JP2000244766A - Synchronization detection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably detect horizontal synchronizing signals even in the case that the composite signals of video signals are disturbed. SOLUTION: An LPF 2 removes chroma signals and noise components from the composite signals and sends them to a minimum value detection circuit 6, a first signal detection circuit 3, a second signal detection circuit 4 and a third signal detection circuit 5. The first signal detection circuit 1, the second signal detection circuit 4 and the third signal detection circuit 5 are informed of the minimum value of the horizontal synchronizing signals from the minimum value detection circuit 6, set a synchronizing signal detection level to be the reference of detection, detect the horizontal synchronizing signals by respectively different detection characteristics and send them to a selection circuit 7 as detection signals. The selection circuit 7 attaches priority to the respective signal detection circuits so as to raise the priority of the signal detection circuit of high detection accuracy, and at the time of receiving the detection signals from the plural signal detection circuits, selects and outputs the detection signals received from the signal detection circuit of the highest priority.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synchronization detection circuit for detecting a horizontal synchronization signal from a composite signal of a video signal, and more particularly to a synchronization detection circuit for enabling stable detection of a horizontal synchronization signal.

[0002]

2. Description of the Related Art As a circuit for detecting a horizontal synchronization signal from a composite signal of a video signal, a synchronization detection circuit as shown in FIG. 7 is used. This synchronization detection circuit
Input terminal 101 and LPF (Low Pass Filter) 102
, A minimum value detection circuit 103, a signal detection circuit 104,
And an output terminal 105.

In such a configuration, the LPF 102
Removes a chroma signal and a noise component from the composite signal input from the input terminal 101, and
03 and the signal detection circuit 104. Minimum value detection circuit 1
03 detects the minimum value (bottom) of the composite signal and notifies the signal detection circuit 104 of it. Signal detection circuit 104
Determines the synchronization signal detection level based on the minimum value of the composite signal received from the minimum value detection circuit 103.
Thereby, the signal detection circuit 104 detects the synchronization signal by comparing the composite signal received from the LPF 102 with the synchronization signal detection level, and specifies and outputs a signal having a detection width larger than a certain width as the horizontal synchronization signal. be able to.

Japanese Patent Application Laid-Open No. 8-275024 discloses a circuit for detecting the width of a horizontal synchronizing signal and a vertical counter for counting the number of horizontal lines for each field. A technique for improving the detection accuracy of a video signal by doing so is disclosed.

[0005]

In the above-described conventional synchronous detection circuit which specifies and outputs a horizontal synchronizing signal based on the synchronizing signal detection level and the detection width, when the horizontal synchronizing signal is attenuated, the synchronizing detection level changes to the video signal. It cannot be detected correctly. When a ghosted video signal is input, the detection width becomes short, noise cannot be distinguished from the horizontal synchronization signal, and correct detection cannot be performed. Further, even when a composite signal having a large noise component is input due to a weak electric field or the like, noise cannot be distinguished from the horizontal synchronization signal, and correct detection cannot be performed.

In the technique disclosed in Japanese Patent Application Laid-Open No. 8-275024, the horizontal synchronizing signal is correctly corrected when the horizontal synchronizing signal is attenuated or when a video signal greatly distorted due to the occurrence of ghost is input. There is a problem that a signal cannot be detected.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide a synchronization detection circuit capable of stably detecting a horizontal synchronization signal even when a composite signal is disturbed. .

[0008]

In order to solve the above-mentioned problems, a synchronization detection circuit according to a first aspect of the present invention is for detecting a horizontal synchronization signal from a composite signal of a video signal. A filter for receiving a composite signal and removing a chroma signal and a noise component, a plurality of signal detection circuits for detecting a horizontal synchronization signal with different detection characteristics from the output signals of the filter, and the plurality of signal detection circuits. A selection circuit for selecting and outputting one of the detected horizontal synchronizing signals.

In such a configuration, the synchronization detection circuit includes a plurality of signal detection circuits having different detection characteristics, so that even when the input composite signal is disturbed, the synchronization detection circuit performs detection in accordance with the disturbance, and the selection circuit performs the detection. It is possible to select and output, and it is possible to stably detect the horizontal synchronizing signal.

The selection circuit assigns priorities to the plurality of signal detection circuits, and when at least two of the plurality of signal detection circuits detect a horizontal synchronization signal, the selection circuit determines the horizontal synchronization signal in accordance with the priority order. It is desirable to select and output a horizontal synchronization signal detected by one of the signal detection circuits that have detected the signal.

The plurality of signal detecting circuits detect a horizontal synchronizing signal from a standard composite signal, and detect a horizontal synchronizing signal from the composite signal in which the horizontal synchronizing signal is attenuated. It is desirable to have a second signal detection circuit capable of detecting a horizontal synchronization signal narrower than a standard signal detection circuit. As a result, various detections according to how the composite signal is disturbed become possible.

[0012] The selection circuit may include, for the first to third signal detection circuits, the first signal detection circuit and the second signal detection circuit.
, And the third signal detection circuit is prioritized so that the priority becomes lower in this order, and one of the first to third signal detection circuits outputs a horizontal synchronization signal. Upon detection, the signal detection circuit outputs the detected horizontal synchronization signal, and when two or all of the first to third signal detection circuits detect the horizontal synchronization signal, the signal is output. It is desirable to select and output the horizontal synchronization signal detected by the signal detection circuit having the highest priority among the signal detection circuits that have detected the horizontal synchronization signal.

Thus, the horizontal synchronizing signal detected by the signal detection circuit with high detection accuracy can be preferentially output, and even if the composite signal is disturbed, the stable horizontal synchronizing signal can be detected.

A synchronization detecting circuit according to a second aspect of the present invention is for detecting a horizontal synchronizing signal from a composite signal of a video signal, and receives the composite signal and passes a certain band. A first filter that removes a chroma signal and a noise component, and a second filter that receives the composite signal and passes a band different from the first filter to remove a chroma signal and a noise component; A first group of a plurality of signal detection circuits each detecting a horizontal synchronization signal with a different detection characteristic from an output signal of the first filter; and a horizontal synchronization signal having a different detection characteristic from the output signal of the second filter. A second group of a plurality of signal detection circuits for detecting signals; a first group of a plurality of signal detection circuits; and a horizontal synchronization signal detected by the second group of a plurality of signal detection circuits. One out by selecting, characterized in that it comprises a selection circuit for outputting.

In such a configuration, the synchronization detection circuit detects the horizontal synchronization signal from the output signals of the two filters that pass different bands, so that when a standard composite signal is input, the synchronization detection circuit accurately detects the horizontal synchronization signal. The sync signal can be detected, and when a disturbed composite signal is input, the presence or absence of the horizontal sync signal can be stably detected.

The selection circuit assigns priority to the plurality of signal detection circuits of the first group and the plurality of signal detection circuits of the second group, and the plurality of signal detection circuits of the first group and the second group. When at least two signal detection circuits among the plurality of detection circuits detect a horizontal synchronization signal, according to the priority order,
It is desirable that one of the signal detection circuits detecting the horizontal synchronizing signal selects and outputs the horizontal synchronizing signal detected.

The first group of a plurality of signal detection circuits includes a first signal detection circuit capable of detecting a horizontal synchronization signal from a standard composite signal, and a horizontal synchronization signal based on a composite signal in which the horizontal synchronization signal is attenuated. A second signal detection circuit capable of detecting a signal; and a third signal detection circuit capable of detecting a horizontal synchronization signal narrower than a standard signal. The signal detection circuit includes a first signal detection circuit capable of detecting a horizontal synchronization signal from a standard composite signal, and a fifth signal detection circuit capable of detecting a horizontal synchronization signal from a composite signal in which the horizontal synchronization signal is attenuated. It is desirable to have a signal detection circuit. As a result, it is possible to perform detection according to various ways of disturbing the composite signal.

[0018] The selection circuit may include, for the first to fifth signal detection circuits, the first signal detection circuit and the second signal detection circuit.
, The fourth signal detection circuit, the fifth signal detection circuit, and the third signal detection circuit in order of decreasing priority in order, and the first to fifth signal detection circuits When one of the signal detection circuits detects a horizontal synchronization signal, the signal detection circuit outputs a horizontal synchronization signal detected by the signal detection circuit, and outputs at least two signals of the first to fifth signal detection circuits. When the detection circuit detects the horizontal synchronization signal, it is desirable to select and output the horizontal synchronization signal detected by the signal detection circuit having the highest priority among the signal detection circuits detecting the horizontal synchronization signal.

Thus, the horizontal synchronizing signal detected by the signal detection circuit having high detection accuracy can be preferentially selected and output, and even if the composite signal is disturbed, the stable horizontal synchronizing signal can be detected. Become.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a synchronization detecting circuit according to an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing a configuration of a synchronization detection circuit according to an embodiment of the present invention. As shown in the figure, the synchronization detection circuit includes an input terminal 1 and an LPF (Low Pass Filtration).
ter) 2, a first signal detection circuit 3, a second signal detection circuit 4, a third signal detection circuit 5, and a minimum value detection circuit 6.
, A selection circuit 7 and an output terminal 8.

The LPF 2 is a filter for removing a chroma signal and a noise component from a composite signal of a video signal input from the input terminal 1 by passing a predetermined band. LPF2 is, for example, about 400 KHz.
Is a cut-off frequency, and a low-pass circuit that passes a band equal to or lower than the cut-off frequency. The LPF 2 sends the composite signal from which the chroma signal and the noise component have been removed to the first signal detection circuit 3, the second signal detection circuit 4, the third signal detection circuit 5, and the minimum value detection circuit 6.

The first signal detection circuit 3, the second signal detection circuit 4 and the third signal detection circuit 5 have different detection characteristics, respectively, for detecting a horizontal synchronizing signal from the composite signal received from the LPF 2. Things.

The first signal detecting circuit 3 is a circuit for detecting a horizontal synchronizing signal from a standard composite signal as exemplified in FIG. The first signal detection circuit 3 synchronizes about 25% of the 40 IRE from the bottom (minimum value) of the horizontal synchronization signal received from the minimum value detection circuit 6 while the amplitude value of the horizontal synchronization signal is 40 IRE as a standard. The signal is set as a signal detection level, and the horizontal synchronization signal is detected by comparing with the composite signal received from the LPF 2. The first signal detection circuit 3 sets the width of the detected horizontal synchronization signal to a standard width (4.
7 μs (microseconds)), and if it is determined that the width is close to the standard width, the detected horizontal synchronization signal is sent to the selection circuit 7 as a detection signal.

The second signal detecting circuit 4 is a circuit for detecting a horizontal synchronizing signal from a composite signal in which the horizontal synchronizing signal is attenuated, as exemplified in FIG. The second signal detection circuit 4 detects that the amplitude value of the horizontal synchronizing signal is 40
In contrast to the IRE, a few% of the 40 IRE from the bottom (minimum value) of the horizontal synchronization signal received from the minimum value detection circuit 6 is set as the synchronization detection level, and is compared with the composite signal received from the LPF 2 to compare the horizontal synchronization signal. Is detected. Second
The signal detection circuit 4 determines whether or not the width of the detected horizontal synchronization signal is close to the standard width (4.7 μs). If it is determined that the width is close to the standard width, the detected horizontal synchronization signal is used as a detection signal. It is sent to the selection circuit 7.

The third signal detection circuit 5 converts a ghosted composite signal as illustrated in FIG. 4 or a composite signal having a large noise component due to a weak electric field or the like as illustrated in FIG. , A circuit for detecting a horizontal synchronizing signal whose width is narrower than the standard. The third signal detection circuit 5 has an amplitude value of the horizontal synchronizing signal of 40 IR as standard.
In contrast to E, about 25% of 40IRE from the bottom (minimum value) of the horizontal synchronization signal received from the minimum value detection circuit 6 is set as the synchronization signal detection level, and the horizontal synchronization signal is detected by comparing with the composite signal. I do. The third signal detection circuit 5 determines that the width of the detected horizontal synchronization signal is the standard width (4.7 μm).
Even if it is very narrow (about 1 μs) as compared with s), it is determined as a horizontal synchronization signal and sent to the selection circuit 7 as a detection signal.

The minimum value detection circuit 6 is a circuit for detecting the minimum value (bottom) of the horizontal synchronizing signal in the composite signal, and measures the minimum value of the composite signal received from the LPF 2 during a period corresponding to the horizontal synchronizing cycle. Then, the minimum value is notified to the first signal detection circuit 3, the second signal detection circuit 4, and the third signal detection circuit 5.

The selection circuit 7 is composed of a multiplexer and the like, selects the detection signals received from the first signal detection circuit 3, the second signal detection circuit 4, and the third signal detection circuit 5, and selects the detection signals as horizontal synchronizing signals. It is for output. The selection circuit 7 includes a first signal detection circuit 3, a second signal detection circuit 4,
The third signal detection circuit 5 is prioritized so that the priority becomes lower in the order, and the detection signal received from the signal detection circuit having the higher priority is preferentially selected and output to the output terminal 8 as a horizontal synchronization signal. I do.

The operation of the synchronization detection circuit according to the embodiment of the present invention will be described below. This synchronization detection circuit can detect an appropriate horizontal synchronization signal according to the degree of disturbance of the composite signal by selecting and outputting a horizontal synchronization signal detected by a plurality of signal detection circuits having different detection characteristics. Circuit.

When a composite signal is input to the input terminal 1, the LPF 2 removes a chroma signal or a noise component from the composite signal, and outputs a first signal detection circuit 3, a second signal detection circuit 4, and a third signal. It is sent to the detection circuit 5 and the minimum value detection circuit 6.

The minimum value detection circuit 6 detects the minimum value (bottom) of the horizontal synchronizing signal in the composite signal received from the LPF 2, and detects the first signal detection circuit 3, the second signal detection circuit 4, and the third signal. The detection circuit 5 is notified.

The first signal detecting circuit 3 receives the minimum value (bottom) of the horizontal synchronizing signal from the minimum value detecting circuit 6, and detects the synchronizing signal detection level set to a value of about 25% of 40 IRE from the bottom and the LPF2. A horizontal synchronizing signal is detected by comparing the received composite signal. First signal detection circuit 3
Determines whether the width of the detected horizontal synchronizing signal is close to 4.7 μs, and if so, sends the detected horizontal synchronizing signal to the selection circuit 7 as a detection signal.

Thus, the first signal detection circuit 3 can detect the horizontal synchronizing signal from the standard composite signal. Further, the first signal detection circuit 3 can detect the horizontal synchronizing signal with higher accuracy than the second signal detection circuit 4 and the third signal detection circuit 5. However, the first signal detection circuit 3 generates a disturbed composite signal such as a composite signal in which the horizontal synchronizing signal is attenuated as shown in FIG. 3 or a ghosted composite signal as shown in FIG. The horizontal synchronizing signal may not be detected from the signal.

The second signal detecting circuit 4 receives the minimum value (bottom) of the horizontal synchronizing signal from the minimum value detecting circuit 6, and receives from the LPF 2 the synchronizing signal detection level set to a value of several% of 40 IRE from the bottom. The horizontal synchronizing signal is detected by comparing with the composite signal. The second signal detection circuit 4
It is determined whether or not the width of the detected horizontal synchronization signal is close to 4.7 μs. If it is determined that the width is close, the detected horizontal synchronization signal is sent to the selection circuit 7 as a detection signal.

Thus, the second signal detection circuit 4 receives a composite signal in which the horizontal synchronizing signal is attenuated as shown in FIG. 3, and the first signal detecting circuit 3 cannot detect the horizontal synchronizing signal. Even at this time, the horizontal synchronization signal can be detected and output as a detection signal.

The third signal detecting circuit 5 receives the minimum value (bottom) of the horizontal synchronizing signal from the minimum value detecting circuit 6, and detects the synchronizing signal detection level set to a value of about 25% of 40 IRE from the bottom and the LPF2. A horizontal synchronizing signal is detected by comparing the received composite signal. Third signal detection circuit 5
Sends the detected horizontal synchronization signal to the selection circuit 7 as a detection signal even if the width is about 1 μs.

As a result, the third signal detection circuit 5 receives a composite signal having a ghost as shown in FIG. 4 or a composite signal having a large noise component as shown in FIG. Thus, even when the first signal detection circuit 3 and the second signal detection circuit 4 cannot correctly detect the horizontal synchronization signal, it is possible to detect the presence or absence of the horizontal synchronization signal.

Upon receiving the detection signal from the first signal detection circuit 3, the selection circuit 7 outputs the detection signal to the output terminal 8 as a horizontal synchronization signal.

On the other hand, when receiving the detection signal from the second signal detection circuit 4, the selection circuit 7 determines whether or not the detection signal has been received from the first signal detection circuit 3.
If it is determined that no detection signal has been received from the second signal detection circuit 4, the detection signal received from the second signal detection circuit 4 is output to the output terminal 8 as a horizontal synchronization signal.

On the other hand, upon receiving the detection signal from the third signal detection circuit 5, the selection circuit 7 receives the detection signal from the third signal detection circuit 5.
And a detection signal is received from at least one of the second signal detection circuit 4 and the second signal detection circuit 4. When the selection circuit 7 determines that no detection signal has been received from any of the first signal detection circuit 3 and the second signal detection circuit 4, the third signal detection circuit 5
Is output to the output terminal 8 as a horizontal synchronization signal.

Thus, when one of the three signal detection circuits outputs a detection signal, the selection circuit 7 can output this detection signal as a horizontal synchronization signal. When two or all of the three signal detection circuits output detection signals, the selection circuit 7 selects the detection signal output by the signal detection circuit having the highest priority, and outputs a horizontal synchronization signal. Can be output as

Accordingly, the selection circuit 7 preferentially selects the detection signal output from the first signal detection circuit 3 having good detection accuracy, and when the first signal detection circuit 3 cannot detect the horizontal synchronizing signal, The second signal detection circuit 4 and the third signal detection circuit 5 can select and output a horizontal synchronization signal detected according to the disturbance of the composite signal.

As described above, this synchronization detection circuit detects the horizontal synchronization signal detected by the first signal detection circuit 3, the second signal detection circuit 4, and the third signal detection circuit 5 having different detection characteristics. The selection circuit 7 can select and output a signal detected according to the disturbance of the composite signal. As a result, even when the composite signal is disturbed, an appropriate detection signal is selected according to the disturbed composite signal, and stable detection of the horizontal synchronizing signal becomes possible.

The present invention is not limited to the above embodiment, and various modifications and applications are possible. For example, in the above-described embodiment, the circuit for removing the chroma signal and the noise signal from the composite signal is described as the LPF 2 only, but the present invention is not limited to this. That is, as shown in FIG. 6, a first LPF 12 and a second LPF 9 having a cut-off frequency different from that of the first LPF 12 are provided, and two LPFs are provided.
The horizontal synchronizing signal may be detected from the composite signal output by the selector 7 and the selecting circuit 7 may select the horizontal synchronizing signal.

In this case, the synchronization detecting circuit operates as the fourth signal detecting circuit 10 which operates in the same manner as the first signal detecting circuit 3.
And a fifth signal detection circuit 11 that operates in the same manner as the second signal detection circuit 4. In this case, for example, the first LPF 12 is a low-pass circuit having a cut-off frequency of about 1 MHz, and the second LPF 9 is a low-pass circuit having a cut-off frequency of about 100 KHz or less. So that the two filters have different characteristics.

The composite signal output from the first LPF 12 corresponds to the first signal detection circuit 3, the second signal detection circuit 4, and the third signal detection circuit 5 described in the above embodiment.
Entered as The composite signal output from the second LPF 9 is input to the fourth signal detection circuit 10 and the fifth signal detection circuit 11. The selection circuit 7 has a priority in the order of the first signal detection circuit 3, the second signal detection circuit 4, the fourth signal detection circuit 10, the fifth signal detection circuit 11, and the third signal detection circuit 5. Prioritize to be lower,
Select and output from more detection results.

By doing so, the first LPF
Although the composite signal output by 12 makes it difficult to detect an accurate synchronization position due to the collapse of the waveform of the synchronization signal itself,
This makes it easier to detect the presence or absence of a horizontal synchronization signal when a video signal containing a large noise component is input. Also,
The composite signal output from the second LPF 9 makes it difficult to detect the presence or absence of a horizontal synchronization signal, but makes it easier to detect an accurate synchronization position.

Therefore, the detection accuracy of the horizontal synchronizing signal in the standard composite signal is improved, and the presence or absence of the horizontal synchronizing signal in the more deteriorated composite signal can be stably detected.

[0049]

As described above, according to the present invention, an appropriate signal can be selected and output from the horizontal synchronization signals detected by a plurality of signal detection circuits having different detection characteristics. Thereby, even when the composite signal is disturbed, the horizontal synchronization signal can be detected stably.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a synchronization detection circuit according to an embodiment of the present invention.

FIG. 2 is a diagram showing a standard composite signal.

FIG. 3 is a diagram illustrating a composite signal in which a horizontal synchronizing signal is attenuated.

FIG. 4 is a diagram illustrating a composite signal with a ghost;

FIG. 5 is a diagram illustrating a composite signal having a large noise component.

FIG. 6 is a diagram showing a modification of the synchronization detection circuit according to the embodiment of the present invention.

FIG. 7 is a diagram illustrating a conventional synchronization detection circuit.

[Explanation of symbols]

1,101 input terminal 2,9,12,102 LPF 3,4,5,10,11,104 signal detection circuit 6,103 minimum value detection circuit 7 selection circuit 8,105 output terminal

Claims (8)

[Claims] 1. A synchronization detection circuit for detecting a horizontal synchronization signal from a composite signal of a video signal, comprising: a filter for receiving the composite signal and removing a chroma signal and a noise component; A plurality of signal detection circuits each detecting a horizontal synchronization signal with a different detection characteristic; and a selection circuit selecting and outputting one of the horizontal synchronization signals detected by the plurality of signal detection circuits. A synchronization detection circuit. 2. The method according to claim 1, wherein the selecting circuit assigns priorities to the plurality of signal detecting circuits, and when at least two of the plurality of signal detecting circuits detect a horizontal synchronizing signal, the selecting circuits determine the priority in accordance with the priorities. 2. The synchronization detection circuit according to claim 1, wherein one of the signal detection circuits detecting the horizontal synchronization signal selects and outputs a horizontal synchronization signal detected. 3. A signal detection circuit comprising: a first signal detection circuit capable of detecting a horizontal synchronizing signal from a standard composite signal; and a horizontal synchronizing signal from a composite signal in which the horizontal synchronizing signal is attenuated. 2. The device according to claim 1, further comprising: a second signal detection circuit capable of detecting the signal; and a third signal detection circuit capable of detecting a horizontal synchronization signal having a width smaller than a standard value. 3. 3. The synchronization detection circuit according to 2. 4. The method according to claim 1, wherein the selecting circuit comprises a first signal detecting circuit, a second signal detecting circuit, and a second signal detecting circuit.
, And the third signal detection circuit is prioritized so that the priority becomes lower in this order, and one of the first to third signal detection circuits outputs a horizontal synchronization signal. Upon detection, the signal detection circuit outputs the detected horizontal synchronization signal, and when two or all of the first to third signal detection circuits detect the horizontal synchronization signal, the signal is output. 4. The synchronization detection circuit according to claim 3, wherein a horizontal synchronization signal detected by a signal detection circuit having the highest priority among the signal detection circuits detecting the horizontal synchronization signal is selected and output. 5. A synchronizing detection circuit for detecting a horizontal synchronizing signal from a composite signal of a video signal, said first synthesizing signal being inputted and passed through a certain band to remove a chroma signal and a noise component. A filter, a second filter that receives the composite signal, passes a different band from the first filter to remove a chroma signal and a noise component, and is different from an output signal of the first filter. A first group of a plurality of signal detection circuits for detecting a horizontal synchronization signal with detection characteristics; and a second group of a plurality of signal detections for detecting a horizontal synchronization signal with different detection characteristics from output signals of the second filter. And a selection circuit for selecting and outputting one of the horizontal synchronizing signals detected by the plurality of signal detection circuits of the first group and the plurality of signal detection circuits of the second group. Comprising the door, the synchronization detecting circuit, characterized in that. 6. The plurality of signal detection circuits of the first group and the plurality of signal detection circuits of the second group are prioritized. When at least two signal detection circuits out of the two groups of the plurality of detection circuits detect the horizontal synchronization signal, according to the priority order,
6. The synchronization detection circuit according to claim 5, wherein one of the signal detection circuits detecting the horizontal synchronization signal selects and outputs a horizontal synchronization signal detected. 7. The first group of a plurality of signal detection circuits includes: a first signal detection circuit capable of detecting a horizontal synchronization signal from a standard composite signal; and a composite signal in which the horizontal synchronization signal is attenuated. A second signal detection circuit capable of detecting a horizontal synchronization signal; and a third signal detection circuit capable of detecting a horizontal synchronization signal having a width smaller than a standard. The plurality of signal detection circuits include a fourth signal detection circuit capable of detecting a horizontal synchronization signal from a standard composite signal and a fourth signal detection circuit capable of detecting a horizontal synchronization signal from a composite signal in which the horizontal synchronization signal is attenuated. 7. The synchronization detection circuit according to claim 5, further comprising: a signal detection circuit. 8. The method according to claim 1, wherein the selecting circuit comprises a first signal detecting circuit, a second signal detecting circuit and a second signal detecting circuit.
, The fourth signal detection circuit, the fifth signal detection circuit, and the third signal detection circuit in order of decreasing priority in order, and the first to fifth signal detection circuits When one of the signal detection circuits detects a horizontal synchronization signal, the signal detection circuit outputs a horizontal synchronization signal detected by the signal detection circuit, and outputs at least two signals of the first to fifth signal detection circuits. The detection circuit detects a horizontal synchronization signal, and selects and outputs a horizontal synchronization signal detected by a signal detection circuit having the highest priority among the signal detection circuits detecting the horizontal synchronization signal. 8. The synchronization detection circuit according to 7.