Abstract:
A color subcarrier generator used with a system for converting data representing color displays into a composite color video signal suitable for display by a visual display apparatus comprises an oscillator which generates a system clock signal with a predetermined frequency, a frequency divider responsive to the system clock signal from the oscillator which generates horizontal and vertical sync signals for the visual display apparatus and a dot clock signal for timing sequential dots of the horizontal lines of the composite color video signal into which the data is converted and a subcarrier generating circuit which generates a color subcarrier signal to be included in the composite color video signal and which is synchronized in phase with the horizontal sync signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to video display apparatuses, and more particularly, to a color subcarrier generator for use with a microcomputer system in which color data is converted into a standard color video signal for display on a visual display apparatus. 
     2. Description of the Prior Art 
     In the NTSC television system, luminance and chrominance signals are interleaved together on a color subcarrier signal included in the composite video signal. The color subcarrier signal is located at the 455th odd harmonic of one-half the horizontal sweep rate, and has an approximate frequency of 3.58 MHz. The following equation expresses the relationship between the frequency of the color subcarrier f sc  and the frequency f H  of the horizontal sweep: ##EQU1## 
     In a microcomputer system in which color data are displayed on a visual display apparatus such as a color cathode ray tube, if the total number of picture elements for one line is N, the following equation expresses the relationship between the frequency f dot  of a dot clock signal used for clocking the dots comprising one line of a character and the frequency f H  of the horizontal sync signal: 
     
         f.sub.dot =N×f.sub.H                                 ( 2) 
    
     In prior art systems, separate oscillators have been used to generate the frequencies f dot  and f sc . It has been difficult to satisfy the relations expressed in equations 1 and 2 by using separate oscillators, however, since the frequencies f dot  and f sc  cannot be easily synchronized. 
     The use of separate, unsynchronized oscillators to generate f dot  and f sc  poses significant problems. The color on the display apparatus can flicker. A moire pattern can also be formed around a displayed figure. 
     One proposal uses a single oscillator instead of two in order to overcome the problems with two oscillators. The output signal from the single oscillator is divided to derive both the clock frequency and the color subcarrier frequency. In other words, the signal from the oscillator is divided by m to derive the color subcarrier frequency f sc , and is divided by n to derive the clock frequency f dot . If the frequency of the oscillator is f 0 , the following equation represents the relationship between the frequency of the oscillator and the frequency of the color subcarrier generator: ##EQU2## The following equation expresses the relationship between the frequency of the oscillator and the frequency of the clock signal: ##EQU3## Substituting in equations 3 and 4, the following relationship is derived: ##EQU4## The following expression for m is derived from equations 1 to 5: ##EQU5## In equation 6, it is to be appreciated that the numbers n and N must be multiples of 5, 7 or 13 so that m is an integer. If N, the number of picture elements on one line, is an arbitrary number, for example, 1024 or 1000, the number n must be an integral multiple of 455 (5×7×13) for m to be an integer. Accordingly, the frequency f 0  of the oscillator must be very high, in the range of 7 to 8 GHz, if N is an arbitrarily large number. However, such a high frequency oscillator is undesirable because of unnecessary radiation, the difficulty of making the necessary frequehcy divisions, and the like. One solution to this problem has been to restrict the total number of picture elements to an integral multiple of 455 (or a subcombination thereof, for example, 65 or 91) so that the number n is an arbitrary value and the frequency f 0  is not quite so high. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a color subcarrier generator which does not place a limit on the total number of picture elements which can be displayed on one line of a visual display apparatus. 
     It is another object of the present invention to provide a color subcarrier generator which does not produce undesirable color flicker when a figure is displayed on a visual display apparatus. 
     It is yet another object of the present invention to provide a color subcarrier generator which does not produce a moire pattern around a figure displayed in color on a visual display apparatus. 
     It is yet another object of the present invention to provide a color subcarrier generator with an oscillator which utilizes a medium level frequency. 
     In accord with the present invention, a color subcarrier generator for use with a system for converting data representing color displays into a composite color video signal suitable for display by a visual display apparatus comprises oscillating means for generating a system clock signal with a predetermined frequency, frequency dividing means responsive to the system clock signal from the oscillator for generating horizontal and vertical sync signals and a dot clock signal for timing sequential dots of the horizontal lines of the composite color video signal into which the data is converted, and subcarrier generating means for generating a color subcarrier signal to be included in the composite color video signal and which is synchronized in phase with the horizontal sync signal. 
     The above, and other objects, features and advantages of the present invention will be apparent from the following detailed description of an illustrative embodiment thereof which is to be read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram illustrating a microcomputer system including an embodiment of a color subcarrier generator in accord with the present invention; and 
     FIG. 2 is a detailed circuit diagram of a portion of the embodiment of the color subcarrier generator of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings in detail, and initially to FIG. 1 thereof, a microcomputer system encodes color display data into an NTSC composite color television signal and includes an embodiment of a color subcarrier generator in accord with the present invention. A central processing unit (CPU) 1 supplies data D 0  ˜D 7  along a data bus 23 and addresses A 0  ˜A 15  along an address bus 22. CPU 1 can be, in one embodiment, a Z80A unit manufactured by Zilog, Inc. A cathode ray tube controller (CRTC) 5 receives data D 0  ˜D 7  and addresses A 0  ˜A 15  from CPU 1 and supplies display addresses through a multiplexer (MPX) 9 to access a random access memory (RAM) 10. CRTC 5 can be, in one embodiment, an HD46505 unit manufactured by Hitachi. Multiplexer 9 selects either the address from CPU 1 or the address from CRTC 5 to access the data in RAM 10, as is known. 
     A crystal oscillator 2 generates an oscillator signal having a frequency f 0  which is, in the preferred embodiment, an integral multiple of a horizontal scanning frequency f H  (where horizontal scanning frequency f H  is approximately equal to the horizontal scanning frequency in the NTSC color television signal). The relationship between oscillator frequency f 0  and horizontal scanning frequency f H  can be expressed as: 
     
         f.sub.0 =m×f.sub.H 
    
     where m is an integer. In the illustrative embodiment, m is selected to be 2048. 
     The oscillator signal from oscillator 2 is supplied to a divider 3 which generates a dot clock signal having a frequency f dot . The dot clock signal is used for clocking or reading out the dots comprising each line of a displayed character. In the illustrative embodiment, divider 3 divides frequency f 0  by two. Thus, the relationship between f 0  and f dot  can be expressed as follows: 
     
         f.sub.o =2×f.sub.dot 
    
     The oscillator signal from oscillator 2 is also supplied to a divider 29 which divides frequency f 0  by 8 and supplies a CPU clock signal having a frequency f cpu  to CPU 1. 
     The dot clock signal having frequency f dot  is supplied to a second frequency divider 4 which generates a character clock signal having a frequency f chr  for clocking or reading out each displayed character. In the illustrative embodiment, frequency divider 4 divides the frequency f dot  by 8. Mathematically, the above relations can be expressed as follows: ##EQU6## In the illustrative embodiment, each character displayed on the visual display apparatus is comprised of 8 horizontally arranged dots on one line, and each horizontal line has 80 characters displayed thereon. 
     The character clock signal having frequency f chr  from frequency divider 4 is supplied to CRTC 5 which includes a 1/64 divider 6, a 1/512 divider 7, and a 1/2 divider 8. Divider 6 and divider 8 generate a horizontal sync signal having frequency f H , while divider 6 and divider 7 generate a vertical sync signal having frequency f V . The horizontal and vertical sync signals are, in the preferred embodiment, approximately equal to the horizontal and vertical sync signals in the NTSC television signal system. 
     A data bus driver 11 supplies data D 0  ˜D 7  from data bus 23 to RAM 10 along data bus 26. It is to be appreciated that address bus 22 and multiplexer 9 supply addresses A 0  ˜A 15 . A corresponding to data D 0  ˜D 7  to be stored in RAM 10. 
     As is known in the art, CRTC 5 reads data stored in RAM 10 by generating a display address and supplying the display address along data bus 24 to multiplexer 9. 
     The data stored in RAM 10 include character data and color data. The character data are supplied to a character generator 12, while the color data are supplied to a video controller 14. Character generator 12 also receives read addresses from CRTC 5 along address bus 25 and supplies dot character signals in parallel which correspond to each character to a parallel to serial converter 13. The parallel dot character signals from character generator 12 are converted into serial dot character signals by parallel to serial converter 13 in response to the dot clock signal having frequency f dot  from frequency divider 3. The dot character signals from parallel to serial converter 13 are supplied to video controller 14. Video controller 14 also receives the horizontal and vertical sync signals with frequencies f H  and f V , respectively, from CRTC 5 and generates red, blue and green color video signals which are supplied to an NTSC encoder 15. The horizontal and vertical sync signals are also supplied to NTSC encoder 15 from video controller 14. NTSC encoder 15 supplies a composite video signal to an output terminal 27. 
     NTSC encoder 15 is also supplied with the color subcarrier signal having frequency f sc . Unlike prior art systems, the color subcarrier signal with frequency f sc  is derived from the horizontal sync signal having frequency f H  from CRTC 5. 
     The color subcarrier generator in accord with the present invention includes a voltage controlled oscillator (VCO) 21 which generates the color subcarrier signal having frequency f sc . A control voltage for voltage controlled oscillator 21 is based upon the frequency f H  of the horizontal sync signal from CRTC 5. In particular, the horizontal synchronizing signal having frequency f H  is supplied to a frequency divider 16 which, in the preferred embodiment, divides frequency f H  by 2. The output signal from frequency divider 16 is supplied to a sample pulse generator 17 which generates control pulses to actuate a sampling circuit 18. Sampling circuit 18 samples the color subcarrier signal and generates a sampled output signal to be supplied to a low-pass filter 19, a DC amplifier 20, and voltage controlled oscillator 21. In the illustrated embodiment, a sample pulse is generated at every other horizontal sync signal, and has a pulse width which is one-quarter of the frequency of the 3.58 MHz color subcarrier signal. It is to be appreciated that sampling circuit 18, low-pass filter 19, DC amplifier 20 and voltage controlled oscillator 21 comprise a phase locked loop (PLL) circuit 55. 
     In FIG. 2, the horizontal sync signal having frequency f H  from CRTC 5 is supplied through an input terminal 30 to a pair of mono-stable multi-vibrators (one-shots) 31, 32. (The corresponding parts or elements of FIG. 1 are indicated in FIG. 2 in parenthese). One-shot 31 divides the frequency f H  of the horizontal sync signal by two. One-shot 32 generates a sample pulse signal having a width one-quarter the frequency of the color subcarrier signal (3.58 MHz) at every other horizontal sync signal. 
     The sampled pulse is supplied to a sampling circuit 33 (indicated by reference numeral 18 in FIG. 1) comprising a differentially connected pair of transistors 40A, 40B and a diode bridge circuit 41. The sampled pulse renders transistor 40A non-conductive when it is supplied thereto. When transistor 40A is non-conductive, transistor 40B and the diodes comprising diode bridge circuit 41 become conductive. In the illustrative embodiment, the color subcarrier signal having frequency f sc  from voltage controlled oscillator 36 (indicated by reference numera1 21 in FIG. 1) is sampled and supplied to low-pass filter 34 (indicated by reference numeral 19 in FIG. 1) through a pair of junctions 41A, 41B in diode bridge circuit 41. 
     Low-pass filter 34 includes a resistor 44 and a capacitor 45. The output signal of low-pass filter 34 is supplied to a voltage controlled oscillator 36 through a buffer amplifier 35 (indicated by reference numeral 20 in FIG. 1). 
     Voltage controlled oscillator 36 includes a crystal oscillator 46, a pair of variable capacitance diodes 47, 48, and an inverter 49. The color subcarrier signal with frequency f sc  is supplied to NTSC encoder 15 through output terminal 50. 
     In the illustrated embodiment, the color subcarrier signal is directly supplied to sampling circuit 18. Since the frequency f sc  of this signal is high (3.58 MHz), PLL circuit 55 (see FIG. 1) can have difficulty in properly locking the signals in phase. Crystal oscillator 46 is included in voltage controlled oscillator 36 to remedy this difficulty. 
     Alternatively, a 1/455 divider 28 can be disposed between sampling circuit 18 and voltage controlled oscillator 21, as illustrated in the phantom lines of FIG. 1. Only a 15.75 KHz signal is supplied to sampling circuit 18 in such an arrangement. Since this frequency is lower then the 3.58 MHz frequency of the color subcarrier signal, the signals can be more easily locked in phase. 
     It is to be appreciated that the color subcarrier generator in accord with the present invention utilizes a single oscillator having a frequency f 0  which is an integral multiple of the frequency f H  of the horizontal sync signal. In the color subcarrier generator, the frequencies f H  and f V  for the horizontal and vertical sync signals, respectively, are generated by dividing the frequency f 0  from the oscillator. The color subcarrier signal with frequency f sc  is derived from the frequency f H  of the horizontal sync signal so that the color subcarrier signal and the horizontal sync signal are synchronized with each other. 
     It is to be further appreciated that a microcomputer system employing a color subcarrier generator in accord with the present invention does not have to have an integral multiple of 455 picture elements on a horizontal line. As another advantage of a color subcarrier generator in accord with the present invention, color flicker of a displayed image is avoided. 
     Although a specific embodiment of the present invention has been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to that precise embodiment, and that various changes and modifications may be effected therein by one skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims