The present invention concerns generally passive matrix displays, in particular to a display device and a method for driving a display device, wherein the display device comprises a liquid crystal material between a first substrate provided with row electrodes and a second substrate provided with column electrodes, in which overlapping parts of the row and column electrodes define pixels, driving means for driving the column electrodes in conformity with an image to be displayed, and driving means for driving the row electrodes.
The display technique will play an increasingly important role in the information and communication technique in the years to come. Being an interface between humans and the digital world, the display device is of crucial importance for the acceptance of contemporary information systems. Notably portable apparatus such as, for example, notebooks, telephones, digital cameras and personal digital assistants cannot be realized without utilizing displays. The passive matrix LCD technology is a very commonly used display technology; it is used, for example in PDA's and in mobile telephones. Passive matrix displays are usually based on the (S)TN (Super Twisted Nematic) effect. A passive matrix LCD consists of a number of substrates. The display is subdivided in the form of a matrix of rows and columns. The row electrodes and column electrodes are arranged on respective substrates und form a grid. A layer with liquid crystals is provided between said substrates. The intersections of these electrodes form pixels. These electrodes are supplied with voltages that orient the liquid crystal molecules of the driven pixels in an appropriate direction so that the driven pixel appears in a different brightness.
Since the size of the displays becomes larger, the significance of the power consumption of the passive matrix LCDs for mobile applications increases all the time. Because such passive matrix displays are often used in portable apparatus, it is particularly important to realize low power consumption.
In general the row electrodes of passive matrix display devices are selected or activated by a row selection voltage for a row selection time, whereas the image data to be displayed is supplied via the column electrodes. There are different schemes for driving display devices. The most common driving scheme is the so called Alt & Pleshko driving scheme. Here each row will be selected separately. At the time the respective row is selected the required column voltages are supplied to the column electrodes. So each pixel in the selected row will show its respective grey scale. After a row has been selected, the next row will be selected until all rows of the display are selected one time. Thereby, a so-called frame is defined as the time it takes to select all rows of the display—in the case of Alt & Pleshko driving—exactly once.
Another more recent driving scheme is the so called multiple row addressing scheme (MRA). Here a group of p rows is simultaneously driven and the encoded image information is applied to the columns. This MRA technique enables a very good optical performance to be achieved in combination with low power consumption. According to said MRA technique a number of p rows are simultaneously driven. A set of orthogonal functions is then applied to the p simultaneously driven rows. A function for the column voltage for driving the corresponding column is calculated from said set of orthogonal functions using a calculation rule. By using this calculation rule for driving the column, a voltage is selected from a plurality of partial column voltage values, said selected voltage being applied to the corresponding column so that the corresponding pixels are switched to a state depending on the image data that is supplied from a memory.
In order to display grey scales a method called Pulse Width Modulation (PWM) can be used. This method that is combined either with Alt & Pleshko driving or MRA driving is based on the splitting of one row selection time into several row sub selection time slots. By variation of the column voltage level between subsequent row sub selection time slots within a certain row selection time, the corresponding pixel can be driven to a grey scale. When splitting one single row selection time in npwm sub selection time slots npwm+1 different grey scales can be generated.
A characteristic of this grey scale method is that for each row sub election time slot the column voltage to be driven has to be calculated. This requires foremost in combination with MRA driving an intensive data processing. Moreover, the column voltage may change its level at most once with each new row sub selection time slot, whereby introducing many transitions into the column voltage waveform. Both, an intensive data processing as well as a high number of transitions have a negative impact on the overall power consumption of the driver.
Therefore it is an object of the invention to provide a display device, a circuit arrangement for driving such display device and a method for driving a display device having low power consumption. In particular it is an object to minimize the number of transitions of the column driving signal.
This object is solved by the subjects of the independent claims.
To this end a display device according to the present invention is provided comprising
a liquid crystal material between a first substrate provided with row electrodes and a second substrate provided with column electrodes, driving means for driving the column electrodes in conformity with an image to be displayed, and driving means for driving the row electrodes, wherein during a row selection time at least one row is selected and column voltages (Gj(t)) are supplied to the column electrodes, wherein the column voltage waveform depends on the grey scale to be displayed by a driven pixel in a certain column and depends on a used selection signal (Fi) supplied to the selected row, wherein a column voltage (Gj(t)) is switchable between at least two different column voltage levels during the row selection time and the column voltage waveform for a following row selection time is mirrored on a mirror axis depending on the column voltage at the end of the current row selection time and the column voltage at the end of the following row selection time.
In a preferred embodiment of the invention the mirroring is performed, if the column voltage at the end of the current row selection time is the same as the column voltage at the end of the following row selection time.
In a preferred embodiment of an inventive display device the mirroring is performed by mirroring the column voltage waveform for every second row selection time. It is also possible to use a different distance between the row selection times. This kind of fix mirroring is very simple to realize.
In an alternative preferred embodiment of an inventive display device the mirroring is performed adaptively. In that embodiment it is not possible to say, which row selection time will be mirrored, because the mirroring is only performed if a transition can be saved. This requires a calculation whether a transition can be saved. By calculating the following column voltage level for the subsequent row selection time or in particularly by calculating the column voltage level of the last row sub slot of the subsequent row selection time already during the current row selection time, then the possibility exists to reduce the number of transitions by mirroring the column waveform of the following row selection time whenever this is of advantage—so-called adaptive mirroring. This reduced number of transition in the column voltage waveform has a positive impact on the power consumption of the driver.
This adaptive column mirroring can be applied to various driving schemes including Alt & Pleshko and MRA as long as they are combined with Pulse Width Modulation. The most important advantage of adaptive column voltage mirroring is that therewith up to 50% of the column transitions can be saved but under no circumstances the number of transitions is increased. Since the number of transitions has a direct impact on the power consumption—the less transitions the lower the power consumption—the presented display device can considerably reduce the power consumption of the driver.
In a further alternative embodiment the mirroring is activated if Vcol (t=n Tr−dt)=Vcol (t=(n+1) Tr−dt) is true for n=1, 2, 3 for the n subsequent row selection times, wherein Tr represents the row selection time and dt one incremental time step.
In a further alternative embodiment the mirroring is performed within a window, whereas the size of the windows to be mirrored has to be at least three row sub selection time slots. By doing this, the mirroring can be performed in a very effective way resulting in a column voltage signal having a significant lower number of transitions. The mirroring within a window, e.g. the window includes three sub election row slots, is only performed whenever the voltage level of the third row sub lot is the same as the one in the first row sub slot, if so—the second and the third row sub slot will be exchange—hence mirrored on the mirror axis between the second and third row sub slot.
In a further preferred embodiment the position of the mirror axis depends n the number of row sub selection time slots comprised in the window. There is a further advantage to calculate the column voltage level of the over-next row sub selection time slot during the current row sub selection time slot.
In a particular preferred embodiment the mirroring is activated if Vcol (t=m Trs−dt)=Vcol (t=(m+2) Trs−dt) for m=1, 2, 3, . . . , M but not for m=M−1, wherein M=Tr/Trs is the number of row sub selection time slots per row selection time slot and, with Trs being the row sub selection time and dt being one incremental time step, wherein m is reset to 1 after having reached M.
The object of the present invention is further solved by a circuit arrangement for driving a display device as claimed in claim 9.
The object of the present invention is further solved by a method for driving a display device as claimed in claim 10.
For a more complete description of the present invention and for further objects and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, in which:
a, 4b illustrate the mirroring of the column voltage waveforms over two row slots for MRA with p=4 and PWM with npwm=4;
a, 5b illustrate the mirroring of the column voltage waveforms over three row slots for MRA with p=4 and PWM with npwm=4;
a, 6b illustrate the mirroring of the column voltage waveforms over four row slots for MRA with p=4 and PWM with npwm=4;
a, 7b illustrate the mirroring of the column voltage waveforms for MRA with p=4 and PWM with npwm=4 over four row slots thereby reducing the number of transition by 50%;
a shows the mirroring criterion in the Alt & Pleshko case with one row selection time subdivided into four sub selection time slots (PWM with npwm=4) when the grey scale table from Table 1 is used.
b shows the mirroring criterion in the MRA with p=4 case with one row selection time slot subdivided into four sub selection time slots (PWM with npwm=4);
a, 10b, 10c, 10d, 10e, 10f, 10g, 10h, 10i are illustrating the mirroring within a window of subsequent row sub slot periods for MRA with p=4 and PWM with npwm=7.
Referring to
In the following
The effectiveness of mirroring heavily depends on the picture to be displayed. The ideal picture of course consists of only one grey scale per column. In this case up to 50% of the transitions can be saved. An example for such a column voltage waveform without mirroring and with mirroring is illustrated in the
a, 8b illustrate the mirroring criterion, wherein
Vcol(t=n Tr−dt)=Vcol(t=(n+1) Tr−dt) must be true (1)
for n=1, 2, 3 for the n subsequent row selection times, wherein Tr represents the row selection time and dt one incremental time step. This means that the column voltage Vcol at the end of the current row selection time (t=n Tr−dt) must be the same as the column voltage Vcol at the end of the following row selection time (t=(n+1) Tr−dt).
In the following the equitation for calculating the column voltage Gj(t) will be given.
whereas Eq. (2) represents the column driving voltage (Gj(t)-function)
for MRA with p=4 for the column with index j for the duration a certain group of p rows is selected and under the assumption that the row index i is given as the row number modulo 4. Note: aij: pixel state of the pixel given by rowi and columnj (pixel in its ON state: aij=−1dec (chosen to be represented by 0 digital), pixel in its OFF state: aij=+1dec (chosen to be presented by 1 digital)).
Fi(t): orthogonal function applied to row; (possible normalized values in case of the walking −1 set of orthogonal functions are: −1dec (chosen to be represented by 0 digital), +1dec (chosen to be represented by 1 digital).
Gj(t): column function to be applied to columnj for the duration the respective group of p rows is selected.
N: number of rows of the display.
Below a grey scale table is shown illustrating the coding of five different grey scales by using a PWM with npwm=4. This means to display a pixel with e.g. grey scale 3, the pixel has to be driven during the first three row sub slots to an on state and in the last row sub slot to an off state respectively.
In the following a further embodiment of the inventive display device will be expected in more detail. It is also possible to mirror the column voltage waveform not only for an entire row selection time. It is also possible to mirror the column voltage waveform within a window of three or more subsequent row sub slots. In the case of a window of three row sub slots, whenever the voltage level of the third row sub slot is the same as the one in the first row sub slot, the second and the third row sub slot will be exchange—hence mirrored on the mirror axis between the second and third row sub slot. This however works only as long as the second and third row sub slots within the windows belong to the same row selection time.
b shows the column voltage waveform before the first mirroring step is performed. The window includes the row sub slots 1-3. Then the mirroring is performed resulting in a column voltage waveform shown in
In the following an example will be given for a window having four row sub slots. When the column voltage level in first row sub slot is identical with the column voltage level in the fourth row sub slot then mirroring is performed. Mirroring means to mirror the waveform given by the second, third and fourth row sub slot on a mirror axis in the middle of the third row sub slot. The order of the row sub slots before mirroring is 1234, wherein the order of the row sub slots after mirroring will be 1432.
In case of a window having five row sub slot window—the mirroring is performed, when the column voltage level in the first row sub slot is identical with the column voltage level in the fifth row sub slot. Mirroring means to mirror the waveform given by the second, third, fourth and fifth row sub slots on a mirror axis between the third and the fourth row sub slot. So the order of the row sub slots before mirroring is 12345, wherein after mirroring: 15432.
It has to be noted, that in both cases no mirroring is allowed when the window crosses the border to the next row selection time AND not all of the row sub slots to be mirrored are in the same row selection time.
Number | Date | Country | Kind |
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03101709.8 | Jun 2003 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB04/50826 | 6/2/2004 | WO | 12/7/2005 |