JPH0374555B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a display signal switching circuit, which converts, for example, a character signal based on data read from an image data memory into a normally transmitted video signal. Used when in super display mode to insert and display in a section.

[Technical background of the invention and its problems]

Television receivers that can receive text multiplex broadcasts are now capable of super display, allowing you to select the text signal input terminal or normal video signal input terminal even within one horizontal period. A display signal switching circuit is provided that can switch the display signal. An example of signal switching performed by this display signal switching circuit will be described with reference to FIG. A in FIG. 1 is a normal video signal, and B is a character signal.
Assuming that the switching circuit selects the character signal B side in the picture portion of the character signal B, and selects the video signal A during the other periods, the combined output will be the combined video signal C or D. The composite video signal C indicates the case where the pedestal level b1 of the character signal B is equal to the pedestal level a1 of the video signal A, and the composite video signal D indicates the case where there is a difference between the pedestal levels a1 and b1. show.

Originally, character signals and video signals come through different paths and are combined, so pedestal levels a1 and b1 rarely match completely. If the pedestal levels a1 and b1 are combined with a difference, the displayed characters will be too bright or too dark compared to the surrounding images, resulting in poor image quality.

In order to solve this problem, as shown in FIG.
Clamp circuits 13 and 14 are provided at the front stage of 1b, respectively. Each clamp circuit 13,
Reference numeral 14 is intended to clamp the clamp levels of the character signal B and the video signal A that are input to each with a common reference voltage V1, to make them the same pedestal level, and to input them to the signal switching circuit 11.

However, the above method requires two identical clamp circuits 13 and 14;
There are problems such as an increase in the number of elements and an increase in the amount of current. Furthermore, since two clamp circuits are used, there is a problem in that the offset of the crank level increases accordingly.

[Purpose of the invention]

This invention was made in view of the above circumstances, and in order to selectively switch two types of first and second input video signals and derive them to a common output terminal to create a third video signal, the first and second input video signals are It is an object of the present invention to provide a display signal switching circuit that can make the pedestal level between second input video signals the same using a circuit with a simple configuration, and can reduce the number of elements and the amount of current.

[Summary of the invention]

In this invention, the input terminals 28a, 2 of the switching circuit 28
An operational amplifier 25 which uses the pedestal level of one input video signal as a reference voltage is provided between 8b and the pedestal level of the other video signal is set to the capacitor 22 by the output of the negative feedback loops 26 and 27.
The pedestal level of both input video signals is made to be the same.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings. In FIG. 3, reference numeral 21 denotes a normal video signal input terminal, which is connected via a capacitor 22 to an input section 23a of a clamp level unification circuit 23 and one input terminal 28a of a switching circuit 28.

On the other hand, 24 is an input terminal for character signals, and is connected to the input section 23b of the clamp level unification circuit 23 and the other input terminal 28b of the signal switching circuit 28.

The clamp level unification circuit 23 includes an operational amplifier 25 to which the character signal is input to its positive input terminal.
has. The video signal is introduced into the negative input terminal of the operational amplifier 25 via the input section 23b, and a switch 26 and a resistor 27 connected in series are connected between the output terminal and the negative input terminal of the operational amplifier 25. A connection path is provided. A clamp pulse P1 is applied to the switch 26, and this clamp pulse P1 is a signal located at the back porch of the video signal. The switch 26 is turned on when the clamp pulse P1 is input, and is turned off during other periods. As a result, in the back porch of the video signal, the DC potential on the character signal side is
It becomes the reference voltage of , and negative feedback is applied.

That is, when the voltage at the negative input terminal of the operational amplifier 25 is lower than the voltage at the positive input terminal, the output voltage becomes higher than the center voltage, causing a charging current to flow into the capacitor 22 via the resistor 27. As a result, capacitor 2
2 is charged, the negative input terminal voltage becomes high and approaches the other positive input terminal voltage (reference voltage). When this is done several times, the positive and negative input terminal voltages become equal.
Therefore, the back porch potentials on the video signal and character signal sides become equal. When the negative input voltage of the operational amplifier 25 is higher than the positive input voltage, the charge in the capacitor 22 is discharged, and the positive,
Operates to equalize negative input terminal voltage.

The signal switching circuit 28 selectively switches and outputs either the video signal A or the character signal B, is controlled by the switching pulse P2, and outputs a third video signal that is a combination of both input signals. .

As described above, according to the present circuit, only one clamper is required, and the number of elements and the amount of current can be reduced accordingly compared to the circuit shown in FIG. Furthermore, there is no need to separately create a reference voltage. Furthermore, since there is only one clamp circuit section and the back porch section of one of the video signals is directly used as the reference voltage, offset is also improved.

FIG. 4 shows the operational amplifier 25, switch 26, etc. of FIG. 3 in more detail, and the same parts as in FIG. 3 are designated by the same reference numerals. First video signal A
is applied to the base of transistor Q3 via input terminal 21 and capacitor 22. On the other hand, the second video signal B (character signal) is input to the input terminal 24,
It is input to the base of transistor Q1 via resistor R8 and transistor Q12. Resistor R9 is a bias resistor. Transistors Q1 to Q4, transistor Q5, and resistors R11 and R12 form a differential amplifier circuit, and the output is from transistor Q.
6. The signal is fed back to the base of the transistor Q3 via a switch formed by transistors Q7 and Q8 and a resistor 27. Clamp pulse P1 is a transistor Q
10, Q11 and an amplifier circuit including resistors R1 to R3, and is applied to the bases of transistors Q7 and Q8 via resistors R7 and R6. Resistance R4, R
5 is a bias resistor between the power supply Vc.c. and ground.
The bases of the transistors Q1 and Q3 are each connected to the input terminal of the signal switching circuit 28.

When the clamp pulse P1 is input, switching transistors Q7 and Q8 are turned on. At this time, the base potential VA of transistor Q1, VB
is higher than the base potential of transistor Q3,
VB>VA and base potential of transistor Q6 VD
becomes higher than when VA=VB. For this reason,
Charging is performed through the path of transistors Q6 and Q8, resistor 27, and capacitor 22, resulting in an operation that brings VA closer to VB. Conversely, if VB<VA,
VD becomes lower than when VA=VB, and the emitter potential of transistor Q6 becomes lower. For this reason,
capacitor 22, resistor 27, transistor Q7,
Discharge occurs through the path of resistor R0, and an operation that brings VA closer to VB is obtained. By this, first,
The pedestal levels of the second video signals A and B can be made equal, and the pattern in the third video signal when combined can be made stable. The purpose of this invention is not only to unify the pedestal levels of a video signal and a character signal, but also to unify the pedestal levels of a transmitted video signal and a video signal from a video tape recorder or an imaging device. It can also be used for

〔Effect of the invention〕

As explained above, according to the present invention, two types of first and second input video signals are selectively switched,
The pedestal levels of the first and second input video signals can be made the same using a circuit with a simple configuration, reducing the number of elements and the amount of current when leading to a common output terminal and creating a third composite video signal. It is possible to provide a display signal switching circuit that obtains a display signal.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of a signal waveform when combining a video signal and a character signal, FIG. 2 is a diagram showing a conventional display signal switching circuit, and FIG. 3 is a circuit diagram showing an embodiment of the present invention. FIG. 4 is a diagram showing a more specific circuit of the circuit shown in FIG. 3. 25... operational amplifier, 26... switch 26,
28...Switching circuit.

Claims (1)

[Claims] 1 A first video signal is applied to the first input terminal via a capacitor, a second video signal is applied to the second input terminal, and the first and second input terminals are switched in accordance with the switching timing pulse. a switching circuit that selectively connects one of the two to the output terminal to obtain a third composite video signal; a second video signal of the second input terminal is applied to the reference voltage input terminal; an operational amplifier to which a first video signal from the first input terminal is applied; an output terminal of the operational amplifier and the first video signal;
is connected between the input terminals of the first video signal and turns on only at the pedestal level portion of the horizontal retrace period of the first video signal,
A display signal switching circuit comprising a switch means for charging and discharging the capacitor with the negative feedback output of the operational amplifier.