JPH06165032A - Video signal switching device - Google Patents

Abstract

PURPOSE:To prevent a video signal containing noise appearing when a frequency is changed from being outputted onto a screen by discriminating the continuity of video signals and muting the video signals based on the output signal. CONSTITUTION:One of plural video signals at different scan frequencies is supplied from a video equipment 1. At a scan frequency change detecting circuit 2, the change of the scan frequency in the video signal is detected, and the output signal is supplied to a video signal muting circuit 3. When the scan frequency is not changed, the output signal of the video equipment 1 is supplied to a monitor device 4 as it is. On the other hand, when the change of the scan frequency is detected, a control signal CS (at an L level, for example,) is supplied from the scan frequency change detecting circuit 2 to the video signal muting circuit 3. Thus, the video signal muting circuit 3 is turned to the active state, and the video signal is muted. Only black, for example, is outputted onto the screen of the monitor device 4 at such a time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal switching device for switching video output signals of, for example, a multi-scan monitor.

[0002]

2. Description of the Related Art A multi-scan monitor capable of inputting a plurality of video signals having different video frequencies has been put into practical use. Computers having different display operation modes can be connected to the multi-scan monitor.
The computer has, for example, 320 dots × 200 dots, 720 dots × 400 dots, 1024 dots × 7.
A mode such as 68 dots is used. When a video signal is input from a computer on the multi-scan monitor, reproduction is performed using the same frequency as that of the computer. Also, the video input signal is switched,
If a video signal is input from another computer,
The frequency for the video signal is used for reproduction. This makes it possible to monitor the input video signal in an optimum state.

[0003]

However, the above-mentioned multi-scan monitor is effective only when the video input signal is switched, and the scan frequency of the video source is changed in the state where an arbitrary video input signal is selected. In this case, there is no effect, and thus the video signal output from the monitor becomes a distorted image due to noise.

Therefore, an object of the present invention is to prevent a video signal containing noise, which appears when the scan frequency is changed, from being output on the screen, and in that case, a video signal in which the monitor screen is a black screen. A switching device is provided.

[0005]

SUMMARY OF THE INVENTION The present invention comprises discriminating means for discriminating the continuity of video signals and muting means to which the output signal of the discriminating means is supplied. This is a video signal switching device in which the starting means mutes the video signal.

[0006]

The change in the scan frequency of the input video signal is detected by the scan frequency change detector. When the scan frequency changes, the video signal mute circuit operates,
Mute the video signal.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a video signal switching device to which the present invention is applied will be described below with reference to the drawings. FIG. 1 is a block diagram of a video system in which a video signal switching device is incorporated. In FIG. 1, a plurality of video signals having different scan frequencies can be input from the video device 1.
One of them is supplied to each of the scan frequency change detection circuit 2 and the video signal mute circuit 3.

The scan frequency change detection circuit 2 detects a change in the scan frequency in the video signal, and the output signal is supplied to the video signal mute circuit 3. When the scan frequency does not change, the output signal of the video equipment 1 is directly supplied to the monitor device 4. Therefore, the video from the video equipment 1 is displayed on the monitor device 4 as it is. On the other hand, when a change in the scan frequency is detected, the control signal CS (for example, L level) from the scan frequency change detection circuit 2 is supplied to the video signal mute circuit 3. As a result, the video signal mute circuit 3 is in the operating state,
The video signal is muted. Only black, for example, is output to the screen of the monitor device 4 at this time.

FIG. 2 shows a scan frequency change detection circuit 2
A detailed circuit block diagram of is shown. In FIG. 2, the video signal supplied from the input terminal 11 is supplied to the horizontal sync separation circuit 12 and the vertical sync separation circuit 13. The horizontal sync separation circuit 12 separates the horizontal sync signal. The horizontal synchronizing signal (signal a) is supplied to the reset terminal of the counter 14 and also the D flip-flop (hereinafter referred to as DFF).
15, DFF16, DFF17, DFF18
And a clock terminal of the DFF 19. A clock is supplied to the counter 14 from the clock generation circuit 20. The counter 14 is reset at the falling edge of the horizontal sync signal from the horizontal sync separation circuit 12, then counts the clock supplied from the clock generation circuit 20, and is reset at the next falling edge of the sync signal.

The count value (time) of the counter 14 is D
It is transferred to FF15. Therefore, the DFF 15 has 1
The horizontal scanning line segment time is held. Further, the DFFs 16 and 17 hold the time before one horizontal scanning line and two horizontal scanning lines before the scanning line held by the DFF 15. That is, the output signal (signal b) of the DFF 15 is supplied to the DFF 16 at the next falling edge of the sync signal. Also, D
The output signal (signal c) of the FF 16 is further supplied to the DFF 17 at the next falling edge of the synchronization signal. DFF1
The signal c from 6 and the output signal (signal d) of the DFF 17 are supplied to the input terminal A and the input terminal B of the comparator 21, respectively. The output signal (signal e) of the comparator 21 is the DFF1
It is supplied to the low active input terminal of the AND gate 22 via 8 and is directly supplied to the other input terminal of the AND gate 22.

The signal b from the DFF 15 and the DFF
The signal d from 17 is supplied to the input terminal A and the input terminal B of the comparator 23, respectively. The output signal (signal g) of the comparator 23 is supplied to the AND gate 24. At the same time, the AND gate 24 is supplied with the output signal (signal f) of the AND gate 22 and the output signal of the vertical blanking signal generator 25. The vertical blanking signal generator 25 is supplied with the output signal of the vertical sync separation circuit 13. The output signal (signal h) of the AND gate 24 is delayed by the DFF 19 and then supplied to the monostable multivibrator 26 as the output signal i. Terminal 1
When the scan frequency of the video signal input from 1 changes, the monostable multivibrator 26 outputs L
A level signal (signal j) is output. The signal j is supplied to the video signal mute circuit 3 shown in FIG.

FIG. 3 shows a timing chart of the signals a to j referred to above. In addition, FIG.
Corresponds to the signal a, and hereinafter, FIGS. 3 (b), 3 (c) ...
(J) corresponds to signal b, signal c ... Signal j. The signal a is a horizontal synchronizing signal input from the input terminal 11, and its frequency is T1. When the frequency of the horizontal synchronizing signal changes from T1 to T2 (the scan frequency of the video signal changes), it means that the video signals having different scan frequencies are input as input signals. In other words, the scan frequency of the video signal is switched at the frequency T2 portion of the signal a. The frequency of the horizontal sync signal is T3.
When changing from T4 to T4, it means that a different video signal having the same scan frequency is input as the input signal. In other words, the video signal is switched in the frequency T4 portion of the signal a.

By the way, as described above, the clock signal output from the clock generation circuit 20 is counted by the counter 14 and then supplied to the DFF 15. DFF1
As shown in FIG. 3B, the output signal b of 5 is delayed by one cycle with respect to the signal a and is output. Also, DFF16
The signal c that is the output signal of
The signal b is delayed by one cycle and supplied to the DFF 17 in the next stage. Further, the output signal d of the DFF 17 is as shown in FIG.
As shown in (d), the signal c is output after being delayed by one cycle. For example, when the signal input to the input terminal A exceeds the range of (1-k) × B ≦ A ≦ (1 + k) × B, the comparator 21 and the comparator 23 output the signal as an H level. To do. Note that “k” is a constant determined by the stability of the synchronization signal.

The output signal e of the comparator 21 and the signal g of the comparator 23 become H level at the timings shown in FIGS. 3 (e) and 3 (g). The signal e is the AN as described above.
It is supplied to the D gate 22 and output as a signal f as shown in FIG. Further, the AND gate 24 is supplied with the signals f, g and the output signal of the vertical blanking signal generation circuit 25, and the output signal h of the AND gate 24 based on these signals is shown in FIG. As shown. As described above, the signal h becomes H level only when the signals f and g are H level, and it is detected that the video signals having different scan frequencies are input. On the other hand, when different video signals having the same scan frequency are input, the signal g is set to the L level, and thus the signal h is set to the L level.
It is a level. By inputting the signal from the vertical blanking signal generating circuit 25 to the AND gate 24, it is possible to prevent the AND gate 24 from malfunctioning with respect to a vertical synchronizing signal whose front and rear synchronisms are deviated. That is, in the vertical blanking period, the output signal of the vertical blanking signal generation circuit 25 is set to L level, which causes the output signal h of the AND gate 24 to be set to L level.

The signal h is delayed by the DFF 19 for a predetermined time and supplied to the monostable multivibrator 26 as a signal i (see FIG. 3 (i)). The monostable multivibrator 26 is triggered by the signal i and is kept at the L level for a certain period of time (see FIG. 3).
(See (j)) is output to the video signal mute circuit 27 via the terminal 27. As a result, the video signal mute circuit 3
Is turned on. Only the synchronizing signal in the video input signal is supplied to the monitor device 4. Therefore, the monitor device 4 has a black screen.

FIG. 4 is a block diagram of an RGB video system to which the video signal switching device according to the present invention is applied. In FIG. 4, the horizontal synchronizing signal HD of the video signal is input from the input terminal 31. Also, input terminal 3
The R, G and B signals of the video signal are input from 2, 33 and 34, respectively. The synchronization signal HD is supplied to the scan frequency change detection circuit 35 and the monitor device 40. The output signal of the scan frequency change detection circuit 35 is supplied to the switches 36, 37 and 38, whereby the switches 36, 37 and 38 are operated in synchronization. In addition, the R signal is the terminal 3 of the switch 36.
6b, the G signal is supplied to the terminal 37b of the switch 37, and the B signal is supplied to the terminal 36b of the switch 36. On the other hand, the terminals 36a and 37a of the switches 36, 37 and 38, respectively.
And 38a are supplied with the power supply voltage 39.

In such a circuit, when there is no change in the frequency of the synchronizing signal supplied to the scan frequency change detecting circuit 35, the R signal, the G signal and the B signal are output from the respective switches 36, 37 and 38. Terminals 36b, 37b and 3
The synchronizing signal HD is supplied to the monitor device 40 while being supplied to the monitor device 40 via 8b. On the other hand, when a change in the sync signal is detected by the scan frequency change detection circuit 35, the control signal C is sent to the switches 36, 37 and 38.
S is applied and the switches 36, 37 and 38 are connected to the terminal 36.
b, 37b and 38b to terminals 36a, 37a and 38
Switched to a. As a result, the monitor device 40 is supplied with a predetermined voltage instead of the R signal, the G signal, and the B signal, and is also supplied with the synchronization signal HD. Therefore,
The monitor device 40 has a black screen.

FIG. 5 is a block diagram of a composite video system to which the video signal switching device according to the present invention is applied. In FIG. 5, from the input terminal 41,
The composite video signal is input and supplied to the terminal 42b of the switch 42 and the sync signal separation circuit 43. The sync signal separation circuit 43 separates the sync signal HD from the composite video signal and supplies it to the terminal 42a of the switch 42 and the scan frequency change detection circuit 44. The scan frequency change detection circuit 44 detects the frequency change of the synchronization signal HD. When the frequency of the synchronizing signal HD changes, the control signal CS is applied from the scan frequency change detecting circuit 44 to the switch 42. The signal selected by the switch 42 is supplied to the monitor device 45 and is output as an image.

In such a circuit, when the scan frequency change detecting circuit 44 detects that the frequency of the synchronizing signal HD has no change, the composite video signal is sent to the monitor device 45 via the terminal 42b of the switch 42. Is supplied to. As a result, a normal image is output via the monitor device 45. On the other hand, when the scan frequency change detection circuit 44 detects a change in the synchronization signal HD, the switch 42 is switched from the terminal 42b to the terminal 42a. As a result, only the synchronization signal HD is supplied to the monitor device 45, and the monitor device 45 has a black screen.

[0020]

According to the present invention, when a change in the scan frequency of the video signal is detected, only the sync signal is supplied to the monitor device instead of the disturbed video in which noise is mixed, and the screen is blacked. To As a result, it is possible to prevent an unpleasant image from being output due to a change in scan frequency.

[Brief description of drawings]

FIG. 1 is a block diagram of a video system incorporating a video signal switching device according to the present invention.

FIG. 2 is a detailed circuit block diagram of a scan frequency change detection circuit.

FIG. 3 is a timing chart of output signals of each circuit included in the scan frequency change detection circuit.

FIG. 4 is a block diagram of an RGB video system to which a video signal switching device according to the present invention is applied.

FIG. 5 is a block diagram of a composite video system to which a video signal switching device according to the present invention is applied.

[Explanation of symbols]

 2, 35, 44 Scan frequency change detection circuit 3 Video signal mute circuit 12 Horizontal sync separation circuit 14 Counter 20 Clock generation circuit

Claims (3)

[Claims] 1. A discriminating means for discriminating the continuity of a video signal, and a muting means to which an output signal of the discriminating means is supplied. The muting means is based on the output signal of the discriminating means. Video signal switching device that mutes signals. 2. The discriminating means, a synchronizing signal separating circuit for separating a synchronizing signal from the video signal, a comparing circuit for comparing lengths of horizontal periods, and the muting means based on an output signal of the comparing circuit. 2. The video signal switching device according to claim 1, further comprising an oscillator that outputs the control signal. 3. The video signal switching device according to claim 1, wherein the video signal is replaced with a predetermined level and output based on the output signal of the discrimination means.