JP3804219B2 - Demodulation system for frequency modulation signal - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a demodulation system that extracts an audio signal from a frequency modulation signal.
[0002]
[Prior art]
For example, in a video tape recorder, audio is recorded together with video, and the audio is recorded as a frequency modulation signal obtained by frequency-modulating a carrier signal with an audio signal. Therefore, at the time of reproduction, demodulation is performed by the demodulation circuit, and the original audio signal is extracted from the frequency modulation signal.
The demodulating circuit outputs the extracted audio signal as a signal whose amplitude changes with a constant center bias voltage as the center, but outputs a voltage far from the center bias voltage when no frequency modulation signal is input.
As a specific example, the frequency modulation signal input to the demodulation circuit may be temporarily interrupted due to sound skipping or may be interrupted for a relatively long time.
Therefore, in such a silent period, the output voltage of the demodulator circuit is a voltage far from the center bias, and when the frequency modulation signal is input again after that, the voltage of the voice signal at that time is changed from the voltage far away. To rise.
In other words, the DC potential changes greatly when the frequency modulation signal is re-input, and the subsequent amplifier circuit cannot quickly follow this change. As a result, even if the frequency modulation signal is input, an audio signal can be output immediately. Therefore, there is a disadvantage that the audio reproduction is delayed.
[0003]
The problem of temporary sound skipping and the problem that the output of the audio signal is delayed later when the frequency modulation signal is re-input to the demodulation circuit can be improved, for example, by the following method.
That is, as shown in the circuit diagram of FIG. 4, the output of the demodulation circuit 102 is input to the capacitor 108 through the buffer 104 and the switch means 106 and is held by the capacitor 108, and the frequency modulation signal input to the demodulation circuit 102. Is interrupted, the switch means 106 is turned off, and the switch means 110 is switched to output the voltage held by the capacitor 108 from the system output terminal 112. Further, the capacitor 108 is connected to the predetermined power supply means 114 that outputs the center bias voltage via the resistor 116, and the charge of the capacitor 108 is discharged through the resistor 116, so that the holding voltage of the capacitor 108 passes over time. At the same time, the center bias voltage is gradually approached.
[0004]
Thus, even if a temporary sound skip occurs, the voltage of the audio signal held by the capacitor 108 immediately before the sound is interrupted by switching the switch means 106 and 110 can be output instead of the audio signal. it can.
Further, when the frequency modulation signal is not input for a long time, the charge accumulated in the capacitor 108 is discharged, and the center bias voltage is output after a certain time has elapsed.
Therefore, even if the frequency modulation signal is input again thereafter, the voltage of the system output terminal 112 only changes from the center bias voltage to the voltage of the audio signal at that time, and the change width of the voltage is small.
[0005]
[Problems to be solved by the invention]
However, in this method, the voltage held in the capacitor 108 that is output instead of the audio signal gradually approaches the center bias voltage by discharging through the resistor 116, so that the output voltage of the system becomes the center bias voltage. The time varies depending on the voltage value initially held by the capacitor 108. For example, when the voltage value is low, the time immediately becomes the center bias voltage.
Conversely, if the time until the center bias voltage is reached is too long, the frequency modulation signal may be input again before the center bias voltage is reached. Depending on the audio signal voltage at that time, the fluctuation range of the voltage becomes large. End up.
[0006]
5A is a waveform diagram showing a frequency modulation signal input to the demodulation circuit 102, and FIG. 5B is a waveform diagram showing an audio signal 117 extracted from the frequency modulation signal. In the figure, the horizontal direction represents time, and the vertical direction represents voltage.
As shown in FIG. 5A, when the frequency modulation signal is interrupted at the timing T1, as shown in FIG. 5B, the voltage 118 held by the capacitor 108 is output as an audio signal. As a result of the discharge described above, the voltage gradually approaches the center bias voltage 119 with time, and finally becomes the center bias voltage 119. The various voltages 120 indicated by dotted lines are those when the voltage held by the capacitor 108 is different when the frequency modulation signal is interrupted. When the holding voltage of the capacitor 108 is different, the time t1 until the center bias voltage is reached varies.
[0007]
6A is a waveform diagram showing in detail the voltage change at the system output terminal 112 when the frequency modulation signal is interrupted at a timing when the voltage of the audio signal 117 is relatively high, and FIG. 6B is a waveform diagram showing the voltage of the audio signal 117. It is a wave form diagram which shows in detail the voltage change of the system output terminal 112 when a frequency modulation signal interrupts at a comparatively low timing. In the figure, the horizontal direction represents time, and the vertical direction represents voltage.
As shown in these figures, when the frequency modulation signal is interrupted at the timing T1, the voltage of the system output terminal 112 gradually decreases due to the discharge of the capacitor 108 and finally becomes the center bias voltage 119. The time t1 until the bias voltage is reached is greatly different between (A) and (B).
[0008]
In addition, since the output capacity of the buffer 104 is limited, it is not possible to use a capacitor with a very large capacitance as the capacitor 108. As a result, the resistor 116 is compared in order to ensure a necessary discharge time. It is necessary to use a high resistance value.
Therefore, the connection point between the capacitor 108 and the resistor 116 has a relatively high impedance, and noise is easily mixed.
In the period t2 (FIG. 5) in which the frequency modulation signal is interrupted, the terminal voltage of the capacitor 108 is output as it is in the entire period. Therefore, noise is easily mixed in this period, and an unpleasant noise sound is emitted from the speaker. Is likely to be emitted.
[0009]
Accordingly, an object of the present invention is to provide a demodulation system for a frequency modulation signal that can reliably cope with sound skipping, has low voltage fluctuation when the frequency modulation signal is re-input, and has high noise resistance during signal interruption. It is to provide.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, the present invention is a demodulation system for extracting an audio signal from a frequency modulation signal by a demodulation circuit, which holds a system output terminal for outputting an output signal of the demodulation circuit and an output voltage of the demodulation circuit A capacitor, first switch means for supplying an output signal of the demodulator circuit to one terminal of the capacitor when in an on state, and a power source for outputting a voltage substantially equal to a center bias voltage of the output signal of the demodulator circuit And second switch means for switching and supplying any one of an output signal of the demodulation circuit, a holding voltage of the capacitor, and an output voltage of the power supply means to the system output terminal, and the first and second A control signal is supplied to the switch means to control the first and second switch means, and the demodulation circuit has a reference amplitude level or higher. When the frequency modulation signal having a width is input, the first switch means is turned on to supply the output signal of the demodulation circuit to the terminal of the capacitor, and the second switch means An output signal of a demodulation circuit is supplied to the system output terminal, and when the frequency modulation signal having an amplitude equal to or higher than a reference amplitude level is not input to the demodulation circuit, the first switch means is turned off; and First, the second switch means supplies the holding voltage of the capacitor to the system output terminal, and then, after a predetermined time elapses, causes the second switch means to supply the output voltage of the power supply means to the system output terminal. And a control means.
[0011]
In the frequency modulation signal demodulation system of the present invention, the control means supplies the control signal to the first and second switch means to control the first and second switch means, and the demodulation circuit has a reference amplitude level or higher. When a frequency modulation signal having an amplitude is input, the first switch means supplies the output signal of the demodulation circuit to the terminal of the capacitor, and the second switch means supplies the output signal of the demodulation circuit to the system output terminal. Supply.
On the other hand, when a frequency modulation signal having an amplitude equal to or higher than the reference amplitude level is not input to the demodulation circuit, the first switch means is turned off, and the holding voltage of the capacitor is first applied to the second switch means as a system output terminal. Then, after a predetermined time elapses, the second switch means supplies the output voltage of the power supply means to the system output terminal.
Therefore, in this frequency modulation signal demodulation system, when the frequency modulation signal input to the demodulation circuit is interrupted, the audio signal held by the capacitor immediately before the frequency modulation signal is interrupted for the predetermined time immediately thereafter. The voltage is output from the system output terminal, and thereafter, the center bias voltage from the power supply means is output until the frequency modulation signal is input again.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing an example of a demodulation system for a frequency modulation signal according to the present invention.
As shown in FIG. 1, the frequency modulation signal demodulation system 2 includes a band pass filter 4, a demodulation circuit 6, and an output control unit 8, and forms an audio reproduction circuit of a video tape recorder.
The band pass filter 4 extracts a frequency modulation signal in a band including the audio signal to be extracted from the signal input to the frequency modulation signal demodulation system 2.
The demodulating circuit 6 receives the frequency modulation signal from the bandpass filter 4 and extracts and outputs an audio signal.
Here, a constant center bias voltage is applied to the audio signal output from the demodulation circuit 6. Therefore, the audio signal is output from the demodulation circuit 6 as a signal whose amplitude changes around the center bias voltage. When no frequency modulation signal is input, the output of the demodulating circuit 6 is a voltage far from the center bias voltage.
[0013]
The output control unit 8 is particularly related to the present invention, and is composed of a capacitor 10, first and second switch means 12 and 14 including semiconductor switch elements, a power supply means 16, a control means 18, and the like.
The capacitor 10 is for holding the output voltage of the demodulating circuit 6, and the terminal 20 of the capacitor 10 is connected to the output terminal 24 of the demodulating circuit 6 through the buffer circuit 22 through the first switch means 12. The other end of the capacitor 10 is connected to a reference potential point.
The power supply means 16 is constituted by a constant voltage source 26 and outputs a voltage substantially the same as the center bias voltage of the output signal of the demodulation circuit 6.
[0014]
The second switch means 14 is for switching and connecting the system output terminal 30 to any one of the output terminal 24 of the demodulation circuit 6, the terminal 20 of the capacitor 10, and the output terminal 28 of the constant voltage source 26. In the embodiment, it is constituted by third and fourth switch means 32 and 34 made of semiconductor switch elements.
Each of the third and fourth switch means 32 and 34 has a configuration of one circuit and two contacts. Both the third and fourth switch means 32 and 34 are switched to the common terminal 36 and the common terminal 36. And the first terminal 38 and the second terminal 38 connected to each other. The common terminal 36 of the third switch means 32 and the first and second terminals 38 and 40 are respectively connected to the system output terminal 30 and the demodulating circuit 6. The output terminal 24 is connected to the common terminal 36 of the fourth switch means 34, and the first and second terminals 38, 40 of the fourth switch means 34 are the terminal 20 of the capacitor 10 and the output terminal of the power supply means 16, respectively. 28.
[0015]
The control means 18 includes a detection circuit 42, a delay circuit 44, and first and second control circuits 46 and 48.
The detection circuit 42 receives the input signal of the demodulation circuit 6 and detects the input signal to output a voltage corresponding to the level of the input signal. The delay circuit 44 delays the output of the detection circuit 42 by a predetermined time and outputs it.
When the detection circuit 42 outputs a voltage indicating that the amplitude of the input signal of the demodulation circuit 6 is equal to or higher than the reference amplitude level, the first control circuit 46 outputs a control signal and outputs the first switch means. 12 is turned on to connect the output terminal of the demodulation circuit 6 and the terminal 20 of the capacitor 10, and to connect the output terminal of the demodulation circuit 6 and the system output terminal 30 to the third switch means 32.
On the other hand, when the detection circuit 42 outputs a voltage indicating that the amplitude of the input signal of the demodulation circuit 6 falls below the reference amplitude level, a control signal is output to turn off the first switch means 12, In addition, the common terminal 36 of the fourth switch means 34 and the system output terminal 30 are connected to the third switch means 32.
[0016]
The second control circuit 48 outputs a control signal and outputs fourth control means 34 when the delay circuit 44 outputs a voltage indicating that the amplitude of the input signal of the demodulation circuit 6 is equal to or higher than the reference amplitude level. Are connected to the terminal 20 of the capacitor 10 and the second terminal 40 of the third switch means 32.
On the other hand, when the delay circuit 44 outputs a voltage indicating that the amplitude of the input signal of the demodulation circuit 6 is below the reference amplitude level, a control signal is output and the output terminal of the power supply means 16 is output to the fourth switch means 34. 28 and the second terminal 40 of the third switch means 32 are connected.
[0017]
Next, the operation of the frequency modulation signal demodulation system 2 configured as described above will be described.
2A is a waveform diagram showing an input signal of the demodulation circuit 6, and FIG. 2B is a waveform diagram showing an output signal from the system output terminal 30. FIG. Hereinafter, these drawings are also referred to as appropriate. In FIGS. 2A and 2B, the horizontal direction represents time and the vertical direction represents voltage.
[0018]
From the frequency modulation signal input to the frequency modulation signal demodulation system 2, a necessary signal component is extracted by the band pass filter 4, and then demodulated by the demodulation circuit 6 to be converted from the frequency modulation signal 51 to the audio signal 52 (FIG. 2). ) Is taken out.
When the frequency modulation signal is input to the demodulation circuit 6 in this way, the detection circuit 42 outputs a voltage indicating that the amplitude of the input signal of the demodulation circuit 6 is equal to or higher than the reference amplitude level. The control circuit 46 outputs a control signal to turn on the first switch means 12 to connect the output terminal 24 of the demodulation circuit 6 and the terminal 20 of the capacitor 10, and the third switch means 32 is connected to the demodulation circuit 6. Are connected to the system output terminal 30.
Therefore, at this time, the audio signal 52 extracted by the demodulation circuit 6 is output from the system output terminal 30 through the third switch means 32.
[0019]
After that, as shown in FIG. 2A, when the frequency modulation signal 51 is interrupted at the timing T1, and therefore the input signal of the demodulation circuit 6 is interrupted, the detection circuit 42 determines the amplitude of the input signal of the demodulation circuit 6. A voltage indicating that falls below the reference amplitude level is output.
As a result, the first control circuit 46 outputs a control signal to turn off the first switch means 12, and the third switch means 32 is connected to the common terminal 36 and the system output terminal of the fourth switch means 34. 30 is connected.
As a result, the voltage 53 held by the capacitor 10 immediately before the frequency modulation signal 51 is interrupted at the timing T1 becomes the fourth switch means 34 and the third switch means 32 as shown in FIG. Through the system output terminal 30.
[0020]
Since the delay circuit 44 outputs the output signal of the detection circuit 42 after being delayed by a predetermined time, when the predetermined time t3 has elapsed, the delay circuit 44 also has the amplitude of the input signal of the demodulation circuit 6 equal to the reference amplitude. Outputs a voltage indicating that the level is below. As a result, the second control circuit 48 outputs a control signal to connect the output terminal 28 of the power supply means 16 and the second terminal 40 of the third switch means 32 to the fourth switch means 34.
Therefore, as shown in FIG. 2B, a center bias voltage 50 having a constant value is output from the system output terminal 30.
[0021]
Note that, until the time t3 elapses after the frequency modulation signal is interrupted, the magnitude of the voltage output from the system output terminal 30 is interrupted as shown by the dotted line in FIG. Various changes occur depending on the voltage held by the capacitor 10 immediately before.
FIG. 3 is a diagram showing in detail this voltage change. FIG. 3A is a waveform diagram showing the output voltage when the capacitor 10 is holding a relatively high voltage, and FIG. It is a wave form diagram which shows an output voltage at the time of hold | maintaining a low voltage.
That is, as shown in FIG. 3A, when the frequency modulation signal is interrupted when the audio signal 52 is at a relatively high voltage, the capacitor 10 holds the voltage, so that a high voltage 53 is output, and the time When t ₃ elapses, the center bias voltage 50 is switched.
On the other hand, as shown in FIG. 3B, when the frequency modulation signal is interrupted when the audio signal 52 is at a relatively low voltage, the capacitor 10 holds the voltage, so that the low voltage 53 is output, and the time When t ₃ elapses, the center bias voltage 50 is switched.
[0022]
After that, when the frequency modulation signal 51 is input again to the demodulation circuit 6 at the timing T2 (FIG. 2), the first control circuit 46 operates based on the output voltage of the detection circuit 42, and the first switch means 12 is turned on. The output of the buffer 22 is turned on and connected to the terminal 20 of the capacitor 10, while the output terminal 24 of the demodulation circuit 6 is connected to the system output terminal 30 in the third switch means.
The second control circuit operates based on the output voltage of the delay circuit 44 and connects the terminal 20 of the capacitor 10 and the terminal 40 of the third switch means 32 to the fourth switch means.
[0023]
As described above, in the frequency modulation signal demodulation system 2 according to the present embodiment, when the frequency modulation signal 51 input to the demodulation circuit 6 is interrupted, the period immediately after the time t3 immediately before the frequency modulation signal is interrupted. The voltage 53 of the audio signal 52 held by the capacitor 10 is output from the system output terminal 30 and then the center bias voltage 50 from the power supply means 16 is output until the frequency modulation signal is input again. .
[0024]
Therefore, even if the frequency modulation signal is temporarily interrupted due to sound skipping or the like, the voltage 53 held by the capacitor 10 immediately before the signal is interrupted is output instead of the audio signal 52, and the time for which this voltage is output Is constant regardless of the magnitude of the voltage (time t3).
Since the center bias voltage 50 is output from the power supply means 16 after the time t3 has elapsed, when the frequency modulation signal is input again later, the voltage at the system output terminal 30 is demodulated as in the prior art. Instead of the lower limit value of the output voltage of the circuit 6, the center bias voltage 50 changes to the voltage of the audio signal 52 at that time.
As a result, the voltage change at the system output terminal 30 is significantly smaller than in the prior art, and the subsequent amplifier circuit can quickly follow the voltage change, thereby eliminating the inconvenience that sound reproduction is delayed when the frequency modulation signal is input again. .
Further, since the center bias voltage 50 is output from the power supply means 16, the center bias voltage 50 can be output with a low impedance. Therefore, noise is not easily mixed, and an unpleasant noise sound can be emitted from the speaker as in the prior art. It is never done.
When a resistor is connected to a capacitor for holding the audio signal 52 and discharging is performed, it is necessary to use a capacitor having a large capacity to secure a necessary discharge time. In the system 2, this is not necessary, and therefore the capacitor 10 and the demodulator circuit 6 and the like can be built in the integrated circuit to save space.
As mentioned above, although this invention was demonstrated based on embodiment, this is an example to the last and this invention is not limited to this example of course.
[0025]
【The invention's effect】
As described above, in the frequency modulation signal demodulation system of the present invention, the control means supplies the control signal to the first and second switch means to control the first and second switch means, and When a frequency modulation signal having an amplitude equal to or higher than the reference amplitude level is input, the first switch means supplies the output signal of the demodulation circuit to the capacitor, and the second switch means outputs the output signal of the demodulation circuit to the system. Supply to the terminal. On the other hand, when a frequency modulation signal having an amplitude equal to or higher than the reference amplitude level is not input to the demodulation circuit, the first switch means is turned off, and the holding voltage of the capacitor is first applied to the second switch means as a system output terminal. Then, after a predetermined time elapses, the second switch means supplies the output voltage of the power supply means to the system output terminal.
Therefore, in this frequency modulation signal demodulation system, when the frequency modulation signal input to the demodulation circuit is interrupted, the audio held by the capacitor immediately before the frequency modulation signal is interrupted for the predetermined time immediately thereafter. The voltage of the signal is output from the system output terminal, and thereafter, the center bias voltage from the power supply means is output until the frequency modulation signal is input again.
Therefore, even if the frequency modulation signal is temporarily interrupted due to sound skipping, the voltage held by the capacitor immediately before the signal is interrupted is output instead of the audio signal, and the length of time that this voltage is output (above The predetermined time is constant regardless of the magnitude of the holding voltage.
Since the center bias voltage from the power supply means is output after a predetermined time has elapsed, when the frequency modulation signal is input again later, the voltage at the system output terminal is not It changes not from the lower limit value of the output voltage but from the center bias voltage to the voltage of the audio signal at that time.
As a result, the voltage change at the system output terminal is significantly smaller than in the prior art, and the subsequent amplification circuit or the like can quickly follow the voltage change, thereby eliminating the inconvenience that sound reproduction is delayed when the frequency modulation signal is input again.
Furthermore, since the center bias voltage is output from the power supply means, the center bias voltage can be output with a low impedance. Therefore, it is difficult for noise to be mixed in and noise noise is not generated as in the prior art.
When a resistor is connected to a capacitor that holds an audio signal for discharging, it is necessary to use a capacitor having a large capacity as a capacitor. However, in this frequency modulation signal demodulation system, this is not necessary. Can be built in an integrated circuit together with a demodulating circuit to save space.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing an example of a demodulation system of a frequency modulation signal according to the present invention.
2A is a waveform diagram showing an input signal of a demodulation circuit, and FIG. 2B is a waveform diagram showing an output signal from a system output terminal.
3A is a waveform diagram showing an output voltage when the capacitor holds a relatively high voltage, and FIG. 3B is a waveform showing an output voltage when the capacitor holds a relatively low voltage. FIG.
FIG. 4 is a circuit diagram showing a circuit provided at the output section of the demodulation circuit in order to cope with the interruption of the frequency modulation signal.
5A is a waveform diagram showing a frequency modulation signal input to the demodulation circuit, and FIG. 5B is a waveform diagram showing an audio signal extracted from the frequency modulation signal.
6A is a waveform diagram showing in detail the voltage change of the system output terminal when the frequency modulation signal is interrupted at a timing when the voltage of the audio signal is relatively high, and FIG. 6B is a waveform diagram showing the voltage of the audio signal being relatively low. It is a wave form diagram which shows in detail the voltage change of a system output terminal when a frequency modulation signal interrupts with timing.
[Explanation of symbols]
2... Demodulation system of frequency modulation signal, 4... Band pass filter, 6, 102... Demodulation circuit, 8... Output control unit 10, 108. ... Second switch means 16, 114 ... Power supply means, 18 ... Control means, 22, 104 ... Buffer circuit, 26 ... Constant voltage source, 30 ... System output terminal, 32 ... Third switch Means 34... Fourth switch means 42... Detector circuit 44. Delay circuit 46. First control circuit 48. Second control circuit 50 119 center bias voltage 51: Frequency modulation circuit, 52, 117: Audio signal, 53, 118, 120: Capacitor holding voltage, 106, 110: Switch means, 112: System output terminal, 116: Resistance

Claims (5)

A demodulation system for extracting an audio signal from a frequency modulation signal by a demodulation circuit,
A system output terminal for outputting an output signal of the demodulation circuit;
A capacitor for holding the output voltage of the demodulation circuit;
First switch means for supplying an output signal of the demodulator circuit to one terminal of the capacitor when in an on state;
Power supply means for outputting substantially the same voltage as the center bias voltage of the output signal of the demodulation circuit;
Second switch means for switching and supplying one of the output signal of the demodulation circuit, the holding voltage of the capacitor, and the output voltage of the power supply means to the system output terminal;
A control signal is supplied to the first and second switch means to control the first and second switch means, and the frequency modulation signal having an amplitude equal to or higher than a reference amplitude level is input to the demodulation circuit. When the first switch means is turned on, the output signal of the demodulation circuit is supplied to the terminal of the capacitor, and the output signal of the demodulation circuit is supplied to the second output means to the system output terminal. When the frequency modulation signal having an amplitude equal to or higher than a reference amplitude level is not input to the demodulation circuit, the first switch means is turned off, and the capacitor is first held in the second switch means. The voltage is supplied to the system output terminal, and the output voltage of the power supply means is supplied to the second switch means after a predetermined time has elapsed. And control means for supplying to the force terminal,
A frequency modulation signal demodulating system comprising: The second switch means includes third and fourth switch means controlled by the control signal from the control means, and both the third and fourth switch means are a common terminal and the common terminal. A first terminal and a second terminal connected to be switched, wherein the common terminal of the third switch means and the first and second terminals are the system output terminal, the output terminal of the demodulation circuit, and the first terminal, respectively. 4 is connected to a common terminal of the switch means, and the first and second terminals of the fourth switch means are connected to the terminal of the capacitor and the output terminal of the power supply means, respectively. The frequency modulation signal demodulation system according to claim 1. The control means receives an input signal of the demodulation circuit, outputs a detection circuit that outputs a voltage corresponding to the amplitude of the input signal, a delay circuit that delays the output of the detection circuit, and the amplitude of the input signal When the detection circuit outputs a voltage indicating that it is equal to or higher than a reference amplitude level, the control signal is output to turn on the first switch means, and the output signal of the demodulation circuit is sent to the terminal of the capacitor. And the third switch means connects the output terminal of the demodulator circuit and the system output terminal, and the detector circuit generates a voltage indicating that the amplitude of the input signal falls below the reference amplitude level. When outputting, the control signal is output to turn off the first switch means, and the third switch means is connected to the common terminal of the fourth switch means and the A first control circuit for connecting a stem output terminal; and when the delay circuit outputs a voltage indicating that the amplitude of the input signal is equal to or higher than the reference amplitude level, the control signal is output to output the control signal A voltage representing that the amplitude of the input signal falls below the reference amplitude level is connected to the terminal of the capacitor and the second terminal of the third switch means to a fourth switch means. A second control circuit that outputs the control signal to connect the output terminal of the power supply means and the second terminal of the third switch means to the fourth switch means, The frequency modulation signal demodulation system according to claim 2, further comprising: 2. The frequency modulation signal demodulation system according to claim 1, wherein the capacitor is built in an integrated circuit including the demodulation circuit. The frequency modulation signal according to claim 1, further comprising a bandpass filter having a center frequency substantially equal to a center frequency of the frequency modulation signal, and the frequency modulation signal is input to the demodulation circuit through a bandpass filter. Demodulation system.

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