JPH0756924B2 - Frequency modulation / demodulation circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency modulation / demodulation circuit suitable for use in a voice recording / reproducing apparatus for recording / reproducing a voice signal by frequency modulation (FM modulation).

[0002]

2. Description of the Related Art Conventionally, a method of FM-modulating and recording a signal has been known as one of signal recording / reproducing methods, and its application range is wide.

Here, a video magnetic recording / reproducing apparatus (hereinafter, referred to as
Simply called VTR. ) Will be described by way of example when applied to the recording of the audio signal.

The improvement in the recording density of the VTR has been remarkable in recent years, and has reached 17 times or more that of about 10 years ago. Further, miniaturization of VTR is also being promoted.
Along with this, the running speed of the magnetic tape and the recording track width have decreased. As a result, in the method of recording the audio signal with the fixed head by the high frequency bias method,
This causes deterioration of reproduction sound quality such as reduction of frequency bandwidth, deterioration of reproduction S / N, deterioration of wow and flutter characteristics. As one of the methods for preventing the reproduction sound quality deterioration, there is known a method in which an audio signal is FM-modulated, frequency-multiplexed with a video signal and recorded by a rotary head (hereinafter, simply referred to as an audio FM multiplex recording method). . The features of this audio FM multiplex recording system are: (1) Wow and flutter characteristics are good because they are not easily affected by fluctuations in the time axis due to uneven magnetic tape running speed.

(2) The reproduction frequency band does not depend on the tape running speed and can be broadened.

And the like.

Generally, in a VTR, a luminance signal is FM-modulated, and a chromaticity signal is low-frequency converted to a lower side of an FM-modulated luminance signal for recording. Therefore, as a method of frequency-multiplexing an FM-modulated audio signal, Is, as shown in A of FIG.
A method of frequency multiplexing between the FM-modulated luminance signal Y and the low frequency conversion chromaticity signal C, a method of frequency multiplexing below the low frequency conversion chromaticity signal C as shown in FIG. Can be considered. As described above, the audio FM multiplex recording method requires an FM modulation circuit and an FM demodulation circuit because of its structure.
The FM modulation / demodulation circuit has been configured by simply combining the FM modulation circuit and the FM demodulation circuit.

FIG. 2 shows a conventional F in the audio FM recording system.
The structural example of the M modulation / demodulation circuit is shown. Input terminal 1 when recording
The audio signal input by the FM signal passes through the automatic gain control circuit (AGC circuit) 2 and the pre-emphasis circuit 3 and then the FM signal.
The signal is input to the modulator 4 and FM-modulated. The output signal from the FM modulator 4 is recorded on the magnetic tape 8 by the magnetic head 7 after the unnecessary band component is removed by the low pass filter (LPF) 5 and the recording level is adjusted by the level adjuster 6. To be done.
The output signal of the pre-emphasis circuit 3 is level-adjusted by the level adjuster 13, and then is output as a recording monitor signal from the output terminal 17 through the switch circuit 15 and the de-emphasis circuit 16.

During reproduction, the magnetic head 7 is moved from the magnetic tape 8.
After the FM wave reproduced by the signal passes through the limiter circuit 9,
It is FM-modulated by a quadrature detection circuit 12 composed of a multiplier 10 and a phase shifter 11. The signal demodulated by the FM in the quadrature detection circuit 12 is a level adjuster 14,
After passing through the switch circuit 15 and the de-emphasis circuit 16, it is output from the output terminal 17 as a reproduced audio signal. The level adjusters 13 and 14 are for adjusting the output level of the monitor signal at the time of recording and the level of the audio signal at the time of reproduction, and the switch circuit 15 is controlled so as to switch between the time of recording and the time of reproduction. .

The FM modulator 4 is a voltage controlled oscillator (VCO) using a multivibrator, a varicap or the like, and the FM demodulation circuit 12 is a phase locked loop in addition to the quadrature detection circuit described above. A detection circuit and a pulse count detection circuit are often used.

[0011]

However, the FM modulation / demodulation circuit, which is simply the combination of the FM modulator and the FM demodulator described above, has no problem in electrical characteristics, but (1) the circuit scale is large.

(2) Considering the IC implementation, the number of required terminals is large. For example, when an IC is formed using an emitter-coupled astable multivibrator as an FM modulator and a quadrature detection circuit as an FM demodulator, the FM modulator has three terminals,
The FM demodulator requires 6 terminals in total of 3 terminals. In addition, there are many peripheral circuit components such as coils and capacitors.

(3) It is necessary to adjust the monitor signal level during recording and the audio output level during reproduction.

There are drawbacks such as

An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art, and to provide an FM modulator / demodulator circuit having a small circuit scale and a small number of external lead terminals when integrated into an IC.

[0016]

According to the present invention, an FM modulator / demodulator circuit is constituted by a voltage controlled oscillator (VCO), a phase detector, a loop filter and a switch circuit, and the V modulator / demodulator circuit is used for recording.
FM modulation is performed by CO and the switch circuit, and at the time of demodulation
It is characterized in that the VCO, the phase detection circuit, the loop filter and the switch circuit perform FM demodulation by PLL detection. By doing so, the circuit scale can be reduced, the number of lead-out terminals when integrated into an IC can be reduced, and the output level adjustment of the monitor signal during recording and the demodulated audio signal can be eliminated. .

[0017]

In the frequency modulation / demodulation circuit of the present invention, the VCO 4 is controlled so that the VCO 4 has the same frequency deviation as the frequency-modulated signal in the VCO 4 during recording during reproduction. That is, since the input signal voltage to the VCO 4 at the time of recording and the input signal voltage to the VCO 4 at the time of reproduction become equal, it is not necessary to adjust the output level of the monitor signal at the time of recording and the reproduced audio signal, and the voltage of the VCO 4 at recording-reproduction Since the frequency conversion characteristic is compensated, the distortion rate can be improved.

[0018]

EXAMPLES The present invention will be described in detail below with reference to examples.

FIG. 3 shows an embodiment of an FM modulation / demodulation circuit of a voice FM multiplex recording system to which the present invention is applied. In this example, a portion surrounded by a broken line has an IC structure. During recording, the audio signal input from the input terminal 1 passes through the AGC circuit 2 and the pre-emphasis circuit 3, and then is output from the terminal 20. The variable resistor 32 adjusts the level for deviation adjustment. Here, resistors and capacitors are connected to the terminals 19 and 20 as shown in the figure to form a pre-emphasis circuit, and the terminal 18 has an AGC circuit control signal obtained by detecting the pre-emphasis signal with a diode. Is applied. The level-adjusted signal is input from the terminal 21 to the VCO 4 using the emitter-coupled multivibrator through the switch circuit 28 and is FM-modulated. The FM-modulated signal is output from the terminal 24, the unnecessary band component is removed by the LPF 5, the recording level is adjusted by the level adjuster 6, and the signal is recorded on the magnetic tape 8 by the magnetic head 7. Here, the terminals 22, 2
3 includes a capacitor 38, resistors 36 and 37, and a variable resistor 3
9 is connected as shown, and the oscillation frequency of the VCO 4 is adjusted. The terminal 27 is connected to the current source resistor of the VCO 4 using the emitter-coupled multivibrator. The switch circuit 2 is used as a monitor signal during recording.
8 output signal, that is, the pre-emphasized signal passes through the de-emphasis circuit 16 including the resistor 34 and the capacitor 35 connected to the terminal 33, and then the output terminal 1
It is output from 7.

At the time of reproduction, the FM wave reproduced by the magnetic head 7 from the magnetic tape 8 is supplied to the limiter circuit 9 from the terminal 25.
Entered in. The FM wave that has passed through the limiter circuit 9 is FM demodulated by the PLL detection circuit including the phase detection circuit 10, the switch circuit 28, the VCO 4, and the loop filter 11. The FM-demodulated audio signal is output from the output terminal 17 after passing through the de-emphasis circuit 16 connected to the terminal 33. Here, a capacitor 40 that constitutes the loop filter 11 is connected to the terminal 26.

Since the recording VCO 4 is also used in the PLL detection circuit as described above, the VCO 4 is controlled during reproduction so that it has the same frequency deviation as the frequency shift of the signal FM-modulated by the VCO 4 during recording. That is, since the input signal voltage to the VCO 4 at the time of recording and the input signal voltage to the VCO 4 at the time of reproduction become equal, it is not necessary to adjust the output level of the monitor signal at the time of recording and the reproduced audio signal, and V
Since the voltage-frequency conversion characteristic of CO4 is compensated, the distortion rate can be improved. Moreover, since the circuit is shared, the circuit scale can be reduced.

Furthermore, when integrated into an IC, in this example, the number of lead terminals is reduced by 2 terminals as compared with an FM modulator / demodulator circuit in which a conventional FM modulator using a VCO and an FM demodulator using quadrature detection are simply combined. In addition, it is possible to reduce the number of terminals by 1 to 3 compared with the FM modulator / demodulator using other combinations.

Next, the switch circuit 28 that allows the VCO to be shared during recording and reproduction will be described.
In the present embodiment shown in FIG. 3, the AGC circuit 2, the pre-emphasis circuit 3, and the VCO 4 are constantly operating at the power supply ALL9 input from the terminal 30, and the limiter circuit 9 and the phase detection circuit 10 are connected to the terminals. The power source PB9 input from 31 is configured to operate only during reproduction,
The switch circuit 28 is controlled by the two power sources so as to be switched during recording and reproduction. FIG. 4 shows an example of a specific configuration of the switch circuit 28.

The base of the transistor Q2 has resistors 41-41.
A potential is applied from a bias circuit composed of 43, diodes D1 to D3 and a transistor Q1, and a recording voice signal input terminal 21 is connected to a transistor Q4 which operates only during reproduction. Further, to the base 45 of the transistor Q3, the signal that has passed through the phase detection circuit 10 (FIG. 3) and the loop filter 11 (FIG. 3) is applied at the time of reproduction, and the same potential as the base potential of the transistor Q2 is applied. The emitters of the transistors Q2 and Q3 are coupled, and the resistor 44 is connected to the coupled emitters.

Therefore, at the time of recording, the transistors Q1,
Since Q2 is on and the transistors Q3 and Q4 are off, the audio signal input from the input terminal 21 passes through the emitter follower circuit composed of the transistor Q2 and the resistor 44, and is output from the terminal 47 to the VCO4. The signal is FM-modulated and is output from the terminal 33 to the de-emphasis circuit 16 to be a recording monitor signal.

During reproduction, the transistors Q1, Q3, Q
4 turns on and transistor Q2 turns off,
FM passing through the phase detection circuit 10 and the loop filter 11
The demodulated signal passes through the emitter follower circuit composed of the transistor Q3 and the resistor 44, and the VCO4 is fed from the terminal 47.
To the de-emphasis circuit 16 from the terminal 33. The output signal of the de-emphasis circuit becomes the reproduced audio signal.

Therefore, the VCO 4 can be shared by the FM modulator and the FM demodulator by the function of this switch circuit. It should be noted that as the specific configuration of the switch circuit that performs the above operation, various configurations other than the above-described embodiments can be adopted. Also, as an example of the VCO 4, an example using an emitter-coupled astable multivibrator has been described, but other VCOs may be used.
But the obvious is clear.

[0028]

As described above, in the present invention, the same V
The CO4 is shared as a frequency modulator during recording and as a variable oscillator of a PLL type frequency demodulation circuit during reproduction. Since the PLL type frequency demodulation circuit operates so that the oscillation frequency of the VCO 4 matches the instantaneous frequency of the input signal (reproduction FM signal), the change in the oscillation frequency of the VCO 4 is completely the same during recording and during reproduction. That is, since the input signal of the VCO 4 at the time of recording and the input signal of the VCO 4 at the time of reproduction are the same, it is not necessary to adjust the level of the monitor signal at the time of recording and the reproduced audio signal. Also, VCO4
Even if the linearity of the voltage-frequency conversion characteristic is poor, since the variable oscillator having the same characteristic is used, it is not affected by the conversion characteristic, and therefore the distortion rate deterioration that occurs during modulation and demodulation can be prevented. Furthermore, since the FM modulator and the variable oscillator, which is a main element of the FM demodulator, are also used, the number of lead terminals can be reduced and the circuit scale can be reduced as compared with the case where the FM modulator and the demodulator are separately provided. Therefore, the present invention
It is also suitable for C conversion.

[Brief description of drawings]

FIG. 1 is a spectrum diagram showing an example of a recording signal frequency spectrum of an audio FM multiplex recording system,

FIG. 2 is a block diagram showing a conventional example of an FM modulation / demodulation circuit in an audio FM multiplex recording system,

FIG. 3 is a block diagram showing an embodiment of an FM modem circuit to which the present invention is applied.

FIG. 4 is a circuit diagram showing a specific example of the switch circuit shown in FIG.

[Explanation of symbols]

 4 ... VCO 10 ... Phase detection circuit 11 ... Loop filter 28 ... Switch circuit

Front page continuation (56) References JP-A-51-130209 (JP, A) JP-A-48-78863 (JP, A) JP-B 2-12047 (JP, B2) "ELCTORONICS" MCGR AW-HILL PUBLICATION Published, December 8, 1977, p. 100-101

Claims (1)

[Claims] 1. A means for adjusting the level of an input audio signal, an input terminal and an output terminal, wherein an output signal having an oscillation frequency according to a control signal supplied to the input terminal is generated at the output terminal. The variable oscillator and the output signal of the level adjusting means at the time of recording are supplied to the input terminal of the variable oscillator, and the output signal of the level adjusting means is externally output as a monitor signal at the time of recording to obtain at the output terminal of the variable oscillator. Recording means for supplying an output signal to the magnetic head, a first input terminal for supplying a reproduction signal obtained from the magnetic head, and a second for supplying an output signal from the variable oscillator.
And a phase detector that generates a phase detection output according to the phase difference between the signals supplied to the first and second input terminals, and the phase detector outputs power only during reproduction. Supplying means, a loop filter for smoothing the phase detection output, the smoothed phase detection output at the time of reproduction to the input terminal of the variable oscillator, and the smoothed phase detection output as an audio signal at the time of reproduction. A frequency modulation / demodulation circuit comprising: