period
in a one-correction-period storage element or memory FM. The output of the first
adder
37 is a value which
10 is the second video signal
34
without optional gamma correction.
The value ΔR is added in the second adder block
38 to the value
of the predicted present luminance FR. The output is the predicted present luminance
for a next correction period.
Although a correction to the second video signal takes several correction
periods, a memory FM of only one correction period (or only one second video signal
frame period) is needed. For each correction period a new correction value is determined
based on the present luminance which was calculated at the start of the previous
correction period and stored during one correction period.
The above described apparatus
32 contains all the above mentioned functional
blocks and connections to change a luminance response in consecutive steps by converting
the first video signal
33 to the second video signal
34. It is however
not always needed to change the luminance response, namely when the luminance response
already follows the slowest response with the first video signal; the apparatus
can work transparently. This can be realized in the processing block
39.
For further explanation reference is made now to FIG. 16 which shows how the
luminance response is built up during three consecutive correction periods from
the time instances T
0 to T
3.
From T
0 to T
1, without correction, the luminance rise would follow
graph
42 and increase from LF to LA
1. According to the invention,
the luminance response should however follow graph
45 and increase from
LF to LB
1. The shape of the rising luminance slope is however not exactly
identical to the opposite of the shape of the falling luminance slope, and so it
is difficult to match the rising luminance to the graph
45 and at the same
time reach luminance LB
1 at time instant T
1. More important however
is that the integrated luminance over the correction period from T
0 to T
1
is correct. Therefore, the luminance should raise so that the integrated luminance
is the same as it would be if graph
45 were followed and LB
1 reached
at T
1. This is so when the luminance follows the exponential graph
47,
whereby the luminance is LD
1 at T
1. The corrected luminance response
is marked as
44 on FIG. 16 (and FIG. 15
a). As to FIG.
13 and
its explanation, ΔC should have an appropriate value to correct the second
video signal so that the luminance increases to LC
1 over a number of correction
periods; LD
1 is the predicted present luminance FR at the end of the correction
period T
0-T
1.
At T
1, a following correction period T
1-T
2 starts. The luminance
should continue to follow as closely as possible graph
45 and at the same
time, the integrated luminance over T
1-T
2 should be substantially
the same as if the luminance response did follow graph
45. Therefore, the
luminance should rise (graph
48) to the luminance LC
2 and rise from
LD
1 to LD
2 within the correction period T
1-T
2. LD
2
is the predicted present luminance after the correction T
1-T
2. If
the video signal would not have been corrected, a luminance LA
2 would have
been reached at T
2.
On the vertical axis
46 in FIG. 16 values are set out with reference to
FIG.
13 and its explanation. The first video signal amplitude value goes
from INF to INT at T
0. At T
1, the difference between the value of
the first video signal and the predicted present luminance FR predicted at T
0,
is Δ
1=INT-;FR
1. The output of the processing block is ΔC
1
and is added to FR
1 to be the new second video signal value. The predicted
rise of luminance after the correction period T
1-T
2 is ΔR
1,
and the predicted present luminance at T
2 is FR
1+ΔR
1=FR
2.
From T
2 on, the luminance response is built up in the same way as described
here before up to a luminance LT. FIG. 15
a shows, that from T
10,
the luminance response follows the slower luminance falling response and no correction
is carried out, the apparatus
32 working transparently.
FIG. 15
b shows the waveform of the RMS drive voltage with reference to
FIG. 14
b, but now in response to the second video signal.
FIG. 15
c shows the second video signal, being the converted first video
signal shown in FIG. 14
a.
FIGS. 17
a,
17b and
17c show similar waveforms
compared to FIGS. 15
a,
15b and
15c but for a
display device of which the luminance rise time is longer than the luminance fall
time. The luminance fall is now made slower from T
10.
In the processing block
39 of FIG. 13, the output values ΔC and
ΔR
are determined as a function of the input values Δ and FR and optional temperature
values and location values. The following C-language function is hereby used.
| |
|
| |
void calc_deltas(int delta_in, int from, int *delta out, int |
| |
*delta_res) |
| |
{ |
| |
float dout, dres; |
| |
if (delta_in > 0) |
/* positive slope */ |
| |
/* no drive correction needed */ |
| |
else |
/* negative slope */ |
| |
( (FRAME_PERIOD - tau-rising * (1 - exp(- |
| |
FRAME_PERIOD/tau_rising))) |
| |
/ (FRAME_PERIOD - tau_falling * (1 - exp(- |
| |
FRAME_PERIOD/tau_falling))) |
| |
) |
| |
* temp function(temperature, FALLING) |
| |
* location_function(screen_x,screen_y); |
| |
/* |
| |
* Predict pixel response. To be used in next frame iteration. |
| |
* Always predict the slowest edge, since that is what we want |
| |
* make the fastest one do as well. |
| |
*/ |
| |
* (1 - exp(-FRAME_PERIOD/tau_rising)) |
| |
* temp_function(temperature, RISING) |
| |
* location_function(screen_x,screen_y); |
| |
*delta_out = (int)rint(dout); |
| |
*delta_res = (int)rint(dres); |
| |
|
In the above shown C-language function, corrections are determined every {fraction
(1/60)} second (frame rate 60 Hz). It is written for the display of an image on
a display device of which the luminance rise time is longer than the luminance
fall time. Values "delta_out" (being ΔC) and "dres" (being ΔR) are
calculated from "delta_in" (being Δ) and "from" (being FR). When "delta_in"
is positive, the luminance should rise (called positive slope) and no correction
is to be made. The calculation of dout (or ΔC) is based on the following
equation wherein T is the correction period:
##EQU1##
The calculation of "dres" (ΔR) is based on the following equation:
##EQU2##
τ
S (or tau_rising) and τ
F (or
tau_falling) are time constants of exponential functions corresponding to luminance
time responses.
The C-program function includes a correction in function of temperature (temp_function)
and location (location_function).
The processing block
39 may be implemented in different ways. It may be
a pre-calculated look-up table with Δ and FR as input values, and ΔC
and ΔR as output values which before being output are sent through multipliers
for temperature and location dependent corrections.
It may be a hardware implementation of the C-program function shown above.
It may consist of a look-up table and a microprocessor to update the value
