Color image display device

Abstract: In a color image display system formed of a monochromatic CRT or like image display device (102) having a screen (103) for display of images and a rotary filter (106) comprising color filter sections (106R, 106G and 106B) of a plurality of colors; image signals are read from memories (130R, 130G, 130B) at a rate higher than they are written in the memories, so as to reduce flickers. The problem of passing of the reading address over the writing address is solved by reading the image signal of prominent color taking a period in which the passing over occurs. High frequency components of other color image signals are extracted and added to the image signal of each color to thereby enhance the definition of the picture.

Description: BACKGROUND OF THE INVENTION The present invention relates to a color image display device comprising a monochromatic display device in combination with a coloring device. The present invention also relates to a video signal processor. Prior Art Example 1 Fig. 1 shows a conventional color image display device using a monochromatic image display means, such as a black-and-white CRT (cathode-ray tube) in combination with a coloring device. As illustrated, it comprises a black-and-white CRT 102 and a coloring device 104 including a disk-shaped rotary filter 106 and a D.C. (direct current) rotary motor 108. The disk-shaped rotary filter 106 is formed of color filter sections 106R, 106G and 106B permitting transmission of light of red (R), green (G) and blue (B) and each extending over an angular range of 120°, as illustrated in Fig. 2. In other words, the disk-shaped rotary filter 106 is divided by radially extending lines 106rg, 106gb and 106br into three equal sectors 106R, 106G and 106B each having a vertex angle of 120° and respectively serving as color filters for red (R), green (G) and blue (B). The rotary filter 106 is fixed to a shaft 108 of the motor 110, and is rotated by the motor 110. As the motor 110 rotates, the filter sections 106R, 106G and 106B sequentially pass over a screen 103 of the CRT 102. A rotation sensor 112, which may comprise a Hall element, detects the rotation phase of the rotary filter 106. A magnet 113 is attached to the motor shaft 108 and serves as a detection member (member to be detected by the rotation sensor 112). That is, each time the motor shaft 108 rotates, the magnet 113 passes by the sensor 112, which thereby detects the passage of the magnet 113, on the basis of the magnetic flux emanating from the magnet 113, and generates a rotation sync pulse RP. The rotation sync pulse RP is supplied to a control circuit 114, which also receives a vertical sync signal V, and supplies the motor 110 with a drive signal. The rotation of the motor 110 is controlled such that the rotary filter 106 rotates once per three vertical periods, and the rotation sync pulse RP is produced at a constant phase angle relative to the vertical sync signal V. The rotation of the rotary filter 106 is such that any given part of the filter passes from the upper part to the lower part of the screen 103 of the CRT 102. For instance, the boundary 106br between the filter sections 106b and 106r first comes to overlie the upper part of the screen 103, and then scans toward the lower part of the screen 103, in the same way as the horizontal scanning lines shifts from the upper edge to the lower edge of the screen 103 and at about the same speed as the vertical scanning (i.e., the shifting of the horizontal scanning lines 103 from the upper edge to the lower edge). A switching circuit 116 receives the color image signals RS, GS and BS, and, being controlled by a selection signal S produced at the control circuit 114 in synchronism with the vertical sync signal V, sequentially and repeatedly selects the color image signals RS, GS and BS. The switching from one of the color image signals RS, GS and BS to another is made every vertical period. The output of the switching circuit 116 is a sequence of color image signals RS, GS and BS each of which last for one vertical period and which are repeatedly altered from one to another every vertical period. The output of the switching circuit 116 is supplied as the luminance signal to the CRT 102. The CRT 102 also receives deflection currents from deflection circuits 118 and 120, which are controlled by the vertical and horizontal sync signals V and H, and displays an image corresponding to the signal supplied thereto. The selection of the color image signals RS, GS and BS is made in time with the rotation of the rotary filter 106. That is, one of the color image signals RS, GS and BS corresponding to the filter section 106R, 106G or 106B which is overlying or passing over the screen 103 of the CRT 102 is supplied from the switching circuit 116 to the CRT 102. This is achieved by control of the control circuit 114, which also detects the rotation of the rotary filter 106 by means of the pulses RP from the rotation sensor 112. Fig. 3A to Fig. 3D show the relationship between the rotation phase of the rotary filter 106, the scan line 103s of the CRT 102 and the switching of the switching circuit 116. The switching circuit 116 is controlled by the selection signal S which is in synchronism with the vertical sync signal V as described above, and is made to select the color image signals corresponding to the filter section that is passing over the screen 103 of the CRT 102, in accordance with the rotation sync pulse RP. The switching is so made that when the front part (as seen in the direction of rotation) of one of the filter sections, e.g., the filter section 106R comes to overlie the upper part of the screen 103 of the CRT 102, supply of the blue color image signal BS to the CRT 102 for one field is terminated and supply of the red color image signal RS to the CRT 102 for one field is commenced (Fig. 3A). Similarly, when the front part of the green filter section 106G comes to overlie the upper part of the screen 103, one vertical period later, supply of the red color image signal RS for one field is terminated and supply of the green color image signal GS for one field is commenced (Fig. 3D). Similarly, when the front part of the blue filter section 106B comes to overlie the upper part of the screen 103, one vertical period later, supply of the green color image signal GS for one field is terminated and supply of the blue color image signal BS for one field is commenced (not illustrated). With the above configuration, the color image signals RS, GS and BS are sequentially supplied from the switching circuit 116 to the CRT 102, while the vertical and horizontal deflection scans are conducted by means of the deflection circuits 118 and 120 in accordance with the vertical and horizontal sync signals V and H, so black-and-white images due to the color image signals RS, GS and BS are sequentially displayed on the screen 103, each in one vertical period. As described above, the selection between the color image signals RS, GS and BS is made in conformity with the filter sections 106R, 106G and 106B of the rotary filter 106 which is passing over the screen 103 of the CRT 102, so that as the image due to the color image signal RS, GS or BS is displayed on the screen 103, the corresponding filter section 106R, 106G or 106B is positioned over the screen 103 of the CRT 102. Accordingly, red, green and blue images by virtue of the color image signals RS, GS and BS are obtained through the filter sections 106R, 106G and 106B of the rotary filter 106, at the vertical period. The net effect is that a color image is seen to the viewer observing the image through the filter sections 106R, 106G and 106B. Problems of the color image display device in the prior art as follows: For displaying one complete color image, video signals of three fields are required. Where the picture contains an area where the image (part of the image) consists solely of one of the three colors, the image signal is of a high (or relatively high) level only during one of the three consecutive fields, and is at a low level during two other fields. This causes flicker, which is objectionable to the viewer. When there occurs a phase discrepancy between the supply of the image signals and the rotation of the rotary filter, the images as seen through the filter have colors different from those of the original image. This may occur at the time of starting the motor 110, or when the video signal supplied to the image display device is switched from one to another. Prior Art Example 2 Fig. 4 shows the configuration of a color image display device of another prior art example (Prior Art Example 2). The components or parts identical or corresponding to those in Fig. 1 are given identical reference numerals. The coloring device 104 and the CRT 102 are omitted. As illustrated, it comprises a black-and-white CRT 102 with deflection circuits 118 and 120, a rotary filter 106, a motor 110 with a shaft 108 and a magnet 113 attached on it, a rotation sensor 112, and a control circuit 114, which are similar to those described with reference to Fig. 1, Fig. 2 and Fig. 3A to Fig. 3D. The functions of the control circuit 114 are partly similar to those described with reference to the Prior Art Example 1, but are partly different, as will be understood from the following description. Fig. 4 also shows a color signal separator 122 and a sync separator 124 both of which receive a composite video signal which comprises luminance and chrominance components, and vertical and horizontal sync signals. The color signal separator 122 receives the composite video signal CV, and produces from the composite video signal, the R, G and B image signals of red, green and blue respectively representing red, green and blue components of a color image. The sync separator 124 extracts vertical and horizontal sync signals V and H from the composite video signal CV. A frequency converter 126 receives the vertical sync signal V and produces therefrom a triple-frequency vertical sync signal V3 having a frequency three times that of the vertical sync signal V. The triple-frequency sync signal V3 is in synchronism with the original vertical sync signal V, and one out of every three sync pulses forming the triple-frequency vertical sync signal V3 are concurrent with the sync pulses of the original vertical sync signal V. Another frequency converter 127 receives the horizontal sync signal H, and produces therefrom a triple-frequency horizontal sync signal H3 having a frequency three times that of the horizontal sync signal H. The triple-frequency vertical sync signal V3 from the frequency converter 126 is in synchronism with the triple-frequency horizontal sync signal H3 from the frequency converter 127. The control circuit 114 of this prior art example receives the rotation sync pulse RP and the vertical sync signal, as well as the triple-frequency vertical sync signal V3 from the frequency converter 126. The control circuit 114 of this prior art example further receives rotation frequency pulses FP from the motor 110, and controls the motor 110 by providing a drive current DA, such that the rotary filter 106 rotates once per vertical period, and in synchronism with the vertical sync signal, i.e., such that the rotation sync pulse RP is produced at a constant phase angle relative to the vertical sync signal V. The control circuit 114 produces control signals and clocks used for control over various circuits in the color image display device. Analog-to-digital (A/D) converters 128R, 128G and 128B receive the color image signals RS, GS and BS from analog to digital form. The resultant digital color image signals, each of which consists of a sequence of digital signals representing the samples, are also denoted by RS, GS and BS. The color image signals RS, GS and BS are illustrated in Fig. 5A, Fig. 5B and Fig. 5C. Image memories 130R, 130G and 130B receive and store color image signals RS, GS and BS. Each of the image memories 130R, 130G and 130B has a capacity of storing one field of corresponding image signals RS, GS and BS, and is formed of a dual port memory capable of concurrent writing and reading. The memories 130R, 130G and 130B are supplied with signals and clocks produced at the control circuit 114 in synchronism with the vertical sync signal V and the horizontal sync signal H, such that the writing of the iamge signals into the memories 130R, 130G and 130B is made in synchronism with the vertical sync signal V and horizontal sync signal H. The memories 130R, 130G and 130B are also supplied with signals and clocks produced at the control circuit 114 and in synchronism with the triple-frequency vertical sync signal V3 and the triple-frequency horizontal sync signal H3 such that the reading of the image signals from the memories 130R, 130G and 130B is made in synchronism with the triple-frequency vertical sync signal V3 and the triple-frequency horizontal sync signal H3. One out of every three read field periods (each of "read field period" is a period for which a field of image signals of each color is read) is commenced in time with the commencement of the write field period (for which a field image signal of each color is written). The writing of the image signals of each color of each field is achieved by designating writing addresses in succession and supplying data representing the samples of the color image signal to the memory to thereby write the sample data in the respective designated writing addresses. The reading of the color image signal from each memory is achieved by designating the reading addresses in succession and reading the data representing the samples of the color image signal from the designated reading addresses to thereby obtain a sequence of the sample data of the color image signal. The color image signals RS, GS and BS are sequentially written in the memories 130R, 130G and 130B as the video signal is supplied from outside of the device and at a rate or interval identical to the rate or interval at which the signals are sampled and digitized by the A/D converters 128R, 128G and 128B. It is noted that the rate of sampling (or A/D conversion) and the rate of writing in a memory, as well as the rate of reading and the rate of D/A conversion which will be mentioned later, are expressed in terms of samples per unit time (e.g., samples per second). As a new field of image signals are written, they are written over the old image signals (of the preceding field). The color image signals RS, GS and BS stored in the memories are repeatedly read. The writing and the reading of the image signals are carried out concurrently. The reading is conducted at a rate or speed three times that of the writing. During a period (one vertical period) which is taken to write one field of signals, the same amount of signals (one field of signals) are read three times. Thus, color image signals RS3, GS3 and BS3 of a triple rate are produced from the memories 130R, 130G and 130B, as shown in Fig. 5D, Fig. 5E and Fig. 5F. The triple-rate color image signals RS3, GS3 and BS3 read out from the memories 130R, 130G and 130B are D/A-converted by D/A converters 132R, 132G and 132B, and the analog color image signals, also denoted by RS3, GS3 and BS3 are supplied to the switching circuit 116, which is controlled by the selection signal S supplied in synchronism with the signal V3. As a result, a time-division multiplexed image signals VS3 are output in sequence and in turn, in which switching of the image signal of one color to the image signal of another color is effected every read field, that is one field of image signal of one color is consecutively output before being taken over by image signal of another color, as illustrated in Fig. 5G. The output of the switching circuit 116 is supplied, via an amplifier not illustrated, to the CRT 102. The triple-frequency signal V3 is supplied from the frequency converter 126 to the vertical deflection circuit 118, and the triple-frequency signal H3 is supplied from the frequency converter 127 to the horizontal deflection circuit 120. Vertical and horizontal deflection currents are supplied from the deflection circuits 118 and 120 to vertical and horizontal deflection coils (not shown) in the CRT 102. The CRT 102 therefore conducts vertical and horizontal deflection scanning at a rate three times that of ordinary scanning. The relationship between the rotation phase of the rotary filter 106, the scan line 103s of the CRT 102 and the switching of the switching circuit 116 is identical to that described with reference to Fig. 3A to Fig. 3D. However, it should be noted that the rotary filter 106 of this prior art example rotates at a rate three times higher than that described in connection with Prior Art Example 1. With the above configuration, the triple-frequency image signals RS3, GS3 and BS3 are sequentially supplied to the CRT 102, while the vertical and horizontal deflection scans are conducted at a triple rate, so black-and-white images due to the color image signals RS3, GS3 and BS3 are sequentially displayed on the screen 103, each in a period one third that of the vertical period. As described above, the selection between the color image signals RS3, GS3 and BS3 is made in conformity with the filter sections 106R, 106G and 106B which is passing over the screen 103 of the CRT 102, so that as the image due to the color image signal RS3, GS3 or BS3 is displayed on the screen 103, the corresponding filter section 106R, 106G or 106B is positioned over the screen 103 of the CRT 102. Accordingly, red, green and blue images by virtue of the color image signals RS3, GS3 and BS3 are obtained through the filter sections 106R, 106G and 106B of the rotary filter 106, at the one-third vertical period. The net effect is that a color image is seen to the viewer observing the image through the filter sections 106R, 106G and 106B. An advantage of Prior Art Example 2 over Prior Art Example 1 is that the images of the respective colors are displayed at a rate three times that of the prior art example. The flicker is therefore much reduced. In Prior Art Example 2, writing into the memories 130R, 130G and 130B reading therefrom are conducted simultaneously, and the reading is performed at a rate three times higher than the rate of writing. As a result, passing-over, i.e., passing of the reading address over the writing address, occurs. This will be explained in further detail with reference to Fig. 6A to Fig. 6D, Fig. 7A to Fig. 7D, and Fig. 8A to Fig. 8D. It is assumed that one of every three read fields is commenced in time with the commencement of the write field, as was also described in connection with Prior Art Example 1. Fig. 6A shows the color image signals RS, GS and BS stored at the end of the first one-third of the write field period, or the period taken for writing one field of signals. Fig. 7A shows the color image signals RS, GS and BS stored at the end of the second one-third of the write field period. Fig. 8A shows the color image signals RS, GS and BS stored at the end of the entire write field period. The areas of the memories are shown in a form corresponding to the image displayed on a display device. The memory area into which the writing with the present field of data has been effected is indicated by hatching, while the area into which the writing of data of the present field has not been effected and therefore still storing the data of the preceding field is unhatched. As will be readily understood, such area is one third (131A) of the entire area for one field (130R, 130G or 130B) at the end of the one-third field period; two third (131A and 131B) at the end of the two-third field period; and the entirety at the end of the entire field. Fig. 6B shows the data read from each of the memories during the first read field period (first cycle of reading operation). Fig. 7B shows the data read from each of the memories during the second read field period. Fig. 8B shows the data read from each of the memories during the third or last read field period. The assembly or set of data read from each memory are shown in a form corresponding to the image on a display device. The region of the image formed of the data of the present write field are indicated by hatching, while the region formed of data of the preceding write field are unhatched. As shown the data read in the first read field period consists of the data of the preceding data. The data read in the third (last) field period consists of the data of the present write field. The first half of the data in the second read field consists of the data of the present write field, while the second half of the data consists of the data of the preceding write field. At the beginning of the first one-third write field period, the writing and reading start with the beginning (starting address) of the field. Since the address for the reading varies or is incremented at a faster rate, the address used for reading is always ahead of the address of the writing. Thus, the data read during this period is of the preceding write field from the beginning to the end, as indicated in Fig. 6B (entire region being unhatched). At the end of the first one-third field period (when the address for the reading is at the end of the entire memory area), the address for the writing is at one third of the full memory area (as counted from the starting address), as indicated by hatching in Fig. 6A. With the writing address at such a position, the second read field starts. In the beginning, the reading address is behind the writing address. But because the reading address advances three times faster than the writing address, the reading address passes over the writing address at the middle of the second read field, which also is the middle of the write field. Thereafter, the reading address is ahead of the writing address. Accordingly, the data read during the first half of the second read field consists of the data of the present field (as indicated by hatching), while the data read during the second half of the second read field consists of the data of the preceding field (as indicated by unhatching). At the end of the second one-third field period (when the reading address is at the end of the entire memory area), the writing address is at two third of the full memory area (as counted from the starting address), as indicated by hatching in Fig. 6B. With the writing address at such a position, the third read field starts. During the third read field, the reading address for reading is maintained behind the writing address and only at the end of the field period, the former catches up with the latter. Accordingly, the data read during the third read field consists of the data of the present field (as indicated by hatching). If the image formed by the data of the present field and the preceding field differs, due to motion of the picture for example, as illustrated in Fig. 9A showing the image of the preceding field, and Fig. 9B showing the image of the present field, the data read during the second one-third write field period will result in an image shown in Fig. 9C. The problem of passing-over will also be described later with reference to Fig. 67, Fig. 68 and Fig. 74. Thus, there is a discontinuity or discrepancy between the upper and lower halves of the picture. The discontinuity or discrepancy of the picture occurs only one out of three images, and where motion occurs. Further prior art examples and their problems are discuussed with reference to Fig. 10 to Fig. 18. Prior Art Example 3 Fig. 10 is a block diagram showing a part of a video signal processor for converting video signal of three primary colors red (R), green (G) and blue (B) into field-sequential signals, used in a display device for displaying the video signals of the three primary colors. Fig. 10 shows that part of the video signal processor which relates to one of the three primary colors. In such field sequential display, the rate of scanning may be higher than the rate of scanning used when the images of the three primary colors are displayed concurrently, e.g., by the use of three beams. Fig. 11A and Fig. 11B are waveform diagrams for illustrating the operation of the circuit of Fig. 10. Fig. 12A to Fig. 12D show the frequency distributions of the signals at various nodes of the video signal processor of Fig. 10. Referring to Fig. 10, the video signal processor comprises an analog-to-digital (A/D converter) 1, a field memory 2, a digital-to-analog (D/A converter) 3, a low-pass filter (LPF) 4 for band-limiting the input signal to thereby preventing occurrence of aliasing noises in the frequency band of the video signal, and another LPF 5 for removing aliasing noises from the output signals. In the following description, controls over the A/D converter 1, the field memory 2 and the D/A converter 3 are all effected at the rising edges of the control clock, and although in practice it is necessary to consider the timing delays of operation due to response delays of the circuit components within each cycle, this is not discussed in the following description. That is, it is assumed that the circuit components operates without delays (except where such delays are specifically mentioned). Discrete time signals obtained by sampling continuous time signals contain signal components of the frequency distribution of the original continuous time signals, and signal components extending, both upward and downward, from the sampling frequency and its multiples, and having a band-width equal to the band-width of the original continuous time signals. If the sampling frequency is less than twice the maximum frequency of the original continuous time signals, the discrete time signal components having a frequency band extending downward from the sampling frequency overlaps the frequency band of the original continuous time signals, separating them from each other by means of a low-pass filter is not possible. To enable separation, the frequency band of the original continuous time signals must be limited to not higher than one half the sampling frequency. This is known as Nyquist's theorem. One half of the sampling frequency is called a Nyquist frequency. In the following description, the discrete time signal components having a frequency band extending downward from the sampling frequency are called the aliasing noises. It is also assumed that the frequency f A/D of the control clock of the A/D converter 1 and the frequency f D/A of the control clock of the D/A converter converter 3 are identical to each other. In the video signal processor of Fig. 10, the LPF 4 band-limits the input signal having a frequency distribution shown in Fig. 12A to produce a signal having a frequency distribution shown in Fig. 12B. The A/D converter 1 samples the input signal at the rising edges of the A/D converter control clock, as shown in Fig. 11A, and performs A/D conversion at the sample points n, n+1, n+2, ... indicated by dots (solid circles). The field memory 2 stores the data from the A/D converter 1 at the rising edges of the memory write control clock. The stored data are output at the memory read control clock. The D/A converter 3 D/A converts the data from the field memory 2 at the rising edges of the D/A converter control clock, and produces a signal indicated by blank circles in Fig. 11B. The output of the D/A converter 3 is maintained until the next rising edge of the D/A converter control clock. The output signal of the D/A converter 3 contains aliasing noises as shown in Fig. 12C. The LPF 5 band-limits the output signal from the D/A converter 3 to a frequency band of not higher than one half the frequency of the D/A converter control clock in order to remove the aliasing noises from the D/A converter control clock frequency f D/A . The output of the LPF 5 has a frequency distribution as shown in Fig. 12D. The signal band-width of the video signal processor described above is not more than the one half the frequency of the A/D converter control clock, i.e., not more than the pass-band of the LPF 4. Fig. 13 is a block diagram showing a video signal processor for converting the video signal of three primary colors of red, green and blue into field sequential signals used in a display device for performing field sequential display of video signals of the three primary colors. In Fig. 13, members identical or corresponding to those in Fig. 10 are denoted by identical reference numerals, with or without suffixes r, g or b, which indicate that the circuit is provided for the red, green or blue video signal. That is, LPFs 4r, 4g and 4b, A/D converters 1r, 1g and 1b, and field memories 2r, 2g and 2b, respectively similar to the LPF 4, the A/D converter 1 and the field memory 2 in Fig. 10 are provided for the input R, G and B signals of red, green and blue. A switch 6 selects the output data from the field memories 2r, 2g and 2b in sequence, for one field period each. The selection is made in accordance with an RGB selection signal. The output of the switch 6 is a sequence of R, G and B signals in cyclic succession, with each of the R, G and B signals continuing for one field. The D/A converter 3 D/A converts the output of the switch 6 and outputs field sequential R, G and B signals. The signal band-width of the video signal processor described above is not higher than one half the control clocks for the A/D converters 1r, 1g and 1b, which is the passband of the LPFs 4r, 4g and 4b. Prior Art Example 4 Fig. 14 is a block diagram showing a part of another conventional video signal processor for converting the video signals of three primary colors, used in a display device for performing field-sequential display of R, G and B video signals. That part which relates to production of the decoded color signal of one color is illustrated. In Fig. 14, members identical or corresponding to those shown in Fig. 10 and Fig. 13 are denoted by identical reference numerals. An inverter 7 inverts the phase of the A/D control clock. Another inverter 8 inverts the phase of the D/A control clock. A switch 9 selects one of the output signals from the D/A converters 3 and 3a. Fig. 15A, Fig. 15B, and Fig. 16A to Fig. 16C show operation of the circuit of Fig. 14. Fig. 17A to Fig. 17F show the frequency distributions at various nodes of the circuit of Fig. 14. It is again assumed that the frequency f A/D of the control clock for the A/D converters 1 and 1a and the frequency f D/A of the control clock for the D/A converter 3 and 3a are identical to each other. The input signal has the frequency distribution shown in Fig. 17A. The LPF 4 band-limits the input signal to a band-width of not higher than the frequency of the control clock for the A/D converters 1 and 1a to produce a signal having a frequency distribution shown in Fig. 17B. The A/D converter 1, the field memory 2 and the D/A converter 3 are similar to those described with reference to Fig. 10. The time points at which the input signal is sampled and A/D-converted by the A/D converter 1 are indicated by solid circles, n, n+1, n+2, n+3, ..., is indicated by output of the A/D converter 1 is shown in Fig. 15A. The output data from the A/D converter 1 and the data written in the field memory 2 are also shown in Fig. 15A. The output data from the field memory 2 and the output signal from the D/A converter 3 are shown in Fig. 16A. The output signal from the D/A converter 3 are indicated by blank circles. The frequency distributions of the output signals of the LPF 4, the A/D converter 1, the D/A converter 3 are also shown in Fig. 17A to Fig. 17C. The A/D converter 1a, the field memory 2a and the D/A converter 3a are similar to the A/D converter 1, the field memory 2 and the D/A converter 3, respectively, but they operate responsive to the control clocks having 180° phase difference from the control clocks for the A/D converter 1, the field memory 2 and the D/A converter 3. The A/D converter 1a therefore samples the input signal at time points indicated by solid squares, at m, m+1, m+2, m+3, ..., and performs A/D conversion, as shown in Fig. 15B. The output data from the A/D converter 1a and the data written in the field memory 2a are also shown in Fig. 15B. The output data from the field memory 2a and the output signal from the D/A converter 3a are shown in Fig. 16B. The output signal from the D/A converter 3 are indicated by blank squares. The frequency distributions of the output signals of the A/D converter 1a and the D/A converter 3a are shown in Fig. 17B and Fig. 17C. As illustrated, the output signals of the D/A converters 3 and 3a contain aliasing noises from the control clock frequency f D/A for the D/A converters 3 and 3a. The phases of the aliasing noises contained in the output signals of the D/A converters 3 and 3a have a phase difference of 180° from each other, because of the phase difference of 180° in the sampling timing. The switch 9 selects one of the output signals from the D/A converters 3 and 3a in accordance with the control clock for the D/A converter 3, and produces a signal having twice the sampling points or sampling frequency as shown in Fig. 16C. The aliasing noises contained in the outputs D/A converters 3 and 3a cancel each other because the phase difference between them is 180°. On the other hand, the output of the switch 9 contains aliasing noises from the frequency twice the frequency of the control clock, i.e., from 2f D/A . As a result, the frequency distribution of the output of the switch is as shown in Fig. 17E. The LPF 5 removes the aliasing noise from the frequency twice the control clock frequency f D/A by limiting the output signal to the control clock frequency f D/A , to thereby outputting the signal having a frequency distribution shown in Fig. 17F. The video signal processor of the Prior Art Example 3 described above has a signal having a bandwidth which extends to the control clock frequency f A/D . However, two A/D converters and two field memories are required, as opposed to one A/D converter and one field memory in the Prior Art Example 1. Fig. 18 shows a video signal processor for converting video signals of the three primary colors into field sequential signals used for a display device performing field sequential display of the video signals of the three primary colors. Members identical or corresponding to those in Fig. 13 are denoted by identical reference numerals. The LPFs 4r, the A/D converters 1r and 1ra, and field memories 2r and 2ra are similar to the LPF 4, the A/D converters 1 and 1a, and the field memories 2 and 2a, but are used for processing the R video signal. The LPFs 4g, the A/D converters 1g and 1ga, and field memories 2g and 2ga are similar to the LPF 4, the A/D converters 1 and 1a, and the field memories 2 and 2a, but are used for processing the G video signal. The LPFs 4b, the A/D converters 1b and 1ba, and field memories 2b and 2ba are similar to the LPF 4, the A/D converters 1 and 1a, and the field memories 2 and 2a, but are used for processing the B video signal. The switch 6 sequentially selects the output from the field memories 2r, 2g and 2b, each for a period of one field, to produce field sequential signals. The switch 6a sequentially selects the output from the field memories 2ra, 2ga and 2ba, each for a period of one field, to produce field sequential signals. The D/A converter 3 D/A converts the output of the switch 6, to produce field sequential signals. The D/A converter 3a D/A converts the output of the switch 6a, to produce field sequential signals. The switch 9 alternately selects the output of the D/A converters 3 and 3a to produce field sequential signals having an extended bandwidth. Thus, the bandwidth of the output signal of the video signal processor can be extended to the frequency of the control clock frequency f A/D . But twice as many A/D converters and field memories are required. In summary, with the video signal processor of the Prior Art Example 3, the input video signal must be limited to one half the control clock frequency. For expanding the bandwidth, it is necessary to increase the control clock frequency, but it is difficult or impossible to provide an A/D converter, a field memory, and a D/A converter capable of operating at such a high frequency. Moreover, the use of the control clocks with higher frequency can cause interference with other circuits. With the video signal processor of the Prior Art Example 4, two A/D converters, two field memories and two D/A converters are required for the video signal of each color, and the size and cost of the overall circuit is increased. FR 2 538 203 relates to a projector for color television pictures. Luminance and chrominance data are written into three memories and are read out of the memories at a rate three times the rate of writing. A rotating filter disc is synchronised with the readout from the memories. US 4,305,092 relates to a video projecting apparatus having a memory device for simultaneously storing at least the red, green and blue color components of the video signal at a first rate during each horizontal line scan period and a transmitting device for sequentially transmitting from the memory device at a second, faster rate during the next horizontal line scan period. SUMMARY OF THE INVENTION It is desirable to prevent flicker in the image. It is also desirable to prevent display of images having different colors from the original video signals. It is also desirable to minimize the capacity of the memory for storing the image signals. It is also desirable to extend the signal bandwidth of a video signal processor while minimizing the number of A/D converters, field memories and D/A converters. The present invention provides an image signal processing circuit as claimed in claim 1. The invention also provides a color image display system as claimed in claim 6 and a video signal processor as set out in claim 8. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram sowing a conventional color image display device using a black-and-white CRT in combination with a coloring device. Fig. 2 is a front view of a disk-shaped rotary filter forming part of the coloring device used in the color image display device of Fig. 1. Fig. 3A to Fig. 3D are diagrams showing the relationship between the rotational phase of the rotary filter, the scan line of the CRT and the switching of a switching circuit used in the color image display device of Fig. 1. Fig. 4 is a schematic diagram showing a color image display device of another prior art example. Fig. 5A to Fig. 5G are schematic time chart showing the color image signals written into memories and color image signals read from the memories and supplied to the CRT. Fig. 6A to Fig. 6D, Fig. 7A to Fig. 7D, and Fig. 8A to Fig. 8D are diagrams showing the progress of writing of data into the memories, in relation to reading of the data from the memories, switching of the read signals and rotation of the rotary filter, for explaining the passing of the reading address over the writing address. Fig. 9A to Fig. 9C are diagrams showing discontinuity in the picture due to the passing over of the address. Fig. 10 is a block diagram showing a part of a video signal processor in the prior art. Fig. 11A and Fig. 11B are diagrams illustrating the waveforms and values of the signals at various points in the processor of Fig. 10. Fig. 12A to Fig. 12D are diagrams showing the frequency distributions of the signals at various points in the processor of Fig. 10. Fig. 12E is a diagram explaining the meaning of each type of hatching in Fig. 12A to Fig. 12D. Fig. 13 is a block diagram showing a video signal processor of another prior art example. Fig. 14 is a block diagram showing a part of a video signal processor of another prior art example. Fig. 15A and Fig. 15B are diagrams showing waveforms and values of the signals at various points in the video signal processor of Fig. 14. Fig. 16A to Fig. 16C are diagrams showing waveforms and values of the signals at various points in the video signal processor of Fig. 14. Fig. 17A to Fig. 17F are diagrams showing the frequency distributions at various points in the video signal processor of Fig. 14. Fig. 17G is a diagram explaining the meaning of each type of hatching in Fig. 17A to Fig. 17D. Fig. 18 is a block diagram showing a video signal processor of another prior art example. Fig. 19A to Fig. 19D are front views of another example of rotary filter. Fig. 20 is a side view showing another example of rotary filter. Fig. 21 is a perspective view of the rotary filter of Fig. 20. DETAILED DESCRIPTION OF THE EMBODIMENTS Embodiment A1 The configuration of the color image display device of this embodiment is also as shown in Fig. 4. This embodiment is directed to the problem of the disontinuity or discrepancy of the picture due to the passing of the read address over the write address, as discussed in the description of Prior Art Example 2. The embodiment is featured by the control, made by the control circuit 114, over the sequence of cyclic selection of the color image signals and the filters such that the second read field is assigned to blue color image signals. That is, at the second read field, the blue image signal BS is selected as shown in Fig. 7C and the B filter section 106B is made to overlie the screen 103 as shown in Fig. 7D. At the first read field, green image signal GS is selected as shown in Fig. 6C and the G filter section 106G is made to overlie the screen 103 as shown in Fig. 6D. At the third read field, the red image signal RS is selected as shown in Fig. 8C and the R filter section 106R is made to overlie the screen 103 as shown in Fig. 8D. The reason for such arrangement is that the blue color is least prominent, and the discontinuity or discrepancy, which may occur, is least objectionable to the human eyes. Stated inversely, the image signals of red and green, which are the two most prominent colors of the three colors, are supplied within the first and the last read fields within each write field. Although not illustrated, it is advantageous if the memories 130R, 130G and 130B are so configured as to use, as a time reference, a horizontal sync signal next to each vertical sync signal V for the writing of the data of each field. Moreover, where the input video signal of an interlaced scanning type, it is advantageous if the output of the frequency converter 126 is shifted by one half of the horizontal period every three fields, the color image signals of three consecutive read fields and the color image signals of the next three consecutive read fields can be offset in the vertical direction on the screen and an effect similar to the interlacing is obtained. Modifications to Embodiment A1 In the embodiment described, the color signal separator is used to produce R, G and B image signals. However, the color signal separator 122 is not required where the color image signals RS, GS and BS are supplied from the outside of the color image display device. In the embodiment described, the sync separator is used to extract the vertical and horizontal sync signals V and H. However, the sync separator 124 is not required where the sync signals V and H are supplied from outside of the color image display device. In the embodiment described above, the image signals are read at a rate three times the rate at which the they are written in the memories. However, the reading may be at a rate other than three time the rate of writing. If the rate of reading is n times (n being an integer) the rate of writing and the rotary filter shown in Fig. 2 (having a single set of filter sections) the rotary filter is rotated at a speed n/3 revolutions per a vertical period of the input video signal. In the embodiment described above, rotary filter 106 has a single set of three filter sections 106R, 106G and 106B of the three primary colors. The rotary filter 106 may alternatively have two sets of filter sections. The first set consisting of three filter sections 106R1, 106G1 and 106B1 of red, green and blue, and the second consisting of three filter sections 106R2, 106G2 and 106B2 of red, green and blue, as illustrated in Fig. 32A. The filter sections 106R1, 106G1, 106B1, 106R2, 106G2 and 106G2 are consecutive in the direction of rotation so that they sequentially and continuously overlie the screen 103 of the CRT 102. In the state shown in Fig. 32A, the scanning line 103s is at or near the upper edge of the screen 103s, and the border line 106br between the filter sections 106B2 and 106R1 is at the center of the screen 103. In step with the downward movement of the scanning line 103s (by vertical deflection or scanning), the rotary filter rotates, in the direction of arrow 106A, as shown in Fig. 32B, Fig. 32C and Fig. 32D. In the state shown in Fig. 32D, the scanning line 103s is again at or near the upper edge of the screen 103, and the border line 106rg between the filter sections 106R1 and 106G1 is at the center of the screen. By the use of the rotary filter 106 having two sets of filter sections, the rotational speed of the rotary filter can be lowered to one half (compared with case where the rotary filter has only one set of filter sections). This is advantageous because the noises due to rotation can be reduced, and the voltage for driving the motor 110 can be lowered. Moreover, the accuracy requirement in mounting the filter sections is alleviated, and the permissible rotational phase error is increased. The number of the sets of the filter sections may be further increased to three, or more. In place of the disk-shaped rotary filter 106, a rotary filter 148 having a shape of a truncated circular cone, as shown in Fig. 33 and Fig. 34 may be used. The rotary filter 148 illustrated in Fig. 33 and Fig. 34 also has two sets of filter sections, 148R1, 148G1, 148B1, 148R2, 148G2 and 148B2. The rotary filter 148 is rotatable about the axis of the cone and is equally divided by generators (straight lines extending along the surface of the cone and passing through the vertex of the cone) of the cone into the red, green and blue filter sections 148R1, 148G1, 148B1, 148R2, 148G2 and 148B2. The screen 103 of the CRT 102 is provided to confront the outer conical surface of the rotary filter 148. By the use of the conical rotary filter, the diameter of the filter can be reduced, and the overall size of the device can be reduced. In connection with the embodiment described, the filter sections of the rotary filter are made to overlie the screen of the display device. However, the invention is not limited to such an arrangement. What is essential that the filter section be made to be interposed in an optical path from the screen to a predefined viewing postion, at which the viewer receives the light from the screen. The display device of the invention having been described above can be used as a view finder in a video camera, or as a projection television set.

Claims: An image signal processing circuit for a color display system in which respective color components of an image are displayed sequentially, the circuit comprising a memory (130) into which the color image signals are written while color image signals are being read for display, the signals relating to each of the respective colors being read out of the memory during an interval shorter than the interval over which those signals are written into the memory, characterised in that the reading from and writing to the memory is so controlled that the color represented by the image signals which are being read from the memory at the time that writing and reading addresses correspond is the predetermined least visually prominent of the respective colors. A circuit according to claim 1, wherein the colors are red, green and blue, and the least visually prominent color is blue. A circuit according to claim 1 or 2, wherein the rate at which the signals are read out of the memory is n times the rate at which the signals are written into the memory, and said control circuit controls the reading in one of n read fields to be performed in time with commencement of the write field. A circuit according to any one of claims 1 to 3, wherein the supply of the image signals of the two most prominent colors is made in a first and last read fields within each write field. A circuit according to claim 4, wherein the colors are red, green and blue, and said two most prominent colors are red and green. A color image display system comprising a monochromatic image display means (102) having a screen (103) for display of images, and a colouring device for sequentially selecting each of a plurality of colors, the system also comprising a circuit as claimed in any one of claims 1 to 5. A color image display system according to claim 6 wherein the coloring device is a rotary filter comprising color filter sections (106R, 106B, 106G), said filter sections being provided to be interposed in an optical path from said screen (103) to a predefined viewing position in turn as the rotary filter rotates. A video signal processor comprising a circuit as claimed in any one of claims 1 to 7.

Inventors: HAYAKAWA MASAHARU, KANAI AKIRA, KODAMA MASAFUMI, MAENO AKIKO, SUGIHARA SHOICHI, YAMAKAWA MASAKI, YAMAMOTO KIYOTAKA, HAYAKAWA, MASAHARU, KANAI, AKIRA, KODAMA, MASAFUMI, MAENO, AKIKO, SUGIHARA, SHOICHI, YAMAKAWA, MASAKI, YAMAMOTO, KIYOTAKA, Hayakawa, Masaharu, c/o Mitsubishi Denki K.K., Kanai, Akira, c/o Mitsubishi Denki, Kodama, Masafumi, c/o Mitsubishi Denki, Maeno, Akiko, c/o Mitsubishi Denki K.K., Sugihara, Shoichi, c/o Mitsubishi Denki K.K., Yamakawa, Masaki, c/o Mitsubishi Denki K.K., Yamamoto, Kiyotaka, c/o Mitsubishi Denki
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: H04N9
IPC codes: H04N9