SU1095450A1 - Device for automatic control of brightness and contrast of television picture - Google Patents

Abstract

DEVICE FOR AUTOMATIC ADJUSTMENT BRIGHTNESS AND CONTRAST TELEVISION PICTURES, comprising series-connected unit adjustable fixing, the input of which is an input video signal, the output of the video amplifier whose output is connected to the cathode of the cathode ray tube, a minimum value detector, a peak value detector whose inputs are combined into outputs Connected to the inputs of the subtraction unit, characterized in that, in order to increase the accuracy of automatic adjustment of brightness and contrast and television image when changing the sweep speed in a wide range, a backlight pulse generator, a pulse width modulator and a backlight pulse amplifier are inserted, connected in series, the output of the backlight pulse amplifier is connected to a cathode ray modulator, the second input of a pulse width modulator is connected with the output of the subtraction unit, and the minimum value detector input is combined with the input of the adjustable latching unit, and its output (L is connected to the care of the control unit uliruemoy fixation.

Description

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The invention relates to radio engineering and can be used in applied television (TV) systems, such as scanning electron microscopes (SEM).

A device for automatically adjusting the brightness for a cathode-ray tube (CRT) with electromagnetic control is known, which provides for adjusting the brightness of a light spot of a CRT depending on the scanning speed, which contains two differentiating amplifiers connected by outputs through squares to the inputs of the adder. The output of the adder is connected via an amplifier to a CRT modulator. The amplifier is covered by a negative feedback circuit containing a switchable precision voltage divider. Signals proportional to line and frame deflection currents 1 are applied to the inputs of differentiating amplifiers.

However, the device is characterized by low accuracy of the analog squaring blocks used in the device, which reduces the accuracy of adjusting the brightness and contrast of the TV image when the scanning speed is changed, as well as the difficulty of manufacturing a precision switchable negative feedback circuit.

A device is known for automatically stabilizing the brightness of a light spot of a cathode ray tube, which contains a CRT, the control inputs of which are connected to the outputs of a video amplifier and a pulse width modulator. The CRT screen through the lens and the beam splitter is connected to the photodetector. The output of the photodetector is connected via an integrator to the first input of the comparator, the second input of which is connected to the source of the reference voltage, and the output is connected to the control input of the pulse-width modulator 2.

However, this device cannot be used to automatically adjust the brightness and contrast of the image, because it has an optical feedback circuit consisting of a lens, a beam splitter, a photodetector. The feedback signal passing through these blocks will have low contrast, since only the lens reduces the contrast by 50-100 times. Consequently, the total luminance adjustment error will be quite large, and the accuracy of exposure control will be low.

In addition, the device is a closed automatic control system. Such systems are subject to self-excitation, in connection with which the phase characteristics of the device are subject to very strict requirements, which complicates its technical implementation. The image obtained on a CRT screen enters the recipient of visual information through a lens and a beam splitter, which drastically reduces

the brightness and contrast of the perceived image is not acceptable for electron microscopes.

The closest to the invention to the technical essence is a device for automatically adjusting the brightness and contrast of a TV image, comprising a series-connected adjustable fixation unit, whose input is a video signal input, an output video amplifier whose output is connected to a cathode of a cathode ray tube, a minimum value detector a maximum value detector, whose inputs are combined, and the outputs are connected to the inputs of subtractor 3.

However, the device uses an automatic back regulation circuit. Thus, there is a closed automatic control system that is subject to self-excitation. This imposes strict requirements on the phase characteristic of the device, which significantly complicates its circuit design.

In addition, the use of feedback control imposes an increased requirement on the dynamic range of all device nodes covered by feedback. At the same time, their dynamic range should be several times larger than with direct control.

The purpose of the invention is to improve the accuracy of automatic adjustment of the brightness and contrast of a television image when changing the scanning speed in a wide range.

The goal is achieved in that a device for automatically adjusting the brightness and contrast of a television image, comprising a series-connected adjustable latching unit whose input is an input of video signals, an output video amplifier whose output is connected to the cathode of a cathode ray tube, a minimum value detector and a maximum value detector, the inputs of which are combined, and the outputs are connected to the inputs of the subtraction unit, a backlight pulse generator is introduced, a pulse width the backlight pulse amplifier and amplifier are connected in series, the output of the backlight pulse amplifier is connected to a modulator of the electron-beam tube, the second input of the pulse-width modulator is connected to the output of the subtraction unit, and the minimum value detector input is combined with the input of the adjustable fixation unit, and the output is connected to the control input of the adjustable latching unit.

FIG. Figure 1 shows a structural electrical circuit for automatically adjusting the brightness and contrast of a TV image; on the flg. 2 - time diagrams of voltages.

The device for automatically adjusting the brightness and image contrast (Fig. 1) contains a minimum video signal detector 1, a maximum video signal detector 2, an adjustable latching unit 3, a subtraction unit 4, an output video amplifier 5, a pulse-width modulator 6, a backlight pulse generator , amplifier 8 backlight pulses, electron beam tube 9 (CRT).

The device works as follows.

The input video signal (Fig. 2a) is fed to the inputs of the detectors 1 and 2 of the minimum and maximum values of the signal, as well as to the block 3 of adjustable latching (Fig. 1). The minimum and maximum values of the video signal allocated in the detectors 1 and 2 (Fig. 26, c) are received at the corresponding inputs of the subtraction unit 4, the output of which is a signal proportional to the swing of the input video signal (Fig. 2e).

In block 3 of the adjustable fixation, the minimum value of the video signal is fixed to the level Uo, which corresponds to the black image (Fig. 2d). After amplification in the output video amplifier 5, it is applied to the cathode of the CRT 9 and modulates the electron beam in intensity. In the pulse-width modulator 6, the modulation is performed in duration in accordance with the magnitude of the output signal amplitude of the backlight pulse subtraction unit 4 (Fig. 2e), which comes from the backlight pulse generator 7.

GW-modulated illumination pulses are amplified in amplifier 8, fed to a CRT modulator 9, and unblocked by a time equal to the duration of the illumination pulse.

As is known, brightness B of each image element on the screen of a CRT 9 is determined by the luminescence time of this element C and the beam current, which, in turn, depends on the size of the video signal corresponding to this element.

B f (U, €,),

In the interval TC-2Ti. (Fig. 2d) the brightness of the darkest (black) element corresponds to the size of the video signal Uo, and the brightness

the brightest - the size and .. At the same time

9 is selected in this way (Fig. 2e),

to get the maximum possible

5 in this CRT image contrast.

Suppose that in the time span preceding (N + l) Ti5, the input signal swing decreased (Fig. 2a). However, the darkness of the darkest element

0 does not change, since the minimum value of a video signal during a frame is fixed to a given UQ level, the same as at TK-2Tjj (Fig. 2d). The brightness of the brightest element will be less

5 (Fig. 2d). However, the brightness and contrast of the image on the CRT 9 screen does not change, since the reduction of the video signal swing is compensated by an increase in the duration of the interaction of the beam with the phosphor ta (the time interval

0 - (N- | -) jjj on -fig. 1e) is proportional to the control signal (Fig. 1e).

When changing the scanning speed, the time of addressing to each element of the image changes. However, the brightness of the image on the CRT screen 9 will be constant, since the duration of the backlight pulse is determined only by the sweep of the input video signal and is limited by the maximum interaction time fa at the highest decomposition rate 0. Thus, at the same time, the image brightness is stabilized at any scanning speed at the brightness level corresponding to the fastest scan.

5 The use of the present invention provides high accuracy of automatic adjustment of the brightness and contrast of a TV image, regardless of the scanning speed, while the brightness and contrast remain unchanged and are optimal for observation as well as for photo registration. The combination of automatic regulation and stabilization. The brightness and contrast of the TV image when changing the scanning speed reduces the cost of the investment.

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Claims (1)

DEVICE FOR AUTOMATIC ADJUSTMENT OF BRIGHTNESS AND CONTRAST OF A TELEVISION IMAGE, containing a series-connected block of adjustable fixation, the input of which is an input of video signals, an output video amplifier, the output of which is connected to the cathode of a cathode ray tube, a minimum value detector, which have a maximum value output, inputs, connected to the inputs of the subtraction unit, characterized in that, in order to improve the accuracy of automatic control of the brightness and contrast of the body when changing the scan speed over a wide range, a backlight pulse generator, a pulse-width modulator and a backlight pulse amplifier are introduced, connected in series, while the output of the backlight pulse amplifier is connected to the cathode ray tube modulator, the second input of the pulse-width modulator is connected to the output of the subtraction unit and the input of the minimum value detector is combined with the input <q of the adjustable fixation unit, and its output is connected to the control input of the adjustable fi sation. SU.,., 1095450 FIG. 1>