Variable scanning frequency multi-color projector

Abstract: A projection television is designed for reproducing a color video image with a variable scanning speed. A projection television circuit monitors the scanning speed for deriving scanning speed data. The circuit controls a color temperature of at least a plurality of mutually distinct color channels (2, 3, 4) on the basis of the scanning speed data so as to establish a predetermined color temperature ratio between respective color channels.

Description: The present invention relates generally to a projection type color television with a variable horizontal line scanning frequency. More specifically, the invention relates to a technology for controlling color temperature. Color projections employing three color cathode ray tubes for projecting R (red), G (green), and B (blue) colors on a projecting screen in overlapping manner for forming a composite color display image are known. Such a color projector generally outputs respective R-, G-, and B-channel video signals to respective cathode ray tubes via amplifiers so that the respective cathode ray tubes each deflect the scanning beam of the respective color over horizontal scanning lines. A luminous characteristic of blue fluorescent elements is different from that of red and green fluorescent elements. Specifically, a persistence characteristic of a blue fluorescent element has a lesser emission decay time than that of the red and green fluorescent elements. For example, in the case of a fluorescent formed of a mixture of ZnS and Ag, the persistence characteristic is τ 1/00 = several tens to several hundreds µs. However, as shown in Figure 4, beam scanning areas S₁, S₂ ... of adjacent scanning lines partly overlap as illustrated by the hatched area. The horizontal line scanning frequency is 15.75 kHz (65 µs) and the scanning period is several tens µs, for example. Since red and green fluorescent elements have persistence characteristics wherein luminance intensity decreases moderately, a variation of the horizontal line scanning frequency may not cause a significant influence over the overlapping area. However, in the case of a blue fluorescent element, because of the rapid decreasing of luminance intensity as shown in Figure 5, a variation of the horizontal scanning line frequency may cause an influence of the luminance intensity at the overlapping area. Specifically, in the example of Figure 5, the luminance intensity at a time t 1' becomes higher than that at a time t₁. This clearly degrades a quality of the reproduced image during a transition in variation of the horizontal line scanning frequency. It is an object of the present invention to provide a variable scanning frequency projection type television having the capability of controlling color temperature for stable reproduction, even when the horizontal line scanning frequency is varied. According to the invention, a variable scanning frequency multi-color projector for projecting a color video image on a display is provided as given in claim 1. Preferred embodiments are given in the dependent claims 2 - 13. The present invention will be understood more fully from the detailed description given herein below and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only. In the drawings: Figure 1 is a block diagram of the preferred embodiment of a variable scanning frequency projection type television according to the present invention; Figure 2 is a chart showing luminous characteristics of red and green fluorescent elements; Figure 3 is a chart showing luminous characteristics of a blue fluorescent element; Figure 4 is an explanatory illustration of scanning beams on adjacent horizontal scanning lines for showing overlapping of pixel areas; and Figure 5 is a chart showing persistence characteristics of the blue fluorescent element during variation of a horizontal line scanning frequency. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Figure 1, a circuit of the preferred embodiment of a projection type color television is illustrated in the form of a block diagram. A composite television signal is input via an input terminal 1. Luminance signal Y is entered through an input terminal 3. On the other hand, chroma difference signals C R and C B are entered via input terminals 2 and 4. The input terminals 2, 3, and 4 also serve as video signal input terminals for respectively receiving an R (red)-channel video signal, a G (green)-channel video signal, and a B (blue)-channel video signal. Another input terminal 5 is provided for accepting an external synchronizing signal. The composite signal input through the input terminal 1 is decoded into R-channel, G-channel and B-channel video signals by a decoder 6. On the other hand, the luminance signal Y and the chroma difference signals C R and C B are converted into R-channel, G-channel, and B-channel video signals by a matrix circuit 7. Respective color channel signals output from the matrix circuit 7 are fed to cathode ray tubes 9R, 9G, and 9B via amplifiers 8R, 8G, and 8B. Respective color channel signals from the decoder are added via summing junctions to the corresponding color channel signals from the matrix circuit 7. Deflector coils 10R, 10G, and 10B are provided for deflecting the scanning beam for periodically or cyclically changing pixels being scanned. As is well known, the synchronizing signal is generally superimposed on the luminance signal Y or, in the alternative, on the G-channel video signal. In order to separate the synchronizing signal from the luminance signal Y or the G-channel signal, synchronizing signal separation circuits 11 and 12 are provided. The synchronizing signal separation circuit 11 is connected to the input terminal 3 to receive therethrough the luminance signal Y or the G-channel video signal on which the synchronizing signal is superimposed. The synchronizing signal separation circuit 11 feeds a separated synchronizing signal component to a synchronizing signal generator 13. The other synchronizing signal separation circuit 12 is connected to the input terminal 1 to receive therethrough the composite signal. The synchronizing signal separator 12 also supplies the separated synchronizing signal component to the synchronizing signal generator 13. The synchronizing signal generator 13 is set to a predetermined priority for selecting one of the inputs from the synchronizing signal separation circuits 11 and 12 for generating the synchronizing signal based thereon. The synchronizing signal generator circuit 13 generates R-channel, G-channel, and B-channel synchronizing signals including horizontal synchronizing signals and vertical synchronizing signals to be fed to respectively corresponding deflection signal generator circuits 14R, 14G, and 14B. The deflection signal generator circuits 14R, 14G, and 14B control the deflector coils 10R, 10G, and 10B for scanning a respective corresponding color ray beam on respective horizontal scanning lines in order that three colors are projected in overlapping manner for reproducing the input television signal. As is well known, registration fluctuation can be adjusted by means of registration adjusters in the respective color-channel cathode ray tubes. On the other hand, distortion of the reproduced image may be corrected by correcting the outputs of the deflection signal generator circuits 14R, 14G, and 14B. The synchronizing signal generator circuit 13 is also connected to a clock counter 15 which counts clock pulses generated by a clock generator 16. The clock counter 15 is designed to be responsive to the horizontal synchronizing signal from the synchronizing signal generator circuit 13 to output the counter value data to a central processing unit (CPU). The clock counter 15 is then reset by the next horizontal synchronizing signal. As can be appreciated, the clock counter 15 is designed to monitor the horizontal line scanning frequency to obtain a counter value corresponding to the horizontal line scanning frequency. Therefore, this counter 15 forms a horizontal line scanning frequency detecting means. The CPU 17 is designed to supply the switching control signal for the matrix circuit 7. The CPU 17 receives the counter value data from the clock counter 15 to derive a gain data in terms of the horizontal line scanning frequency represented by the counter value data. The CPU 17 thus derives the gain data and feed to the B-channel amplifier 8B. Here it should be noted that the gain data is set in the form of a table or map or any other appropriate form in the CPU on the basis of results of experiments, and is adapted to values for maintaining luminance of reproduced blue color (color temperature) constant irrespective of the horizontal line scanning frequency. Therefore, one of a plurality of preset gain data is selected in terms of the horizontal line scanning frequency indicated by the counter value data. It should be appreciated that, though the illustrated embodiment is designed to control only B-channel color temperature, it is possible to design the circuit for controlling not only B-channel color temperature but also R-channel and G-channel color temperatures. As can be appreciated, the CPU 17 forms variable gain/bias means for adjusting a relative gain ratio in the B-channel amplifier 8B versus the R-channel and G-channel amplifiers 8R and 8G. It is also possible to adjust both the relative gain ratio between the respective color channel amplifiers and gains of the respective amplifiers. In operation, the television signal or video signal is selectively input through the input terminals 1 through 5. Then, respective R-channel, G-channel, and B-channel video signals are fed to associated cathode ray tubes 9R, 9G, and 9B via respective color channel amplifiers 8R, 8G, and 8B. Simultaneously, the synchronizing signal generator circuit 13 outputs synchronizing signals. The clock counter 15 counts the clock pulse from the clock pulse generator 16 during the interval between horizontal synchronizing signals for detecting the horizontal line scanning frequency to feed the horizontal line scanning frequency indicative signal to the CPU 17. The CPU 17 derives an optimal gain value of the B-channel amplifier 8B for feeding a gain control signal to the associated amplifier 8B. The B-channel amplifier 8B is responsive to the gain control signal to vary the amplifier gain to establish a gain ratio relative to the R-channel and G-channel amplifiers 8R and 8G, commanded by the gain control signal. With the illustrated construction, the color temperature of the blue image can be adjusted in relation to that of the red and green images for successfully avoiding the influence of a variation of the horizontal line scanning frequency.

Claims: A variable scanning frequency multi-color projector for projecting a color video image on a display, comprising a plurality of cathode ray tubes (9R, 9G, 9B) respectively adapted to generate predetermined color rays to scan on said display for projecting the color video image; deflecting means (10R, 10G, 10B) associated with each of said cathode ray tubes for deflecting said color rays for scanning on a plurality of scanning lines; a video signal source (7) for providing respective color channel video signals in respective color channels to said cathode ray tubes on receiving a multi-color video signal containing a plurality of mutually distinct color channel signal components; characterized in that there is further provided a scanning frequency detect (15) for monitoring the scanning period on each scanning line to produce a scanning frequency signal; and a color temperature adjusting means (8R, 8G, 8B) in at least one color channel for adjusting the color temperature of a color ray beam generated by the corresponding cathode ray tube, said color temperature adjusting means being responsive to said scanning frequency signal so as to derive a color temperature magnitude adjusting signal based on the scanning frequency. A multi-color projector as set forth in claim 1, wherein said color temperature adjusting means derives said color temperature magnitude adjusting signal for said at least one color channel so as to establish a desired color temperature ratio relative to color temperatures associated with the other color channels. A multi-color projector as set forth in claim 1, wherein said color temperature adjusting means comprises an amplifier in each of the color channels, and further comprises means for performing the color temperature adjustment by adjusting the gain of the amplifier so as to establish a predetermined relative gain ratio of the amplifiers compared to one another, said predetermined relative gain ratio being variable depending upon scanning frequency. A multi-color projector as set forth in claim 1, wherein said at least one color channel is the color channel for blue image reproduction. A multi-color projector as set forth in claim 1, wherein said plurality of cathode ray tubes comprises first, second, and third cathode ray tubes (9R, 9G, 9B) each for projecting a different color with color rays on the display. A multi-color projector as set forth in claim 5, wherein said projector is a projection television with a variable scanning frequency, and wherein said first cathode ray tube (9R) is a cathode ray tube for projecting a red ray and which has a fluorescent element; said second cathode ray tube (9G) is a cathode ray tube for projecting a green ray and which has a fluorescent element; said third cathode ray tube (9B) is a cathode ray tube for projecting a blue ray and which has a fluorescent element having a shorter emission decay time than fluorescent element decay times of the other cathode ray tubes; said color temperature adjusting means comprises three amplifiers (8R, 8G, 8B) for amplifying red, green and blue channel signals respectively being input to said cathode ray tubes; said scanning frequency detector (15) comprises a counter for counting and detecting the horizontal scanning frequency associated with said frequency deflecting signals; said projector further comprising : deflecting signal generator circuits (14R, 14G, 14B) for generating deflecting signals for controlling said deflecting means (10R, 10G, 10B); and control means (17) for controlling at least one of the parameters comprising gain and bias of at least one of said three amplifiers based on the counting of said counter. A projection television with variable scanning frequency as set forth in claim 6, wherein said control means (17) comprises gain control means for controlling gain of said three amplifiers based on the counting of said counter. A projection television with variable scanning speed as set forth in claim 7, wherein said gain control means comprises means for setting a gain for the amplifier for the blue channel signals higher than a gain for the other amplifiers in response to an increasing deflection frequency. A projection television with a variable scanning frequency, as set forth in claim 6, wherein said control means (17) comprises gain control means for controlling a gain of one of said three amplifiers based on the counting of said counter. A projection television with variable scanning frequency as set forth in claim 9, wherein said one amplifier is for the blue channel signals. A projection television with a variable scanning frequency, as set forth in claim 6, wherein said control means (17) comprises bias control means for controlling biases of said three amplifiers based on the counting of said counter. A projection television with a variable scanning frequency, as set forth in claim 6, wherein said control means (17) comprises bias control means for controlling a bias of one of said three amplifiers based on the counting of said counter. A projection television with variable scanning frequency as set forth in claim 12, wherein said one amplifier is for the blue channel signals.

Inventors: FUNADO SHIGETO C O SONY CORPOR, FUNADO, SHIGETO, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: H04N9
IPC codes: G09G1, G09G5, H04N9, H04N5