INCREASED-FREQUENCY ADDRESSING OF DISPLAY SYSTEM EMPLOYING REFLECTIVE LIGHT MODULATOR

Abstract: In a scrolling color projection system for displaying images represented by continuously updated frames of information, e.g., video images, and employing a single, reflective electro-optic light modulator such as an AMLCD, flicker in the image caused by asymmetric electrical behavior of the light modulator is reduced without substantially reducing the brightness of the display, by addressing the modulator array at least twice during each frame period.

Description: Increased- frequency addressing of display system employing reflective light modulator. This invention relates to display systems for displaying continuously updated image information signals arranged in successive frames, employing electro-optic light modulators, and more particularly relates to a scrolling color projection display system in which color bars are repetitively scrolled across a single electro-optic light modulator panel to produce a color display. The invention also relates to a method for addressing a display system for displaying continuously updated image information signals arranged in successive frames, the system employing a reflective electro-optic light modulator (10) comprising a row-and- column matrix array of pixels (20), for modulating light during successive frame periods. Such single panel scrolling color display systems are known. See, for example, commonly assigned U.S. patent 5,532,763, incorporated herein by reference. These display systems are particularly suitable for displaying color information in the form of continuously updated image information signals arranged in successive frames representing full color frames, each frame comprised of component color sub-frames, such as color video information. These systems employ an electro-optic light modulator comprised of a row-and-column matrix array of pixels, for modulating light in accordance with the image information signals during successive frame periods. The signal information is applied to the pixel rows of the array a line at a time during each frame period for each component color. Such display systems employing a single reflective high-density active-matrix liquid crystal display (AMLCD) on silicon as the electro-optic light modulator are being considered for high resolution color projection systems, due to the fact that the matrix structure of row and column electrodes, switches and storage capacitors can all be integrated on the silicon substrate below the reflective pixel electrodes, thus enabling the high pixel density required for such high resolution displays. Unfortunately, the location of the reflective pixel electrodes in the LC cell structure leads to an asymmetric electrical behavior during driving, in the form of a DC off-set in the dπve voltage, even in the case of AC dπving. This DC off-set is perceived as flicker by the viewer of a continuously updated display image, such as a video image. One way to suppress this flicker is to adjust the dπve voltage to compensate for the DC off-set. However, such a compensation scheme is difficult to implement, since the off- set voltage tends to be non-uniform over the pixel array, and also tends to vary with time. Another way to suppress this flicker would be to operate the system at a frame rate well above the limit where flicker can be perceived. However, such an operation would reduce the bπghtness of the display. The reason for this is that guard bars (30, 34 and 38 in Figs 2 and 3A) separate the color bars (32, 36 and 40), in order to accommodate the finite switching time of the LC mateπal, and thus preserve color puπty. Operating the system at a higher frame rate) would reduce the size of the color bars (46,50,54,58,62.66 in Fig 3B) in the scrolling direction, since the time peπod for addressing the pixel array with each new frame of color information would be reduced. However, the size of the guard bars (44,48,52,56,60,64) would remain the same, because the switching time of the LC mateπal remains the same. Thus, the propoπion of dead time in which the pixel array is not being addressed with color information would be reduced. Accordingly, it is an object of the invention to provide a display system for displaying continuously updated image information signals arranged in successive frames, and employing a reflective electro-optic light modulator, in which flicker of the display is reduced without substantially reducing the bπghtness of the display In accordance with the invention, a display system is provided for displaying continuously updated image information signals arranged in successive frames representing color frames, the system employing a reflective electro-optic light modulator compπsed of a row-and-column matπx array of pixels, for modulating light in accordance with the image information signals duπng successive frame peπods, wherein each row of the array is addressed at least twice duπng each frame peπod with the image information A further advantage of the system in accordance with the invention is that in a projection system employing scrolling bars the flicker can be reduced without substantially reducing the bπghtness of the display. A further advantage of the system in accordance with the invention is that when a scrolling bar projection system employs a reflective active matπx LCD modulator, flicker due to electπcal asymmetry in the active matπx LCD is reduced without substantially reducing the bπghtness of the display Further advantageous embodiments of the system in accordance with the invention are defined in the dependent claims It is also an object of the invention to provide a method of addressing a display system employing a reflective electro-optic modulator in which flicker of the display is reduced without reducing the bπghtness of the display Thereto, the method according to the invention is characteπzed in that the method compπses addressing each row of the array at least twice duπng each frame peπod In accordance with a preferred embodiment of the invention, the matπx array is illuminated with scrolling color bars corresponding to the color signal information being addressed In accordance w ith another embodiment of the invention, each frame compπses two or more component color sub-frames, and each row of the array is addressed at least twice duπng each frame peπod with the image information signals for at least one of the component color sub-frames, and preferably for all of the component color sub-frames Preferably, the reflective electro-optic light modulator is an active matπx liquid crystal display (AMLCD) device Fig 1 is a simplified plan view of a typical AMLCD device of the pπor art. Fig 2 is a schematic representation of the illumination of an electro-optic light modulator panel in a scrolling color svstem, Figs 3A an 3B are timing diagrams with different frame rates for addressing a row of a pixel matπx of an electro-optic light modulator of a scrolling color projection system, and Fig 4 is a timing diagram for a scrolling color projection system of the invention, in which the frequency of addressing is increased without increasing the frame rate Referπng to Fig 1, there is shown schematically a block diagram of a LCD-TV display system of the pπor art including a display device which compπses an AMLCD panel 10 The panel 10 consists of m lines (1 to m) with n hoπzontal display (picture) elements or pixels 20 (1 to n) in each line In practice, the total number of display elements (m x n) in the matπx array may be 100,000 or more (up to 1.3 million for HDTV). Each display element 20 (representing one pixel of the display) has an associated IGFET transistor acting as a switching element. The gates of all the IGFETs in each line are connected to a row (Y) electrode 14 and the source electrodes of each IGFET in a column are connected to a column (X) electrode 15, there being m row electrodes 14 and m column electrodes 15. The drains of the IGFETs 11 are connected to respective electrodes of the display elements in a manner which will be descπbed. A common counterelectrode for the display elements is earned by a substrate spaced from the substrate carrying the IGFETs and the associated electrodes of the display elements with liquid crystal mateπal disposed therebetween. The liquid crystal mateπal modulates light according to voltage applied there across. Color filters in registration with the picture elements may be employed to produce a color display using known pπnciples Addressing of each line of the matπx array of display elements 20 is achieved by applying a gate voltage to the row electrode 14 for that line for an addressing time Ta. This turns on all IGFETs in that row of the matπx, allowing video information to be transferred to the display elements via the column electrodes 15. The row electrodes are sequentially addressed in this manner to provide hne-at-a-time scanning of the entire matπx array. One completed scan of the matπx array represents one frame of video information, after which the array is readdressed with the next frame of information Duπng the time Ta in which the IGFETs in one line are turned on to allow addressing, all of the other IGFETs in the remaining rows of the array are switched off, thus isolating their associated picture elements from stray voltages which would otherwise introduce display-degrading cross-talk. Duπng this time interval, these isolated picture elements must maintain their picture information by viπue of their natural capacitance and any auxiliary storage capacitance (not shown in this figure). As can be seen in Fig. 1, the row (Y) electrodes 14 are dπven by a digital shift register 21 supplied with regular timing pulses from a clock circuit 22 which is fed with line synchronizing pulses from a synchronization separator 23 deπved from the incoming signals via a tuner 24, LF circuits 25 and video amplifier 26. Video information signals are supplied to the column (X) electrodes 15 simultaneously from an analog shift register circuit 28, compπsing one or more shift registers, supplied with video signals from the video amplifier 26 and timing pulses from the clock circuit 22 in sync with line addressing The shift register circuit provides seπal-to-parallel conversion appropπate to the hne-at-a-time addressing of the panel 10, samples the corresponding line in the video signal and places the appropπate voltages on the column (X) electrodes 15, thence to the source side of the IGFETs in the column. When the IGFETs in the line being addressed are turned on, the voltage at each source is transferred to the dram, and thence to the picture element connected to the drain, whereby the liquid crystal associated with the element is charged with the source voltage representing the video information for that element. Referπng now to Fig. 2, there is shown a schematic representation of the illumination of an electro-optic light modulator panel in a scrolling color system. Such a panel is typically an AMLCD composed of a matπx of rows (or lines) and columns of pixels defined by individually addressable reflective pixel electrodes (not shown), addressed in a hne-at-a- time manner as descπbed above. Red, blue and green color bars (32,36,40) continuously scroll down the matπx array (represented by box 42) in the direction of the arrow. Red color bar 32, blue color bar 34 and green color bar 36 are shown on the array at instant of time t Addressing of the rows with the color signal information corresponding to that of the next advancing color bar leads the color bar by a distance corresponding to the switching time needed for the LC mateπal to switch to the state corresponding to the new color signal, this distance referred to herein as the guard bar. Thus, at the instant of time t, row 341 is being loaded with green signal information, row 683 is being loaded with blue signal information, and row 1024 is being loaded with red signal information The spaces between the color bars 32, 36 and 40 represent guard bars 30. 34 and 38. Referπng to the timing diagram of Fig. 3A. which represents the timing of events for any paπicular line in the AMLCD at a frame rate of 60Hz, it is seen that a frame peπod of 16.66 mS is divided into three equal sub-frame peπods for the addressing of the red, blue and green pπmary color components of one frame of full color video information The arrows at t = 0; t = 5.53 mS; and t = 11.06 mS mark the beginning of addressing of the green (G+), red (R+) and blue (B+) component color signal information. The curves A, πsing at the beginning of addressing, represent the optical response of the LC mateπal to the signal information, showing a delay in achieving the transmission state representative of the signal information. Thus, the curves A represent the bπghtness increase of the display with time. The curves B, falling at the beginning of addressing, represent the optical relaxation of the LC mateπal after addressing by the previous color component's signal information (?). The shaded blocks 32, 36 and 40 represent the illumination of the matrix array with the respective green, red and blue color bars. The spaces 30, 34 and 38 between the shaded blocks represent the guard bars. Fig. 3B, a timing diagram similar to that of Fig. 3A, illustrates graphically the effect on system performance of increasing the frame rate from 60Hz to 95Ffz. The frame rates 60Hz and 95Hz are selected for compaπson because, when the guard bar is kept constant at 600 uS, the illumination time of one color bar on the matπx array at 60Hz is equivalent to the illumination time of two color bars at 95Hz. There are now twice as many color bars (46,50,54,58,62,66) and guard bars (44,48,52,56,60,64) as in Fig. 3A. A loss of nearly 10% of light throughput is realized due to the additional guard bars required at 95Hz. A further loss of about 34% of light throughput is realized due to the response time of the LC mateπal, for a 0-90% optical relaxation time of 1.2 mS This loss was determined by integrating the area under each bπghtness response curve A of the LC mateπal duπng their respective illumination cycles. Fig. 4 shows a timing diagram similar to those of Figs. 3 A and 3B, in which the frequency of addressing each color component, i.e., the refresh rate, is doubled, while maintaining the same overall frame rate, in order to reduce flicker without decreasing light throughput of the system. Thus, the green component color signals are addressed beginning at the arrow marked G+, and again beginning at the arrow marked G- (- indicates inverted polaπty of the G signals), and so on for R and B. Thus, each row is addressed twice per color sub-frame, with the polaπty being inverted each time. As in Figs. 3A and 3B, the shaded blocks 70, 74 and 78 represent the green, red and blue color bars, respectively, while spaces 68, 72 and 76 represent the guard bars.

Claims: CLAIMS: 1. A display system for displaying continuously updated image information signals arranged in successive frames representing full color frames, the system employing a reflective electro-optic light modulator (10) compπsed of a row-and-column matπx array of pixels (20), for modulating light in accordance with the image information signals duπng successive frame peπods, and means (21-26) for addressing the array with the image information signals, characteπzed in that each row of the array is addressed at least twice duπng each frame peπod. 2. The display system of claim 1 in which each frame compπses at least two component color sub-frames, and each row of the array is addressed at least twice duπng each frame peπod with the image information signals for at least one of the component color sub-frames. 3 The display system of claim 1 in which each row of the array is addressed at least twice duπng each frame peπod with the image information for all of the component color sub- frames. 4. The display system of claim 1 in which the reflective electro-optic light modulator is an active matπx liquid crystal display devιce(lθ). 5. The display system of claim 2 in which the matπx array is illuminated with scrolling color bars (32,36.40) corresponding to the color component sub-frame signal information being addressed 6. The display system of claim 1 in which the frame rate is 60Hz, and the rows of the pixel array are addressed twice duπng each frame with the sub-frame signal information. 7. A method for addressing a display system for displaying continuously updated image information signals arranged in successive frames representing full color frames, the system employing a reflective electro-optic light modulator (10) compπsed of a row-and-column matπx array of pixels (20), for modulating light in accordance with the image information signals duπng successive frame peπods, characteπzed in that the method compπsing addressing each row of the array at least twice duπng each frame peπod. 8. The method of claim 7 in which each frame compπses at least two component color sub-frames, and each row of the array is addressed at least twice duπng each frame peπod with the image information signals for at least one of the component color sub-frames. 9. The method of claim 7 in which each row of the array is addressed at least twice duπng each frame peπod with the image information for all of the component color sub-frames. 10 The method of claim 1 in which the reflective electro-optic light modulator is an active matπx liquid crystal display devιce(lθ) 11. The method of claim 7 in which the matπx array is illuminated with scrolling color bars (32,36,40) corresponding to the color component sub-frame signal information being addressed. 12. The method of claim 7 in which the frame rate is 60Hz, and the rows of the pixel array are addressed twice duπng each frame with the sub-frame signal information

Inventors: JANSSEN PETER, MELNIK GEORGE, JANSSEN, PETER, MELNIK, GEORGE
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: G09G3
IPC codes: G02F1, G09G3, H04N5, H04N9