JPH0641278U - Focus control device - Google Patents

Abstract

(57) [Abstract] [Purpose] It is possible to prevent the focus shift due to the change of the level of the input video signal and reproduce the clear image. A video level detection circuit 13 for detecting the level of an input video signal displayed on a CRT picture tube 1, and a focus control voltage for the picture tube 1 according to the level of the input video signal based on the detection level of the detection circuit 13. And a focus correction unit 14 that variably corrects.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a focus control device that corrects a focus control voltage of a CRT picture tube.

[0002]

[Prior art]

 Conventionally, a monochrome or color CRT picture tube such as a viewfinder of a video camera or a television receiver is provided with a focus electrode (grid) in order to focus an electron beam and focus it on a phosphor screen. Then, when a focus control voltage of an appropriate magnitude is applied to the focus electrode, the electron beam focuses the electron beam in a spot shape on the phosphor screen by the action of the electron lens. The focus control voltage is usually formed by dividing the anode voltage.

[0003]

[Problems to be solved by the device]

 Due to the characteristics of this type of CRT picture tube, if the level of the input video signal to be projected fluctuates, the anode current fluctuates and the anode voltage changes, which changes the characteristics of the electron lens and changes the focus state. Fluctuates. Therefore, even if the focus control voltage is set so that the optimum focus state is achieved at an appropriate brightness level of the input video signal, for example, a bright screen (white screen) and a dark screen (black screen) are out of focus, and a clear image is displayed. There is a problem that it cannot be reproduced.

It is an object of the present invention to prevent a shift in focus due to a change in the level of an input video signal so that a clear video can be reproduced.

[0005]

[Means for Solving the Problems]

To achieve the above object, in the focus control device of the present invention, a video level detection circuit for detecting the level of an input video signal projected on a CRT picture tube, and the input based on the detection level of the detection circuit. And a focus correction unit that variably corrects the focus control voltage of the picture tube according to the level of the video signal.

[0006]

[Action]

 In the focus control device of the present invention constructed as described above, the brightness of the screen of the picture tube based on this video signal is detected from the level of the input video signal by the video level detection circuit.

Further, based on the detection level of the video level detection circuit, the focus correction unit variably corrects the focus control voltage according to the brightness (luminance) of the screen. With this variable correction, the shift of the focus due to the change in the level of the input video signal is prevented, and a clear video can be reproduced.

[0008]

【Example】

 One embodiment will be described with reference to FIG. FIG. 1 shows the configuration when applied to a black and white CRT picture tube of a viewfinder circuit of a video camera. The monochrome input video signal of the monitor terminal 2 displayed on the CRT picture tube 1 is input to the synchronizing circuit 3 and the video circuit 4. Supplied in parallel.

Then, the synchronizing circuit 3 separates and extracts the vertical synchronizing signal and the horizontal synchronizing signal of the input video signal, the vertical synchronizing signal is shaped into a sawtooth wave for vertical deflection by the vertical oscillating circuit 5, and then the vertical output circuit. It is amplified by 6 and supplied to the deflection yoke 7 of the picture tube 1. The horizontal synchronizing signal of the synchronizing circuit 3 is converted into an oscillating signal for horizontal deflection by the AFC / horizontal oscillating circuit 8 and then amplified by the horizontal output circuit 9 and supplied to the deflection yoke 7.

Further, the horizontal output circuit 9 forms a pulse during the horizontal blanking period and supplies the pulse to the high voltage circuit 10. The high voltage circuit 10 boosts the input pulse by a flyback transformer and then rectifies it to generate a direct current of several KV. A high voltage anode voltage is formed and this anode voltage is supplied to the anode terminal A of the picture tube 1. The flyback transformer of the high voltage circuit 10 has a plurality of taps, and a screen voltage obtained by dividing the anode voltage from a suitable tap to the screen electrode G2, the focus electrode G3, and the cathode K for electron acceleration of the picture tube 1. , Focus voltage and cathode voltage are supplied.

On the other hand, the input video signal supplied to the video circuit 4 is subjected to processing such as level clamping and amplification by the video circuit 4, and then is supplied to the control electrode G1 of the picture tube 1 via the video output circuit 11. Supplied. In this embodiment, the input video signal is supplied to the control electrode G1 to make the cathode K a constant voltage. However, the amount of electrons (luminance) from the cathode K causes the relative potential difference between the electrode G1 and the cathode K. Since the input video signal is supplied to the cathode K and the control electrode G1 has a constant voltage, the same result can be obtained.

Further, the input video signal via the video circuit 4 is also supplied to the video level detection circuit 13 of the focus control device 12, and this detection circuit 13 extracts, for example, an AC component of the input video signal and outputs the sync chip. The level (luminance) of the input video signal is detected from the level, and a detection signal proportional to the detection level is supplied to the time constant circuit 15 of the focus correction unit 14.

The time-constant circuit 15 integrates the supplied detection signal with an appropriate time constant to smooth its waveform fluctuation, and detects, for example, an input video signal which alternates between white and black. Prevents sudden changes in the signal. Then, the detection signal via the time constant circuit 15 is supplied to the focus control signal generation circuit 16, which generates a binary correction control signal for focus control voltage variable correction based on the detection signal.

That is, the focus control voltage required for optimum focusing becomes high and low when the level of the input video signal becomes high, based on the variation of the anode voltage according to the level change of the input video signal. When the voltage becomes low, the generation circuit 16 uses, for example, a detection signal corresponding to APL 50% as a reference signal, and when the input video signal is 50% or more of APL based on the comparison between this signal and the actual detection signal, When the level of the input video signal is less than 50% of the APL, the correction control signal which becomes the high level is generated.

Then, the correction control signal is supplied to the base of the switching transistor 17, and the transistor 17 is turned off at the low level and turned on at the high level of the correction control signal. By the way, the transistor 17 has its emitter grounded and its collector connected to the focus electrode G3 via the resistor 18.

The focus control voltage of the high voltage circuit 10 is supplied to the focus electrode G3 from a sliding piece of a variable resistor 19 for voltage adjustment provided between the high voltage circuit 10 and the ground. If the input video signal is a bright-screen signal with APL of 50% or more, the transistor 17 is turned off, so that the focus control voltage is divided by only the variable resistor 19 and supplied to the focus electrode G3.

Next, if the input video signal is a dark screen signal with an APL of less than 50%, the transistor 17 is turned on, and the resistor 18 is connected in parallel to the resistor between the sliding piece of the variable resistor 19 and the ground. The focus control voltage that is connected and supplied to the focus electrode G3 is lower than that in the bright screen.

Then, the variable resistor 19 is adjusted in advance so that the focus control voltage of the focus electrode G3 becomes an optimum voltage for an input video signal of 75% APL, and the resistor 18 is set to an input video signal of 25% APL, for example. On the other hand, the focus control voltage of the focus electrode G3 is set to be optimum.

Therefore, the focus control voltage of the picture tube 1 is variably corrected to high and low according to the level of the input video signal, the focus shift due to the level change of the input video signal is corrected, and clear video reproduction can be performed. Although the focus control voltage is variably corrected by the switching of the transistor 17 in the above-described embodiment by switching the so-called two-stage, for example, a plurality of sets of the transistor 17 and the resistor 18 are provided to switch and correct the focus control voltage in multiple stages. Alternatively, instead of the transistor 17 and the resistor 18, an electronic variable resistor may be provided to continuously variably correct the focus control voltage.

Of course, if multi-step switching correction or continuous variable correction is performed, the accuracy of defocus correction can be further improved. Further, in the above embodiment, the present invention is applied to the focus control of the black and white CRT picture tube, but it is needless to say that it can be applied to the focus control of the color CRT picture tube.

[0021]

[Effect of device]

 Since the present invention is configured as described above, it has the following effects. The image level detection circuit 13 detects the brightness of the screen of the CRT picture tube 1 based on the level of the input video signal based on this video signal, and the focus correction unit 14 determines the brightness of the screen based on the detected level. Since the focus control voltage is variably corrected by this, it is possible to correct and prevent the focus shift of the picture tube 1 due to the level change of the input video signal, and clear image reproduction without focus shift can be performed.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of a focus control device of the present invention.

[Explanation of symbols]

 1 CRT picture tube 13 video level detection circuit 14 focus correction unit

Claims (1)

[Scope of utility model registration request] 1. A video level detection circuit for detecting the level of an input video signal projected on a CRT picture tube, and a focus control voltage for the picture tube according to the level of the input video signal based on the detection level of the detection circuit. A focus control device including a focus correction unit that variably corrects.