EP1471496A1

EP1471496A1 - Driving method for a liquid crystal display

Info

Publication number: EP1471496A1
Application number: EP03425252A
Authority: EP
Inventors: Santo Ilardo
Original assignee: STMicroelectronics SRL; SGS Thomson Microelectronics SRL
Current assignee: STMicroelectronics SRL
Priority date: 2003-04-23
Filing date: 2003-04-23
Publication date: 2004-10-27

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 (c_j(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 f_ik(t) are time functions which are orthogonal to each other, A_k 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 c_j(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 f_nk(t) are time functions which are orthogonal to each other, B_k is a numeric coefficient, I_njk 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 r_i(t) is the i-th first signal as a function of the time t, the functions f_ik(t) are time functions which are orthogonal to each other, A_k 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

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:

Figure 1 shows a schematic block diagram of a liquid crystal display according to the prior art;

Figures 2a, 2b and 2c show a conceptual diagrams and wave form diagrams explaining the multiple line simultaneous selection addressing, according to the prior art;

Figure 3 shows an explanatory waveform for a multiple gray scale formation in a frame rate control (FRC) procedure, according to the prior art;

Figure 4 shows an explanatory codification table of the gray levels in a frame rate control (FRC) procedure, according to the prior art;

Figure 5 shows a magnified portion of the waveform of Figure 3;

Figure 6 shows the table 1 of the values of the coefficients A_k and B_k and the table 2 shows words with six bits, the correspondent rms voltage values and the correspondent transmittance values;

Figure 7 shows the time diagrams of orthomormal functions of two row signals and of one column signal in the case of line by line addressing;

Figure 8 shows the voltage diagram as a function of the time of a column signal in the case of a row i and a row i+1;

Figure 9 shows the time diagrams of orthornormal functions of two row signals and of one column signal in the case of multiline addressing.

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 2^G 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...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 r_i(t) where

wherein the functions f_ik(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 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.

The functions r_i(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 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 value of the function I_nkj is given by the k-th bit of the row n and of the column j and I_nkj is equal to 1 if the bit b_k=0 while is equal to-1 if b_k=1.

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:

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 V_min and the maximum level V_max 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 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 which are present in table 1 in Figure 6 give the maximum voltage range.

The coefficients A_k and B_k 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 f_0,0, f_0,1 and f_0,2 are employed for the row 0 r₀(t) while the orthonormal functions f_1,0, f_1,1 and f_1,2 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 I_0,0 the gray shade of the pixel finding on the row 0 and on the column 0 and with I_1,0 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 c_j(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 p_0,0, p_0,1 and p_0,2 are used for the row 0 r₀(t) while the orthonormal functions p_1,0, p_1,1 and p_1,2 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 I_0,0 the gray shade of the pixel finding on the row 0 and on the column 0 and with I_1,0 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

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 (r_i(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 (c_j(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 r_i(t) is the i-th first signal as a function of the time t, the functions f_ik(t) are time functions which are orthogonal to each other, A_k 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 c_j(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 f_nk(t) are time functions which are orthogonal to each other, B_k is a numeric coefficient, I_njk 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 f_ik(t) are orthonormal to each other.
Method according to claim 1, characterized in that the functions f_nk(t) are orthonormal to each other.
Method according to claim 1, characterized in that the coefficients A_k and B_k must submit to the following function relationship Ak = N Bk and
wherin V_th is the threshold of the liquid crystal display.
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.