JP2000341556A - Circuit and method for adjusting image signal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce costs by simplifying the configuration of a circuit regarding the generation of a reference signal for an adjustment circuit at an input direct current level of a cathode-ray tube. SOLUTION: This circuit has a current detection circuit 4, which detects current made to flow in a display device (cathode-ray tube 100), a duty factor changing circuit (e.g., an output circuit of a signal oscillator 3 for on-screen) which inputs a prescribed signal, changes the duty factor and outputs it to the display device, and a control circuit 6 which changes the control parameters of an image signal applied to the display device, on the basis of the detected results of current caused to flow in the display device on the basis of an output signal of the duty factor changing circuit. The circuit 6, for instance, compares the detection value of current smoothed by an integration circuit 5 with initial value detected in advance and changes the amplification degree or direct current voltage level of an amplifier circuit 2 on the basis of the results.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an adjustment circuit for adjusting the amplitude of an image signal input to a display device such as a cathode ray tube or the cutoff level of an electrode, and a method of adjusting the adjustment circuit.

[0002]

2. Description of the Related Art A color cathode ray tube (CRT) is provided with three independent electron guns corresponding to three colors of red (R), green (G), and blue (B). A beam current corresponding to the input signal flows from the anode provided to the cathode of each electron gun. The beam current changes its current value due to the temperature drift or aging of the CRT or the circuit even when the same input signal is applied. If the balance of the beam current of each electron gun with respect to the input signal is lost, correct colors cannot be reproduced. In addition, various input / output devices for video signals have slightly different color expressions, and there are cases where it is desired to reproduce an image close to the original by adjusting the balance of each beam current from the input signal of the CRT. This balance adjustment of each beam current with respect to the input signal is called “white balance adjustment” because it reproduces a beautiful white color without bluish or reddish tint. In the white balance adjustment, the DC voltage level (cutoff level) and the amplitude level (gain) of the input signal are adjusted for each electron gun.

[0003] The white balance is usually adjusted at the time of factory shipment of a monitor device having a built-in CRT, and is automatically or manually adjusted at hand after long use by a user. The monitor device includes a CRT white balance adjustment circuit in a housing. Of the white balance adjustments, the adjustment of the cutoff level is based on inputting a signal close to the black level, detecting the beam current at that time, and providing feedback so that the cutoff level is changed according to the detected beam current amount. You just need to do it. However, a beam current due to a signal close to the black level is weak, and noise is easily picked up from a high voltage used in a deflection circuit or the like. For this reason, in the conventional cutoff level adjustment, a calibration video signal (usually referred to as a cutoff level adjustment reference signal) of about 20% (20 IRE) of the peak value of the video signal level is used as an input signal.

FIG. 7 shows a portion related to cutoff level adjustment together with a CRT in a conventional white balance adjustment circuit. 3 corresponding to R, G, B on CRT100
Cathode 101R, 101G, 101B of book electron gun
And an anode 102. A switch, an amplifier having a cutoff level adjusting function, and an output transistor (emitter follower) are connected between the input terminals of the video signals R, G, and B and each cathode. That is, the emitter of the PNP transistor Q (R) is connected to the cathode 101R, and the switch 103 is sequentially connected to the base of the PNP transistor Q (R) from the video signal R input side.
(R) and the amplifier 104 (R) are connected in series. Similarly, a PNP transistor Q is connected to the cathode 101G.
(G) is connected to the PNP transistor Q
At the base of (G), a switch 103 (G) and an amplifier 104 (G) are connected in series from the video signal G input side. The emitter of a PNP transistor Q (B) is connected to the cathode 101B, and a switch 103 (B) and an amplifier 104 (B) are connected in series to the base of the PNP transistor Q (B) from the video signal B input side. Have been.

[0005] Each of the switches 103 (R), 103 (G),
103 (B) shows a reference signal S for cutoff level adjustment.
_{The BK} generation circuit 105 is connected so that the video signal can be switched to the reference signal _SBK .

A detection resistor R1 is connected between the collector of the PNP transistor Q (R), Q (G) or Q (B) and the ground potential. A beam current detection circuit 10 is provided at a connection point between the PNP transistor Q (R) and the detection resistor R1.
6 (R) is connected. Similarly, a beam current detection circuit 106 (G) is connected to a connection point between the PNP transistor Q (G) and the detection resistor R1. Also, PNP
At the connection midpoint between the transistor Q (B) and the detection resistor R1,
The beam current detection circuit 106 (B) is connected.

The amplifiers 104 (R), 104 (G), 10
4 (B), beam current detection circuits 106 (R), 106
(G), 106 (B) and reference signal generation circuit 105
Is controlled by the microcomputer 107.

In the cutoff adjusting circuit having such a configuration, each of the amplifiers 104 (R), 104 (G), 1
It is assumed that the cutoff level of 04 (B) is held at the value after white balance adjustment at the time of shipment from the factory. When the cut-off level adjustment by the user is instructed, under the control of the microcomputer 107, the reference signal from the reference signal generating circuit 105 of the brightness degree 20 IRE S _BK switch 103 (R), 103 (G ) and 103 (B ) And are switched to video signals R, G, B. This signal switching is usually performed at a timing when a certain time has elapsed after the end of the vertical blanking period. The reference signal _SBK is _supplied to the amplifier 104 (R), 104 (G) or 1
04 (B) and PNP transistors Q (R), Q (G)
Alternatively, the voltage is amplified by the DC voltage level at the time of shipment from the factory by Q (B), and the cathodes 101R, 1 of the corresponding electron gun are amplified.
01G, 101B And a beam current flows through the CRT. The beam current corresponding to each of R, G, and B is converted into a voltage by the corresponding detection resistor R1, and
It is input to the beam current detection circuit 106 (R), 106 (G) or 106 (B). Beam current detection circuit 106
In (R), 106 (G), and 106 (B), the timing is controlled by the microcomputer 107, the beam current is sampled and held, the beam current value is read, and after being A / D converted as necessary. Are sent to the microcomputer 107. Microcomputer 10
7 generates a cutoff level adjustment signal Sc based on the detected value of the beam current (or causes the beam current detection circuit to generate the cutoff level adjustment signal Sc), and outputs the signal to the amplifiers 104 (R) and 104.
(G) and 104 (B) to adjust the cutoff level.

In the case of gain adjustment, a white level reference signal _SW is generated by the reference signal generation circuit 105, a beam current is detected by the white level reference signal SW, and the gain adjustment signal _SG is converted to a microcomputer based on the detected value. The outputs of the amplifiers 107 balance the amplification degrees of the three amplifiers 104 (R) to 104 (B).

In the case of automatic adjustment, for example, the microcomputer 107 compares the detected beam current value with a reference level, and determines a cutoff level adjustment amount and a gain adjustment amount according to the comparison result. In the case of manual adjustment, the monitor screen is measured by an image measuring device, and the cutoff level and the gain are adjusted to optimal values based on the measurement result.

[0011]

When the conventional cutoff level adjusting circuit is used, the switch 103 (R),
It is necessary to incorporate a reference signal generation circuit 105 in addition to 103 (G) and 103 (B), or to provide a reference signal S _BK (and _SW ) from the outside. This reference signal S _BK (and S _W ) must be one in which the level and the low level are accurately set. Therefore, there is a disadvantage that the configuration of the reference signal generation circuit 105 is complicated and the price of the display device is accordingly increased. In addition, it is troublesome for the user to prepare the reference signal from the outside, and there is a disadvantage that the reference signal generator and the like are expensive.

An object of the present invention is to provide an image signal adjustment circuit which can simplify the circuit configuration and reduce the cost, and an adjustment method therefor.

[0013]

An image signal adjusting circuit according to the present invention adjusts an image signal to be applied to a display device in which a current flows according to the voltage value of the applied image signal. An adjustment circuit, a current detection circuit that detects the current, a duty ratio change circuit that inputs a predetermined signal, changes a duty ratio, and outputs the same to the display device, and a signal output from the duty ratio change circuit A control circuit for changing a control parameter of an image signal applied to the display device based on a result obtained when the current detection circuit detects the current based on the control signal.

Preferably, the apparatus further comprises a smoothing circuit for receiving the current from the display device, smoothing the current, and outputting the smoothed current to the current detection circuit. Alternatively, there is further provided a smoothing circuit for smoothing a signal after the current of the display device is detected by the current detection circuit. Preferably, the apparatus further includes a storage circuit for storing the pattern of the predetermined signal or description data relating to the predetermined signal. The storage circuit further stores an initial value of a current generated and detected when the predetermined signal is used in the display device in an initial state. Further preferably, the apparatus further includes an amplifier circuit for amplifying the image signal, wherein the control circuit compares a detected value of the current from the current detection circuit with an initial value of the current read from the storage circuit, and Based on the comparison result, the amplification degree of the amplifier circuit or the DC voltage level of the output is controlled.

Preferably, the duty ratio changing circuit controls the timing of switching the digital signal from the first level to the second level. Preferably, the duty ratio changing circuit lowers the duty ratio such that the DC voltage level decreases to a predetermined level after smoothing while the amplitude of the input signal remains unchanged.

Preferably, the predetermined signal is a signal indicating an on-screen display pattern previously stored in the storage circuit or reproduced based on the description data previously stored in the storage circuit. is there.

The method of adjusting an image signal according to the present invention is a method of adjusting an image signal to be applied to a display device in which a current flows according to an applied voltage value. And outputs a current to the display device to detect a current flowing through the display device, and changes a control parameter (for example, amplification degree or DC voltage level) of an image signal applied to the display device based on the detection result.

In the image signal adjustment circuit and the adjustment method having such a configuration, for example, a so-called black level reference signal is generated by changing a duty ratio of a predetermined signal. As the predetermined signal, for example, a white level reference signal used for gain adjustment in the on-screen display pattern held in the storage circuit can be used. When the display device is a cathode ray tube, first, the duty ratio changing circuit changes the duty ratio and outputs the same while keeping the amplitude of the input predetermined signal. The signal after the change is input to, for example, a cathode ray tube after amplification, and accordingly, a beam current flows for each of R, G, and B colors. The beam current flows out of the cathode of the electron gun of the cathode ray tube and is detected by a current detection circuit. Before and after that, the current is converted into a direct current by a smoothing circuit, whereby a beam current detection value is obtained. The beam current detection value is compared with the initial value of the beam current detected in advance in the initial state such as at the time of shipment from the factory using the same pattern, and the control circuit responds to the comparison result to determine whether or not the image signal is to be subsequently input. The degree of amplification or the DC voltage level is set.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below by taking as an example the adjustment of a DC voltage level (hereinafter, cut-off level) of an electrode of a cathode ray tube.

FIG. 1 is a diagram functionally showing an outline of an image signal adjustment circuit according to an embodiment of the present invention. The image signal adjustment circuit 1 includes a video amplification circuit 2 connected to a cathode 101 of an electron gun of a cathode ray tube (CRT) 100, and an on-screen signal generator 3 for supplying an on-screen video signal to the video amplification circuit 2. , A beam current detection circuit 4 for detecting a beam current flowing from the cathode of the CRT 100, an integration circuit 5 for integrating the detected value of the beam current, a microcomputer 6, and a memory 7.

Here, although not particularly shown, an output circuit capable of changing the duty ratio of the signal is provided at the output stage of the signal generator 3 for on-screen. Circuit ".

The on-screen signal generator 3 operates in a normal operation, for example, in a function setting of a television (setting of brightness, color, etc.) or a function setting of a video cassette recorder (reservation, etc.), such as a character set in advance. This is for generating a signal indicating an on-screen pattern.

Since the on-screen signal is unique to the device and is not changed, the signal for on-screen is usually stored and held in a memory (ROM) in many cases.

FIG. 2 shows an on-screen display IC in which an on-screen signal is held in an internal memory area in advance.
FIG. 3 is a diagram showing a more practical circuit configuration using the above. FIG.
Has a video preamplifier 2a, a video output amplifier 2b, an on-screen display IC 11, an integrating circuit 5, and a central processing unit (CPU) 12.

The video amplifier 2 shown in FIG. 1 is constituted by the two amplifiers 2a and 2b. Two amplifiers 2a and 2b
Are input to the corresponding cathodes 101R, 101G, and 101B of the CRT 100 through the DC cut capacitor C. In FIG. 2, the beam current is first smoothed by the integrating circuit 5, and then sent to an on-screen display IC for detection.

The on-screen display IC 11 includes the on-screen signal generator 3 and the beam current detection circuit 4 shown in FIG.
And a function of the memory 7, and performs various other processes related to on-screen display.

FIG. 3 is a block diagram showing the configuration of the main part inside the on-screen display IC. The inside of the on-screen display IC 11 includes an oscillator 20 and a PLL circuit 2.
1, timing analyzer 22, beam current detection circuit 2
3, display controller 24, RAM 25, character
A font ROM 26 and an I ² C bus interface 27 are provided.

A crystal XT is connected to the input of the oscillator 20, and a reference clock pulse
The signal is output to the LL circuit 21, the timing analyzer 22, and the beam current detection circuit 23.

A capacitor C 'constituting the filter is externally connected to the PLL circuit 21, and an output pulse from the PLL circuit 21 is input to the display controller 24.

The display controller 24 supplies a deflection pulse HF
LY and a vertical synchronization pulse VS are supplied. The display controller 24 outputs gain and offset adjustment signals ROUT, GOUT, and BOUT to the video preamplifier 2a in FIG.

A vertical synchronizing pulse VS and a horizontal synchronizing pulse HS are supplied to the timing analyzer 22.

The integrated beam currents RCI, GCI, and BCI from the integrating circuit 5 (FIG. 2) are added to the beam current detecting circuit.
Is entered.

The I ² C bus interface 27 has a CP
The control signal (system data SD) from U12 (FIG. 2)
A) and a system clock signal SCL for synchronizing between other ICs. The I ² C bus interface 27, the timing analyzer 22, the beam current detection circuit 23, the display controller 24, the RAM 25, and the character / font ROM 26 are connected so that data can be exchanged in both directions.

In the RAM 25, for example, data describing an instruction for determining an on-screen pattern is written. In the character font ROM 26, for example, the on-screen pattern itself is written and held.

In the present embodiment, white balance adjustment is performed by utilizing the function of the on-screen display IC having such a configuration or the function of the on-screen signal generator of FIG. Hereinafter, an operation example of the circuit according to the present embodiment will be described with reference to the configurations illustrated in FIGS. 2 and 3. Here, FIG. 4 is a waveform diagram of the reference signal.

In the gain adjustment, the character font R
Among on-screen signals that are stored in advance in the OM 26 or reproduced based on the description data in the RAM 25, a predetermined character signal whose white level repeatedly appears at a predetermined cycle, for example, is used as a so-called white level reference signal. FIG. 4A shows an example of the white level reference signal.
Under the control of the CPU 12, for example, the white level reference signal is read from the ROM 26 and
4 to the video preamplifier 2a. The white level reference signal amplified by the video preamplifier 2a is further amplified by the video output amplifier 2b and applied to the cathodes 101R to 101B of the CRT 100 via the capacitor C. Then, in the CRT 100, a beam current corresponding to the white level reference signal is supplied to the anode 1 for each of the RGB colors.
02 flows to the cathodes 101R to 101B.

Each of the beam currents is smoothed by the integrating circuit 5 and converted into a DC voltage, and then input to the on-screen display IC 11. On-screen display IC
Inside 11, the input integrated beam current is detected by the beam current detection circuit 23, whereby a detected value of the beam current is obtained. The detected value of the beam current is
The data is sent to the CPU 12 through the I ² C bus, and is compared with an initial value of the beam current at the time of shipment from a factory stored in, for example, an external nonvolatile memory (EEPROM or the like). In accordance with the comparison result, a gain control signal is output from the display controller 24 to the video preamplifier 2a, whereby the gain is adjusted while maintaining a balance among the R, G, and B colors.

FIG. 5 is a graph showing the relationship between the input signal level of the CRT and the beam current. In the figure, a curve (0) represents the time of adjustment at the factory (initial value), a curve (1) represents a case where the cut-off level drifts due to a change with time, and a curve (2) represents the gain of the amplifiers 2a and 2b and the electron of the CRT 100. This shows a case where the emission current value of the gun has decreased and the white level (high level) of the input signal has been returned to the initial value by performing gain adjustment by the method described above.

If the cut-off level changes, the high-level current V _H also changes with the change. If the current flowing at the high level is made equal to the initial value by the above-described method, theoretically, The cutoff level also returns to the initial value. For example, in the case where the cutoff level simply drifts as shown by the curve (1) in FIG. 5, the curve (1) substantially overlaps the curve (0) only by adjusting the high level, that is, by adjusting the gain. And white balance adjustment including the cutoff level.

However, in practice, the gain of the amplifier may decrease, or the emission current may decrease due to the deterioration of the electron gun. In such a case, if the high level _VH is simply adjusted, the curve (2) in FIG. ), The cutoff level may be shifted.

Therefore, it is necessary to adjust the cutoff level. In this adjustment, it is originally desired to adjust the beam current when there is no signal to near zero. However, as shown in FIG. 6 (A), when noise is added to the zero level and swings positively and negatively, the average value is obtained by the beam current detection circuit that detects only the positive level. In the case of FIG. 6B where is a negative value, the beam current value is detected to be zero as in the case of FIG. 6A. Therefore, conventionally, as shown in FIG.
The cutoff level was adjusted using a black level signal of 0% (30 IRE).

On the other hand, in the present invention, as shown in FIG. 4C, a black level reference signal having the same amplitude as that of the white level reference signal and reduced only in the duty ratio is generated from the white level reference signal and cut off. Used for level adjustment. Hereinafter, a method of generating the black level reference signal and a method of adjusting the cutoff level will be described with reference to FIGS.

Under the control of the CPU 12, in the on-screen display IC 11, among the on-screen signals stored in advance in the character font ROM 26 or reproduced based on the description data in the RAM 25, the white level reference signal A character signal to be read is read out from, for example, the ROM 26 and output to the timing analyzer 22. The timing analyzer 22 corresponds to the “duty ratio changing circuit” of the present invention, and generates the black level reference signal by reducing the duty ratio of the white level reference signal to, for example, 1/50. Specifically, a black level reference signal having a waveform as shown in FIG. 4C is generated by changing the on / off timing of the white level reference signal. The generated black level reference signal is output to the video preamplifier 2a via the display controller 24. The black level signal amplified by the video preamplifier 2a is further amplified by the video output amplifier 2b,
Is applied to the cathodes 101R to 101B of the CRT 100. Then, in the CRT 100, R, G, B
For each of the colors, a beam current corresponding to the black level reference signal flows from the anode 102 to the cathodes 101R to 101B.

Each beam current is smoothed by the integration circuit 5 and converted into a DC voltage, and then input to the on-screen display IC 11. On-screen display IC
Inside 11, the input integrated beam current is detected by the beam current detection circuit 23, whereby a detected value of the beam current is obtained. The detected value of the beam current is
The data is sent to the CPU 12 through the I ² C bus, and is compared with an initial value of the beam current at the time of shipment from a factory stored in, for example, an external EEPROM. Depending on the result of this comparison,
A cutoff level control signal is output from the display controller 24 to the video preamplifier 2a, whereby the cutoff level is adjusted while maintaining a balance among the R, G, and B colors.

In the cutoff level adjusting circuit according to this embodiment, the existing on-screen signal is used as the white level reference signal, and the duty ratio of the white level reference signal is changed to generate the black level reference signal. There is no need to provide a dedicated circuit for generating a reference signal as in the past,
The circuit configuration is simple. That is, since a D / A converter or a gain control amplifier is not used as a signal source, the cost can be reduced and the price can be reduced.

Further, the cutoff level adjusting circuit according to the present embodiment has an advantage that it is resistant to noise.

That is, a D / A converter or a gain control amplifier is used as a signal source for generating a reference signal as in the related art, and the generated signal has some error. Assuming that the reference signal generated from the signal source includes an error of 1% of the amplitude, the white level reference signal having the amplitude of 100% has an error of 1% from the beginning. If the error of the same amplitude is also superimposed during the generation of the black level reference signal, the reference value constantly fluctuates due to the black level reference signal, causing a decrease in cutoff level adjustment accuracy.

On the other hand, in the present embodiment, for example, when the duty ratio of the white level reference signal is reduced to 1/50 to generate the black level reference signal, the DC voltage level becomes 1/50 and the amplitude is reduced. The error of 1% is reduced to 1/50. Therefore, the adjustment accuracy of the cutoff level becomes higher than before.

It should be noted that the white level reference signal may be generated by a reference signal generation circuit in the same manner as in the related art without using an on-screen signal. According to the present invention, the black level reference signal is generated by changing the duty ratio of the white level reference signal in this case as well, so that the circuit load can be reduced, and the accuracy of cutoff level adjustment due to noise can be reduced. I will not invite you.

Further, if a function for changing the width of a character built in for on-screen in units of dots is provided, the duty ratio can be easily changed in units of one dot. For example, by appropriately combining five types of characters having widths of 0, 1, 2, 3, and 4 dots, a pulse can be generated with any width having a minimum unit of 1 dot.

In the above description, the case where the black level reference signal is generated by lowering the duty ratio of the white level reference signal and the cutoff level is adjusted based on the black level reference signal has been described. The present invention is also applicable to a case where a black level reference signal is extracted from a screen signal or the like, a duty ratio thereof is increased to generate a white level reference signal, and the amplification (gain) of the video signal is increased based on the white level reference signal. Included in scope.

[0052]

According to the image signal adjusting circuit and the adjusting method according to the present invention, the predetermined signal of the existing repetition period is used as one of the high-level and low-level reference signals, and the duty ratio is changed. Since the other reference signal is generated, the circuit configuration for generating the reference signal can be simplified. Also, if this predetermined signal is used in advance for the on-screen signal built in the storage circuit, it is not necessary to generate one of the reference signals, and the high-level and low-level reference signals are generated only by the duty ratio changing circuit. The circuit configuration for generating the reference signal can be further simplified. Further, the noise level of the other reference signal generated by changing the duty ratio is reduced, and the accuracy of cutoff level adjustment is increased. As described above, a simple circuit, a low cost, and a highly accurate image signal adjustment circuit can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram functionally showing an outline of an image signal adjustment circuit according to an embodiment of the present invention.

FIG. 2 is a diagram showing a more practical circuit configuration using an on-screen display IC in which an on-screen signal is held in an internal memory area in advance.

FIG. 3 is a block diagram illustrating a main configuration inside an on-screen display IC.

FIG. 4 is a waveform diagram of a reference signal.

FIG. 5 is a graph showing a relationship between an input signal level of a CRT and a beam current.

FIG. 6 is a diagram showing a mode in a case where noise is placed on a zero level of a beam current.

FIG. 7 is a diagram illustrating a configuration of a portion related to cutoff level adjustment together with a CRT in a conventional white balance adjustment circuit.

[Explanation of symbols]

1, 10: image signal adjustment circuit, 2: video amplifier circuit (amplifier circuit), 2a: video preamplifier (amplifier circuit),
2b: video output amplifier, 3: on-screen signal generator, 4, 23: beam current detection circuit (current detection circuit), 5: integration circuit (smoothing circuit), 6: microcomputer (control circuit), 7 ... Memory (storage circuit), 11
... On-screen display IC, 12 ... Central processing unit (control circuit), 20 ... Oscillator, 21 ... PLL circuit, 22
... Timing analyzer (duty ratio changing circuit), 2
4 ... Display controller (control circuit), 25 ... RAM (storage circuit), 26 ... Character font ROM (storage circuit), 27 ... I ² C bus interface.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (reference) H04N 9/73 H04N 9/73 EF term (reference) 5C021 PA02 PA17 PA52 PA58 PA76 RA07 RB03 RC03 SA11 XA02 XA13 XA15 5C058 AA01 BA35 BB11 CA12 CA20 5C066 AA03 BA20 CA01 DA09 EA07 GA01 GA11 KA12 KD02 KE05 LA02

Claims (17)

[Claims] An image signal adjustment circuit for adjusting an image signal and applying the same to a display device in which a current flows in accordance with a voltage value of the applied image signal, wherein the current detection circuit detects the current. A duty ratio changing circuit for inputting a predetermined signal and changing the duty ratio and outputting the same to the display device; and a result when the current detection circuit detects the current based on the signal output from the duty ratio changing circuit. A control circuit for changing a control parameter of the image signal applied to the display device based on the control signal. 2. The image signal adjusting circuit according to claim 1, further comprising a smoothing circuit that receives the current from the display device, smoothes the current, and outputs the smoothed current to the current detection circuit. 3. The image signal adjustment circuit according to claim 1, further comprising a smoothing circuit for smoothing a signal after the current of said display device is detected by said current detection circuit. 4. The image signal adjustment circuit according to claim 1, further comprising a storage circuit for storing the pattern of the predetermined signal or description data relating to the predetermined signal. 5. The adjustment of an image signal according to claim 4, wherein the storage circuit further stores an initial value of a current generated and detected when the predetermined signal is used for a display device in an initial state. circuit. 6. An amplifying circuit for amplifying the image signal, wherein the control circuit reads a current detection value from the current detection circuit from an initial value of a current read from the storage circuit or an external storage means. 6. The image signal adjusting circuit according to claim 5, wherein the control circuit controls the amplification degree or the output DC voltage level of the amplifier circuit based on the comparison result. 7. The image signal adjusting circuit according to claim 1, wherein said duty ratio changing circuit controls a timing of switching a digital signal from a first level to a second level. 8. The image signal adjusting circuit according to claim 1, wherein said duty ratio changing circuit reduces the duty ratio so that the DC voltage level decreases to a predetermined level after smoothing while the amplitude of the input signal remains unchanged. 9. The image signal adjustment circuit according to claim 4, wherein said predetermined signal is a signal indicating an on-screen display pattern held in said storage circuit in advance. 10. The image signal adjustment circuit according to claim 4, wherein said predetermined signal is a signal indicating an on-screen display pattern reproduced based on description data held in said storage circuit in advance. . 11. The image signal adjustment circuit according to claim 1, wherein the display device is a cathode ray tube, and the current is a beam current that flows from the anode to the cathode of the cathode ray tube and can be monitored from an external terminal. 12. A method for adjusting an image signal to be applied to a display device in which a current flows according to an applied voltage value, wherein a predetermined input signal is output to the display device while changing a duty ratio. A method for adjusting an image signal, wherein a current flowing through the display device is detected, and a control parameter of the image signal applied to the display device is changed based on the detection result. 13. The method according to claim 12, wherein the signal after or before the detection of the current is smoothed, and the DC voltage level of the image signal is adjusted according to the signal level after the smoothing. . 14. A method for comparing the detected value of the current with an initial value of a current generated and detected when the predetermined signal is used in a display device in an initial state, and based on the comparison result, the image signal The image signal adjustment method according to claim 12, wherein the degree of amplification or the DC voltage level of the image signal is changed when amplifying the image signal. 15. The image signal adjusting method according to claim 13, wherein in changing the duty ratio, the duty ratio is reduced so that the DC voltage level decreases to a predetermined level after smoothing while the amplitude of the input signal remains unchanged. 16. The image signal adjusting method according to claim 12, wherein the predetermined signal is a signal indicating an on-screen display pattern held in advance therein. 17. The image signal adjusting method according to claim 12, wherein the predetermined signal is a signal indicating an on-screen display pattern reproduced based on descriptive data held therein in advance.