JPH07240898A - Stabilizing circuit for modulation frequency of frequency modulator - Google Patents

Abstract

PURPOSE:To suppress fluctuation in a modulation frequency due to various causes, to stably control the modulation frequency with high-accuracy and to allow the circuit to cope with the frequency modulator having two or more than two modulation frequencies by the use of simple circuit configuration. CONSTITUTION:A horizontal synchronization part extract circuit 4 extracts a horizontal synchronization tip of a modulation signal 3a obtained by frequency modulating a video signal 0a at a frequency modulation circuit 3 after being subject to horizontal synchronization tip clamping at a clamp circuit, a frequency comparison detection circuit 5 compares the frequency of the horizontal synchronization tip with a reference frequency generated by a reference frequency generating circuit 6 and an output voltage level of a DC voltage generating circuit 7 is varied to reduce a deviation between both the frequencies to adjust automatically a clamp voltage of the clamp circuit 2 as a closed loop thereby making the modulation frequency stable. The reference frequency generating circuit 6 selects any of plural reference frequency signals 6a having different frequencies corresponding to plural modulation frequencies and provides an output of the selected signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to an information recording / reproducing apparatus such as a magnetic recording / reproducing apparatus for frequency-modulating a video signal and recording it on a recording medium, and stabilizes the modulation frequency to suppress the modulation frequency fluctuation of a frequency modulator. It is related to the digitalization circuit.

[0002]

2. Description of the Related Art In a magnetic recording / reproducing apparatus for recording a video signal, it is general that the video signal is frequency-modulated as shown in FIG. 5 and recorded on a magnetic recording medium.

The information signal recorded on the magnetic recording medium is reproduced not only by one magnetic recording / reproducing apparatus but also by another apparatus of the same standard. At that time, the quality of the reproduced video signal in each device must be kept constant, so that the frequency of the information signal recorded on the magnetic recording medium (hereinafter referred to as the recording signal) is exactly the specified value. Must be controlled to become.

However, fluctuations in the frequency of the recording signal also occur due to variations in the performance of circuit components that make up the apparatus, fluctuations in power supply voltage, environmental changes such as temperature and humidity, and various changes over time. In the magnetic recording / reproducing apparatus, the use place is not limited, and the magnetic recording / reproducing apparatus is more likely to be affected by environmental changes.

In order to suppress such a variation in the modulation frequency of the recording signal, conventionally, a signal obtained by extracting the horizontal synchronization leading end portion of the modulation signal obtained by frequency-modulating the video signal and demodulating this is extracted. Is compared with the reference voltage level, and the clamp voltage of the horizontal synchronizing tip portion of the video signal is controlled based on the error voltage (see Japanese Patent Laid-Open No. 62-115923). A modulation frequency stabilizing circuit of such a conventional frequency modulator will be described below with reference to FIG.

When a video signal is input to the frequency modulator,
The horizontal sync separation circuit 51 separates the horizontal sync signal from the video signal, and the clamp circuit 52 clamps the horizontal sync tip of the video signal based on the horizontal sync signal. The signal output from the clamp circuit 52 is frequency-modulated in the frequency modulation circuit 53.

The modulated signal output from the frequency modulation circuit 53 is sent to the recording amplifier circuit 59 to be recorded on a magnetic recording medium (not shown), and is also sent to the horizontal synchronizing portion extracting circuit 54. This horizontal synchronizing portion extracting circuit 54
Based on the horizontal sync signal from the horizontal sync separation circuit 51,
The demodulation circuit 55 extracts the horizontal sync tip of the modulation signal.
Send to. The DC voltage level of the signal demodulated by the demodulation circuit 55 and the DC voltage level of the reference voltage output from the reference voltage generation circuit 56 are compared with each other by the voltage comparison / detection circuit 57.
Compared with. Then, the modulation frequency is stabilized by varying the output voltage level of the DC voltage generating circuit 58 according to the comparison result and adjusting the clamp voltage of the clamp circuit 52.

[0008]

However, in the above-mentioned conventional configuration, since the fluctuation of the modulation frequency is detected from the level change of the DC voltage obtained by demodulating the modulated signal by the demodulation circuit 55, the influence of the fluctuation of the power supply voltage is detected. There is a problem that it cannot be eliminated.

Further, in order to detect the minute fluctuation of the modulation frequency without being influenced by the noise component in the equipment in the above-mentioned conventional structure, the demodulation gain is increased to increase the variation width of the obtained DC voltage level. Therefore, a considerably large voltage width is required to deal with two or more types of modulation frequencies, and the circuit configuration becomes complicated.

The present invention has been made in view of the above, and an object thereof is to suppress frequency fluctuations due to various factors, to accurately control the modulation frequency of a recording signal to a prescribed value, and to further control two or more types of modulation frequencies. Frequency modulator with
It is an object of the present invention to provide a modulation frequency stabilizing circuit of a frequency modulator that can be handled with a simple circuit configuration.

[0011]

According to a first aspect of the present invention, there is provided a modulation frequency stabilizing circuit for a frequency modulator, comprising a horizontal synchronizing signal separating circuit for separating a horizontal synchronizing signal from a video signal, and a horizontal synchronizing signal separated from the video signal. A clamp circuit that specifies a flat portion of the video signal having a constant voltage level of the video signal based on the synchronization signal and sets the DC voltage level of the flat portion of the video signal to the clamp voltage level, and a clamp circuit provided after the clamp circuit. In order to solve the above-mentioned problems, a frequency modulator having a frequency modulation circuit for frequency-modulating a video signal is provided to stabilize the modulation frequency,
It is characterized in that the following measures are taken.

That is, the modulation frequency stabilizing circuit extracts the clamp portion of the modulation signal corresponding to the flat portion of the video signal clamped by the clamp circuit based on the horizontal synchronizing signal separated from the video signal. The output circuit, the reference frequency generation circuit that generates the reference frequency signal, the frequency of the modulation signal corresponding to the clamp part from the clamp part extraction circuit, and the frequency of the reference frequency signal are compared to clamp the modulation signal. Depending on the detection result of the frequency comparison detection circuit and the frequency comparison detection circuit that detects the deviation of the modulation frequency of the part from the specified value, the clamp voltage level of the clamp circuit so that the deviation from the specified value becomes smaller. And a clamp voltage variable circuit that changes the voltage. It should be noted that the flat portion of the video signal serving as the clamp point includes the horizontal synchronization tip portion or the pedestal portion of the video signal.

According to a second aspect of the present invention, there is provided a modulation frequency stabilizing circuit for a frequency modulator, wherein a plurality of different modulation frequencies are switched to change the frequency of a video signal like a frequency modulator for a consumer video cassette recorder. A frequency modulator equipped with a frequency modulation circuit capable of modulating,
In order to solve the above problems, in the configuration of the invention of claim 1, the reference frequency generation circuit is configured to switch and output a plurality of reference frequency signals having different frequencies corresponding to each modulation frequency. It is characterized by being.

[0014]

According to the structure of the invention described in claim 1, when the video signal is input to the frequency modulator, the horizontal sync signal is separated from the video signal in the horizontal sync separation circuit, and based on the horizontal sync signal, The clamp circuit clamps the flat portion of the video signal. Then, the clamp voltage at this time is set by the clamp voltage variable circuit.

The clamped video signal is frequency-modulated in a frequency modulation circuit provided after the clamp circuit. At this time, in the clamp part extraction circuit, only the clamp part (the part corresponding to the flat part of the video signal) of the modulation signal output from the frequency modulation circuit is extracted. Then, the frequency comparison and detection circuit compares the frequencies of both the modulation signal corresponding to the clamp part output from the clamp part extraction circuit and the reference frequency signal output from the reference frequency generation circuit, The deviation of the modulation frequency of the clamp part from the specified value (whether the frequency of the clamp part of the modulation signal is higher, lower, or equal to the specified frequency) is detected. The clamp voltage varying circuit varies the clamp voltage level of the clamp circuit based on the detection result so that the deviation of the modulation frequency from the specified value is reduced.

As described above, it is caused by various factors,
The frequency deviation of the modulation signal output from the frequency modulation circuit is detected by comparing the frequency of the clamp part of the modulation signal with the specified reference frequency, and the clamp voltage level of the clamp circuit is automatically adjusted according to the detection result. By configuring such a closed loop, the frequency of the modulation signal can be accurately controlled to a specified value, and the modulation frequency can be stabilized.

In particular, instead of comparing the modulated signal with the reference voltage after demodulating the modulated signal as in the conventional case, the frequency of the clamp portion of the modulated signal is compared with the reference frequency to detect the modulation frequency shift, so that it is eliminated in the conventional case. The modulation frequency shift due to the fluctuation of the power supply voltage, which cannot be cut off, can be surely eliminated.

By the way, a consumer video cassette recorder generally has a plurality of recording modes, and a frequency modulation circuit capable of frequency-modulating a video signal by switching a plurality of different modulation frequencies according to the recording mode is provided. A provided frequency modulator is used. Also for such a frequency modulator having two or more types of modulation frequencies, as in the invention of claim 2, the reference frequency generating circuit is provided with a plurality of reference frequencies having different frequencies corresponding to the respective modulation frequencies. It is possible to easily cope with the problem by simply switching the signals and outputting the signals, and the circuit structure can be simplified as compared with the conventional case.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following will describe one embodiment of the present invention with reference to FIGS.

As shown in FIG. 1, the frequency modulator according to the present embodiment includes a horizontal sync separation circuit 1 for separating the horizontal sync signal from the video signal 0a, and a horizontal sync signal output from the horizontal sync separation circuit 1. 1a, a clamp circuit 2 for setting the DC voltage level of the horizontal sync tip of the video signal 0a to a clamp voltage level, a frequency modulation circuit 3 for frequency modulating the output signal of the clamp circuit 2, and this frequency modulation circuit. Modulation frequency stabilizing circuit 10 for controlling the clamp voltage of the clamp circuit 2 so that the modulation frequency of the horizontal synchronizing tip portion of the modulation signal 3a output from 3 becomes a specified value.
It has and.

The video signal modulated by the frequency modulation circuit 3 of the frequency modulator is sent to the recording amplifier circuit 8 and recorded on a recording medium such as a magnetic recording medium (not shown).

The clamp voltage of the clamp circuit 2 is set by the DC voltage generation circuit 7 of the modulation frequency stabilizing circuit 10, and the frequency of the modulation signal 3a output from the frequency modulation circuit 3 is determined by this set voltage.

The modulation frequency stabilizing circuit 10 extracts a horizontal synchronization tip portion of the modulation signal 3a based on the horizontal synchronization signal 1a output from the horizontal synchronization separation circuit 1 and outputs a horizontal synchronization portion extraction circuit (clamp portion extraction). Circuit 4), a reference frequency generation circuit 6 for generating a reference frequency signal 6a, the horizontal synchronization partial modulation signal 4a from the horizontal synchronization partial extraction circuit 4 and the reference frequency signal 6a are compared in frequency for horizontal synchronization. The frequency comparison detection circuit 5 for detecting the deviation of the modulation frequency of the partial modulation signal 4a from the specified value, and the DC voltage 7a having a voltage level corresponding to the detection result (detection signals 5a and 5b) of the frequency comparison detection circuit 5 are described above. A DC voltage generation circuit (clamp voltage variable circuit) 7 that outputs the voltage to the clamp circuit 2 and variably sets the clamp voltage level is provided.

An example of the frequency comparison / detection circuit 5 will be described below with reference to FIGS. 2 and 3.

As shown in FIG. 2, the frequency comparison / detection circuit 5 basically has an enable pulse generation circuit 50.
1, edge pulse generation circuits 502 and 503, first gate pulse generation circuit 504, second gate pulse generation circuit 505, first counter circuit 506, second counter circuit 5
07 and a magnitude comparator circuit 508.

The horizontal synchronization partial modulation signal 4a from the horizontal synchronization partial extraction circuit 4 is the enable pulse generating circuit 50.
1 and the edge frequency generating circuit 502, the reference frequency signal 6a from the reference frequency generating circuit 6
Is input to the edge pulse generation circuit 503. The enable pulse creation circuit 501 creates an enable pulse 5c indicating a comparison detection range of frequencies based on the horizontal synchronization partial modulation signal 4a. The edge pulse generation circuit 502 generates the modulated edge signal 5d corresponding to the pulse leading edge (rising edge) of the horizontal synchronization partial modulation signal 4a, while the edge pulse generation circuit 503 generates the reference frequency signal 6
A reference edge signal 5e corresponding to the pulse leading edge (rising edge) of a is generated (see FIG. 3). Each of the circuits 501 to 5
The enable pulse 5c, the modulated edge signal 5d, and the reference edge signal 5e created in 03 are sent to the first and second gate pulse creation circuits 504 and 505, respectively.

The first gate pulse generation circuit 504 is
After the rising edge of the enable pulse 5c, a first gate pulse 5f that rises at the first input reference edge signal 5e and falls at the subsequently input modulated edge signal 5d is generated and output to the first counter circuit 506. To do. On the other hand, the second gate pulse generation circuit 505 is
After the rising edge of the enable pulse 5c, a second gate pulse 5g that rises at the second input reference edge signal 5e and falls at the subsequently input modulated edge signal 5d is generated, and is sent to the second counter circuit 507. Output.

A clock signal 5h having a predetermined frequency is input from an oscillator (not shown) to the first counter circuit 506 and the second counter circuit 507, and the first counter circuit 506 uses the first gate pulse 5f as a clock signal. 5h is gated (see signal 5i shown in FIG. 3), the clock signal 5h is counted and the first count data 5k is output only during the period of the first gate pulse 5f, while the second
The counter circuit 507 gates the clock signal 5h with the second gate pulse 5g (see the signal 5j shown in FIG. 3), counts the clock signal 5h only during the period of the second gate pulse 5g, and outputs the second count data 5l. .

Since the frequency (cycle) of the clock signal 5h determines the detection limit value of the deviation of the modulation frequency, it is desirable to set the frequency as high as possible than the modulation frequency.

The above-mentioned first count data 5k and second count data
The count data 5l is compared by the magnitude comparator circuit 508. Here, the first count data 5
When k is larger, the modulation frequency is higher than the reference frequency, while when the second count data 5l is larger, conversely, the modulation frequency is lower than the reference frequency. When the first count data 5k and the second count data 5l are equal, the modulation frequency and the reference frequency are in the same state. In this way, the deviation of the modulation frequency from the specified value can be detected by comparing the both data 5k and 5l. The magnitude comparator circuit 508 determines whether the modulation frequency is higher than the reference frequency as a comparison result of both data 5k and 5l.
Two detection signals 5a and 5 are determined as low or equal.
It is represented by b (2-bit detection data) and output. Then, the DC voltage generating circuit 7 of FIG. 1 controls the output level of the DC voltage 7a based on the detection signals 5a and 5b.

In an actual circuit, depending on the phase relationship between the modulated edge signal 5d and the reference edge signal 5e shown in FIG.
Since the lengths of the first gate pulse 5f and the second gate pulse 5g change extremely and correct data may not be obtained in some cases, a correction circuit is required to prevent the detection data from being output in such a case. However, the correction circuit is omitted here.

If the video signal as the modulated signal and the modulated signal are positive clamps in the relationship as shown in FIG. 5, for example, when the modulation frequency is higher than the reference frequency, FIG.
When the modulation frequency is lower than the reference frequency, the output level of the DC voltage 7a is increased, and the output level of the DC voltage 7a is increased so that the horizontal synchronizing tip is clamped by the clamp circuit 2. Adjust the voltage
The frequency of the modulated signal 3a is set to a specified value.

Looking at the VHS video cassette recorder, which is the most popular as a magnetic recording / reproducing apparatus for consumer video signals, a VH for performing normal video recording / reproduction will be described.
S-VH that records and plays back images in S mode and high resolution
S mode and H for recording / playback of high-definition video
There is a standard called W-VHS which has three recording modes of D mode in one unit.

In such a VHS video cassette recorder, since the recording modulation frequency in each recording mode is different,
It is necessary to detect the deviation from the specified frequency for a plurality of different modulation frequencies, but even in such a case, it is not necessary to provide a plurality of modulation frequency stabilizing circuits of the present invention, and it is possible to correspond to the modulation frequency of each recording mode. Then, if a plurality of reference frequency signals 6a having different frequencies are switched and output from the reference frequency generation circuit 6, it is possible to cope with a plurality of modulation frequencies without increasing the circuit scale so much.
An example of this reference frequency generation circuit 6 is shown in FIG.

The reference frequency generating circuit 6 is basically a first to a third reference frequency generating circuit 6 for generating a reference frequency signal 6a corresponding to the modulation frequency of each recording mode.
01.602.603 and the modulation frequency switching signal 9a by the selection of the recording mode from the system controller 9 (see FIG. 1) as the reference frequency switching instruction means,
It is composed of a switch 604 for switching the output of the reference frequency generation circuit 6 to the output of any of the first to third reference frequency generation circuits 601, 602, 603.

Although the reference frequency generating circuit 6 having three kinds of modulation frequencies is illustrated in FIG. 4, the case of having two kinds or four or more kinds of modulation frequencies can be similarly considered.

Further, in the above description, in order to facilitate understanding of the present invention, the first to third reference frequency generation circuits 601, 6 are provided.
Although the configuration in which the output of 02.603 is switched by the switch 604 is shown, the present invention is not limited to this, and in an actual circuit, the control voltage, the time constant of the peripheral circuit, etc. are switched in one reference frequency generation circuit. As a result, the frequency of the output signal can be changed.

The operation of the frequency modulator having the above structure will be described below.

As shown in FIG. 1, when the video signal 0a is input to the frequency modulator, the horizontal sync separation circuit 1 separates the horizontal sync signal 1a from the video signal 0a, and based on the horizontal sync signal 1a, Clamp circuit 2 is video signal 0
Clamp the horizontal sync tip of a. The clamp voltage at this time is set by the DC voltage 7a from the DC voltage generating circuit 7 of the modulation frequency stabilizing circuit 10. The signal output from the clamp circuit 2 is frequency-modulated in the frequency modulation circuit 3.

The modulated signal 3a output from the frequency modulation circuit 3 is sent to the recording amplifier circuit 8 to be recorded on a recording medium such as a magnetic recording medium, and is also sent to the modulation frequency stabilizing circuit 10. In this modulation frequency stabilizing circuit 10, the horizontal synchronization part extraction circuit 4 extracts the horizontal synchronization leading end portion of the modulation signal 3a based on the horizontal synchronization signal 1a from the horizontal synchronization separation circuit 1. The frequency comparison / detection circuit 5 then outputs the horizontal synchronization partial modulation signal 4a output from the horizontal synchronization partial extraction circuit 4 and the reference frequency generation circuit 6 to each other.
The frequency of the modulation frequency is compared with the reference frequency signal 6a output from the detection circuit 5 to detect the deviation from the specified value of the modulation frequency, and the detection signals 5a and 5b are output to the DC voltage generation circuit 7 as the detection result. The DC voltage generation circuit 7 varies the output level of the DC voltage 7a based on the input detection signals 5a and 5b to adjust the clamp voltage of the clamp circuit 2 so that the deviation from the specified value of the modulation frequency is reduced. adjust. As a result, the modulation frequency of the recording signal is accurately controlled to the specified value.

When a recording mode is selected by a mode selector (not shown), the system controller 9 causes the modulation frequency switching signal 9a corresponding to the selected recording mode.
Is output to the frequency modulation circuit 3 and the reference frequency generation circuit 6, the modulation frequency of the frequency modulation circuit 3 is switched, and the frequency of the reference frequency signal 6a output from the reference frequency generation circuit 6 is also switched correspondingly. Also in this case, the modulation frequency of the recording signal is stably controlled by the same operation as described above.

As described above, the modulation frequency stabilizing circuit 10 of the frequency modulator according to the present embodiment is based on the horizontal sync separation circuit 1 for separating the horizontal sync signal 1a from the video signal 0a and the horizontal sync signal 1a. A clamp circuit 2 for setting the DC voltage level of the horizontal synchronizing tip portion of the video signal 0a to the clamp voltage level, and a frequency modulation circuit 3 provided after the clamp circuit 2 for frequency-modulating the video signal 0a. It is provided in a frequency modulator to stabilize its modulation frequency, and based on the horizontal synchronizing signal 1a separated from the video signal 0a, the horizontal synchronizing tip portion of the modulating signal 3a output from the frequency modulating circuit 3 described above. The horizontal synchronization part extracting circuit 4, the reference frequency generating circuit 6 for generating the reference frequency signal 6a, and the horizontal synchronization part extracting circuit 4 The frequencies of both the modulation signal (horizontal synchronization partial modulation signal 4a) corresponding to the tip portion and the reference frequency signal 6a are compared to detect a deviation of the modulation signal from the specified value of the modulation frequency of the horizontal synchronization tip portion. The clamp voltage level of the clamp circuit 2 is changed according to the frequency comparison detection circuit 5 and the detection result (detection signals 5a and 5b) of the frequency comparison detection circuit 5 so that the deviation from the specified value becomes small. This is a configuration including a DC voltage generating circuit 7.

As a result, it is caused by various factors,
The frequency shift of the modulation signal 3a output from the frequency modulator is calculated by measuring the horizontal synchronization tip portion (clamp portion) of the modulation signal 3a.
Detected by comparing the frequency of
Since the modulation frequency stabilizing circuit 10 forms a closed loop capable of automatically adjusting the clamp voltage level of the clamp circuit 2 based on the detection result, it is possible to accurately control the frequency of the modulation signal 3a to a specified value. Therefore, the modulation frequency can be stabilized. In particular, although the influence of the power supply voltage fluctuation cannot be completely eliminated by the conventional technique, the modulation frequency stabilizing circuit 10 of the present embodiment can surely eliminate the modulation frequency shift due to the power supply voltage fluctuation.

Further, the frequency modulation circuit 3 of the frequency modulator of the present embodiment is capable of frequency-modulating the video signal 0a by switching a plurality of different modulation frequencies, and the reference frequency generation circuit 10 changes the modulation frequency to each modulation frequency. Correspondingly, a plurality of reference frequency signals 6a having different frequencies are switched and output.

As a result, even if there are two or more types of modulation frequencies, a simple circuit configuration can be used, and thus it is common to have a plurality of recording modes, such as magnetic recording for consumer video cassette recorders. It can be easily applied to a playback device.

With regard to the clamp of the video signal, in the present embodiment, the horizontal synchronizing front end portion of the video signal is used as the clamp point at which the DC voltage level can be always set regardless of the content (state) of the video signal. However, the same can be applied to the case where the clamp is applied to the other part of the video signal. The part of the video signal that can be clamped is a flat part of the video signal (the part where the waveform of the video signal is flat) in which the voltage level of the video signal is constant, and there is a pedestal part in addition to the horizontal sync tip part shown in this embodiment. . When the pedestal portion is clamped, the pedestal portion, which is the clamp point, can be specified by delaying the horizontal synchronization signal 1a obtained by the horizontal synchronization separation circuit 1 by a predetermined time.
In this case, instead of the horizontal synchronization part extraction circuit 4, a pedestal part extraction circuit (clamp) for extracting the pedestal part of the modulation signal based on the horizontal synchronization signal (based on the signal obtained by delaying the horizontal synchronization signal 1a by a predetermined time). Partial extraction circuit) is used.

The above-mentioned embodiments are intended to clarify the technical contents of the present invention, and should not be construed in a narrow sense by limiting only to such specific examples. Various modifications can be made within the scope of the claims.

[0048]

As described above, the modulation frequency stabilizing circuit of the frequency modulator according to the first aspect of the present invention includes the horizontal synchronizing signal separating circuit for separating the horizontal synchronizing signal from the video signal and the horizontal synchronizing signal separated from the video signal. A clamp circuit that specifies a flat portion of the video signal having a constant voltage level of the video signal based on the synchronization signal and sets the DC voltage level of the flat portion of the video signal to the clamp voltage level, and a clamp circuit provided after the clamp circuit. A modulation signal that is provided in a frequency modulator that includes a frequency modulation circuit that frequency-modulates the video signal, and that corresponds to the flat portion of the video signal that is clamped by the clamp circuit based on the horizontal synchronizing signal that is separated from the video signal. Clamp part extraction circuit for extracting the clamp part, a reference frequency generation circuit for generating a reference frequency signal, and a clamp part extraction circuit for the clamp part extraction circuit. Frequency comparison and detection circuit that compares the frequencies of both the modulation signal corresponding to the reference section and the reference frequency signal to detect the deviation of the modulation frequency of the clamp section of the modulation signal from the specified value, and the frequency comparison and detection circuit. And a clamp voltage variable circuit for varying the clamp voltage level of the clamp circuit so that the deviation from the specified value becomes small in accordance with the detection result.

Therefore, the frequency deviation of the modulation signal output from the frequency modulator, which is caused by various factors including the power supply voltage fluctuation, is detected by comparing the frequency of the clamp portion of the modulation signal with the specified reference frequency. , The closed loop is constructed so that the clamp voltage level of the clamp circuit can be automatically adjusted according to the detection result, and it becomes possible to control the frequency of the modulation signal to the specified value with high accuracy and stabilize the modulation frequency. There is an effect that can be realized.

As described above, the modulation frequency stabilizing circuit of the frequency modulator according to the invention of claim 2 is the same as the frequency modulating circuit of the frequency modulator according to the configuration of the invention of claim 1.
A video signal can be frequency-modulated by switching a plurality of different modulation frequencies, and the reference frequency generating circuit switches a plurality of reference frequency signals having different frequencies according to each modulation frequency and outputs the reference signal. Has become.

Therefore, in addition to the effect of the first aspect of the invention, the effect that the frequency modulator having two or more kinds of modulation frequencies can be easily dealt with by a simple circuit configuration is also obtained.

[Brief description of drawings]

FIG. 1 shows an embodiment of the present invention and is a block diagram showing a configuration of a main part of a frequency modulator.

FIG. 2 is a block diagram showing a configuration example of a frequency comparison and detection circuit of the frequency modulator.

FIG. 3 is a signal waveform diagram showing a signal waveform of each part in the frequency comparison and detection circuit.

FIG. 4 is a block diagram showing a configuration example of a reference frequency generation circuit of the frequency modulator.

FIG. 5 is an explanatory diagram showing a relationship between a video signal and a modulation signal in frequency modulation.

FIG. 6 is a block diagram showing a configuration of a main part of a conventional frequency modulator.

[Explanation of symbols]

 1 Horizontal sync separation circuit 2 Clamp circuit 3 Frequency modulation circuit 4 Horizontal sync extraction circuit (clamp extraction circuit) 5 Frequency comparison detection circuit 6 Reference frequency generation circuit 7 DC voltage generation circuit (clamp voltage variable circuit) 10 Modulation frequency Stabilizer 1a Horizontal sync signal 3a Modulation signal 4a Horizontal sync partial modulation signal 5a Detection signal 5b Detection signal 6a Reference frequency signal 7a DC voltage

Claims (2)

[Claims] 1. A horizontal sync signal separation circuit for separating a horizontal sync signal from a video signal, and a flat part of the video signal having a constant voltage level of the video signal is specified based on the horizontal sync signal separated from the video signal,
The frequency modulator is provided with a clamp circuit for setting the DC voltage level of the flat portion of the video signal to a clamp voltage level, and a frequency modulation circuit provided in the latter stage of the clamp circuit for frequency-modulating the video signal. A modulation frequency stabilization circuit for a frequency modulator that stabilizes the modulation frequency, which clamps the modulation signal corresponding to the flat portion of the video signal clamped by the clamp circuit based on the horizontal synchronizing signal separated from the video signal. Compare the frequencies of both the clamp part extraction circuit that extracts the part, the reference frequency generation circuit that generates the reference frequency signal, the modulation signal corresponding to the clamp part from the clamp part extraction circuit, and the reference frequency signal. The frequency comparison detection circuit that detects the deviation of the modulation frequency of the clamp part of the modulation signal from the specified value. And a clamp voltage variable circuit for varying the clamp voltage level of the clamp circuit according to the detection result of the number comparison detection circuit so as to reduce the deviation from the specified value. Modulation frequency stabilization circuit. 2. The frequency modulation circuit of the frequency modulator is capable of frequency-modulating a video signal by switching a plurality of different modulation frequencies, and the reference frequency generation circuit corresponds to each modulation frequency.
2. A modulation frequency stabilizing circuit for a frequency modulator according to claim 1, wherein a plurality of reference frequency signals having different frequencies are switched and output.