JPS6062285A - Video signal switching circuit - Google Patents

Abstract

PURPOSE:To switch and display video signals precisely without any variation in pedestal level by switching the 1st and the 2nd video signals, i.e. two kinds of video signal and their contrast values selectively through a single changeover switch. CONSTITUTION:A control signal corresponding to a display mode is applied to control signal input terminals 5 and 6 of a video signal switching circuit 60, and a TV video signal and a character video signal to be switched are applied to terminals V1 and V2 respectively. Those video signals are converted by voltage- current converters 3 and 4 of the circuit 60 int current values, which are inputted to common emitters of differential couples of transistor (TRs) Q1 and Q4, Q2 and Q3 constituting current changeover switches. Control signals 5 and 6 are inputted to the bases of TRs Q1 and Q4, Q2 and Q3. The 1st and the 2nd video signals and their contrast values are switched selectively through the single circuit 60 to switch and display the video signals precisely without any variation in pedestal level.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention provides a system for switching between two different video signals, a text multiplex signal and a video signal, such as when receiving a text multiplex broadcast.
Show one or the other or not yet.

The present invention relates to a video signal switching circuit for inserting and displaying a text multiplex signal, which is another video signal, into a part of one video signal.

[Technical background of the invention and its problems]

FIG. 1 is a block diagram of a general teletext broadcasting receiving adapter, in which a teletext multiplex signal multiplexed with a video signal is applied to a cheese 10 and a video intermediate frequency amplification circuit 20. After passing through the video signal, the video signal is detected by the video detection circuit 30. The output of the video detection circuit 30 is applied to a decoder section 40 having a function of extracting and decoding a character multiplex signal,
This decoder section generates a character signal to be displayed and a color signal for the character. Thereafter, the above five character signals to be displayed and color signals are combined in a video signal synthesis circuit 50, and a character video signal to be displayed is obtained at the output of the video signal synthesis circuit 50. On the other hand, the television video signal detected by the video detection circuit 30 is applied to the video signal switching circuit 60, which selects whether to display the television video signal, the character video signal, or the television video signal. Control of whether or not to superimpose the text video signal on the video signal and display it is performed by a control signal from the decoder section 40. In this case, the control signal is transmitted through the delay line 70 in order to prevent the Sugi echo caused by the time difference occurring between the time for decoding the text video signal in the decoder section 40 and the time for extracting the control signal from the text multiplex signal. It is added to the video signal switching circuit ω.

An adapter that can receive teletext as shown in Figure 1 is equipped with a trap that allows full-screen fixed display, TV display, and super display, and it is possible to not only select one of the video signals but also to display it within one horizontal period. 60 video signal switching circuits are provided which can select a character signal input terminal or a normal video signal input terminal.

An example in which the video signal switching circuit 60 performs signal switching in the case of super display will be described with reference to FIG.

FIG. 2 is a waveform diagram illustrating a control operation when superimposing a text video signal B on a television video signal A to generate a superimposed signal C to synthesize a signal in a so-called super mode.

In FIG. 2, it is assumed that the switching circuit selects the character video signal side in the picture area of the single character video signal B, and selects the television video signal A in other periods. The control signals in the text multiplex signal sent from the broadcasting station are shown in Figure 2 (D, E, F).
) will lower the contrast of the television video signal near the characters.

To reduce the contrast of the TV video signal.

For example, if the television video signal has one character in white and the video signal also has white characters, it will be difficult to read the characters.

The output synthesized in this manner becomes a synthesized video signal C in FIG.

This composite video signal (Figure 2C) includes a character position signal indicating the position of the displayed character, a television contrast down signal E indicating the position for lowering the contrast of the television video signal around the character signal, and a fixed display on the entire surface. It is synthesized by control signals related to three types of video signal switching, including a full-screen fixed display signal F indicating whether the image is displayed or not. Here, Table 1 summarizes the relationship between the display mode and the control signal for each display mode.

(Margin below) As shown in Table 1 above, the TV video signal is shown in Figure 2A.

Two video signals of the character video signal (FIG. 2B) and the full fixed display signal (FIG. 2) F9 character position signal (second diagram).

TV CONTRAST DOWN SIGNAL In order to obtain the composite video signal C in FIG. 2 from the three switching signals shown in FIG. 2 E, as shown in FIG. a circuit that lowers
A changeover switch for a TV video signal with reduced contrast and a TV video signal without reduced contrast, and an OR logic circuit are required.

That is, the video signal switching circuit shown in FIG. Switching is controlled by When this control mode is super mode, the video signal switching circuit ω
As shown in FIG. 3, for the television video signal, the contrast of the abstract television video signal is lowered by a contrast down circuit 61 in the vicinity of the character video signal to be displayed in response to a control signal (E in FIG. 2).

Then, the TV video signal whose contrast has been lowered for the text display area is converted into a TV video signal by a changeover switch 62 using a control signal (FIG. 2, F).

Superimposed signal by selectively switching the character video signal (Figure 2C)
is output to the output terminal of the video signal switching circuit ω.

Note that whether or not to lower the contrast of the television video signal is controlled by the contrast selection switch 63 in response to a control signal (E in FIG. 2) indicating the peripheral portion of the character video signal.

Here, in FIG. 3, even if the pedestal levels of the character video signal B and the TV video signal A match 'L,' the TV video signal H whose contrast has been lowered by the contrast down circuit and the TV video signal whose contrast has not been lowered are There is an offset in the pedestal level to the video signal, and the pedestal level of the TV video signal G whose contrast has been changed is still the same as the TV video signal A or the TV video signal A which is the input of the switch. TV video signal H with reduced contrast
The problem occurs when the pedestal level becomes higher or lower than the pedestal level. Further, the pedestal level may deviate between the period of the television video signal A and the television video signal H(7)M with lower contrast.

In addition, due to the offset of the changeover switch 62 for switching between the television video signal and the text video signal, the pedestal level may deviate between the period of the television video signal G and the period of the text video signal B whose contrast has been switched.

As a result, for example, the brightness of the characters is not transmitted correctly, as shown in Figure 4 Composite Video Signal Figure 4 1゜J, and the displayed characters are too bright or too dark compared to the surrounding image, or the contrast is reduced. During the television video signal period, there is a problem that the brightness may drop more than necessary or may not fall at all, resulting in poor image quality.

Also, the selection switch 63 in FIG. 3 above. This is due to the off-cent of the changeover switch 62. TV video signal.

In addition to deteriorating the image quality of displayed characters due to mismatch in the pedestal levels of character video signals, conventional video signal switching circuits have delay circuits (not shown) in each of the three control signal paths to decode character multiplex signals. time and control signals (D) transmitted to the video signal switching circuit 60.

This is necessary in order to match the transmission time of E and F).

If the characteristics of the delay circuits do not match, it may cause image deterioration such as inadvertently lowering the contrast of characters to be displayed.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems by providing two types of first . selectively the second video signal,
To provide a video signal switching circuit capable of not only accurately switching the pedestal level without changing it, but also selectively and easily lowering the contrast.

[Summary of the invention]

That is, the present invention includes two voltage-current converters having the same characteristics and a transistor that switches between two video signals converted into currents or lowers the contrast. Consists of a current switch, the first of two. converting each second video signal into a current by a voltage-current converter;
A current switch selectively accepts either signal.
K can be easily switched, and when one video signal is in a no-signal state, that is, at a pedestal level, K can selectively lower the contrast of the other video signal.

[Embodiments of the invention]

FIG. 5 shows an embodiment of the video signal switching circuit according to the present invention, which corresponds to the video signal switching circuit 60 of the teletext receiving adapter.

In FIG. 5, the control signals (D, E, F in FIG. 2) corresponding to the display modes shown in Table 1 above are controlled as step-wave multi-level signals by one control signal path by the means described later. It is applied to the signal input terminal 5.6. In addition, the television video signal No. 6 and 9 character video signals to be switched are applied to the terminals V, V2, respectively.

That is, the first video signal V+ (for example, a television video signal)
, the second video signal v2 (for example, a character video signal) is converted into a voltage-current by a voltage-current converter 3.4, and the transistors Q and Q2 of a differential pair of transistors constituting a current switch
is input to the common emitter of Qs and Q4Φ.

Here, the first video signal is Vl, and the second video signal is
．． The characteristics of voltage-current converters 3 and 4 are K CA/v :)
The emitter area ratio between Q and Q2 is equal to the emitter area ratio between Q4 and Q, which is El:E2. v,,v,
has the same pedestal level P and is expressed as:

V+ = ft(t) + P...Song ■V2
= '2(t) + P...Song... ■If the control signal 5 is higher than the sum voltage that can sufficiently switch the differential pair of transistors compared to 6, the current amount cuff will be the emitter area ratio of the differential pair. Regardless, KV+ = K Cft(t) +
P! ) = ---- River @, and if the control signal 5 is low enough to switch the differential pair of transistors compared to 6, the current cuff will be KV2 = regardless of the emitter area ratio of the differential pair. K Cf2(S+P) -・・・・
...... ■. In the section where the television video signal and text video signal are superimposed and displayed, the voltages of control signals 5 and 6 are equal, and the current amount cuff is KV, , KV for each differential pair.

It is the sum of the divided flows, = −CE2 ft (t>+ E2P + B+ ft
(t)+B+P)E, +1!2. f2(t)=0, that is, V2=P, and the second
When the video signal 2 is in a no-signal state, that is, at the pedestal level, Equation 0 becomes as follows, and the output is obtained by lowering the contrast of the first video signal, and the pedestal level is the same as when the first video signal is output or at the pedestal level. This is exactly the same as when the second video signal is output.

As can be seen from equations ① to 0 above, in teletext broadcasting, etc., in the contrast reduction mode of the TV video signal when switching between the text video signal and the TV video signal for super display, the contrast is lowered to reduce the contrast of the characters on the screen. Among the surrounding TV video signals, the character video signal is sharp and the pedestal level is high during the period when the contrast is lowered, so the first video signal vl is used as the TV video signal, and the second video signal is
If the video signal ■2 is a character video signal, by equalizing the voltages of the control signal terminals 5 and 6, the contrast of the TV video signal can be easily adjusted to an arbitrary level by adjusting the emitter area ratio of the differential pair of transistors. can be lowered.

The circuit shown in FIG. 6 is a concrete example of the circuit shown in FIG. 5, in which the output is current-voltage converted by a resistor R and outputted with negative polarity. The voltage-to-current converter 3 in FIG. 5 that converts the first video signal ■ into a current is composed of transistors Q5 + Q6 +
The first video signal is applied to the base of the input transistor Q of the differential amplifier via the resistor R9, and the second video signal is applied to the base of the input transistor Q of the differential amplifier. The voltage in Figure 5 that converts the signal V2 into a current -
Current converter 4B, transistor Q? Resistor R8 consists of a differential amplifier consisting of + Qs + resistors R, , R,
The second video signal is applied to the base of the input transistor Q8 of the differential amplifier. Regarding the current source, I
,=I2. About resistance Rt-Rs "R4" Rs
It's Toshide. Further, it is assumed that the pedestal levels and amplitudes of the two types of video signals are equal. One input of the differential amplifier constituting each voltage-current converter, transistor Q
6 and Q7 are connected to a voltage source B1 having a potential equal to the pedestal level of the video signal through resistors R6, R, , respectively. The positive polarity side outputs of the differential amplifiers constituting the voltage-current converter, that is, the collectors of transistor Q6 and transistor Q7, are connected to the power supply, and the negative polarity side outputs are connected to transistor Q! in each converter.
A differential pair of transistors, Q+, which is taken out in the form of current from I, Q, and constitutes a current switch.

Common emitter of Q2, Q3. Q, is connected to the common emitter of Q. The common base of transistors Q1 and Q4 and the common base of transistors Q2 and Q3 are connected to control signal terminals 5 and 6, respectively, and the control signal switches the first and second video signals converted into current, and the resistor R0 switches the current. - Performs voltage conversion and outputs in voltage form. Here, when trying to obtain a positive output, the positive output of the differential amplifier that constitutes the voltage-current converter, that is, the current output from the collectors of transistors Q6 and Q7, is It is only necessary to switch and output the current using a current switch composed of Q4. At this time, the negative output of the differential amplifier constituting the voltage-current converter, that is, the collectors of the transistors Q, , Q, may be connected to the power supply.

In addition, in teletext broadcasting, as shown in Figure 1, a character video signal synthesis circuit is required before the 5 video signal switching circuit ω, and the 1 character video signal synthesis circuit converts the luminance signal and color signal of the characters. They are combined, added together, and output as a character video signal. Here, for example, the first video signal
When is a TV video signal, the second video signal is a character video signal, a character luminance signal is added in place of the second character video signal, and a character color signal is converted to a differential signal that constitutes a voltage-current converter. By inputting the input to the base of the transistor Q7 of the amplifier via a capacitor, the differential amplifier that constitutes this voltage-current converter can also have the function of adding the luminance signal and color signal of one character, as shown in Fig. 1. In the character video signal synthesis circuit, an adder for luminance signals and color signals is not required, and the circuit scale can be reduced accordingly, making it suitable for integration.

In addition, the first video signal and the second video signal transmitted in FIG.
If the pedestal level of the video signal of Q6. Even if there is a slight offset between the voltage source B1 applied to the base of Q through resistors R6 and R7 and the pedestal level of the video signal, the current at the output of the voltage-to-current converter caused by the offset Since the offset amount is the same in the two voltage-current converters, the pedestal level will be the same as the first or second video signal regardless of whether the output is switched to the first or second video signal. There is also an advantage that there is no difference between the second video signals.

FIG. 7 shows an example in which the power supply voltage is lowered and the output is taken out as a current output. In Figure 7, 3 in Figure 5,
The two voltage-to-current converters corresponding to 4 are transistors Q
A differential amplifier consisting of lo, Q, I, resistors R, o, R, , and a transistor Q+2.
+ Q10 + Resistors R, 2, R, . . . A current source 4. Consists of a differential amplifier. Then, the video signals converted into each current are passed through the transistor Q+4 + Q+a
A current is applied to a current switch consisting of transistors Q and -Q by a current mirror operation consisting of a current mirror consisting of +resistors R and 8J2Q, a transistor Q+p + Q10, and resistors R, , , R, and 2. The video signal selectively switched by this current switch is transferred to the transistor Q+s+
Q+. .

A current mirror circuit consisting of resistors R22 and R23 outputs a video signal in the form of a current. By converting the video signal into a current by converting it into a current in this way, it is possible to handle a large video signal by using a current mirror or the like even at a low power supply voltage.

It's a moat, and it can also have an amplification effect in the current mirror. The output is a current from the output terminal 7, and can be easily converted into a voltage by connecting a resistor between the ground potential and the output terminal 7. When integrated into an integrated circuit, the power consumption inside the integrated circuit can be reduced by outputting a current and converting it into a voltage externally, especially when the output is a large signal.

As described above, the video signal switching circuit 60 according to the present invention shown in FIGS. 5 to 7 does not require the contrast down circuit 61 like the conventional video signal switching circuit shown in FIG. The contrast of the television video signal corresponding to the vicinity of the 5-character video signal can be controlled. Furthermore, the changeover switch 62 in FIG. Contrast selection switch 6
Since the function of 3 is performed by a differential switch using transistors Q1 to Q4, the DC level offset between the TV video signal, which is the second video signal, and the character video signal, which is the second video signal, is changed between the two signals. This prevents the deterioration of the displayed image that sometimes occurs.

As described above, according to the video signal switching circuit according to the present invention, when switching between the first and second video signals, no pedestal level offset is generated between the two signals. If the pedestal levels of both the first and second video signals are made the same, it will have the effect of further improving the reproduced image.

FIG. 8 shows an example of a clamp circuit that clamps the pedestal levels of two types of video signals input to the video signal switching circuit to the same level. In Fig. 8, the pedestal level of one of the video signals 1 and 2 is used as a reference, and the pedestal level of the other video signal is clamped. The first video signal is input to the terminal Vt, and the second video signal is input to the terminal K.

The second video signal is input to the positive input terminal of the operational amplifier Opl, and the first video signal is input to the negative input terminal via a capacitor. A series connection path of a switch S and a resistor R24 is provided between the output terminal and the negative input terminal of the operational amplifier Opl, and the switch SI has a clamp pulse P.
, is added. This clamp pulse P1 is a signal located at the bank porch of the video signal, and the switch S is turned on when the clamp pulse P is input, and is turned off during other periods. In such a circuit configuration, the pedestal level at the back porch of the first video signal and the pedestal level at the bank porch on the second video signal side become the reference voltage of the operational amplifier Opt, and negative feedback is applied. That is, when the voltage at the negative input terminal of the operational amplifier Op1 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 C1 via the resistor R24. As a result, the capacitor C1 is charged and the negative input terminal voltage becomes high, approaching the positive input terminal voltage which is the terminal v1 to which the first video signal is applied. When this operation is performed several times, the voltages at the positive and negative input terminals become equal. Therefore, the pedestal levels of both the first and second images (back porch of No. 8) become equal.On the other hand, if the negative input terminal voltage of the operational amplifier Opl is higher than the positive input voltage, the above-mentioned reverse occurs. In this operation, the charge in the capacitor C1 is discharged, and the voltages at the positive and negative input terminals are made equal.In this way, the pedestal level of one video signal is used as the reference, and the pedestal level of the other video signal is set as the pedestal level of the other video signal. If Y7 is used, the t7 cent between the pedestal levels of both video signals will be improved, and only one clamp circuit will be required, and by using it together with the video signal switching circuit described above, video signals can be switched with more precision. Ru.

Fig. 9 shows Fig. 8 in more detail, and the first video signal before the pedestal level is clamped is connected to the terminal ■,
from a current source I3. through a capacitor C1. Transistors Q24-Q28. It is connected to the base of the negative input terminal Q2H of the operational amplifier Op+, which is composed of resistors R8 and R1, through the terminal (), and the second video signal is transmitted from the terminal (2) to the operational amplifier via the transistor Q2. positive input terminal Q
Connected to the base of 28. In addition, the output of the operational amplifier is switched by the transistor Q22 + Qxs, and the resistor R
It is fed back to the base of transistor Q211 via Q24. Clamp pulse P, is transistor Qto +Q2
8. Resistors R and o at the base of transistor Q221 and Q230 forming a switch through an amplifier circuit including resistors R25 to R27, 1. It is added via L, . Here, the resistors R128 and R12g are bias resistors for the transistor Q22 + Qh3 that divides the voltage between the power supply VOO and the ground to make the switch +11 (+11).Then, the j-th and second video signals are inputted. The base of transistor Q2!l + Q28 is connected to terminal V, respectively.
, V2 pedestals are aligned, and the video signal switching circuit 6
o is input. Otherwise, when the clamp pulse P1 is input, the switching transistor Qn+Q23, which functions as a bidirectional switch, is turned on. This time,
The base potential of transistor Q28 is the same as that of transistor Q25.
, the emitter potential of the transistor Q24 is higher than the base potential of the transistor Q2y and the base potential of the transistor Q2y.
J becomes higher than when the base potentials are equal, and transistor Q7. .

Charging is performed through the path of resistor R24 and capacitor C7,
It operates in the direction of equalizing the base potentials of transistors Q2s and Q2g. Conversely, when the base potential of the transistor Q25 is higher than the base potential of the transistor Q28, the emitter potential of the transistor Q24 becomes lower than when the base potential of the transistor Q2cI and the base potential of the transistor Q28 are equal. Q22. Discharge occurs along the path R=, and the transistors operate in the direction of making the base potentials of transistors Q2B and Q26 equal.

The pedestal levels of the 1st and 3rd g2 video signals are made equal.

In this way, the first and second video signal input terminals V of the video signal switching circuit 6o shown in FIGS. 5 to 8 above,
By applying the same pedestal level to V2, the deterioration of the image due to the difference in the pedestal level after switching by the video signal switching circuit mark is prevented.

Note that, as described above, video signal switching control is performed by a control signal transmitted through a single control signal terminal. As shown in Figure 2, video signal switching control is conventionally controlled using three types of signals (E and F in the second diagram), but the video signal switching circuit according to the present invention uses these three types of signals. The control signal is converted into one staircase wave control signal and applied to the control signal terminals 5 and 6. In this way, converting multiple types of control signals into a single control signal as a multi-level signal using a staircase wave etc. eliminates the time difference between the control signals when they are transmitted to the video switching circuit, and also reduces the time difference between the control signal and the character video signal. This is to prevent image deterioration due to a time difference in the amount of time applied to the video switching circuit.

In other words, as shown in Figure 1, in a teletext receiver, etc., it is necessary to delay the control signal related to video signal switching by the time required to synthesize the video signal from the signal read from the image memory. However, when delaying using a shift register or the like in the display control section, the clock frequency is a problem, and a highly accurate delay cannot be expected. Further, when delaying with an inverter or the like, variations in the delay time become a problem, making it impractical, and it is necessary to provide some kind of delay circuit externally to delay the control signal. At this time, if a delay circuit is provided to delay each control signal, not only will delay circuits be required for the number of control signals, but also variations in delay time of each delay circuit will become a problem. Therefore, from the control signal related to video signal switching, there are three states of the control signal actually used for switching in the video signal switching circuit: first video signal display, second video signal display, first video signal display with reduced contrast, and Three-value video switching signals with levels depending on the state are synthesized into a staircase wave. The signal is delayed by one delay circuit, and the signal is input and decoded to generate current switch control signals 5 and 6 of the video signal switching circuit. As a result, since there is only one delay circuit, there is no delay variation between control signals related to video signal switching, and current switch control signals 5 and 6 with high accuracy can be generated, which can be used together with the video signal switching circuit described above. For example, more accurate switching can be performed. However, the means for transmitting control signals requires only one delay line, which is advantageous in terms of cost.

FIG. 10 shows a control signal conversion circuit that converts a plurality of control signals for performing switching control in the video switching circuit (a) according to the present invention into, for example, a staircase wave-like multivalued signal. In FIG. 10, terminals T, T2. T, is F in Figure 2.

Control signals corresponding to the control signals indicated by D and E are applied. That is, in FIG. 10, F is a control signal indicating whether or not the entire screen is fixedly displayed. E' is a control signal that limits the amplitude of the television contrast down signal E and is level-shifted, and indicates the position at which the contrast of the television video signal is lowered; D is a control signal that limits the amplitude of the character position signal and is also a level-shifted signal. This is a control signal that indicates the position of the picture part of the character.

In FIG. 10, a video switching signal synthesis circuit is constructed by transistors Q (capital) to Qo and resistors Rs6 to R42, and at the emitter of transistor Q38, Ll,
B2. A video switching control signal having a three-value level of B3 is synthesized. Here, the Ll level is a control to display a character video signal which is a second video signal, the L2 level is a control to display a TV video signal which is a first video signal with a reduced contrast, and B3 is a control to display a TV video signal. It corresponds to For example, these three values are Ll > B2 >
B3, and the delay circuit is realized by a delay line DLI. The decoder that decodes the current switch control signal of the video signal switching circuit is a transistor Q.
so~Q42. Current source Ifi+I7+ and resistor R,3,
R44, and a bias voltage source B4. B
5 are the three values 1i'H, L' of the output from the delay circuit, respectively.
2, L'3 (L'1>L'2>L'B)
For L′1〉B〉L′.

The relationship is set as >B', >L; Here, L', , L'2゜L' are respectively, B2.
It corresponds to L. When the output of the delay circuit is at the level L', transistors Qso and Q4B turn on,
Current switch control signal 5.5? I, 5 is a high level, and 6 is a low level. When the output of the delay circuit is at the level of B3, the transistors Q41 + Q4□ are turned on, and the current switch control signals 5 and 6 have a low level at 5 and a high level at 6. Also, when the delay circuit output is at the level of L'2, 1
．． Transistor Q'! 9 + Q41 is turned on and control signal 5.・Both numbers 6 and 6 are at a high level. Switching control for switching video signals is performed depending on the states of these three control signals.

In this embodiment i9+-1, the teletext receiving adapter was explained, but the present invention is applicable not only when switching or superimposing a TV video signal and a text video signal, but also when combining a TV video signal and a video output from a personal computer. It can also be used for switching and superimposing signals.

〔Effect of the invention〕

As described above, according to the present invention, a single video signal changeover switch is used to selectively switch between two types of video signals, which are the first and second video signals. Since switching and contrast reduction are performed,
Accurate video signal switching is possible without changing the pedestal level.

Moreover, when the first and second video signals are superimposed and displayed, it can be done in the video switching circuit itself without providing a separate contrast down circuit, and the circuit scale can be reduced significantly, so it is especially easy to use integrated circuits. In this case, it is advantageous in terms of power consumption, device area, product cost, and reliability. Furthermore, since the output format is current, there is the advantage that large signals can be handled even when integrated circuits are used with low voltage power supplies.

Furthermore, since the video signal switching circuit according to the invention is driven after decoding a plurality of control signals into a single multi-valued control signal, it also requires a delay circuit to adjust the transmission time of the control signal. It also has the effect that only one is enough.

[Brief explanation of the drawing]

Figure 1 is a circuit block diagram showing a general teletext receiving adapter, Figure 2 is a waveform diagram to explain the operation of Figure 1, and Figure 3 is a circuit block diagram showing a conventional video signal switching circuit. FIG. 4 is a waveform diagram for explaining the operation of FIG. 3, and FIGS. 5 to 10 are video signal switching circuit diagrams according to the present invention. Patent attorney Norichika: J Figure 6 Figure 7

Claims (1)

[Claims] A first video signal input terminal into which a first video signal is input; and a second video signal input terminal whose current value changes according to the first video signal applied to the first video signal input terminal. 1 signal current source, a second video signal input terminal into which a second video signal is input, and a current value changes according to the second video signal applied to the second video signal input terminal. and a second signal current source. a first differential switch device driven by a control signal input terminal to which a control signal for selectively outputting the first and second video signals is applied, and a current corresponding to the second signal current source; a balanced differential switch that is driven; and a balanced differential switch that is driven, and the first and a video signal output terminal provided on the balanced differential switch that selectively outputs the second video signal, and outputs the first and second video signals according to the input level of the control signal. A video signal switching circuit characterized in that at least one signal is obtained at the video signal output terminal.