Liquid crystal display apparatus and method of performing liquid crystal display

Abstract: A liquid crystal display apparatus in which color display information is converted into information about luminance of liquid crystal display used for driving liquid crystal so as to provide a liquid crystal display. The apparatus comprises a decider unit (6) for deciding whether the color display information takes a display form pursuant to the dither method, and a converter unit (8, 10, 12, 14) for converting the color display information into liquid crystal luminance display information in accordance with a frame base thin-out method corresponding to the color display information when the decision results indicate that the color display information takes a display form pursuant to the dither method but for converting the color display information in accordance with a dot base thin-out method when the decision results are negative.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a liquid crystal display apparatus according to the preamble of claim 1 and a method of performing liquid crystal display according to the preamble of claim 5. This invention relates also to a liquid crystal display apparatus in which display information is converted into information about luminance of liquid crystal display used for driving liquid crystal so as to provide a liquid crystal display and a method of performing a liquide crystal display, and more particularly to this type of liquid crystal display apparatus and method capabale of alleviating flicker in picture and display flow which occur when display data of a display form pursuant to the dither method is displayed in accordance with gradation display of luminance. The dither method referred to herein signifies a technique of realizing binary display of graded images of various half tones by making use of human visual characteristics, as well known in the art. Description of the Prior Art As technique of realizing gradation display of luminance of display images in the liquid crystal display apparatus, a socalled dot base frame thin-out method has been proposed as described in JP-A-61-205983, according to which where a given number of frames are thinned out to provide some taken-out frames at which the image is turned on and lit so that a half tone display may be effected, a set of dots within a taken-out frame and another set of dots X-directionally and Y-directionally adiacent to the former dots in X and Y directions of Carte-sian coordinates on the screen within another taken-out frame are turned on at different timings. More particularly, liquid crystal is driven in accordance with an effective value of voltage applied to one dot and luminance of display is proportional to the effective value. Therefore, effective value control based on a frame thin-out method has been employed to achieve gradation display. In the frame thin-out method, within a predetermined number of frames, some frames at which "turn on" (lit) is effected and the remaining frames at which "turn off" (unlit) is effected are determined at a rate (called a thin-out rate) and "turn on" and "turn off" operations at that rate are repeated to control the effective value of applied voltage, thereby performing half tone display of luminance proportional to the effective value. Disadvantageously, in this method, flicker occurs in image display each time "turn on" operation is switched to "turn off" operation or vice versa and besides when dots are thinned out at the same timing over the entire screen, flicker in image display is aggravated to degrade display quality of image. As method for elimination of the flicker defect, the dot base thin-out method has been contrived. With reference to Figs. 4 and 5A to 5E, the dot base thin-out method will be described in greater detail. Fig. 4 illustrates a fundmental dot set in the dot base and Figs. 5A to 5E show thin-out operation effected at a period of four frames. Referring to Fig. 4, in the dot base thin-out operation is carried out in unit of a fundamental dot set 18 of 4 dots consisting of 2 dots in X direction (one odd dot and one even dot) and 2 dots in Y direction (one odd dot and one even dot) on the screen of a liquid crystal pannel 17. Within a frame defined by the fundamental dot set 18, dots are thinned out at different timings for two groups of which one has two staggered dots represented by α and the other has two staggered dots represented by β. This thin-out operation will be described specifically with reference to Figs. 5A to 5E. In these Figures, turned-on dots are depicted by hatched dots. Fig. 5B shows a display mode at a thin-out rate of 1/4 in which any one of the four dots constituting the fundamental dot set is turned on once over 4 frames. At a first timing for the dots α, dots α1 (the dots α particularly indicated for a first frame) are turned on within the first frame and dots α2 to α4 are turned off within the remaining frames. At a second timing for the dots β, dots β3 (the dots β particularly indicated for a third frame) are turned on within the third frame and the dots β1, β2, β4 are turned off within the remaining frames. Figs. 5C and 5D illustrates similar thin-out modes. More specifically, in Fig. 5C, at the first timing for the dots α, the dots α1 within the first frame and the dots α2 within the second frame are turned on with the dots α3 and α4 within the remaining frames turned off, and at the second timing for the dots β, the dots β3 within the third frame and the dots β4 within the fourth frame are turned on with the dots β1 and β2 within the remaining frames turned off. In Fig. 5D, at the first timing for the dots α, the dots α1 within the first frame, the dots α2 within the second frame and the dots α4 within the fourth frame are turned on with the dots α3 within the remaining frame turned off and at the second timing for the dots β, the dots β2 within the second frame, the dots β3 within the third frame and the dots β4 within the fourth frame are turned on with the dots β1 within the remaining frame turned off. Figs. 5A and 5E illustrate perfect turn-off display and perfect turn-on display, respectively. A prior art employing the staggered type frame thin-out method described as above does not take the aforementioned dither technique and conventionally, when the dither method and the dot base frame thin-out method are used in combination, there arises a problem that the display flow is increased and the picture quality is aggravated. JP-A-63-287828 discloses the generation of a gradation of each frame when a gradation display is made. Gradation display data is selected in accordance with the value of the display information in order to obtain different gradation. The liquid crystal display controller disclosed therein makes use of two methods which are applied to gradation patterns having different levels of brightness. For example, one method is used for generating a pattern of brightness A, while another method is used for generating a pattern of brightness B. However, there is no possibility to determine whether the display information takes a form that would lead to a degraded image quality and to adjust the conversion accordingly. SUMMARY OF THE INVENTION The present invention contemplates elimination of the above conventional drawbacks and has for its object to provide liquid crystal display apparatus and method capable of suppressing the display flow and preventing the picture quality degradation from occurring even when displaying data pursuant to the dither method. This object is achieved by the liquid crystal display apparatus according to claim 1 and the method of performing liquid crystal display according to claim 5. According to a preferred embodiment of the invention, color display information is decided as to whether to take a display form pursuant to the dither method, and when the decision results indicate that the color display information takes the display form pursuant to the dither method, the color display information is converted into liquid crystal luminance display information in accordance with a frame unit thin-out method corresponding to the color display information but when the decision results are negative, the color display information is converted into liquid crystal luminance display information in accordance with a dor base thin-out method. The present invention having the above construction can ensure exellent liquid crystal display without inviting the picture quality degradation. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing the essential part of an embodiment of the invention; Figs. 2A to 2E are diagrams useful to explain thin-out operation in unit of frame; Figs. 3A to 3F are diagrams useful to explain the principle of dither decision together with an example of decision results; Fig. 4 is a diagram useful to explain a fundamental dot set in the dot base thin-out method; and Figs. 5A to 5E are diagrams useful to explain dot base thin-out operation at a period of 4 frames. DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to being engaged in accomplishing the present invention, the present inventors have confirmed the following. More particularly, in display based on the dot base frame thin-out method, display flow (which gives a visual feeling that the image runs in the right or left direction) is slightly generated in a display in all-over painting fashion of uniform luminance (at a uniform thin-out rate) but therein, such a flicker as generated in the frame base thin-out method by which thin-out operation is effected at the same timing over the entire screen is not found and the display can be stable. Specifically, the dither method is classified into two types. In one type (first type), when adjacent dots on the screen are to be thinned out at the same thin-out rate, for example, when one of the adjacent dots is turned on within one of two frames and turned off within other frame, the preceding dot is turned on within the first frame while being turned off within the second frame and phase inversion is effected for the succeeding dot so that the succeeding frame is turned off within the first frame while being turned on within the second frame. In the other type (second type), adjacent two dot sets on the screen are thinned out at different thin-out rates. In this manner, the number of expressible kinds of color can be increased. Thus, by changing the thin-out rate between adjacent two dot sets such that the thin-out rate for the first dot set corresponds to red color and the thin-out rate for the second dot set corresponds to white color, a thin-out rate can be realized which corresponds to pink visually. Then, the present inventors have found that when display data of a display form pursuant to the second type of the dither method undergoes the dot base frame thin-out operation, the aforementioned display flow is aggravated and the picture display becomes unstable, thus aggravating the picture quality degradation. The present invention has been achieved to eliminate the above problems. The invention will now be described by way of example with reference to the accompanying drawings. Fig. 1 is a block diagram illustrating the essential part of an embodiment of the invention. Referring to Fig. 1, reference numeral 1 designates serial input display data to be subjected to gradation display, 2 a serial/parallel converter adapted to convert the input serial display data into 4-bit parallel data and deliver the parallel data as designated at 5, 4 a latch circuit, 10 a dot base thin-out data generator for generating data 11 pursuant to the aforementioned dot base thin-out method, and 8 a frame base thin-out data generator for generating frame base thin-out data 9. The frame unit thin-out data generator 8 and dot base thin-out data generator 10 are responsive to a head line clock signal and a line clock signal (see Fig. 4) which constitute a liquid crylstal panel display drive signal and the serial input display data 1 (color display information) to generate thin-out data corresponding to the input display data as shown in Figs. 2A to 2E and Figs. 5A to 5E. For example, in the case of 8-gradation display, each of the generators generates 8 kinds of thin-out data. This operation is in general well known and will not be detailed herein. Reference numeral 6 designates a dither decider for deciding whether the 4-bit parallel data 5 is dither display data and generating, in accordance with the decision results, a select signal 7 which controls a thin-out data selector 12. The selector 12 is responsive to the select signal 7 to select the thin-out data so that either one of the frame unit thin-out data 9 and dot base thin-out data 11 is selected and delivered as selected thin-out data 13. A data decoder 14 responds to the 4-bit parallel data 5 to decode the selected thin-out data 13 and decoded data is delivered as 4-bit gradation display data 16 through a latch 15. More particularly, the data decoder 14 selects one kind of data from, for example, 8 kinds of selected thin-out data in accordance with the 4-bit parallel data. In other words, one kind of thin-out data is selected from various kinds of thin-out data generated from the data generators 8 and 10 under the direction of the color display information and delivered to the liquid crystal panel. The latches 4 and 15 operate in synchronism with a latch clock signal 3. It will be appreciated that the X driver for driving the liquid crystal panel in X-axis direction typically fetches display data for determination of turn-on/off of display 4 bits by 4 bits or 8 bits by 8 bits simultaneously. Therefore, by performing decision as to whether the display form is pursuant to the dither method, that is, dither decision in unit of 4 bits, the decision results can be applied to either one of the 4-bit type and 8-bit type X drivers and the display mode can be switched to one commensurate with the X driver in use. In order to perform control operation in unit of 4 bits, ordinary serial data must be converted into 4-bit parallel data. During an interval of time beginning with conversion of initial serial data of 4 dots into 4-bit parallel data and ending with completion of serial/parallel conversion of the following 4-dot display data, dither decision is carried out for the initial 4 dots, thin-out display data for a display of 4 dots is selected in accordance with the decision results and the selected data is delivered as 4-bit display data to the X driver, thereby preventing the X driver from operating erroneously. Since liquid crystal is activated to provide a display of luminance proportional to the effective value of applied voltage, displays of identical luminance can be obtained in different thin-out modes as far as the effective values of applied voltage are identical in these modes and therefore no problem is encountered in display luminance. The staggered type thin-out data generator 10 and frame base thin-out data generator 8 shown in Fig. 1 will now be described. The dot base thin-out method has already been described by referring to Fig. 4 and Figs. 5A to 5E but reference should again be made to Figs. 4 and 5A to 5E in explaining the dot base thin-out data generator 10. As has been described, Fig. 4 shows the fundamental dot set in the dot base and Figs. 5A to 5E show thin-out operation effected at a period of four frames. As will be seen from Fig. 4, the fundamental dot set 18 is constructed of 4 dots of which two are in X-axis direction and two are in Y-axis direction on the liquid display panel 17 and provides dot base thin-out data. The dot base thin-out data generator 10 generates display data 11 for each frame of the fundamental dot set 18 at different two timings, one for the dots α and the other for the dots β, as shown in Figs. 5A to 5E. On the other hand, the frame base thin-out data generator 8 generates display data 9 for each frame by thinning out all dots of the fundamental dot set 18 at the same timing as shown in Figs. 2A to 2E. The dot base thin-out data and frame base thin-out data will now be described with reference to Figs. 5A to 5E and Figs. 2A to 2E, respectively. In these Figures, hatched dots indicate turned-on dots. Referring in particular to Fig. 5B, within the first frame, an X-directionally odd and Y-directionally odd dot and an X-directionally even and Y-directionally even dot are turned on at the timing for the dots α and an X-directionally even and Y-directionally odd dot and an X-directionally odd and Y-directionally even dot are turned off at the timing for the dots β, within the second frame all the dots are turned off, within the third frame the state within the first frame is inverted, and within the fourth frame all the dots are turned off as within the second frame. In connection with each one dot of the fundamental dot set 18, it is turned on once and turned off three times throughout the four frames. In other words, 1/4 turn-on is achieved in a display in unit of four frames. In the case of the staggered type thin-out method shown in Figs. 5A to 5E, flicker can be eliminated almost completely but display flow in the right or left direction is slightly generated in a display in all-over painting fashion at a uniform thin-out rate, as described previously. When the staggered type is applied to display data pursuant to the dither method, the display flow is aggravated. Referring now to Fig. 2B, in this example of frame base thin-out data, all dots of the fundamental dot set 18 are thinned out at the same timing so that within the first frame, all the dots are turned on and within the second to fourth frames, all the dots are turned off, indicating that in conneciton with each one dot of the fundamental dot set, it is turned on once through out the four frames. In the case of the frame unit thin-out operation as shown in Figs. 2A to 2E, flicker is large but no display flow is caused in a display in all-over painting fashion. When the frame base type is applied to display data pursuant to the dither method, the flicker can be suppressed and no display flow is found. As will be seen from the foregoing, evaluation of display quality of the all-over painting like display obtained in accordance with the two kinds of thin-out method and display quality of the dither display indicates that gradation display of higher quality can be realized by using the frame base thin-out method in combination with data of dither display form and the dot base thin-out method in combination with data of other display form. The dither decider shown in Fig. 1 will now be described with reference to Fig. 3. The dither decider 6 decides whether the 4-bit parallel data 5 is of dither display form and produces the desision results, an example of which is shown in Figs. 3A to 3F. In these Figures, A, B and C represent different pieces of data and the dither display form is settled when the following conditions are satisfied: (1) Lefthand and righthand dot data pieces about a central dot data piece, which stands for second or third one of the four dot data pieces, differ from the central data piece. (2) The lefthand and righthand data pieces are identical to each other. In Fig. 3, "○" in the decision results denotes coincidence with the dither display and "×" denotes non-coincidence with the dither display. First data and second data in Figs. 3A and 3B do not satisfy condition (1) and "× " results. In Fig. 3C, conditions (1) and (2) are satisfied and "○" results. In Fig. 3D, lefthand and righthand dot data pieces about the second dot data piece B are A and the conditions are satisfied. The fourth data piece C has no relation to the conditions. In Fig. 3E, first data satisfies the conditions but second data does not satisfies the condition and there result "○" and "×" in the decision results. In Fig. 3F, condition (2) is not satisfied. In this manner, display data of 4 dots satisfying the conditions (1) and (2) for the dither display is determined to be of dither display form by means of the dither decider 6 and a select signal 7 for the data selector 12 is produced from the decider 6. In accordance with the select signal 7, either frame base thin-out data 9 or dot base thin-out data 11 is selected and delivered as selected thin-out data 13. In the data decoder 14, the selected thin-out data 13 is decoded under the direction of the 4-bit parallel data to provide gradation display data 16 which is delivered through the latch 15. Through the method set forth so far, only when display data of four dots takes a dither display form, gradation display can be carried out in accordance with unit frame thin-out data, and for data of the other display form, gradation display can be carried out in accordance with dot base thin-out data. Returning to Fig. 1, the operation of the circuit will be summarized. Display data 1 received serially is converted into 4-bit parallel data 5. The 4-bit parallel data is decided as to whether to be of a dither display form by means of the dither decider 6 and decision results are sent to the data selector 12. The data selector 12 selects either frame unit thin-out data or staggered type thin-out data in accordance with the decision results and the data decoder 14 decodes the selected data under the direction of the 4-bit parallel data 5 to produce gradation display data 16 which is delivered to the liquid crystal panel. It will be appreciated that the dither decision is carried out in unit of 4-bit parallel data and that the dither decider 6 can be constructed of a simple decoder. Thus, in accordance with the present invention, a specified display method is used for a specified display pattern to prevent degradation of the display quality and ensure stable display. The number of the kinds of specified display pattern is not limited to one. Assuming that N kinds of specified display pattern are available, display methods dedicated to the respective kinds may be employed. According to the present invention, in the liquid crystal display apparatus, when the display pattern is a specified A pattern such as a dither display form, gradation display is carried out in accordance with a specified C method such as the frame unit thin-out method but when the display pattern is a B pattern other than the dither display form, gradation display is carried out in accordance with a D method such as the dot base thin-out method different from the C method, thereby ensuring that any display pattern can be displayed with high display quality.

Claims: A liquid crystal display apparatus in which display information is converted into luminance information of a liquid crystal display for driving the liquid crystal according to a thin-out method, whereby a dot-based thin-out method consists in switching on/off particular dots of a group of dots for a plurality of frames, and whereby a frame-based thin-out method consists in simultaneously switching on/off all the dots of a group for a plurality of frames, characterized in that the liquid crystal display apparatus comprises decider means (6) which is responsive to the display information for deciding which of a plurality of thin-out methods is to be applied for a group of dots, each group consisting of an array of 2 x 2 dots, the plurality of thin-out methods consisting of a dot-based thin-out method and a frame-based thin-out method, the decision for each group being based on evaluation of said array of 2 x 2 dots in the group, whereby if two vertically adjacent dots in said array have the same luminance, and said 2 x 2 dots have not all the same luminance, then the frame-based thin-out method is decided to be applied, otherwise the dot-based thin-out method is decided to be applied, and providing an output indicative thereof, converter means (8, 10, 12, 14) responsive to the output of the decider means (6) for converting the display information into liquid crystal display luminance information in accordance with a frame-based thin-out method for a plurality of frames when the output of the decider means (6) is indicative of the frame-based thin-out method and for converting the display information into liquid crystal display luminance information in accordance with a dot-based thin-out method when the output of the decider means (6) is not indicative of the frame-based thin-out method. A liquid crystal display apparatus according to claim 1, wherein said converter means includes a timing generator unit (8, 10) for generating a timing for said dot-based thin-out method and a timing for said frame-based thin-out method, a selector unit (12) for selecting one of said timings in accordance with the output of the decider means, and a decoder unit (14) for decoding data of selected timing. A liquid crystal display apparatus according to claim 1, wherein said converter means includes a timing generator (8) for generating liquid crystal display luminance information in accordance with the frame-based thin-out method and a second generator (10) for generating liquid crystal display luminance information in accordance with the dot-based thin-out method and selecting means (12) for selecting liquid crystal display information an accordance with the output of said decider means. A liquid crystal display apparatus according to one of claims 1 to 3, wherein display information is color display information. A method of performing liquid crystal display in which display information is converted into luminance information of a liquid crystal display for driving the liquid crystal display, according to a thin-out method, whereby a dot-based thin-out method consists in switching on/off particular dots of a group of dots for a plurality of frames, and whereby a frame-based thin-out method consists in simultaneously switching on/off all the dots of a group for a plurality of frames, said method being characterized by the steps of: deciding which of a plurality of thin-out methods is to be applied for a group of dots, each group consisting of an array of 2 x 2 dots, the plurality of the thin-out methods consisting of a dot-based thin-out method and a frame-based thin-out method, the decision for each group being based on evaluation of said array of 2 x 2 dots in each group, whereby if two vertically adjacent dots in said array have the same luminance, and said 2 x 2 dots have not all the same luminance, then the frame-based thin-out method is decided to be applied, otherwise a dot-based thin-out method is applied, providing an output indicative thereof, and converting the display information into liquid crystal display luminance information in accordance with a frame-based thin-out method for a plurality of frames when the output of the decider means is indicative of the frame-based thin-out method and for converting the display information into liquid crystal display luminance information in accordance with a dot-based thin-out method when the output of the decider means is not indicative of the frame-based thin-out method.

Inventors: FUJISAWA KAZUHIRO, INUZUKA TATSUHIRO, KONUMA SATOSHI, MANO HIROYUKI, FUJISAWA, KAZUHIRO, INUZUKA, TATSUHIRO, KONUMA, SATOSHI, MANO, HIROYUKI, FUJISAWA, KAZUHIRO, HITACHI JONAN HAITSU 101, INUZUKA, TATSUHIRO, HITACHI KAMAKURARYO, KONUMA, SATOSHI, HITACHI SHIMOKURATARYO, MANO, HIROYUKI, HITACHI ISOGORYO
Applicants: HITACHI LTD, HITACHI VIDEO ENG, HITACHI VIDEO ENGINEERING, INC., HITACHI, LTD.
Application year: 1990
Main classification: G09G3
IPC codes: H04N3, G09G3