JPH0332153B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM modulation/demodulation device, and particularly to an FM modulation/demodulation device in an information recording/reproducing system.

For example, a home video tape recorder (hereinafter referred to as
It is called VTR. The audio information recording and reproducing system in ) employs an alternating current bias system, similar to ordinary audio tape recorders. The AC bias type is a method that straightens the relationship between the recording current and the residual magnetism on the magnetic tape, which is the recording medium, by superimposing an alternating current of 50 KHz to 200 KHz on the signal current to prevent distortion. In the long-time mode of a home VTR that records audio information using this AC bias method, the S/N of the reproduced audio signal obtained is 45 to 50 dB and the frequency band is approximately 5 to 7 KHz. It is no longer possible to perform so-called high-fidelity recording and reproduction using information recording and reproduction systems. At the same time, the magnetic recording and reproducing technology using the AC bias method is a technology that has been improved over many years, and improvements in the recording and reproducing head and the tape itself are progressing, so the S/N and frequency band can be dramatically improved. There is no hope for improvement. Therefore, magnetic tape with a relatively narrow width (for example, 8 mm) is used as a recording medium.
In an audio information recording and reproducing system for a VTR, consideration is being given to adopting a method in which a carrier wave is frequency-modulated based on the signal to be recorded and the resulting FM signal is recorded, similar to the recording of video signals.

An object of the present invention is to provide an FM modulation/demodulation device that is simple in configuration and stable in operation when such a system is adopted.

An FM modulator/demodulator according to the present invention has a center frequency of an FM signal obtained by frequency modulating a carrier wave based on a signal containing information to be recorded by an AFC (automatic frequency control) circuit including a frequency discriminator. The frequency discriminator included in the AFC circuit is used as an FM demodulator for demodulating the FM signal read from the recording medium.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

In FIG. 1, for example, an audio signal supplied to input terminal IN ₁ as a signal containing information to be recorded on a recording medium is connected to capacitor C ₁ and resistor R _{1 .}
The signal is supplied to the input terminal of a voltage controlled oscillator 2 via a high-pass filter 1 consisting of the following. An FM signal whose instantaneous frequency changes according to the instantaneous level of the audio signal is output from the voltage controlled oscillator 2 and is supplied to the output terminal OUT ₁ and also to one input terminal of the changeover switch circuit 3. The other input terminal of the changeover switch circuit 3 is supplied with the FM audio signal read from the recording medium via the input terminal _IN2 . The control input terminal of the changeover switch circuit 3 is supplied with a mode control signal from a mode changeover switch (not shown) or the like, which has a high level during recording and a low level during playback, for example. The changeover switch circuit 3 switches the voltage controlled oscillator 2 supplied to one input terminal when a high level signal is supplied to the control input terminal, that is, during recording.
selectively outputs the output of one and when a low level signal is supplied to the control input terminal, i.e. during playback, the output is read from the recording medium supplied to the other input terminal.
It is configured to output an FM audio signal. The output of this changeover switch circuit 3 is supplied to a discriminator 4. The discriminator 4 discriminates the instantaneous frequency of the FM audio signal selectively output from the switch circuit 3 or the FM signal output of the voltage controlled oscillator 2, and outputs a signal having an instantaneous level corresponding to the instantaneous frequency. The output of this discriminator 4 is supplied to the output terminal OUT ₂ and is also sent to a differential amplifier 6 via a low-pass filter 5 consisting of an integrating circuit, for example.
is supplied to the negative input terminal of A reference voltage E _R is applied to the positive input terminal of the differential amplifier 6.
The output of the differential amplifier 6 is supplied to the input terminal of the voltage controlled oscillator 2 as the output of the AFC circuit 7 via the resistor R ₁ in the high-pass filter 1 .

In the above configuration, during recording, the output of the voltage controlled oscillator 2 is selectively supplied to the discriminator 4, where it is FM demodulated and then passed through the low pass filter 5.
The low frequency components are extracted by . Then, a signal obtained by amplifying the difference signal between this low frequency component and the reference voltage E _R is outputted from the differential amplifier 6 and passed through the resistor R ₁
The center frequency of the voltage controlled oscillator 2, that is, the frequency of the carrier wave of the FM signal output from the voltage controlled oscillator 2 is kept constant.

Next, during playback, the FM read from the recording medium
The audio signal is selectively discriminator 4
The reproduced audio signal obtained is supplied to the output terminal OUT _{2 and} demodulated.

The FM modulation/demodulation device according to the present invention as described above can be replaced with an equivalent circuit as shown in FIG. 2 if attention is paid to the effect on the DC component. Note that R ₂ in FIG. 2 is a resistance in the integrating circuit as the low-pass filter 5. Now, voltage controlled oscillator 2
The voltage-frequency conversion gain of the discriminator 4 is Ko [Hz/V], and the frequency-voltage conversion gain of the discriminator 4 is K _D
[V/Hz], and if the gain of the differential amplifier 6 is Af, then
As is clear from FIG. 2, the center frequency of the output of the voltage controlled oscillator 2, that is, the frequency fc of the carrier wave of the FM signal outputted from the voltage controlled oscillator 2, is expressed by the following equation.

AfKo ₍ E _R −K _D Fc)=fc ……(1) ∴fc=E _R・(KoAf)/ (1+KoK _D Af) ……(2) Here, each If the gain is set, equation (2) can be transformed as shown in the following equation.

fc≒E _R /K _D ...(3) FM output from voltage controlled oscillator 2 from equation (3)
It can be seen that the frequency fc of the carrier wave of the signal is uniquely determined by the reference voltage _ER and the frequency-voltage conversion gain _KD of the discriminator 4. Further, the control voltage Vc supplied to the voltage controlled oscillator 2 is expressed by the following equation.

Vc≒E _R /KoK _D ...(4) Next, focusing on the effect on the AC component, the first
The device shown in the figure can be replaced with an equivalent circuit as shown in FIG. Now, as in the case in Fig. 2, the voltage-frequency conversion gain of voltage controlled oscillator 2 is
Ko [Hz/V], frequency of discluminator 4 -
If the voltage conversion gain is K _D [V/Hz], the gain of the differential amplifier 6 is Af, the signal to be recorded is e _i , the FM audio signal is e _p , and the output of the differential amplifier 6 is e _x , as is clear from FIG. 3, the following equation holds.

{(e _i −e _x )SC ₁ R ₁ /(1+SC ₁ R ₁ )+e _x }KoK _D =
e _p ...(5) -Afe _p /(1+SC ₂ R ₂ )=e _x ...(6) Here, S is a complex variable.

Now, when 1〓o ₀ K _D Af is substituted into equation (6), the following equation is obtained.

e _p /e _i = (1/Af) {SC ₁ R ₁ (1 + SC ₂ R ₂ )} / {(S ² C ₁ C ₂ R ₁ R ₂ /KoK _D Af +S(C ₁ R ₁ +C ₂ R ₂ )/KoK _D Af+1} ...(7) Here, by setting ω ₁ = 1/C ₁ R ₁ and ω ₂ = 1/C ₂ R ₂ , equation (7) can be transformed as shown in the following equation.

e _p /e _i (jω)=−(1/Af) {ω/ω ₁ (ω/ω ₂ −j
)} /[{1−(1/KoK _D Af)(ω ² /ω ₁ ω ₂ )} +(jω/KoK _D Af)(1/ω ₁ +1/ω ₂ )] ……(8) (8 ) From the formula, (e _p /e _i ) → 0 (ω → 0), so
No modulation/demodulation is performed in the DC region. Furthermore, since e _p /e _i (jω)→−Ko·K _D (ω→∽), the modulation and demodulation frequency characteristic of the FM modulation and demodulation device according to the present invention becomes a high-pass characteristic.

Figure 4 shows an 8mm FM modulator/demodulator according to the present invention.
FIG. 2 is a block diagram showing a case where the present invention is used in a recording and reproducing system for audio information recorded superimposed on a video signal by a rotating head in a VTR. In FIG. 4, the output terminal OUT ₁ , the changeover switch circuit 3 and the discriminator 4 are connected in the same way as in FIG. 1. However, in this example, the other input terminal of the changeover switch circuit 3 has a signal read from the recording medium and supplied to the input terminal IN ₂ .
An FM audio signal is fed through a bandpass filter 8 and a limiter 9. The bandpass filter 8 has a center frequency equal to the frequency of the carrier wave of the FM audio signal, for example, 1.4MHz, and a bandwidth of about twice the maximum frequency deviation, for example, 400KHz.
It is configured so that Further, the output of the voltage controlled oscillator 2 is supplied to one input terminal of the changeover switch circuit 3 via a bandpass filter 10 having the same configuration as the filter 8.

On the other hand, an audio signal supplied to the input terminal IN ₁ as a signal containing information to be recorded is amplified by an input amplifier 11 and then supplied to a compression amplifier 12 forming a noise suppression system. The compression amplifier 12 is configured to reduce the dynamic range of the input signal so as to lower the peak level of the input signal below the saturation level of the tape, and to increase the small signal so that the signal is larger than the noise level. ing. The output of the compression amplifier 12 is supplied to the input terminal of the voltage controlled oscillator 2 via a high pass filter 1 and a level shifter 13, each consisting of a capacitor C ₁ and a resistor R ₁ . Note that the level shifter 13 is configured to shift the instantaneous level of the output of the high-pass filter 1 so that the level of the DC component in the output of the high-pass filter 1 becomes a level near the optimum operating point of the voltage-controlled oscillator 2.

In addition to the configuration shown in FIG. 1, the discriminator 4 has a configuration including a muting command means for generating a muting command signal when an overmodulated FM signal is supplied. The output of this discriminator 4 is passed through a carrier filter 14 for removing the FM carrier component and a buffer amplifier 15.
The signal is supplied to an expansion amplifier 16 which forms a noise suppression system via an operational amplifier 17, a capacitor C ₃ , resistors R ₃ and R ₄ , and a low-pass filter 19 consisting of a reference voltage _ER . The output of the low-pass filter 19 is supplied to the input terminal of the level shifter 13 via the resistor R1 in the high-pass filter 1 as the output of the AFC circuit 7' formed by the discriminator 4, carrier filter ₁₄ , buffer amplifier 15, and low-pass filter 19. Ru. The expansion amplifier 16 is configured to perform signal processing opposite to that of the compression amplifier 12, that is, to restore the peak level to the original level and to return the minute signal to the original minute level signal. The output of the expansion amplifier 16 is amplified by an output amplifier 21 and then supplied to an output terminal OUT ₂ via a muting circuit 22. Muting circuit 2
2 is configured to stop supplying the output of the output amplifier 21 to the output terminal OUT ₂ in response to a muting command signal output from the discriminator 4.

In the above configuration, during recording, the audio signal is supplied to the voltage controlled oscillator 2 via the input amplifier 11, the compression amplifier 12, the high pass filter 1, and the level shifter 13, and the FM signal output from the voltage controlled oscillator 2 is transmitted to the band pass filter 10. and is supplied to the AFC circuit 7' via the changeover switch circuit 3. Then, the output of this AFC circuit 7' is supplied to the control input terminal of the voltage controlled oscillator 2 via the resistor _R1 and the level shifter 13, and the center frequency of the output of the voltage controlled oscillator 2 is set to a predetermined value, similar to the device shown in FIG. It is assumed that the frequency is maintained. The output of the voltage controlled oscillator 2 is supplied to the output terminal OUT ₁ via the bandpass filter 10 as an FM audio signal that is input to a recording equalizer amplifier or the like.

Also, during playback, the data read from the magnetic tape
The FM audio signal is supplied via the bandpass filter 8, limiter 9, and switch circuit 3 to the discriminator 4 in the AFC circuit 7', similar to the device shown in FIG. 1, where it is FM demodulated. A playback audio signal is output. This reproduced audio signal is transmitted to a carrier filter 14, a buffer amplifier 15, an expansion amplifier 16,
The signal is supplied to the output terminal OUT ₂ via the output amplifier 21 and the muting circuit 22.

FIG. 5 is a circuit diagram showing another embodiment of the invention, in which only the low-pass filter 19 is shown. Other circuit blocks 1, 2, 3, 7', 8, 9,
10, 11, 12, 1, 16, 21, 22 are the 4th
They are omitted because they are configured and connected in the same way as in the figure. In this example, the constant voltage source E _R is removed, and the output voltage of the frequency/voltage converter 23 is used as the operational amplifier 17 instead of the output voltage of the constant voltage source E _R.
is supplied to the positive input terminal of This frequency
The voltage converter 17 is supplied with a horizontal synchronizing signal output from, for example, an 8 mm VTR video signal recording/reproducing device (not shown).

Even in this configuration, since the repetition frequency of the pulses forming the horizontal synchronization signal is stable, a stable voltage is applied to the positive input terminal of the operational amplifier 17, and as in the device shown in FIG. 4, equation (3) is satisfied. As shown in FIG. 2, the center frequency of the output of the voltage controlled oscillator 2 is controlled to be constant at a predetermined frequency, and the frequency of the FM carrier component of the FM audio signal becomes stable. It is clear that the signal supplied to the frequency/voltage converter 23 is not limited to the horizontal synchronization signal as long as it has a stable frequency.

FIG. 6 is a circuit block diagram showing still another embodiment of the present invention, in which only filter 1 is shown. Other circuit blocks 2, 3, 7', 8, 9, 1
0, 11, 12, 13, 16, 19, 21, 22
are omitted because they are constructed and connected in the same way as in FIG. In this example, filter 1
are a capacitor _C4 and _a resistor R5 connected in series between the output terminal of the compression amplifier 12 and the input terminal of the level shifter 13, and a capacitor _C5 connected in parallel with the resistor _R5 .
and a resistor _R6 connected between the input terminal of the level shifter 13 and the output terminal of the low-pass filter 19.

In this configuration, the same effect as the device shown in FIG. 4 can be obtained, but in this example, if C ₄ 〓C ₅ , the filter 1 has an angular frequency ω ₁ as shown in FIG. In the range, the transmission amount increases as the angular frequency increases, and in the range from angular frequency ω ₁ to ω ₂ , the transmission amount remains constant,
In the range of angular frequency ω ₂ or higher, the frequency characteristic is such that the amount of transmission increases as the angular frequency becomes higher. Note that ω ₁ and ω ₂ are as shown in the following equation.

ω ₁ = 1/C ₄ (R ₅ + R ₆ ) ...(9) ω ₂ = 1/τ ... (10) τ = C ₅ R ₅ R ₆ / (R ₅ + R ₆ ) ... (11) Then, capacitor C ₄ and resistor R ₅ are connected so that ω ₁ corresponds to the lowest frequency of the audio band.
By setting the value of R ₆ , high-frequency components above the frequency corresponding to the angular frequency ω ₂ are emphasized, making it possible to suppress so-called triangular noise.

As described in detail above, the FM modulation/demodulation device according to the present invention has a center frequency of an FM signal obtained by frequency modulating a carrier wave based on a signal containing information to be recorded by an AFC circuit including a frequency discriminator. In addition, the frequency discriminator included in the AFC circuit is used as an FM demodulator for demodulating the FM signal read from the recording medium, which simplifies the configuration and reduces the circuit size. Obtained by frequency modulating a carrier wave based on a signal containing information that can be recorded as well as information to be recorded.
This means that the center frequency of the FM signal can be stabilized. In addition, when the center frequency of the FM signal is controlled by the signal obtained by amplifying the difference signal between the low-frequency component of the output of the frequency discriminator and the predetermined voltage in the AFC circuit, the method of generating the predetermined voltage may be used. Since the stability of the center frequency of the FM signal changes, the stability of the center frequency of the FM signal can be set depending on the grade of the system in which the FM modulator/demodulator according to the present invention is used.

[Brief explanation of drawings]

FIG. 1 is a circuit block diagram showing an embodiment of the present invention, FIGS. 2 and 3 are circuit block diagrams showing an equivalent circuit of the device shown in FIG. 1, and FIG. 4 is an FM modulation/demodulation system according to the present invention. A block diagram showing a case in which the apparatus is used in an audio information recording/reproducing system for an 8 mm VTR, FIG. 5 is a circuit block diagram showing another embodiment of the present invention, and FIG. 6 is a circuit block diagram showing another embodiment of the present invention. FIG. 7, a circuit diagram showing an embodiment, is a graph showing the frequency characteristics of the filter 1 in the device shown in FIG. Explanation of symbols of main parts, 2...voltage controlled oscillator, 3...changeover switch circuit, 4...discriminator, 7...AFC circuit.

Claims (1)

[Claims] 1. In an information recording and reproducing system, an FM signal to be recorded on a recording medium is generated by frequency modulating a carrier wave based on a signal containing information to be recorded, and an FM signal that is read from the recording medium. An FM modulation/demodulation device that demodulates an FM signal, the voltage controlled oscillator having an instantaneous output frequency that changes depending on the instantaneous level of the signal containing the information to be recorded, and the output of the voltage controlled oscillator and reading from the recording medium. and a frequency discriminator that performs FM demodulation by discriminating the instantaneous frequency of the output of the signal selection circuit. After the output of the voltage controlled oscillator is selectively supplied to the frequency discriminator, the center frequency of the voltage controlled oscillator is controlled by the low frequency component of the output of the frequency discriminator, and the center frequency of the voltage controlled oscillator is controlled. The output of the frequency discriminator is outputted as an FM signal recorded on the recording medium, and when demodulating, the FM signal read from the recording medium is selectively supplied to the frequency discriminator. FM
An FM modulation/demodulation device characterized in that it outputs a demodulated signal. 2. The FM modulator and demodulator according to claim 1, wherein the information is audio information.