Patent Grant
8081179
This application claims benefit of Indian Patent Application No. 328/CHE/2008 filed Feb. 7, 2008 and the text of application 328/CHE/2008 is incorporated by reference in its entirety herewith.
Instant invention is related to a method to display gray shades in any RMS responding displays and more specifically passive matrix liquid crystal displays such as twisted nematic (TN) and super twisted nematic (STN) displays is disclosed. This method reduces the number of time intervals to complete a cycle and achieve more number of gray shades with simple waveforms having less number of voltages as compared to that of pulse-height modulation, amplitude modulation, successive approximation and wavelets based techniques.
Quality of image improves with the number of gray shades pulse width modulation [1] and frame modulation [2] add gray shade capability to liquid crystal displays. The number of gray shades that can be displayed with these techniques is limited because the number of time intervals in a cycle increases linearly with the number of gray shades. In a matrix display with N address lines, N.(G−1) time intervals are necessary to display G gray shades. Flicker will be observed in the display if a large number of gray shades are displayed using frame modulation. The smallest time interval in pulse width modulation may be comparable or even less than the RC time constant (product of output resistance of drivers and equivalent capacitance of pixels) when the number of gray shades is large. Error in the RMS voltage across pixels due to distortion in the addressing waveforms will result in poor brightness uniformity among pixels that are driven to the same state in pulse width modulation when the number of gray shades is large. Another important consideration is the error in the RMS voltage across pixels as described next. The difference of RMS voltages across ON and OFF pixels is small in passive matrix displays. For example, the ON pixels get a voltage that is about 10% higher than that of OFF pixels in a matrix display with 100 address lines. The difference in RMS voltage across pixels that are driven to any two adjacent gray shades is even smaller and it decreases with increase in number of gray shades. The difference in RMS voltages of neighboring gray shades is about 0.625% for 16 gray shades, 0.156% for 64 gray shades and about 0.039% for 256 gray shades in a display where in 100 address lines are multiplexed. It is obvious that the error in the RMS voltage across the pixels has to be small as the number of grayscales is increased to ensure good brightness uniformity among pixels that are driven to the same gray shade. Error in the RMS voltages is primarily due to the following reasons:
While the error in voltages of the addressing waveforms can be almost eliminated with a well-designed voltage level generator (VLG), the distortions in the addressing waveforms cannot be eliminated but can be minimized as described in the following text.
Output resistance of the driver circuit can be decreased either by buffering each output of the driver integrated circuit or by reducing the ON resistance of the analog switches in the multiplexers that select the voltages of the addressing waveforms. Both will increase the die size of the driver integrated circuit. It is expensive to decrease the output resistance or the ON resistance because of the large number of stages in the driver integrated circuit (A matrix display with N rows and M columns needs (N+M) drivers). It is preferable to reduce the number of intervals in a cycle to reduce the error due to distortion in the addressing waveforms so that the select time will increase (for a given refresh rate) and therefore RC time constant will be small as compared to the duration of the select time and thereby reduce the error in the RMS voltage. Amplitude modulation [3] and pulse height modulation [4] can display a large number of gray shades with a minimum number of time intervals. However, the number of voltages in the data waveforms is large. For example, the amplitude modulation that is based on line-by-line addressing has the least number of voltages in the addressing waveforms (i.e.2(G−1)to display G gray shades) among these techniques. It is much higher for the pulse height modulation that is based on multi-line addressing. Either the hardware complexity of the drivers is high as in case of digital type drivers with analog multiplexers and digital to analog converters or the power consumption is high as in case of analog type data drivers when amplitude modulation and pulse height modulation are used for displaying gray shades. Successive approximation [5]-[6] technique can be used to display a large number of gray shades with simple drivers. The number of time intervals is equal to the smallest integer value that is equal to or greater than logarithm of the number of gray shades i.e. log2 G. Similarly wavelet based addressing techniques can display large number of gray shades. Number of time intervals necessary is about the same order for the wavelets based techniques for displaying gray shades [7]-[12]. Both the techniques have less number of voltages in the addressing waveforms as compared to amplitude and pulse height modulation techniques and therefore the hardware complexity of the drivers is also less as compared to amplitude and pulse height modulation. It is preferable to meet the following conditions when gray shades are displayed in passive matrix liquid crystal displays:
The successive approximation technique and the wavelets based addressing techniques meet this criterion to some extent.
Accordingly the invention provides for a method to display gray shades in RMS responding matrix display, comprising acts of: selecting each row of the display matrix with a set of ‘s’ discrete select voltages in a sequential or random manner and applying a set of discrete data voltages to all the columns of the display matrix wherein the data voltages are of same or opposite polarity to that of select voltages with data voltage of each magnitude occurring a predetermined number of times in the ‘s’ time durations.
The main objective of the invention is to achieve a large number of gray shades with simple waveforms having less number of voltages and a small number of time intervals in a cycle.
Another main object of the present invention is to develop a method to display gray shades in RMS responding display matrix.
Another main object of this method is to improve brightness uniformity among pixels that are driven to the same state.
Another main object of this invention is to reduce the hardware of the driver circuit by having just a few voltages in the addressing waveforms.
Another main objective of this invention is to increase duration of application of each select voltage without causing flicker in the display.
Another main objective of this invention is to ensure that the energy delivered to the pixels in a row during the select and non-select duration of a cycle is substantially same as that of energy delivered to pixels each column during a cycle within practical limits for all the pixels in all the rows of the matrix display.
Another main objective of this invention is to ensure that the energy delivered to the pixel during ‘s’ time intervals of data sequence is substantially same for all the gray shades in all the pixels in all the columns of the display.
Still another object of the present invention is to select a row of the display matrix with a set of discrete select voltages.
Yet another main object of the present invention is to apply a set of discrete data voltages to columns of the display matrix wherein the data voltages are of either polarity (i.e., same or opposite polarity to that of the select voltage)and the number of occurrence of each magnitude (in the data voltage sequence of length-s) is same for all data voltage sequences set to display gray shade in a RMS responding display matrix.
However the invention should not be considered to be restricting the scope of the method to the above-mentioned objectives. It is possible that this invention can meet other objectives as well that falls within the scope of this disclosure.
The primary embodiment of invention is a method to display gray shades in RMS responding display matrix comprising acts of:
In yet another embodiment of the present invention the polarity of the select and the data voltages are changed periodically to achieve dc-free operation.
In still another embodiment of the present method each data voltage of specified amplitude has a select voltage that is √{square root over (N)} times the amplitude (magnitude) of the data voltage to achieve the maximum difference in RMS voltages of pixels that are driven to the two extreme gray shades i.e. ON and OFF states.
In still another embodiment of the present invention the amplitude of select voltages are suitable chosen to provide uniformly spaced RMS voltages from RMS voltage of OFF pixels to the RMS voltage of the ON pixels.
In still another embodiment of the present invention the select and data voltages are suitably chosen to provide for maximum number of RMS voltages for a given set of select and data voltages.
In still another embodiment of the present invention the select voltages in the s-time intervals are arranged to form a ascend voltage profile followed by a descending voltage profile to reduce power dissipation in driver circuit.
In still another embodiment of the present invention the select voltages are applied for equal durations.
In still another embodiment of the present invention the duration is longer than RC time constant of the driver circuit.
In still another embodiment of the present invention wherein varying amplitude and/or sign of the select and the data voltages to control the energy of the select and data waveforms during a cycle and there by vary RMS voltage across each pixel of the display.
In still another embodiment of the invention a subset of all the possible data voltage sequences is applied to develop RMS voltages that are useful to correct the non-linearity of electro-optic response and/or human eye response.
In still another embodiment of the present invention the number of gray shades is greater than that of successive approximation technique with same number of time intervals in a cycle for a given matrix display.
Still another embodiment of the invention is to achieve the maximum selection ratio and display more number of gray shades in the display than possible with successive approximation technique with same number of time intervals in a cycle for a given matrix display.
In still another embodiment of the present invention the display is passive matrix liquid crystal display.
In still another embodiment of the present invention the row of the display matrix is can be randomly selected with the sequence of ‘s’ select voltages by ensuring that each row is selected just once in a cycle instead of the sequential selection of rows in the conventional methods of matrix addressing.
The method is based on selecting one of the N address lines (rows) in a matrix display at a given instant of time with one of the voltages from a set of ‘s’ voltages {+r1, +r2, +r3, . . . , +rs} of different amplitudes as shown in
Thus the energy delivered to the pixels can be controlled with:
Energy delivered to a pixel is small when the sign of the data voltage is same as the select voltage as compared to the case when the sign of data voltage is opposite to that of select voltage. For a specific select voltage say r1 that is applied to a row during t1, data voltages can be any one of 2·s values i.e. {+d1 or,−d1 or +d2 or,−d2 or +d3 or,−d3 or . . . +ds or,−ds}. Hence the energy delivered during the first time interval can be any one of the 2·s values depending on the choice of the data voltage from the set of 2·s values and optionally it can be tuned to a desired value by varying t1. Let the select voltage during the second time interval (i.e. t2) be r2. Choice of data voltages is restricted to one of 2(s−1) in the second time interval because a data voltage of specific amplitude is used just once during the s-time intervals. Hence the energy delivered to a pixel during the second time interval can be one of the 2(s−1) values. Duration of the select and data voltages can also be varied to control the energy delivered to the pixels. The number of data voltages that can be applied to the pixels diminishes as one progress from the first to the final select pulse and it is just two voltages for a specific pixel in the sth (last) time interval. Energy delivered to the pixels can be computed by substituting values of select and data voltages that are applied to the pixels during the s-time intervals) as shown in the following expression.
The first term corresponds to the energy delivered during the select time and the second term corresponds to the energy delivered to the pixel when (N−1) rows (excluding the one in which the pixel is located) are selected. The root-mean-squared (RMS) voltage across the pixel is given by the following expression.
The first and last terms are constant value for specific sets of select and data voltages because each select and data voltage of specific amplitude is used just once during the s-select intervals. RMS voltage will be one of the s !2s values depending on the choice of data voltages during the s-time intervals. It is more by a factor s ! as compared to the 2sRMS voltages that is achievable by using successive approximation technique. It is the maximum number of unique voltages that can be achieved with the technique of this invention. The actual number of RMS voltages may be lower under certain conditions. Number of unique RMS voltages will reduce by a large factor when the condition for the maximum selection ratio viz., ri=√{square root over (N)}·di for all values of i; i.e., i =1,2,3, . . . ,s is imposed. Number of unique combinations will decrease if certain product terms i.e. ri·dk,i has the same value for more than one i and k. However, the maximum number of unique RMS voltages is larger than that of successive approximation or any other technique known so far for displaying gray shades in RMS responding displays for a specific matrix display and specific number of voltages and specific number of time intervals. Number of gray shades that can be achieved without any compromise on the selection ratio is shown in Table I. The maximum selection ratio of
is achieved with the technique and even then the number of gray shades in higher than that of successive approximation technique as shown in the right most column of Table 1.
Although, the number of RMS voltages that is achievable during the s time intervals will be less than the maximum (2ss!); it is much higher than the successive approximation technique depending on the values assigned to the select and data voltages. Typical row and column waveforms of one embodiment of the present invention are shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 00328/CHE/2008 | Feb 2008 | IN | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 5233447 | Kuribayashi et al. | Aug 1993 | A |
| 5675353 | Tanaka et al. | Oct 1997 | A |
| 5861869 | Scheffer et al. | Jan 1999 | A |
| 7245282 | Jones | Jul 2007 | B2 |
| 20060114209 | Kim et al. | Jun 2006 | A1 |
| 20070257946 | Miller et al. | Nov 2007 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20090201241 A1 | Aug 2009 | US |
