EP1471496A1 - Driving method for a liquid crystal display - Google Patents
Driving method for a liquid crystal display Download PDFInfo
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- EP1471496A1 EP1471496A1 EP03425252A EP03425252A EP1471496A1 EP 1471496 A1 EP1471496 A1 EP 1471496A1 EP 03425252 A EP03425252 A EP 03425252A EP 03425252 A EP03425252 A EP 03425252A EP 1471496 A1 EP1471496 A1 EP 1471496A1
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
- G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
- G09G3/2007—Display of intermediate tones
- G09G3/2011—Display of intermediate tones by amplitude modulation
-
- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
- G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
- G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
- G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
- G09G3/3611—Control of matrices with row and column drivers
- G09G3/3622—Control of matrices with row and column drivers using a passive matrix
-
- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
- G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
- G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
- G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
- G09G3/3611—Control of matrices with row and column drivers
- G09G3/3674—Details of drivers for scan electrodes
- G09G3/3681—Details of drivers for scan electrodes suitable for passive matrices only
-
- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
- G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
- G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
- G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
- G09G3/3611—Control of matrices with row and column drivers
- G09G3/3685—Details of drivers for data electrodes
- G09G3/3692—Details of drivers for data electrodes suitable for passive matrices only
-
- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
- G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
- G09G3/2007—Display of intermediate tones
- G09G3/2018—Display of intermediate tones by time modulation using two or more time intervals
- G09G3/2022—Display of intermediate tones by time modulation using two or more time intervals using sub-frames
-
- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
- G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
- G09G3/2007—Display of intermediate tones
- G09G3/2077—Display of intermediate tones by a combination of two or more gradation control methods
Definitions
- the present invention relates to a driving method for a liquid crystal display.
- the liquid crystal displays are actually used in a more and more increasing number of products as the cellular phones, the notebooks, etc.
- the displays which may be of black and white, or gray shade or colour type, are typically made up of a matrix of rows and columns electrodes which, suitably driven by application of a voltage signal, determine at the crossing points, the so-called pixels, a modification in optic behavior of the liquid crystal interposed.
- the image visualized on the display is obtained by means of different driving methods of the rows and of the columns.
- FIG. 1 a schematic block diagram of a liquid crystal display 1 is shown which has a flat panel structure in which a liquid crystal layer is interposed between a group 2 of N row electrodes and a group 3 of M column electrodes.
- a Super Twisted Nematic (STN) or a Twisted Nematic (NT) liquid crystal can be used as liquid crystal layer.
- a control means 6 is connected with a circuit stage 4 adapted to drive the row electrodes 2 and it is also connected with a circuit stage 5 adapted to drive the column electrodes 3.
- a voltage circuit 7 supplies a voltage level necessary for generating a column signal by means of the circuit stage 5 and it supplies a voltage level for generating a row signal by means of the circuit stage 4.
- a known driving scheme, implemented by the control means 6, is the so called line by line addressing, wherein the N rows 2 of the matrix display 1 are sequentially selected one at a time for a time period T.
- an orthonormal function generating means 8 generate a plurality of orthonormal functions which are orthonormal to each other and said means 8 supply them sequentially to the circuit stage 4.
- the circuit stage 4 adequately selects a voltage level, provided by the voltage level circuit 7, in accordance with the orthonormal functions and supplies it to the group 2 of row electrodes as row signal.
- LCDs are slow devices, with response time in the range of a few tens to few hundred milliseconds; for this reason the scanning time NT must be short as compared to the response times of the LCD display.
- RMS Root Mean Square
- MLA Multi Line Addressing
- the MLA method causes the simultaneous selection of a plurality of row electrodes 2 at the same time period.
- the column electrodes 3 can independently be controlled by means of the period NT.
- Said plurality of waveforms r1, ..., rn represents the voltage levels in correspondence with respective column elements of the liquid crystal display panel 1.
- the plurality of waveforms r1, ..., r4 of row electrodes 2 represents a set of the entirety of the wave forms r1, ..., m.
- the column electrodes voltage series are determined by the sequence of one and zero of said plurality of waveforms r1, ..., r4.
- FRC Frame Rate Control
- many frames are required for a multiple gray scale information.
- seven frames F1, F2, ..., F7 are required in FRC for codifying the gray scales because three memory bits for each pixel are needed to codify the eight gray levels, wherein, particularly, the first four frames, that is F1, F2, F3, and F4, codify the most significant bit (MSB), the fifth and sixth frames, that is F5 and F6, codify the medium significant bit (mSB) and the seventh frame, that is F7, codifies the less significant bit (LSB), according to Figure 3.
- MSB most significant bit
- mSB medium significant bit
- LSB less significant bit
- FIG 4 a table 13 wherein the stored data in a read access memory (RAM) for each pixel of the flat display 1, is shown.
- RAM read access memory
- the first frame F1 represents symbolically the sequence of four scanning steps over all the row electrodes, based each one on a different row pattern (four columns of matrix R1 of Figure 2c).
- the maximum time distance among the frames wherein the value of the said memory RAM is evaluated in the case of the LSB is of six frames, in the case of the mSB is of five frames and in the case of the MSB is of three frames. Such a time distance produces a phenomenon called flickering.
- PAM Pulse Width Modulation
- This technique suffers the cross-talk problem which is due to an increase of the all the charge quantity which is transferred onto the adjacent pixels instead that onto the destination pixel. This is due to the different factors: to the fact that a waveform deformation of the approximatively exponential type is associated to each transition, to the fact that the queue of a row pulse in a period of elementary time T1 is partially overlapped to the column pulses of the successive time period T1, to the transient induced by the commutations of the row electrodes on the column electrodes.
- the cross-talk effect causes a contrast reduction in the display LCD and a decreasing of the transmittance of the off pixels.
- a driving method for a liquid crystal display having a multiplicity of display elements arranged at the intersections of a matrix provided with N row electrodes and M column electrodes with N and M entire numbers, said method comprising a first phase for scanning all the row electrodes of said matrix in a scanning time period, said first phase comprising the sequential generation of a plurality of first signals each one adapted to excite at least one row electrode of said matrix for a first prefixed time period, the generation of second signals respectively adapted to excite each column electrode of said matrix simultaneously to the excitation of the at least one row electrode, said second signals being adapted to determine the gray shade of each display element of the excited row electrode, said first prefixed time period being smaller than the scanning time period, characterized in that a word formed by G bits corresponds to each gray shade and in that each one of said first signals is defined by means of the following equation wherein r i (t) is the i-th first signal as a function of the time t, the functions f i
- the method comprises a first phase for scanning all the row electrodes of said matrix in a scanning time period NT.
- the first phase comprises the sequential generation of a plurality of first signals each one adapted to excite at least one respective row electrode of said matrix for a first prefixed time period T and the generation of second signals adapted to excite each one column electrode of said matrix simultaneously to the excitation of the at least one row electrode; the second signals determine the gray shade of each display element of the excited row electrode.
- gray shades can be generated wherein the gray shades can be represented by words composed by G-bits, for example with words composed by six bits as illustrated in table 2 in Figure 6; a transmittance value and a rms voltage value are associated to each word composed by six bits.
- Each pixel has a different gray shade determined by a word composed by G-bits; for example the word "00...00” represents the full white pixel and the word “11...11” represents the full black pixel; the intermediate words "b G-1 ...b 0 " represent the intermediate gray shades.
- This driving method comprises the driving of the N rows by means of NG orthogonal functions.
- coefficients A K modulate the amplitude of the functions f ik (t) for generating the gray shades and are chosen suitably, for example as shown in table 1 in Figure 6.
- Each column j-th is driven by means of a signal c j (t) of the this type: , where the coefficients B k modulate the amplitude of the functions f nk (t) for generating the gray shades; the functions f nk (t) are the NG orthogonal functions, preferably orthonormal.
- the voltage level number is determined by the coefficients A k and B k and by the choice of the orthogonal functions f nk (t).
- the root mean square value of the electric field between the two plates of the i-th row and the j-th column is given by:
- V th the display threshold, that is the voltage wherein it has the 50% of transmittance, it is given by: Therefore the dynamic range of the voltage (given by is limited, in fact: from which: where has its maximum if In such case where
- the products A k B k can be fixed by using the least squares values with all the 2 G points and a function modelling the transmittance.
- the coefficients A k and B k can be applied to each group of orthonormal functions.
- each row For example eight gray shades can be obtained by using three orthonormal functions by each row, as shown in Figure 9 in the case of line by line addressing.
- the orthonormal functions f 0,0 , f 0,1 and f 0,2 are employed for the row 0 r 0 (t) while the orthonormal functions f 1,0 , f 1,1 and f 1,2 are employed for the row 1 r 1 (t); the functions r 0 (t) and r 1 (t) assume the time waveform shown in the respective graphics.
- a column signal c 0 (t) is obtained which is given by the waveform shown in Figure.
- the pulses of the orthonormal functions have the same time duration it is obtained that the frequency and the voltage level number grows in logarithmic manner with respect to the gray shade number, as shown in Figure 8 where the possible voltage level V of a column signal c j (t) are shown in the case of a row i or a row i+1.
- the orthonormal functions p 0,0 , p 0,1 and p 0,2 are used for the row 0 r 0 (t) while the orthonormal functions p 1,0 , p 1,1 and p 1,2 are used for the row 1 r 1 (t); the functions r 0 (t) and r 1 (t) assume the time waveform shown in the respective graphics, as shown in Figure 9.
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- Crystallography & Structural Chemistry (AREA)
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Abstract
A driving method for a liquid crystal display having a multiplicity of
display elements arranged at the intersections of a matrix provided with N
row electrodes and M column electrodes with N and M entire numbers is
described. The method comprises a first phase for scanning all the row
electrodes of said matrix in a scanning time period (NT); the first phase
comprises the sequential generation of a plurality of first signals (r,(t)) each
one adapted to excite at least one row electrode of said matrix for a first
prefixed time period (T), the generation of second signals (cj(t)) respectively
adapted to excite each column electrode of said matrix simultaneously to the
excitation of the at least one row electrode. The second signals are adapted
to determine the gray shade of each display element of the excited row
electrode and the first prefixed time period (T) is smaller than the scanning
time period (NT). A word formed by G bits corresponds to each gray shade
and each one of said first signals is defined by means of the following
equation
wherein r,(t) is the i-th first signal as a function
of the time t, the functions fik(t) are time functions which are orthogonal to
each other, Ak is a numeric coefficient and k is a index changing from 0 to
G-1. Each one of said second signals is defined by means of the following
equation
wherein cj(t) is the j-th second signal as a
function of the time, n is a index changing from 1 to N where N is the row
number of the matrix, k is a index changing from 0 to G-1, the functions
fnk(t) are time functions which are orthogonal to each other, Bk is a numeric
coefficient, Injk is a number which assumes value 1 if the k-th bit of the word
formed by G bits is zero or assumes value -1 is the k-th bit is 1.
Description
The present invention relates to a driving method for a liquid crystal
display.
The liquid crystal displays (LCD) are actually used in a more and
more increasing number of products as the cellular phones, the notebooks,
etc. The displays, which may be of black and white, or gray shade or colour
type, are typically made up of a matrix of rows and columns electrodes
which, suitably driven by application of a voltage signal, determine at the
crossing points, the so-called pixels, a modification in optic behavior of the
liquid crystal interposed.
The image visualized on the display is obtained by means of different
driving methods of the rows and of the columns.
In Figure 1 a schematic block diagram of a liquid crystal display 1 is
shown which has a flat panel structure in which a liquid crystal layer is
interposed between a group 2 of N row electrodes and a group 3 of M
column electrodes. A Super Twisted Nematic (STN) or a Twisted Nematic
(NT) liquid crystal, by way of example, can be used as liquid crystal layer.
A control means 6 is connected with a circuit stage 4 adapted to drive
the row electrodes 2 and it is also connected with a circuit stage 5 adapted to
drive the column electrodes 3.
A voltage circuit 7 supplies a voltage level necessary for generating a
column signal by means of the circuit stage 5 and it supplies a voltage level
for generating a row signal by means of the circuit stage 4.
A known driving scheme, implemented by the control means 6, is the
so called line by line addressing, wherein the N rows 2 of the matrix display
1 are sequentially selected one at a time for a time period T. In fact, an
orthonormal function generating means 8 generate a plurality of orthonormal
functions which are orthonormal to each other and said means 8 supply them
sequentially to the circuit stage 4. The last applies a plurality of row signals
represented by the orthonormal functions to all the row electrodes 2 in a
period NT, also called scanning time.
Particularly, the circuit stage 4 adequately selects a voltage level,
provided by the voltage level circuit 7, in accordance with the orthonormal
functions and supplies it to the group 2 of row electrodes as row signal.
It is known that LCDs are slow devices, with response time in the
range of a few tens to few hundred milliseconds; for this reason the scanning
time NT must be short as compared to the response times of the LCD
display.
An important parameter for determining the state of the pixel is the
ratio of Root Mean Square (RMS) voltage across an ON pixel to that across
an OFF pixel.
Another technique used is the active addressing technique, particularly
the Multi Line Addressing (MLA) technique.
The MLA method causes the simultaneous selection of a plurality of
row electrodes 2 at the same time period. According to this method the
column electrodes 3 can independently be controlled by means of the period
NT. In this case, it is necessary to apply pulse voltages having different
polarities to the row electrodes 2 to simultaneously and independently
control the display pattern in the column direction, as shown in Figures 2a,
2b, and 2c.
Particularly, in Figure 2a, it is possible to note a plurality of
waveforms r1, r2, ..., m-1, rn for driving the row electrodes 2 of the liquid
crystal display panel 1 and a horizontal axis representing the time
subdivided into a plurality of intervals t0, t1, ..., tn.
Said plurality of waveforms r1, ..., rn represents the voltage levels in
correspondence with respective column elements of the liquid crystal
display panel 1.
In fact, as shown in Figure 2b, the plurality of waveforms r1, ..., r4 of
row electrodes 2 represents a set of the entirety of the wave forms r1, ..., m.
The column electrodes voltage series are determined by the sequence of one
and zero of said plurality of waveforms r1, ..., r4.
Referring now to Figure 2c, indicating the plurality of waveforms r1,
..., r4 of Figure 2b as R1, an image of a matrix which corresponds to the
waveforms r1, ..., r4 is shown.
What described in such Figures 2a, 2b and 2c is well known to a
skilled person.
It has also known in the state of the art to use a Frame Rate Control
(FRC) in a gray scale of the multiple line simultaneous selection method.
FRC is a system in which the pixels ON and OFF are dispersed among a
plurality of frames and the gray scale is expressed by the average brightness,
as shown in Figure 3.
As shown in this Figure 3, many frames are required for a multiple
gray scale information. By way of example seven frames F1, F2, ..., F7 are
required in FRC for codifying the gray scales because three memory bits for
each pixel are needed to codify the eight gray levels, wherein, particularly,
the first four frames, that is F1, F2, F3, and F4, codify the most significant
bit (MSB), the fifth and sixth frames, that is F5 and F6, codify the medium
significant bit (mSB) and the seventh frame, that is F7, codifies the less
significant bit (LSB), according to Figure 3.
In Figure 4 a table 13 wherein the stored data in a read access memory
(RAM) for each pixel of the flat display 1, is shown.
In fact in the table 13 of Figure 4, there is the codification of each
pixel according to the gray scale in object. In fact, the codification foresees a
pixel completely white in the case of the MSB, mSB and LSB bits are equal
to zero, indicated as 14 in Figure 4, whereas said codification foresees a
pixel completely black in the case of the MSB, mSB and LSB bits are equal
to one, which are indicated as 15 in the Figure 4, and the gradation of the
other levels of gray are a combination of said MSB, mSB and LSB bits,
which are indicated as "g1", ..., "g6" in Figure 4.
For example, the first frame F1, as magnified in Figure 5, represents
symbolically the sequence of four scanning steps over all the row electrodes,
based each one on a different row pattern (four columns of matrix R1 of
Figure 2c).
It is to be noted that the maximum time distance among the frames
wherein the value of the said memory RAM is evaluated in the case of the
LSB is of six frames, in the case of the mSB is of five frames and in the case
of the MSB is of three frames. Such a time distance produces a phenomenon
called flickering.
In order to solve such a problem, a plurality of solutions have been
proposed, such as the solution wherein the MSB, mSB and LSB bits in a
frame are evaluated in a way the most equidistant each other inside the same
plurality of frame.
However, by applying this solution the flat display panel 1 still suffers
of a remarkable flickering due to the high number of frame and moreover to
visualize the gray indicated as "g1" in the box 13 according to the above
method the LSB memory would be repeatedly evaluated with a time
distance of six frames.
Another technique used is that of the Pulse Width Modulation (PAM)
of the waveforms of the signals applied to the columns for generating gray
shades. This technique suffers the cross-talk problem which is due to an
increase of the all the charge quantity which is transferred onto the adjacent
pixels instead that onto the destination pixel. This is due to the different
factors: to the fact that a waveform deformation of the approximatively
exponential type is associated to each transition, to the fact that the queue of
a row pulse in a period of elementary time T1 is partially overlapped to the
column pulses of the successive time period T1, to the transient induced by
the commutations of the row electrodes on the column electrodes. The cross-talk
effect causes a contrast reduction in the display LCD and a decreasing
of the transmittance of the off pixels.
In view of the state of the art described, it is an object of the present
invention to provide a driving method for a liquid crystal display which is
not affected by cross-talk effects and not has a high frame frequency.
According to the invention, such object is achieved by means of a
driving method for a liquid crystal display having a multiplicity of display
elements arranged at the intersections of a matrix provided with N row
electrodes and M column electrodes with N and M entire numbers, said
method comprising a first phase for scanning all the row electrodes of said
matrix in a scanning time period, said first phase comprising the sequential
generation of a plurality of first signals each one adapted to excite at least
one row electrode of said matrix for a first prefixed time period, the
generation of second signals respectively adapted to excite each column
electrode of said matrix simultaneously to the excitation of the at least one
row electrode, said second signals being adapted to determine the gray shade
of each display element of the excited row electrode, said first prefixed time
period being smaller than the scanning time period, characterized in that a
word formed by G bits corresponds to each gray shade and in that each one
of said first signals is defined by means of the following equation
wherein ri(t) is the i-th first signal as a function of the
time t, the functions fik(t) are time functions which are orthogonal to each
other, Ak is a numeric coefficient and k is a index changing from 0 to G-1,
and in that each one of said second signals is defined by means of the
following equation
wherein cj(t) is the j-th second
signal as a function of the time, n is a index changing from 1 to N where N
is the row number of the matrix, k is a index changing from 0 to G-1, the
functions fnk(t) are time functions which are orthogonal to each other, Bk is a
numeric coefficient, Injk is a number which assumes value 1 if the k-th bit of
the word formed by G bits is zero or assumes value -1 is the k-th bit is 1.
The features and the advantages of the present invention will be made
evident by the following detailed description of an embodiment thereof
which is illustrated as not limiting example in the annexed drawings,
wherein:
wherein:
Referring to the present invention a driving method for a liquid crystal
display having a multiplicity of display elements arranged at the
intersections of a matrix provided with N row electrodes and M column
electrodes with N and M entire numbers is described. The method comprises
a first phase for scanning all the row electrodes of said matrix in a scanning
time period NT. The first phase comprises the sequential generation of a
plurality of first signals each one adapted to excite at least one respective
row electrode of said matrix for a first prefixed time period T and the
generation of second signals adapted to excite each one column electrode of
said matrix simultaneously to the excitation of the at least one row electrode;
the second signals determine the gray shade of each display element of the
excited row electrode.
With this method according to invention 2G gray shades can be
generated wherein the gray shades can be represented by words composed
by G-bits, for example with words composed by six bits as illustrated in
table 2 in Figure 6; a transmittance value and a rms voltage value are
associated to each word composed by six bits. Each pixel has a different
gray shade determined by a word composed by G-bits; for example the word
"00...00" represents the full white pixel and the word "11...11" represents
the full black pixel; the intermediate words "bG-1...b0" represent the
intermediate gray shades.
This driving method comprises the driving of the N rows by means of
NG orthogonal functions. Particularly, the i-th row is driven by the signal
ri(t) where
wherein the functions fik(t) are the N*G
orthogonal functions, preferably orthonormal, that is functions wherein:
is equal to 1 if i=m and k=n while is equal to 0 in
all the other cases.
The coefficients AK modulate the amplitude of the functions fik(t) for
generating the gray shades and are chosen suitably, for example as shown in
table 1 in Figure 6.
The functions ri(t) are N orthogonal functions, that is it occurs that
is equal to
if i=j otherwise it is equal to 0.
The rows are driven by functions which are independent to each other.
However, the informations determining the state of the pixel are present in
the driving functions of the M columns. Each column j-th is driven by means
of a signal cj(t) of the this type:
, where the
coefficients Bk modulate the amplitude of the functions fnk(t) for generating
the gray shades; the functions fnk(t) are the NG orthogonal functions,
preferably orthonormal. The value of the function Inkj is given by the k-th
bit of the row n and of the column j and Inkj is equal to 1 if the bit bk=0 while
is equal to-1 if bk=1.
The voltage level number is determined by the coefficients Ak and Bk
and by the choice of the orthogonal functions fnk(t).
The root mean square value of the electric field between the two plates
of the i-th row and the j-th column is given by:
From the above mentioned equation it is possible to verify that the root
mean square value of the pixel belonging to the i-th row and to j-th column
does not depend on the state of the other pixels, therefore said driving
method presents zero cross-talks.
From the above mentioned equation it is possible to obtain the
minimum level Vmin and the maximum level Vmax of the root mean square
value of the voltage of the pixel placed between the two plates of the i-th
row and the j-th column:
and
By indicating with Vth the display threshold, that is the voltage
wherein it has the 50% of transmittance, it is given by: Therefore the dynamic range of the voltage (given by is limited, in fact: from which: where has its maximum if In such case where
wherein it has the 50% of transmittance, it is given by: Therefore the dynamic range of the voltage (given by is limited, in fact: from which: where has its maximum if In such case where
The products Ak Bk can be fixed by using the least squares values with
all the 2G points and a function modelling the transmittance.
The coefficients Ak and Bk which are present in table 1 in Figure 6
give the maximum voltage range.
The coefficients Ak and Bk can be applied to each group of
orthonormal functions.
For example eight gray shades can be obtained by using three
orthonormal functions by each row, as shown in Figure 9 in the case of line
by line addressing. In fact the orthonormal functions f0,0, f0,1 and f0,2 are
employed for the row 0 r0(t) while the orthonormal functions f1,0, f1,1 and f1,2
are employed for the row 1 r1(t); the functions r0(t) and r1(t) assume the time
waveform shown in the respective graphics.
By fixing with I0,0 the gray shade of the pixel finding on the row 0 and
on the column 0 and with I1,0 the gray shade of the pixel finding on the row
1 and in the column 0 a column signal c0(t) is obtained which is given by the
waveform shown in Figure. In such case six level row voltages and six
column voltages are obtained. If the pulses of the orthonormal functions
have the same time duration it is obtained that the frequency and the voltage
level number grows in logarithmic manner with respect to the gray shade
number, as shown in Figure 8 where the possible voltage level V of a
column signal cj(t) are shown in the case of a row i or a row i+1.
In the case of multiline addressing (MLA) where, for example, two
rows (the row 0 and the row 1) are simultaneously addressed, the
orthonormal functions p0,0, p0,1 and p0,2 are used for the row 0 r0(t) while the
orthonormal functions p1,0, p1,1 and p1,2 are used for the row 1 r1(t); the
functions r0(t) and r1(t) assume the time waveform shown in the respective
graphics, as shown in Figure 9. By fixing always with I0,0 the gray shade of
the pixel finding on the row 0 and on the column 0 and with I1,0 the gray
shade of the pixel finding on the row 1 and in the column 0 a column signal
c0(t) is obtained which is given by the waveform shown in Figure. In such
case six level row voltages and six column voltages are obtained.
Claims (6)
- Driving method for a liquid crystal display having a multiplicity of display elements arranged at the intersections of a matrix provided with N row electrodes and M column electrodes with N and M entire numbers, said method comprising a first phase for scanning all the row electrodes of said matrix in a scanning time period (NT), said first phase comprising the sequential generation of a plurality of first signals (ri(t)) each one adapted to excite at least one row electrode of said matrix for a first prefixed time period (T), the generation of second signals (cj(t)) respectively adapted to excite each column electrode of said matrix simultaneously to the excitation of the at least one row electrode, said second signals being adapted to determine the gray shade of each display element of the excited row electrode, said first prefixed time period (T) being smaller than the scanning time period (NT), characterized in that a word formed by G bits corresponds to each gray shade and in that each one of said first signals is defined by means of the following equation wherein ri(t) is the i-th first signal as a function of the time t, the functions fik(t) are time functions which are orthogonal to each other, Ak is a numeric coefficient and k is a index changing from 0 to G-1, and in that each one of said second signals is defined by means of the following equation wherein cj(t) is the j-th second signal as a function of the time, n is a index changing from 1 to N where N is the row number of the matrix, k is a index changing from 0 to G-1, the functions fnk(t) are time functions which are orthogonal to each other, Bk is a numeric coefficient, Injk is a number which assumes value 1 if the k-th bit of the word formed by G bits is zero or assumes value -1 is the k-th bit is 1.
- Method according to claim 1, characterized in that the functions fik(t) are orthonormal to each other.
- Method according to claim 1, characterized in that the functions fnk(t) are orthonormal to each other.
- Method according to claim 1, characterized in that said first signal excites one row electrode.
- Method according to claim 1, characterized in that said first signal excites more than one row electrode.
Priority Applications (1)
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EP03425252A EP1471496A1 (en) | 2003-04-23 | 2003-04-23 | Driving method for a liquid crystal display |
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EP03425252A EP1471496A1 (en) | 2003-04-23 | 2003-04-23 | Driving method for a liquid crystal display |
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Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0522510A2 (en) * | 1991-07-08 | 1993-01-13 | Asahi Glass Company Ltd. | Driving method of driving a liquid crystal display element |
EP0569974A2 (en) * | 1992-05-14 | 1993-11-18 | In Focus Systems, Inc. | Gray level addressing for LCDS |
EP0598913A1 (en) * | 1992-05-08 | 1994-06-01 | Seiko Epson Corporation | Method and circuit for driving liquid crystal device, etc., and display device |
EP0618562A1 (en) * | 1993-03-30 | 1994-10-05 | Asahi Glass Company Ltd. | A display apparatus and a driving method for a display apparatus |
-
2003
- 2003-04-23 EP EP03425252A patent/EP1471496A1/en not_active Withdrawn
Patent Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0522510A2 (en) * | 1991-07-08 | 1993-01-13 | Asahi Glass Company Ltd. | Driving method of driving a liquid crystal display element |
EP0598913A1 (en) * | 1992-05-08 | 1994-06-01 | Seiko Epson Corporation | Method and circuit for driving liquid crystal device, etc., and display device |
EP0569974A2 (en) * | 1992-05-14 | 1993-11-18 | In Focus Systems, Inc. | Gray level addressing for LCDS |
EP0618562A1 (en) * | 1993-03-30 | 1994-10-05 | Asahi Glass Company Ltd. | A display apparatus and a driving method for a display apparatus |
Non-Patent Citations (1)
Title |
---|
SEUNG-WOO LEE ET AL: "Realization of video-rate STN display using row voltage modulation method", MICROELECTRONICS AND VLSI, 1995. TENCON '95., IEEE REGION 10 INTERNATIONAL CONFERENCE ON HONG KONG 6-10 NOV. 1995, NEW YORK, NY, USA,IEEE, US, 6 November 1995 (1995-11-06), pages 464 - 467, XP010160162, ISBN: 0-7803-2624-5 * |
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