Video signal process for two-panel liquid crystal projector

Abstract: A video signal processing apparatus for use in a two-panel liquid crystal projector includes a matrix circuit (54) for receiving a luminance signal and colour signals output from a luminance/chroma processor (53) and generating colour signals in a different way from that of an existing matrix circuit. A matrix circuit (54) generates a new luminance signal by adding the minimum value of the colour difference signals to the luminance signal. The minimum value is subtracted from the respective colour difference signals to generate new colour signals. Another matrix circuit (54) generates colour signals and a luminance signal which are applied to a liquid crystal panel (57, 58) based on the minimum value of the colour signals which are obtained by the existing matrix circuit (54) instead of using the colour difference signals. The colour signals and luminance signal which are obtained by the respective matrix circuits (54) are used for light modulation for a colour liquid crystal panel (58) and white/black liquid crystal panel (57). Then, amplitude of the newly obtained luminance signal is controlled by an amplifier (66, 75) having an amplification factor based on a difference between light utilization efficiencies of the luminance signal and the colour signals which are displayed on a screen. Thus, an unnatural image phenomenon is removed and resolution and brightness can be improved. Further, disharmonization between the luminance signal component and the colour signal component can be prevented in view of the optical and signal processing conditions.

Description: Field of the Invention The present invention relates to a liquid crystal projector using a liquid crystal panel, and particularly, though not exclusively, to a video signal processing apparatus for a liquid crystal projector in which a video signal applied to a liquid crystal panel is controlled to improve brightness and resolution of an image displayed. Description of Related Art Generally, a liquid crystal projector uses a liquid crystal panel in which light modulation is controlled according to an applied voltage to display an image on a screen. The liquid crystal projector uses a one-panel system, a two-panel system or a three-panel system. Referring to Figures 1 through 3, conventional liquid crystal projectors adopting the one-panel, two-panel and three-panel systems will be described below. In figures 1 through 3, values in the parentheses of the respective reference numerals represent light transmission factors in corresponding components. The light transmission factors correspond to those of currently commercialized components. Figure 1 shows a conventional one-panel liquid crystal projector. When the light from a lamp 11 passes through a hot mirror 12, light excluding a visible ray is intercepted. The visible ray is collected by a focus lens 13 and then the collected visible ray is incident on a polarizing filter 14. Only forty percent of the incident light passes through polarizing filter 14 and is incident on a liquid crystal panel 15 for R, G and B primary colour signals. Only ten percent of the incident light of liquid crystal panel 15 passes therethrough and then is incident on polarizing filter 16. Ninety percent of the light incident on polarizing filter 16 passes therethrough and then is projected on a screen via a projection lens 17. As described above, since the one-panel liquid crystal projector has a small number of components, it enables the manufacture of a compact, low-price and easily assembled product. However, since a colour signal liquid crystal panel composed of a combination of R, G and B subpixels has been used, the efficiency of light utilization is low, down to 3.06%, to thereby lower the brightness of the image. Figure 2 shows an optical arrangement of a conventional three-panel liquid crystal projector. The three-panel liquid crystal projector uses three liquid crystal panels for R, G and B colour signals, respectively. When the light from a lamp 21 passes through a hot mirror 22, light excluding a visible ray is intercepted. The visible ray is incident on a dichroic mirror 23. The dichroic mirror 23 separates colour components of the incident light. The R component reflected from dichroic mirror 23 reflects from a mirror 24 and passes through a polarizing filter 25 and is incident on a liquid crystal panel 26 for an R signal. The G and B components transmitted through dichroic mirror 23 are incident on another dichroic mirror 27. The B component reflected from dichroic mirror 27 passes through a polarizing filter 28 and is incident on a liquid crystal panel 29 for a B signal. The G component transmitted through dichroic mirror 27 passes through a polarizing filter 30 and is incident on a liquid crystal panel 31 for a G signal. The light passing through R signal liquid crystal panel 26 and the light passing through B signal liquid crystal panel 29 are incident on dichroic mirror 34 via polarizing filters 32 and 33, respectively. The dichroic mirror 34 transmits the R component of the incident light and reflects the B component thereof, to transfer the incident components to dichroic mirror 35. The light passing through G signal liquid crystal panel 31 passes through polarizing filter 36 and then is reflected from mirror 37. The light reflected from mirror 37 is incident on dichroic mirror 35. The dichroic mirror 35 for combining the colours transmits the R and B components and reflects the G component, to apply the R, B and G components to a projection lens 38. The projection lens 38 projects the incident light on a screen. As described above, since the three-panel liquid crystal projector uses a liquid crystal panel for each colour component, a quality of picture is more excellent and an efficiency of light utilization of total 4.23% is higher than those of the one-panel system. Thus, the brightness of the image is slightly increased. However, due to the higher number of components, the structure is complicated. Also, use of the three liquid crystal panels results in increased costs. A two-panel liquid crystal projector has been developed to solve the drawbacks of the above-described one-panel system and the three-panel system. A conventional two-panel liquid crystal projector is shown in Figure 3. The two-panel liquid crystal projector uses two liquid crystal panels for a luminance signal and a colour signal. In Figure 3, when the light from lamp 41 passes through hot mirror 42, the light excluding a visible ray is intercepted. The visible ray is divided into an S polarized component and a P polarized component by polarizing filter 43. The S polarized component reflected from polarizing filter 43 is reflected again by mirror 44 and is input to a white/black liquid crystal panel 45. The P polarized component transmitted from polarizing filter 43 is reflected by mirror 46 and is input to a colour liquid crystal panel 47. The S polarized component is optically modulated as a white and black image by white/black liquid crystal panel 45 and is incident to polarizing filter 48. The P polarized component is optically modulated as a colour image by colour liquid crystal panel 47 and is incident on polarizing filter 48. The polarizing filter 48 for combining both polarized components transmits the input white and black image and reflects the colour image, so as to be supplied to a projection lens 49. The projection lens 49 supplies the white/black image and the colour image to a screen, so as to be overlapped and displayed thereon. The structure of the white/black liquid crystal panel which is used in the two-panel liquid crystal projector is shown in Figure 4 and that of the colour liquid crystal panel is shown in Figure 5. The pixel structure of the conventional liquid crystal panel has a delta-type pixel array (a triangular structure) in which an even column and an odd column are offset by a 1/2 pixel. The pixel structure of the white/black liquid crystal panel is a field quincunx (QT) structure in space in view of a sampling operation, having a resolution area such as a frequency spectrum shown in Figure 6. In the pixel structure of the colour liquid crystal panel as shown in Figure 5, the respective subpixels of R, G and B are offset to each other, and are arranged as being a rhombic shape. Here, the subpixels of the colour liquid crystal panel also have QT structures in space with respect to the respective R, G and B components. However, the horizontal resolution is decreased. Figures 7A through 7D show sampling point of time with respect to the signals input to the liquid crystal panels. In the case of the colour liquid crystal panel, sampling of the colour signals has a polyphase type in which a sampling period of one colour signal R, G or B is moved by a 1/3 period to sample another component. Meanwhile, in the case of the white/black liquid crystal panel, sampling of the luminance signal is performed by three times the sampling frequency with respect to the colour signal as shown in Figure 7D. Accordingly, the resolution of the image is three times that of the colour liquid crystal panel. Such a two-panel liquid crystal projector has a merit that the resolution is as high as the three-panel system. That is, the one-panel system uses a single liquid crystal panel in which the R, G and B are separately displayed, with a result that the resolution is lowered. The two-panel system has nearly the same resolution as that of the three-panel system. Among the three types of the liquid crystal projector, the efficiency of light utilization is highest at 7.34% in the case of a luminance signal. Also, the two-panel liquid crystal projector is compact and relatively inexpensive. A video signal processing circuit for use in such a two-panel liquid crystal projector is disclosed in a Japanese laid-open patent publication No. 03201695 on September 3, 1991, entitled "Projection Display Apparatus." The projection display apparatus includes a white/black liquid crystal display device for modulating a light transmission factor of a first linear polarized component based on a luminance signal separated by a YC separation circuit, and a colour liquid crystal display device for modulating a light transmission factor of a second linear polarized component based on the separated colour signal. However, the apparatus in the prior art processes the luminance signal and the colour signal so that they may be just overlapped on a screen. Thus, an unnatural image is displayed on a screen because the black level on the screen is higher than that of an original signal and the contrast is low. JP-A-04 267 246 (Fujitsu) discloses a video signal processing apparatus for use in a two-panel liquid crystal projector comprising: a luminance signal generator for receiving first red, green and blue signals and generating a second luminance signal which is used for a luminance signal liquid crystal panel based on one of three first colour signals; and means for receiving the second luminance signal, and the first red, green and blue signals and generating second red, green and blue signals which are used in a colour signal liquid crystal panel. Summary of the Invention It is an aim of embodiments of the present invention to provide a video signal processing apparatus in a two-panel liquid crystal projector capable of removing an unnatural image by generating a luminance signal applied to a luminance signal liquid crystal panel and a colour signal applied to a colour signal liquid crystal panel based on one of colour difference signals. According to the present invention, there is provided a video signal processing apparatus for use in a two-panel liquid crystal projector comprising: a matrix circuit for receiving a first luminance signal, chroma difference signals and generating a second luminance signal based on one of the colour difference signals; means for receiving and processing the second luminance signal and the colour signals, so as to be used for a luminance signal liquid crystal panel and a colour signal liquid crystal panel, respectively; characterised in that the apparatus further comprises a luminance/chroma processor for receiving a luminance signal and the chroma signals and generating the first luminance signal and colour difference signals; and the matrix circuit receives the colour difference signals and generates a second luminance signal and a second plurality of colour signals using a minimum one of the colour difference signals as a reference signal, wherein each of said plurality of colour signals is generated by subtracting the reference signal from a respective colour difference signal. Preferably, the matrix circuit generates the second luminance signal and the colour signals based on the minimum value of the colour difference signals. Accordingly, the matrix circuit may comprise a minimum value detector for detecting the minimum value among the colour difference values input from said luminance/chroma processor; a first subtracter for subtracting the minimum value from the R-Y colour difference signal output from said luminance/chroma processor to generate a red colour signal; a second subtracter for subtracting the minimum value from the G-Y colour difference signal output from said luminance/chroma processor to generate a green colour signal; a third subtracter for subtracting the minimum value from the B-Y colour difference signal output from said luminance/chroma processor to generate a blue colour signal; and an adder for subtracting the minimum value from the first luminance signal to generate the second luminance signal. Suitably, said apparatus further comprises an amplifier for amplifying the second luminance signal output from said adder. Suitably, said amplifier has a signal amplification factor based on a difference between light utilization efficiencies of the luminance signal and the colour signals which are displayed on a screen. Brief Description of the Drawings The preferred embodiments are described, by way of example only, with reference to the drawings wherein: Figure 1 is an optical arrangement diagram of a conventional one-panel liquid crystal projector. Figure 2 is an optical arrangement diagram of a conventional three-panel liquid crystal projector. Figure 3 is an optical arrangement diagram of a conventional two-panel liquid crystal projector. Figure 4 shows a structure of a white/black liquid crystal panel used in the two-panel liquid crystal projector. Figure 5 shows a structure of a colour liquid crystal panel used in the two-panel liquid crystal projector. Figure 6 shows a frequency spectrum of the white/black liquid crystal panel. Figures 7A through 7D show sampling points of time with respect to the signals input to the liquid crystal panels. Figure 8 is a block diagram of a signal processing system in a two-panel liquid crystal projector according to the present invention. Figure 9 is a detailed circuit diagram of the matrix circuit according to one embodiment of the present invention. Figure 10 is a view for explaining a change of a signal level by the matrix circuit of Figure 9. Figure 11 is a detailed circuit diagram of the matrix circuit according to another embodiment of the present invention. Figure 12 shows a view of the Figure 9 matrix circuit to which an amplifier is added. Figure 13 shows a view of the Figure 11 matrix circuit to which an amplifier is added. Detailed Description of the Preferred Embodiments Preferred examples of the present invention will be described below in more detail with reference to the accompanying drawings Figures 8 through 13. In Figure 8, when a composite video signal (CVS) is input to a sync separator 51, sync separator 51 separates a sync signal S ync , a first luminance signal Y and a chroma signal C from the input signal, respectively. A timing generator 52 generates drive signals DR1 and DR2 and an invert control signal synchronized to the sync signal Sync output from sync separator 51. The respective drive signals DR1 and DR2 are determined considering that the luminance signal has three times the sampling frequency as those of the respective colour signals. Drive signal DR1 is applied to a white/black liquid crystal panel 57, while drive signal DR2 is applied to a colour liquid crystal panel 58. On the other hand, a luminance/chroma processor 53 connected to the output of sync separator 51 receives first luminance signal Y and chroma signal C from sync separator 51 and generates colour difference signals R-Y, G-Y and B-Y. When matrix circuit 54 has a constitutional structure shown in Figure 9, matrix circuit 54 receives first luminance signal Y and colour difference signals R-Y, G-Y and B-Y, and generates a second luminance signal Y' and colour signals R 1 , G 1 and B 1 . On the other hand, when matrix circuit 54 has a constitutional structure shown in Figure 11, matrix circuit 54 receives colour signals R', G' and B' generated by an existing matrix circuit (not shown) and generates a third luminance signal Y 2 ' and colour signals R 2 , G 2 and B 2 . The luminance signal Y' is applied to a first invert amplifier 55 while colour signals R, G and B are applied to a second invert amplifier 56. First invert amplifier 55 inverts and amplifies luminance signal Y' according to an invert control signal applied from timing generator 52 and supplies the result to white/black liquid crystal panel 57. Second invert amplifier 56 inverts and amplifies colour signals R, G and B according to the invert control signal and supplies the result to colour liquid crystal panel 58. Respective X-axis and Y-axis driver integrated circuits (not shown) in liquid crystal panels 57 and 58 operate by corresponding drive signals DR1 and DR2, respectively. Thus, white/black liquid crystal panel 57 samples the luminance signal applied from first invert amplifier 55 according to drive signal DR1, while colour liquid crystal panel 58 samples the colour signals applied from second invert amplifier 56 according to drive signal DR2. Figure 9 is a detailed circuit diagram of the matrix circuit according to one embodiment of the present invention. The matrix circuit shown in Figure 9 receives colour difference signals R-Y, G-Y and B-Y and first luminance signal Y from luminance/chroma processor 53, and generates colour signals R 1 , G 1 and B 1 and second luminance signal Y 1' . Such a matrix circuit includes a minimum value detector 61, subtracters 62, 63 and 64 and an adder 65. When colour difference signals R-Y, G-Y and B-Y and first luminance signal Y from luminance/chroma processor 53 are input to the Figure 9 circuit, minimum value detector 61 detects the minimum value among the input colour difference signals and supplies the minimum value to subtracters 62, 63 and 64 and adder 65. First subtracter 62 subtracts the minimum value from colour difference signal R-Y and supplies the obtained red signal R 1 to second invert amplifier 56 of Figure 8. Second subtracter 63 subtracts the minimum value from colour difference signal G-Y and supplies the obtained green signal G 1 to second invert amplifier 56 of Figure 8. Third subtracter 64 subtracts the minimum value from colour difference signal B-Y and supplies the obtained blue signal B 1 to second invert amplifier 56 of Figure 8. Here, adder 65 subtracts the minimum value from first luminance signal Y and supplies the obtained second luminance signal Y 1' to first invert amplifier 55 of Figure 8. First invert amplifier 55 inverts and amplifies input second luminance signal Y 1' and supplies the result to white/black liquid crystal panel 57. Second invert amplifier 56 inverts and amplifies the input colour signals R 1 , G 1 , and B 1 and supplies the result to colour liquid crystal panel 58. Figure 10 is a view for explaining a change of a signal level by the matrix circuit of Figure 9. For example, if the values of the respective colour signals R, G and B prior to transmission are 0.5 for R, 0.8 for G and 0.6 for B, the value of luminance signal Y becomes 0.668 from a general relationship between luminance signal Y and colour difference signals R-Y, G-Y and B-Y, that is, an equation of Y=0.59G+0.3R+0.11B. Thus, the value of R-Y becomes -0.188 (=0.5-0.688), the value of G-Y becomes 0.112 (0.8-0.688), and the value of B-Y becomes -0.088 (0.6-0.688). Since the minimum value of the calculated colour difference signals is -0.188, minimum value detector 61 outputs the minimum value of -0.188. Thus, first subtracter 62 outputs colour signal R 1 having the value of 0. Second subtracter 63 outputs colour signal G 1 having the value of 0.3. Third subtracter 63 outputs colour signal B 1 having the value of 0.1. Adder 65 outputs second luminance signal Y 1 . having the value of 0.5. Finally, the minimum value of 0.5 among the original colour signals (R=0.5, G=0.8 and B=0.6) is supplied to first invert amplifier 55, while the difference values between the minimum value and the colour signals are supplied to second invert amplifier 56. The second example of the present invention which makes signals to be supplied to invert amplifiers 55 and 56 using colour signals will be described with reference to Figure 11. Figure 11 is a detailed circuit diagram of part of the matrix circuit 54 according to a second embodiment of the present invention. The Figure 9 circuit produces colour signals R 1 , G 1 and B 1 and second luminance signal Y 1' to be supplied to invert amplifiers 55 and 56 using colour difference signals R-Y, G-Y and B-Y and first luminance signal Y, while the Figure 11 circuit produces colour signals R 2 , G 2 and B 2 and third luminance signal Y 2' to be supplied to invert amplifiers 55 and 56 using colour signals R', G' and B' which are produced by an existing matrix circuit (not shown). The conventional matrix circuit for generating colour signals R', G' and B' produced by the existing method is not shown. However, according to the design selection, the conventional matrix circuit can be installed between luminance/chroma processor 53 and matrix circuit 54 of Figure 8, or can be constituted so as to be included in matrix circuit 54. The colour signals R', G' and B' produced by the conventional method are applied to minimum value detector 71 and subtracters 72, 73 and 74, respectively. The detected minimum value is used as third luminance signal Y 2' which is applied to first invert amplifier 55 and is also applied to subtracters 72, 73 and 74, respectively. Fourth subtracter 72 subtracts the minimum value from the input colour signal R' to generate colour signal R 2 . Fifth subtracter 73 subtracts the minimum value from the input colour signal G' to generate colour signal G 2 . Sixth subtracter 74 subtracts the minimum value from the input colour signal B' to generate colour signal B 2 . The signals R 2 , G 2 and B 2 produced in subtracters 72, 73 and 74 are applied to second invert amplifier 56. The matrix circuits which are explained referring to Figures 9 and 11 use the minimum value of the colour difference signals or the colour signals to process the luminance signal which is used for displaying the luminance component of the image. Accordingly, the resolution and brightness of the image can be enhanced. For example, when the displayed image is a single red colour, the image is displayed only through colour liquid crystal panel 57, while when only the luminance component of the image exists, the image is displayed through only white/black liquid crystal panel 58 having higher resolution and brightness. Then, when an image has both luminance component and chroma component, most of the luminance component is displayed through white/black liquid crystal panel 57, and a pure chroma signal component is displayed through colour liquid crystal panel 58. If the R-Y component is the minimum value, there is no R component which is displayed through colour liquid crystal panel 58. However, since the minimum value is substantially related to the luminance signal component, the G and B components obtained by subtracting the minimum value therefrom are displayed on colour liquid crystal panel 58. The luminance signal component which determines most of the resolution in the two-panel processing is displayed through white/black liquid crystal panel 57, thereby improving the entire resolution and brightness. Since the black level of the image is higher than the original signal due to the overlapping of the luminance signal and the colour signals, a problem that the contrast becomes lower can be solved. Thus, an unnatural image phenomenon can be removed so that an image close to the original image can be displayed. However, the general two-panel liquid crystal projector has a considerable difference between a light utilization efficiency of an optical luminance signal path being a light path used for light modulation of a luminance signal and that of an optical colour signal path being a light path used for light modulation of colour signals. Referring to Figure 3 in which the light transmission factors corresponding to the respective components are represented, the light utilization efficiency of the optical luminance signal path is approximately 7.34%, and that of the optical colour signal path is approximately 2.45%. When the Figures 9 and 11 circuits are used in the two-panel liquid crystal projector, the luminance signal, the colour difference signals or the colour signals are produced by adder 65, or subtracters 62, 63 and 64 (or 72, 73 and 74) in view of the signal processing condition. The newly obtained colour signals become relatively smaller than the newly obtained luminance signal, in view of the magnitude of the signal. To further improve the resolution and brightness of the image displayed by solving the above-described problems in relation to the optical condition and the signal processing condition, it is necessary to control an amplification factor with respect to the luminance signal applied to first invert amplifier 55 or the colour signals which are applied to second invert amplifier 56. The circuits constituted by adding a function for controlling the amplification factor to the Figures 9 and 11 circuits are shown in Figures 12 and 13. Figure 12 shows an amplifier 66 which is added to the output end of adder 65 in the Figure 9 matrix circuit. Figure 13 shows an amplifier 75 which is added to the output end of minimum value detector 71 in the Figure 11 matrix circuit. The amplification factor of amplifier 66 or 75 which decreasingly amplifies the input luminance signal and supplies the amplified result to first invert amplifier 55 is set considering the above-described signal processing and the optical viewpoints, which has a value for decreasing amplitude of the luminance signal. Also, the amplification factor can be set so that the amplitude of the luminance signal is not abruptly decreased. Then, when the components of which the light utilization efficiencies of the optical luminance signal path and the optical colour signal path are the same as each other are used in consideration of the only optical condition, an amplification factor can be set to one. If the amplification factor of amplifier 66 or 75 is set based on the above-described content or contents, the Figure 12 amplifier 66 decreases the amplitude of third luminance signal Y 2' applied from adder 65 according to a predetermined amplification factor to generate a fourth luminance signal Y 3 ,. The Figure 13 amplifier 75 decreases the amplitude of the minimum value applied from minimum value detector 71 according to a predetermined amplification factor to generate a fifth luminance signal Y 4' . The fourth luminance signal Y 3' and the fifth luminance signal Y 4' are applied to first invert amplifier 55, respectively. If the Figures 12 and 13 circuits are adapted in the existing two-panel liquid crystal projector, the resolution and brightness can be improved. Also, the signal input to the luminance signal liquid crystal panel is attenuated in a proper ratio, to prevent disharmonization between the luminance signal component and the colour signal components in consideration of the optical and signal processing conditions. While only certain examples of the invention have been specifically described herein, it will be apparent that numerous modifications may be made thereto without departing from the scope of the invention.

Claims: A video signal processing apparatus for use in a two-panel liquid crystal projector comprising: a matrix circuit (54) for receiving a first luminance signal, chroma difference signals and generating a second luminance signal based on one of the colour difference signals; means (55, 56) for receiving and processing the second luminance signal and the colour signals, so as to be used for a luminance signal liquid crystal panel (57) and a colour signal liquid crystal panel (58), respectively; characterised in that the apparatus further comprises a luminance/chroma processor (53) for receiving a luminance signal and the chroma signals and generating the first luminance signal and colour difference signals; and the matrix circuit receives the colour difference signals and generates a second luminance signal and a second plurality of colour signals using a minimum one of the colour difference signals as a reference signal, wherein each of said plurality of colour signals is generated by subtracting the reference signal from a respective colour difference signal. A video signal processing apparatus for use in a two-panel liquid crystal projector according to claim 1, wherein said matrix circuit (54) comprises: a minimum value detector (61) for detecting the minimum value among the colour difference values input from said luminance/chroma processor (53); a first subtracter (62) for subtracting the minimum value from the R-Y colour difference signal output from said luminance/chroma processor (53) to generate a red colour signal; a second subtracter (63) for subtracting the minimum value from the G-Y colour difference signal output from said luminance/chroma processor (53) to generate a green colour signal; a third subtracter (64) for subtracting the minimum value from the B-Y colour difference signal output from said luminance/chroma processor (53) to generate a blue colour signal; and an adder (65) for subtracting the minimum value from the first luminance signal to generate the second luminance signal. A video signal processing apparatus for use in a two-panel liquid crystal projector according to claim 2, further comprising an amplifier (66) for amplifying the second luminance signal output from said adder (65). A video signal processing apparatus for use in a two-panel liquid crystal projector according to claim 3, wherein said amplifier (66) has a signal amplification factor based on a difference between light utilization efficiencies of the luminance signal and the colour signals which are displayed on a screen.

Inventors: KO HAN-IL, LEE MYEONG-HWAN, SONG DONG-IL, KO, HAN-IL, LEE, MYEONG-HWAN, SONG, DONG-IL
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1995
Main classification: H04N9
IPC codes: G02F1, H04N9, G03B33