JPS5816791B2 - raster blanking circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a raster blanking circuit in a television receiver.

More specifically, in American television receivers for hotels and hospitals, in addition to channels that receive regular television signals containing both video wave components and audio wave components, special broadcasts on CATV, such as There are channels, such as music broadcasts, that receive signals containing only audio wave components.

The present invention relates to a circuit that automatically blanks raster lines without providing a mechanical switch when a channel that does not include a video wave component in such a composite video signal is selected.

In general, in this type of television receiver, when trying to receive a television signal that is only an audio signal and does not include a video wave component, if the screen is left in operation,
The noise component causes the screen to flicker, leading to the misunderstanding that there is a malfunction.

To prevent this, it is desirable to completely blank the raster.

Conventionally, in such television receivers, in order to perform raster blanking, a mechanical switch is provided for all channels, and for each channel that requires raster blanking, when that channel is selected, Each switch was set to work.

This type of specially provided mechanical switch not only requires extremely troublesome settings for each channel that requires raster blanking, but also has the drawbacks of having a complex structure that can cause malfunctions. was there.

The present invention was made to improve such conventional drawbacks, and detects the amplitude or level of a composite video signal, and adjusts the horizontal synchronization signal period of the composite video signal and the output from the horizontal circuit according to this level. The level of the horizontal pulse can be shifted to a reference voltage, and the video circuit is controlled depending on the presence or absence of this level shift.

Therefore, according to the present invention, when the composite video signal does not contain any video wave components or contains only a small video wave component that is not reflected as an image on the picture tube, and the level is low, the picture tube is cut off. When the level is high because it contains sufficient video wave components, the picture tube is allowed to operate normally without being canted off.

An embodiment of the present invention will be described based on the drawings.

1 is a video circuit that outputs a composite video signal, and this video circuit 1 is connected to a level detection circuit 2 via a capacitor C1.

This level detection circuit 2 consists of two resistors R1 and R2, and at the connection point A, the amplitude or level of the composite video signal is detected, and the detection signal is transmitted to the transistor TR1.
and a diode D are input to the gate circuit 3.

Reference numeral 4 denotes a gate pulse generation circuit, specifically a horizontal circuit, and the gate pulse, that is, the horizontal pulse, from this horizontal circuit 4 is introduced into the gate circuit 3 via a capacitor C2.

The output terminal of the gate circuit 3 is connected to a switching circuit 6 via an integrating circuit 5 consisting of a resistor R3 and a capacitor C3.

This switching circuit 6 consists of a transistor TR2 and a base resistor R4, and an output terminal 7 connected to the collector of this transistor TR2 is connected to a video circuit, and when a signal from the switching circuit 6 is received, the picture tube is cut off. The video signal is now fixed at the black level by switching the picture tube heater off or turning off the picture tube heater.

It goes without saying that the control point of the video circuit connected to this output terminal 6 is connected after the input point of the composite video signal of the video circuit 1.

Next, the present invention will be explained in detail.

First, it is assumed that a composite video signal containing a video wave component is input via a capacitor C4 as shown in FIG. 2a.

The level va at point A set by the resistors R1 and R2 of the level detection circuit 2 is as follows.

Since the areas of the shaded areas X and Y are equal with this level va as the center, when a video wave component is included, the composite video signal waveform is in a state where the entire waveform has fallen, and the lower end of the horizontal synchronizing signal is connected to the transistor. It falls below the conduction level of TR.

Therefore, the transistor TR1 of the gate circuit 3 is turned on only during the period of the horizontal synchronization signal.

On the other hand, the horizontal pulse signal synchronized with this horizontal synchronization signal is
Supplied through capacitor C2.

This horizontal pulse signal becomes V when the transistor TR1 is turned on.

00 reference voltage.

This is converted into a direct current by an integrating circuit 5 consisting of a resistor R3 and a capacitor C3, and at this time the voltage Vb at point B becomes as shown at C.

The voltage vb at this time is precisely expressed by the following equation.

Here, VD is the drop caused by the diode D.

The voltage vb at the point B is applied to the transistor TR2 of the switching circuit 6.

Since the resistors R3 and R4 are set so that the transistor TR2 is turned off when the transistor TR1 is turned on, the following equation holds true.

Here, VBE is the base-emitter voltage of transistor TR2.

From the above, the transistor TR2 is turned off and no output is provided to the output terminal 7.

Therefore, the video circuit operates normally and no raster blanking is performed.

Next, as shown in FIG. 3a, it is assumed that a composite video signal containing no video wave component is input via the capacitor C1.

Note that a television signal that is only an audio signal for special broadcasting by CATV may or may not include a horizontal synchronizing signal even if it does not include a video wave component.

Further, even if the signal does not contain a video wave component, there are cases in which a small video signal that is not displayed on the picture tube is substantially supplied.

Even when such a video wave component is not included, as described above, the area of the shaded portions X/ and Y' are equal with the set level Va as the center, so the lower end of the horizontal synchronizing signal is connected to the transistors TR,
does not reach the conduction level of , so transistor TR1 is turned off.

Therefore, the horizontal pulse signal input through capacitor C2 is not level-shifted to the reference voltage of voo, so as shown in Figure 3C, the voltage at point B becomes approximately zero as the set voltage point, but the set voltage The operating voltage on the actual circuit is V.

c' BE voltage, and transistor TR2 is turned on.

By turning on this transistor TR2, 1. An output is applied to the output terminal γ, and the video circuit is driven to cut off the picture tube, or the heater of the picture tube is cut off to blank the raster.

Note that the resistors R1 and R2 forming the level detection circuit 2
If at least one of them is made into a variable resistor, as shown in Fig. 4, the setting level va shown by the solid line can be changed to the dotted line val, or conversely, it can be changed from ■a/ to Va. You can freely adjust the level for raster ranking.

As described above, the present invention detects the composite video signal level using the level detection circuit, turns on and off the gate circuit depending on this level, and controls the video circuit depending on whether or not the level of the horizontal pulse is shifted to the reference voltage. Therefore, if a channel that does not have a video wave component is selected, raster blanking can be automatically performed.

Therefore, compared to the conventional method of providing mechanical switches for all channels and setting the switches for individual channels that require raster blanking, there is no need for troublesome settings, and there is no mechanism required. It has excellent effects such as being simplified, having fewer causes of failure, and being able to be provided at low cost.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram showing an embodiment of the raster blanking circuit according to the present invention, FIG. 2 is an illustration of the operation when there is a video signal, and FIG. 3 is an explanation of the operation when there is no video signal. 4 are explanatory diagrams for level adjustment. 1... Video circuit, 2... Level detection circuit, 3... Gate circuit, 4... Gate pulse generation circuit (horizontal circuit), 5... ...Integrator circuit, 6...Switching circuit, 7...Output terminal.

Claims (1)

[Scope of Claims] 1. A video circuit that outputs a composite video signal, a level detection circuit that is connected to this video circuit and detects the level of the composite video signal, and a horizontal circuit that generates horizontal pulses in synchronization with the composite video signal. and a gate circuit connected to the level detection circuit and the horizontal circuit and turned on and off depending on whether a video wave component is included in the composite video signal to control whether to level shift the horizontal pulse to the reference voltage. A circuit consisting of an integrating circuit that is connected to this gate circuit and converts the horizontal pulse into a DC voltage regardless of whether or not the horizontal pulse is level-shifted to a reference voltage, and a switching circuit that is switched by the DC voltage of this integrating circuit and outputs a raster blanking signal. Stub ranking circuit. 2 The level detection circuit consists of two resistors connected in series with one end connected to a reference power source and the other end grounded, and detects the level by inputting a composite video signal to the connection point of the resistors via a capacitor. A raster blanking circuit according to claim 1. 3. The raster blanking circuit according to claim 2, wherein at least one of the two resistors of the level detection circuit is a variable resistor.