JPH0750916B2 - Spot killer circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention prevents spot burning by blanking the video output when the power source of a television receiver rises and falls. It concerns killer circuits.

[Prior Art] FIG. 3 shows a wiring diagram of a conventional spot killer circuit. In FIG. 3, (1) is a pulse generation circuit, and this pulse generation circuit (1) includes a grounded collector transistor (Tr),
When the base voltage (C) of the transistor (Tr) rises and falls, the resistor (R1) and capacitor (C1) that determine the time constants and the emitter voltage (R) of the transistor (Tr) fall It is composed of a diode (D1) and a capacitor (C2) that hold a specified voltage, and the collector output voltage (D) of the transistor (Tr) goes high when the voltage (A) of the power supply (12) rises and falls. And output. That is, a pulse is generated.

(2) is a signal processing IC for converting a video signal (hereinafter referred to as Y signal) and a chroma signal (hereinafter referred to as C signal) into R, G, and B
The signal is demodulated into the respective signals of Y, the Y signal input terminal (7), the C signal input terminal (8), the R, G, B signal output terminals (14), (15), (16), and blanking. An input terminal (3), a contrast control voltage input terminal (6) and a power supply voltage input terminal (5) are provided. (4) is a blanking pulse output terminal from a flyback transformer (hereinafter referred to as FBT) (not shown), which is the signal processing IC (2).
Is connected to the blanking input terminal (3).

(9) is a cathode ray tube (hereinafter referred to as CRT), (10) is a contrast control voltage input terminal, and (11) is an ACL control voltage input terminal.

Next, the operation of the above configuration will be described.

When the power supply (12) is turned on, the power supply voltage (A) rises as shown by the solid line in FIG. 4 (a). At the same time, the emitter voltage (B) of the transistor (Tr) of the pulse generation circuit (1) rises as shown by the dotted line in FIG. 4 (a), while the base voltage (C) of the transistor (Tr) changes. Due to the time constant of the resistor (R1) and the capacitor (C1), it rises slowly as shown by the one-dot chain line in Fig. 4 (a). Therefore, the transistor (Tr) continues to be turned on until the rise of the base voltage (C) is completed, and the collector output voltage (D) of the transistor (Tr) becomes high as shown in FIG. 4 (b). That is, a pulse is output, and this pulse is input to the bun-ranking input terminal (3) of the signal processing IC (2) to blank the video output. This allows Y until the operation of the deflection system circuit starts.
Since no signal is input to the cathode of the CRT (9), spot burning is prevented.

Moreover, the electrode (12) is turned off, and the power supply voltage (A) becomes the fifth
At the same time as falling as shown by the solid line in FIG. 5A, the base voltage (C) of the transistor (Tr) of the pulse generating circuit (1) also falls as shown by the alternate long and short dash line in FIG. On the other hand, the emitter voltage (B) of the transistor (Tr) is the fifth
As shown by the dotted line in the figure (a), the diode (D1) and the capacitor (C2) keep the prescribed voltage and the transistor (Tr) ON.
The collector output voltage (D) is as shown in FIG. 5 (b). As a result, a high signal, that is, a pulse is sent to the blanking input terminal (3) to blank the video output before the operation of the deflection system circuit is stopped, and spot burning is prevented.

[Problems to be Solved by the Invention] The conventional spot killer circuit configured as described above is
At power-up and power-down, the video output is only blanked during the period determined by the time constant of the circuit, as already explained, so during the video output blanking sweep, that is, at power-up and power-up. The electron beam current of the CRT does not flow during the falling period. Therefore, when the electron beam current increases, it detects the current and controls the current.
The mitter, that is, the ACL, controls to increase the contrast in response to the electron beam current of the CRT not flowing. As a result, the contrast control voltage (F) rises as shown by the dotted line in FIG. 2, and as a result, the contrast control voltage rises above a predetermined voltage during steady operation. In such a situation, the operation of the deflection system circuit of the CRT has already stopped and the electron beam generation capability has been reached when the pulse of the pulse generation circuit (1) depending on the time constant has disappeared, especially at the fall of the power supply. However, if it is not stopped, there is a drawback that a large spot-shaped electron beam current flows in the CRT in response to the extremely large contrast control voltage as described above, and the CRT is burned out.

The present invention has been made to solve the above-mentioned problems, and the contrast control voltage at the time of blanking sweep due to the ACL control for which the conventional measures have been overlooked when the power supply rises and falls. It is an object of the present invention to provide a spot killer circuit which can surely prevent spot burning by suppressing the rise of the.

[Means for Solving the Problems] A spot killer circuit according to the present invention includes a control voltage for controlling the contrast of a video signal and an ACL (Automatic Curre).
nt Limitter) A control voltage generation circuit that generates a contrast control voltage from a control voltage, a pulse generation circuit that generates a pulse depending on a predetermined time constant when the power supply rises and falls, and the pulse generation circuit. A blanking input terminal for blanking the video output by the pulse generated by the pulse generation circuit, and a contrast for inputting the contrast control voltage generated by the control voltage generation circuit and connected to the control voltage generation circuit. A signal processing circuit having at least a control voltage input terminal, a time constant circuit for giving a predetermined time constant to a transmission path of the contrast control voltage to the contrast control voltage input terminal of the signal processing circuit, and the signal. Generated by the pulse generator circuit, connected to the contrast control voltage input terminal of the processing circuit By turning on and off in response to the generated pulse, the contrast control voltage is changed to the pulse regardless of the ACL control voltage that acts to increase the contrast control voltage during blanking sweep of the cathode ray tube when the pulse is generated. And a switching circuit that lowers the level to a predetermined level or less during a certain period and during a predetermined period determined by a predetermined time constant of the time constant circuit even when the pulse disappears.

[Operation] According to the present invention, a pulse is generated by detecting the power supply voltage when the power supply is turned on and off, the image output is blanked by this pulse, and the contrast control voltage of the signal processing IC is received by this pulse. A switching circuit is newly provided in order to forcibly decrease the contrast control voltage, and in the conventional example, the contrast control voltage that rises during blanking sweep is forcibly reduced. Even if this pulse disappears, the contrast control voltage cannot rise immediately because of the time constant of the time constant circuit. For this reason, even if the pulse disappears, the contrast can be kept low for a predetermined period determined by the time constant, so that the spot does not appear again on the CRT after the operation of the spot killer circuit in the conventional circuit.

[Embodiment of the Invention] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a connection diagram of a spot killer circuit according to an embodiment of the present invention. In FIG. 1, (13) is a switching circuit, and this switching circuit (13) is an O of a power supply (12).
At N, OFF, that is, when the power supply voltage (A) rises and falls, the contrast control voltage is lowered by the pulse generated by the pulse generation circuit (1).
It has a switching transistor (Tr1) whose base is connected to the output side of the pulse generation circuit (1), and a capacitor (C3) and a contrast control voltage input terminal (6) are provided on the collector side of this switching transistor (Tr1). ) Is connected. Since other configurations are the same as those of the conventional example shown in FIG. 3, corresponding parts are designated by the same reference numerals, and detailed description thereof will be omitted.

Next, the operation of the above configuration will be described.

The operation of generating a pulse when the power supply voltage (A) rises and falls when the power supply (12) is turned on and off and the operation of blanking the video output by the generated pulse are the same as the conventional case. Since it is the same, the description is omitted.

Here, the base voltage of the transistor (Tr1) of the switching (13) rises due to the pulse output (D) generated by the pulse generation circuit (1) when the power supply voltage (A) rises and falls. Then, this transistor (Tr1) is turned on. In this way, the transistor (Tr1)
When turned on, the collector potential is lowered, so that the contrast control voltage (E) is rapidly lowered and the contrast is narrowed. As a result, the input voltage (F) to the contrast control voltage input terminal (6) of the signal processing IC (2) is held low even when the power supply (12) is turned on and off.
It is possible to prevent spot burning of the CRT (9). Especially after the pulse output (D) from the pulse generation circuit (1) disappears and the transistor (Tr1) turns off, the input terminal ((3) due to the time constant formed by the capacitor (C3) and the resistor connected to the F point, etc. The contrast control voltage applied to 6) is kept low for a predetermined period of time, so that the pulse generation circuit (1) is turned on especially when the power supply falls.
Even if the operation of the deflection system circuit of the CRT is already stopped when the pulse output (D) from the CRT disappears and the electron beam generation capability is not yet stopped, the spotted electron beam current almost flows through the CRT. No, the CRT is not burned out.

[Effects of the Invention] As described above, according to the present invention, not only the video output is blanked by the pulses generated when the power supply rises and falls, but also the contrast of the display screen of the television receiver is controlled. By lowering the contrast control voltage to be applied, spot burn can be surely prevented even when the ACL control works. Further, since it suffices to add a simple switching circuit to the conventional circuit, it is possible to realize a highly accurate countermeasure against spot burning at low cost.

[Brief description of drawings]

FIG. 1 is a connection diagram of a spot killer circuit according to an embodiment of the present invention, and FIG. 2 is a characteristic diagram showing a correlation between a contrast control voltage and an input voltage when the power supply rises and falls and in a steady operation. Is a wiring diagram of a conventional spot killer circuit, FIG. 4 is a voltage waveform diagram for explaining the operation of the pulse generation circuit at the rise of the power supply, and FIG. 5 is a voltage waveform diagram for explaining the operation of the pulse generation circuit at the fall of the power supply. Is. (1) …… Pulse generation circuit, (2) …… Signal processing IC,
(3) …… Blanking input terminal, (6) …… Contrast control voltage input terminal, (9) …… CRT, (11) …… ACL
Control voltage input terminal, (13) ... Switching circuit. The same reference numerals in the drawings indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A control voltage generation circuit that generates a contrast control voltage from a control voltage that controls the contrast of a video signal and an ACL (Automatic Current Limitter) control voltage, and a predetermined time constant when the power supply rises and falls. A pulse generating circuit for generating a pulse depending on the pulse generating circuit, a blanking input terminal connected to the pulse generating circuit for blanking a video output by the pulse generated by the pulse generating circuit, and connected to the control voltage generating circuit A signal processing circuit having at least a contrast control voltage input terminal for inputting the contrast control voltage generated by the control voltage generation circuit, and the contrast control voltage applied to the contrast control voltage input terminal of the signal processing circuit. Constant circuit for giving a predetermined time constant to the transmission path of the signal, and the signal processing circuit By connecting to the contrast control voltage input terminal and turning on / off in response to a pulse generated by the pulse generating circuit, it works to increase the contrast control voltage at the time of blanking sweep of the cathode ray tube when the pulse is generated. Regardless of the ACL control voltage, switching for decreasing the contrast control voltage to a predetermined level below during the pulse and during the predetermined period determined by the predetermined time constant of the time constant circuit even when the pulse disappears. Circuit with a spot killer circuit.