JPH0233298A - Video signal detector - Google Patents

Abstract

PURPOSE:To improve the detection accuracy of an input video signal by separating a signal with a frequency equal to that of a synchronizing signal from a level difference signal between a frequency signal following to the synchronizing signal of a video signal and the synchronizing signal obtained from a synchronizing signal separator circuit and discriminating the level of the signal component. CONSTITUTION:When a separation synchronizing signal is obtained from a video signal by a synchronizing separator circuit 1, since an AFC circuit 10 generates an output synchronously with it, the output of a subtractor 13 is a low level output. When the waveform of the separated synchronizing signal and the output waveform of the AFC circuit 10 are kept the same, the output of the subtractor 13 is nearly 0. Thus, the output of a BPF 14 is at a low level and the presence of the video signal is detected by a level detector 15. When only noise exists in the output of the synchronizing separator circuit 1 and no separation synchronizing signal is obtained, the horizontal synchronizing frequency component of the output of the AFC circuit 10 is extracted by the BPF 14 and the noise component is eliminated at the same time, the level detection circuit 15 surely detects the absence of the video signal.

Description

TECHNICAL FIELD The present invention relates to a video signal detection circuit that detects the presence or absence of a video signal.

BACKGROUND ART A video device such as a television receiver having a so-called parent and child screen is
It has a video signal processing device to perform video signal synthesis processing and the like. Such video signal processing devices include a video signal detection device that determines the presence or absence of a video signal supplied to a circuit, or whether the supplied video signal is normal, and performs processing according to the signal state. Some are equipped with equipment.

An example of such a video signal detection device is shown in FIG.

I will explain with reference.

In FIG. 3, a video signal is supplied to the synchronization separation circuit 1 from a signal source such as a television tuner or a video disc player (not shown). A synchronization separation circuit 1 separates a horizontal synchronization signal from a video signal and supplies it to a monostable multi-by-break 2 as a trigger signal. Monostable multibibreak 2. The pulse width is shorter than one period of the horizontal synchronizing signal, and is set to, for example, 1/2 period. The output of the monostable multivibrator 2 is smoothed by a low pass filter (hereinafter referred to as LPF) 3 to become a level signal according to the duty ratio of the multivibrator output, and the level signal is sent to the level detection circuit 4.
is supplied to The level detection circuit 4 is constituted by, for example, a window comparator, and generates a discrimination signal indicating the presence of a video signal when the output level of the LPFB is within a predetermined range, and supplies it to a video signal processing device (not shown).

Note that it is also possible to use a similar configuration using a vertical synchronization signal instead of the horizontal synchronization signal.

In this configuration, when the video signal supplied to the sync separation circuit 1 is normal and the output of the sync separation circuit 1 is as shown in FIG. 4(A), the output of the monostable multivibrator 2 is constant. The duty ratio becomes , and the output of the LPF 3 becomes a constant level corresponding to this.

On the other hand, if there is no video signal and noise is supplied due to an out-of-tune state of the tuner or the like, noise will occur in the output of the synchronization separation circuit 1 as shown in FIG. 4(B).

Due to this noise, the number of times the monostable multi-bi break 2 is trigged increases, and the duty ratio of its output increases.

Therefore, the output level of the LPF 3 increases compared to the normal case described above.

Furthermore, in a state where no video signal is supplied to the synchronization separation circuit 1 as shown in FIG.
No output is generated, the duty ratio is 0, and the output of the LPFB is at a lower level than when the video signal is normal.

In this way, since the output level of the LPF 3 differs depending on the state of the video signal, the presence or absence of the video signal can be detected by determining this using the level detection circuit 4.

By the way, noise caused by detuning noise, interference, etc. is not always random noise; for example, there may be a partial no-signal state in a series of noises. In such a noise state, the average value output of the LPF 3 transitions between a high level and a low level with the detection level in the normal state in between, and the level detection circuit 4 detects an input state where noise is occurring or a state where no video signal is present. Despite this, there may be cases where a judgment is made that the condition is normal. Furthermore, when the noise includes low-frequency components, the output level of the LPF 3 may fluctuate, causing a similar problem.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a video signal detection device that improves detection accuracy for input video signals.

In order to achieve the above object, the video signal detection device of the present invention determines the presence or absence of a video signal based on a separated synchronization signal separated from a video signal. signal generating means for generating a frequency signal with a followed frequency and phase; subtracting means for obtaining a difference signal representing a level difference between the separated synchronizing signal and the frequency signals; and a level determining means that determines that a video signal exists when the output level of the extracting means is small.

EXAMPLE Hereinafter, an example of the present invention will be described with reference to FIG.

In FIG. 1, a horizontal synchronization signal (hereinafter referred to as a separated synchronization signal) separated from a video signal by a synchronization separation circuit 1 is sent to one comparison input terminal of a phase comparator 11 and a subtracter 13.
is supplied to one input terminal of

The output signal of the variable frequency oscillator 12 is supplied to the other comparison input of the phase comparator 11. The phase comparator 11 obtains a phase difference signal representing the phase difference between both signals, and uses the phase difference signal as a frequency control signal to control the frequency of the variable frequency oscillator 12 via an integrating circuit (LPF) that removes noise (not shown). feed the input.

The oscillator 12 is constituted by, for example, a multivibrator, and supplies a pulse output to the other input terminal of the subtracter 13 and the other comparison input terminal of the phase comparator 11. Phase comparator 11
The oscillator 12 constitutes an AFC circuit 10 that generates a frequency output that follows the frequency and phase of the separated synchronization signal.

The subtracter 13 is constituted by, for example, an operational amplifier, and mixes the output signal of the oscillator 12 and the separated synchronization signal at an appropriate level ratio with opposite polarities to obtain a difference signal representing the level difference between the two signals. The level ratio is adjusted so that, for example, the areas of the separated synchronization signal and the output pulse of the oscillator 12 are equal, and when both signals are present, the output of the band pass filter (hereinafter referred to as BPF) 14, which will be described later, is Set it to the minimum. Note that when the output signal of the oscillator 12 and the separated synchronization signal have opposite polarities, the subtracter 13
is used as an adder to obtain the stated difference signal.

The difference signal is supplied to the BPF 14, and a signal component having the same frequency as the separated synchronization signal in the difference signal is extracted. The extracted signal component is supplied to a level detection circuit 15, which is configured with, for example, a level detector and a level comparator, and determines the effective value of the signal, and its level is determined. The level detector 15 generates a determination output indicating the presence of a video signal when the level of the signal component is below a predetermined reference level.

In such a configuration, when a video signal exists and a separated synchronization signal is obtained as shown in FIG. Become. If the separated synchronization signal waveform and the output waveform of the AFC circuit 10 are made the same, the output of the subtracter 13 will be approximately 0. Therefore, the output of the BPF 14 will be at a low level and the presence of the video signal will be detected by the level detector.

As shown in FIG. 2(B), when only noise exists in the output of the synchronization separation circuit and no separated synchronization signal is obtained, the input of the subtractor 13 becomes the output of the AFC circuit 10 and the noise output.
The output of the subtracter 13 includes the horizontal synchronization frequency component of the AFC circuit output. When this signal is extracted by the BPF 14, noise is removed at the same time, and level detection is performed, a relatively large signal level is obtained. Therefore, the level detection circuit 15 reliably detects the absence of a video signal. In addition, phase comparator 1
Since the noise component inputted to 1 is removed by the above-mentioned integrating circuit, fluctuations in the oscillation frequency are suppressed.

Furthermore, when there is no video signal and no synchronization separation circuit output is generated as shown in FIG. I get a signal. This signal level is relatively large and the level detection circuit reliably detects the absence of a video signal.

If there is a certain phase shift between the separated synchronization signal and the AFC circuit output, a bipolar pulse will be generated in the output of the subtracter 13, and a high frequency component will be present. However, the signal components of the horizontal synchronization frequency are still canceled out, and the high frequency components are removed by the BPF 14, so the device functions normally even when there is the above-mentioned phase shift.

In this way, when a video signal is present, the BPF14
Since the output of is small and large when no video signal is present, the level difference between the two states is large enough to ensure detection of both. In addition, the output of the BPF 14 is at approximately the same level in the noise input state and the unmanned state, and the output level to be determined when transitioning between the noise input state and the unmanned state as in the conventional example is the video signal detection level. There is also the advantage that there will be no false positives because the signal will not pass through.

Note that if the synchronization separation circuit is not configured to remove the equivalent pulse and vertical synchronization signal, a relatively large amplitude will occur at the subtracter output during the signal period even when a video signal is present. In this case, it is possible to remove this by providing an LPF with an appropriate time constant after the level detector described above.

Further, in the embodiment, a horizontal synchronization signal is used for video signal detection, but a configuration may also be adopted in which a vertical synchronization signal is used.

As described in detail, the video signal detection device of the present invention includes a signal source that generates a frequency signal whose frequency and phase follow the synchronization signal of the video signal, and extracts the frequency signal and the synchronization signal from the synchronization signal separation circuit. The signal component with the same frequency as the synchronizing signal is separated from the level difference signal with the synchronizing signal, and
Since the configuration is such that the level of this signal component is discriminated, the difference in signal level to be discriminated between the presence and absence of the synchronization signal is large, and the video signal can be detected reliably, which is preferable.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 (
A) to (C) are diagrams for explaining the operation of the embodiment, FIG. 3 is a block diagram showing a conventional example, and FIG. 4 (A) to (C)
FIG. 2 is a diagram for explaining the operation of a conventional example. Explanation of symbols of main parts 1...Synchronization separation circuit 10...AFC circuit 13...Subtractor 14...Band pass filter 15...・Level detection circuit applicant Pioneer Corporation

Claims (1)

[Claims] A video signal detection device that determines the presence or absence of a video signal based on a separated synchronization signal separated from a video signal, the frequency signal having a frequency and a phase that follow the frequency and phase of the separated synchronization signal. subtraction means for obtaining a difference signal representing a level difference between the separated synchronization signal and the frequency signal; and extraction means for extracting a signal component of a frequency equal to the separated synchronization signal from the difference signal. and level determination means for determining that a video signal exists when the output level of the extraction means is small.