Horizontal Retrace Time Adjustment Pulse Generating Circuit for a Display Device

1. Field of the Invention

The present invention relates to a display device and, more particularly, to a horizontal retrace time adjustment pulse generating circuit for a display device, in which the retrace time of the horizontal deflection circuit is properly adjusted with respect to the flyback pulse.

2. Discussion of Related Art

An exemplary display device comprises a video preamplifier which amplifies a video input by means of a low-voltage amplifier and maintains the amplified signal at a predetermined level; a video main amplifier which amplifies the output from the video preamplifier and supplies the amplified signal energy for each pixel of a cathode-ray tube (CRT); a microcomputer which receives horizontal and vertical synchronizing signals to produce screen control signals; vertical and horizontal deflection circuits which receive the screen control signals and generate vertical and horizontal deflection signals for output to a deflection yoke mounted on the CRT; and a high voltage circuit which supplies the anode voltage for the CRT, using a flyback pulse generated by the horizontal deflection circuit.

The vertical and horizontal deflection signals control the deflection of an electron beam generated from an electron gun within the CRT, to thus form a displayed image on the screen or the display device. The horizontal scanning in which the electron beam is deflected horizontally and vertical scanning in which the electron beam is deflected vertically are performed concurrently.

In horizontal scanning, after completing the scanning of a horizontal line in the display area, the next line is scanned from a point just below that previously scanned. The time duration from the completion of the previous line to the beginning of the next is commonly called retrace time, or horizontal flyback time. To display a clear image void of retrace lines, the video signal is blanked during the retrace time. Thus, the display device is provided with a retrace time signal blanking circuit for generating a blanking signal to remove the retrace line caused by the horizontal flyback of the electron beam for scanning the next line. If the retrace line signal is not properly blanked, video noise appears on the display.

General design specifications set the retrace time to approximately 17-20% of one horizontal period and the blanking time to approximately 20-25% of one horizontal period. These percentages are not appropriate, however, for high-resolution monitors which use a high horizontal scanning frequency, due to a characteristic limitation of the output transistor of the horizontal deflection circuit.

It is therefore important to determine and maintain the proper blanking time with respect to retrace time, which is dependent on the horizontal frequency employed. For example, should the retrace time be reduced for a higher horizontal frequency or should the blanking time be increased for a lower horizontal frequency, the horizontal deflecting current increases which increases power consumption and tends to overheat the output transistor. Also, shorter retrace times result in high voltage peaks in the output of the horizontal deflection circuit, which also is harmful to the output transistor. Meanwhile, the retrace time should be set with regard to the highest horizontal frequency used in a given display device, such that the deflection current, which is increased in a relatively low frequency mode to achieve full video deflection, may be too high for the output transistor.

One prior horizontal deflection circuit for providing a horizontal retrace time adjustment is contemplated by U.S. Pat. No. 4,705,993 to Shigeru Kashiwagi and entitled Horizontal Deflection Circuit Having A Variable Retrace Period utilizes a switching device which has a drawback of requiring user intervention by necessarily requiring the user to manipulate a switch for intermittently coupling a second capacitor to a first capacitor in a resonant circuit to vary the retrace period.

Accordingly, the present invention is directed to a circuit that generates a retrace time adjustment pulse which is temporally positioned in the center of the flyback pulse, so that a proper blanking signal may be realized.

It is an object of the present invention to provide a horizontal retrace time adjustment pulse generating circuit that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

Another object of the invention is to provide a horizontal retrace time adjustment pulse generating circuitry which eliminates undue stress on the output transistor of a horizontal defection circuit.

Yet another object of the invention is to provide a horizontal retrace time adjustment pulse generating circuit which reduces video noise resulting from retrace lines.

To accomplish these and other objects of the present invention, there is provided a horizontal retrace time adjustment pulse generating circuit, comprising; a microcomputer for outputting a horizontal flyback signal and an analog voltage based on an input horizontal synchronizing frequency; oscillation duty adjusting means for outputting a duty-cycle adjusting signal based on the analog voltage of the microcomputer; a monostable multivibrator for outputting a rectangular wave based on the horizontal flyback signal of the microcomputer and the duty-cycle adjusting signal of the oscillation duty adjusting means; feedback signal detecting means for detecting a horizontal flyback pulse from a horizontal deflection circuit and outputting the detected signal; and signal coupling means for coupling the rectangular wave of the monostable multivibrator and the detected signal from the feedback signal detecting means and outputting a result of the coupling as a time control pulse for horizontal retrace.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary and are intended to provide further explanation of the invention as claimed.

A more complete appreciation of the present invention, and many of the attendant advantages thereof, will become readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:

FIG. 1 is a generalized block diagram of an exemplary display device;

FIG. 2 is a block diagram of the display device including a retrace time adjustment pulse generating circuit according to the present invention;

FIG. 3 is a more-detailed block diagram of the relevant elements of FIG. 2;

FIG. 4 is a circuit diagram of the retrace time adjustment pulse generating circuit according to the present invention; and

FIGS. 5A-5I shows the waveforms of various signals present at respective points of the circuit of FIG. 4.

An exemplary display device, as shown in FIG. 1, comprises a video preamplifier 60 which amplifies a video input by means of a low-voltage amplifier and maintains the amplified signal at a predetermined level; a video main amplifier 70 which amplifies the output from the video preamplifier to 40-60 Vpp and supplies the amplified signal energy for each pixel of a cathode-ray tube (CRT) 80; a microcomputer 20 which receives horizontal and vertical synchronizing signals to produce screen control signals; vertical and horizontal deflection circuits 30 and 40 which receive the screen control signals and generate vertical and horizontal deflection signals for output to a deflection yoke (DY) mounted on the CRT; and a high voltage circuit 50 which supplies the anode voltage for the CRT, using a flyback pulse generated by the horizontal deflection circuit.

The display device of FIG. 2 similar to the device of FIG. 1 and is further provided with a horizontal retrace time adjustment pulse generating circuit 90, according to the present invention, for controlling a retrace time control circuit 100 by using microcomputer output signals derived from the horizontal synchronizing signal. A complete description of the operation and construction of the retrace time control circuit can be found in allowed U.S. patent application Ser. No. 08/785,972, incorporated by reference herein.

Referring to FIG. 3, microcomputer 20.sup.N converts a received horizontal synchronizing signal (H-SYNC) into an analog output using an internal digital-to-analog converter (not shown) and outputs the D/A result. An oscillation duty adjusting portion 91 uses the received output voltage (D/A) to adjust the oscillation duty of a monostable multivibrator 92 which generates a square wave signal based on a horizontal flyback signal (HBF) output by microcomputer 20.sup.N. Horizontal deflection circuit 40.sup.N outputs an amplified horizontal drive signal and a feedback signal detecting portion 93 detects a flyback pulse from horizontal deflection circuit 40.sup.N. A signal coupling portion 94 couples the rectangular wave output signal from monostable multivibrator 92 and the output signal of feedback signal detecting portion 93.

In the detailed construction of the above circuit, as shown in FIG. 4, the oscillation duty adjusting portion 91 is comprised of an operational amplifier U1 which receives, via an input resistor R1, the analog output voltage (D/A) from microcomputer 20.sup.N at its inverting input terminal and compares it with a reference voltage, from a zener diode D5, applied at its non-inverting input terminal. A capacitor C1 is connected between the inverting and non-inverting input terminals of operational amplifier U1. A feedback resistor R2 is connected between an output terminal and the inverting input terminal of operational amplifier U1. An output resistor R3, which limits the output voltage of operational amplifier U1, is connected between the output terminal of operational amplifier U1 and to a node connecting a resistor R4 to an anode of a circuit protection diode D4 which prevents reverse current flow. Resistor R4 is further connected to a supply voltage Vcc. A cathode of diode D4 is connected to monostable multivibrator 92 directly and via a capacitor C2. Capacitor C2 is an external oscillation capacitor, and together with the direct input from diode D4, determines the duty cycle of the rectangular wave output of monostable multivibrator 92.

Horizontal output circuit 40.sup.N is comprised of a horizontal driving transistor Q1 having a base for receiving a horizontal drive signal an emitter connected to a ground terminal and a collector connected to an output of retrace time control circuit 100. The collector provides a horizontal deflection signal to a deflection yoke DY. A damping diode D3 and a capacitor C3 are connected in parallel between the collector and the ground terminal. A deflection transformer TI and a S-correction capacitor C4 are connected in series between deflection yoke DY and the ground terminal.

Feedback signal detecting portion 93 is comprised of voltage-dividing resistors R6 and R7 which divide an induction voltage from the secondary coil of deflection transformer TI, a rectifying diode D1, and a reference point diode D2.

Signal coupling portion 94 is comprised of an input transistor Q2 having a first load resistor R8 connected between its collector and supply voltage Vcc. The base of transistor Q2 receives the detected feedback signal from feedback signal detecting portion 93. An inverter U2 is connected to the collector of transistor Q2 for inverting the signal input therefrom. An AND gate U3 receives the inverted output of inverter U2 at one input terminal and another input terminal of AND gate U3 is connected to the Q output terminal of monostable multivibrator 92 and performs an AND operation on the received inputs. An output of AND gate U3 is provided to the gate of an output transistor Q3 having a second load resistor R9 connected between its drain and supply voltage Vcc and the source of transistor Q3 is connected to a the ground terminal. The drain of transistor Q3 supplies a retrace time adjustment pulse to retrace time adjustment circuit 100.

The operation of the circuit shown in FIG. 4 will now be explained in conjunction with the waveforms of FIGS. 5A-5I. At point a, the HFB signal, FIG. 5A, output from microcomputer 20.sup.N is applied to monostable multivibrator 92 at input terminal B. Meanwhile, the D/A output signal of microcomputer 20.sup.N is applied to the inverting input of the operational amplifier U1 of oscillation duty adjusting portion 91 via resistor R1. Thus, for example, if the input voltage to operational amplifier U1 is high the output voltage thereof is low, and the low output at point b, FIG. 5B, increases the duty cycle of the output of monostable multivibrator 92 at point c, FIG. SC. Likewise, the converse is true for a low input voltage to the operational amplifier U1.

In horizontal deflection circuit 40.sup.N, a horizontal driving signal is amplified and induced at point d at the output of the deflection transformer T1 as a horizontal flyback pulse, FIG. 5D, which is applied to rectifying diode D1 of feedback detecting portion 93 through voltage dividing resistors R6 and R7. Here, negative components are removed and the rectified signal is applied to the base of transistor Q2 of signal coupling portion 94 with respect to reference diode D2. The applied signal is amplified and appears at point e, FIG. 5E, and is input to inverter U2 where it is inverted to get the waveform of FIG. 5F at point f.

The two waveforms applied to the AND gate U3, that is, the rectangular waveform of FIG. 5C at point c output from the Q output terminal of monostable multivibrator 92 and the inverted output waveform FIG. 5F at point f from inverter U2, are AND-operated, resulting in the synthesized waveform of FIG. 5G at point g, which is then amplified and inverted by output transistor Q3 as shown in FIG. 5H at point h. The final output waveform is accurately centered in the flyback pulse shown in FIG. 5I and can be employed as a retrace time adjustment pulse.

The illustrated dotted lines indicate how a change in the output, FIG. 5B, of oscillation duty adjusting portion 91 influence the output, FIG. 5C, of monostable multivibrator 92 and consequently, the respective outputs, FIGS. 5G and 5H, of AND gate U3 and output transistor Q3.

As above, there is provided a horizontal retrace time adjustment pulse generating circuit in which an operational amplifier uses a microcomputer output voltage to adjust the duty cycle of a monostable multivibrator, and a pulse-detected deflection signal is compared with the adjusted rectangular wave output, to thereby generate a retrace time adjustment pulse which is temporally centered in the flyback pulse.

It will be apparent to those skilled in the art that various modifications can be made in the horizontal retrace time adjustment pulse generating circuit of the present invention without departing from the spirit of the invention. Thus, it is intended that the present invention cover such modifications as well as variations thereof within the scope of the appended claims and their equivalents.