JPH0424780B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides frequency modulation (FM) for audio signals, for example.
The present invention relates to a frequency modulator/demodulator suitable for use in an audio recording/reproducing device that performs recording/reproduction by modulating the frequency.

Conventionally, one of the methods for recording and reproducing signals is
A method of recording a signal by FM modulating it is known, and its application range is wide-ranging.

Here, the video magnetic recording and reproducing device (hereinafter simply referred to as
It's called VTR. ) will be explained by taking as an example the case where it is applied to the recording of audio signals.

The recording density of VTRs has improved dramatically in recent years, reaching more than 17 times that of about 10 years ago. In addition, efforts are being made to make VCRs smaller. As a result, the running speed of the magnetic tape and the recording track width have decreased. As a result, in the method of recording an audio signal using a high frequency bias method using a fixed head, reproduction sound quality deteriorates, such as a reduction in reproduction frequency bandwidth, deterioration in reproduction S/N, and deterioration in feather and flutter characteristics. One known method for preventing these deteriorations in playback quality is to perform FM modulation on an audio signal, frequency multiplex it with a video signal, and record it using a rotating head (hereinafter simply referred to as audio FM multiplex recording method). . this audio
The characteristics of the FM multiplex recording system are as follows: (1) It is less susceptible to time axis fluctuations due to uneven magnetic tape running speed, so it has good wow and flutter characteristics.

(2) The playback frequency band does not depend on the tape running speed, and a wide band is possible.

etc. can be mentioned.

Generally, in a VTR, the luminance signal is FM-modulated and the chromaticity signal is low-frequency converted to the lower side of the FM-modulated luminance signal for recording, so the method of frequency multiplexing the FM-modulated audio signal is as follows: As shown in A of FIG. 1a, a method of frequency multiplexing between the FM modulated luminance signal Y and the low-pass converted chromaticity signal C;
A method of frequency multiplexing the lower side of the low frequency converted chromaticity signal C as shown in FIG. 1b may be considered. In this case, the audio FM multiplex recording system requires an FM modulation circuit and an FM demodulation circuit due to its configuration.

Figure 2 shows an example of the configuration of an FM modulation/demodulation circuit in the audio FM multiplex recording system. During recording, the audio signal input from the input terminal 1 passes through an automatic gain control circuit (AGC circuit) 2, a pre-emphasis circuit 3, and an amplitude clip circuit 20 to prevent overmodulation, and then undergoes debiasing adjustment by a level adjuster 21. After that, the signal is input to the FM modulator 4 and subjected to FM modulation. The output signal of the FM modulator 4 is filtered by a low pass filter (LPF) 5 to remove unnecessary band components, and a level adjuster 6 adjusts the recording level.
8, it is added to the FM modulated video signal input from the input terminal 19. The output signal of the adder 18 is recorded on the magnetic tape 8 by the magnetic head 7. Also,
The input signal to the FM modulator 4 is sent to the switch circuit 15.
The signal is then passed through the de-emphasis circuit 16 and outputted from the output terminal 17 as a recording monitor signal. During reproduction, only the FM audio signal of the FM signal reproduced from the magnetic tape 8 by the magnetic head 7 is extracted by a band pass filter (BPF) 13. The extracted FM audio signal passes through a limiter circuit 9 and then is FM demodulated in a quadrature detection circuit 12 comprising a multiplier 10 and a phase shifter 11.
The signal FM demodulated by the quadrature detection circuit 12 passes through a level adjuster 14, a switch circuit 15, and a de-emphasis circuit 16, and then is outputted from an output terminal 17 as a reproduced audio signal. Here, the level adjuster 14 is for matching the output level of the monitor signal during recording with the level of the output audio signal during playback, and the switch circuit 15 is controlled to be switched during recording and playback.

Note that the FM modulator 4 may be a voltage controlled oscillator (VCO) using a multivibrator, varicap, etc., and the FM demodulation circuit 12 may be a
In addition to the quadrature detection circuit described above, a phase locked loop (PLL) detection circuit, a pulse count detection circuit, etc. can also be used.

However, simply combining it with the FM demodulator mentioned above
FM modulation/demodulation circuits are: (1) The circuit scale is large.

(2) When considering IC implementation, the number of required terminals is large. For example, if an emitter-coupled unstable multivibrator is used as an FM modulator, and a quadrature detection circuit is used as an FM demodulator, it will be integrated into an IC.
Terminals are also required. Furthermore, there are many peripheral circuit components such as coils and capacitors.

(3) Level adjustment is required between the monitor signal level during recording and the audio output level during playback.

There are drawbacks such as.

An object of the present invention is to eliminate the drawbacks of the above-mentioned techniques, and to provide an FM modulator/demodulator that has a small circuit scale, has a small number of external lead-out terminals when implemented as an IC, and has a stable oscillation frequency.

In order to achieve the above object, the present invention includes an FM modulator, a phase detector, a loop filter, and a switch circuit equipped with an automatic frequency control circuit (AFC) that detects the amount of deviation between the output signal of the FM modulator and a reference signal. in
It constitutes an FM modulator/demodulator, and during recording, the FM modulator equipped with AFC performs FM by operating a switch circuit.
FM with the above AFC when modulating and demodulating
The feature is that a modulator, a phase detection circuit, a loop filter, and a switch circuit constitute a PLL detection circuit to perform FM demodulation. By doing this, the circuit scale can be reduced, and
When integrated into an IC, the number of lead-out terminals may also be small.
Further, there are advantages such as no need to adjust the output level of the monitor signal and the demodulated audio signal during recording, and variations in oscillation frequency and changes due to temperature characteristics can be ignored.

Hereinafter, the present invention will be explained in detail with reference to Examples. FIG. 3 shows an embodiment of an FM modulator/demodulator in an audio FM multiplex recording system to which the present invention is applied,
The part surrounded by the broken line is the configuration of an integrated circuit (IC). During recording, an audio signal input through the input terminal 1 is pre-emphasized through the AGC circuit 2 and the pre-emphasis circuit 3, and then output from the terminal 33. Here, terminals 32, 33
A resistor and a capacitor are connected as shown in the figure to form a pre-emphasis circuit. Further, an AGC circuit control signal obtained by detecting the pre-emphasized audio signal with a diode is applied to the terminal 31. As mentioned above, this AGC circuit performs operation control using the output signal of the pre-emphasis circuit 3, and also functions as a clip circuit to prevent overmodulation. The pre-emphasized audio signal outputted from the terminal 33 is level-adjusted for debiasing adjustment by the variable resistor 21, and then inputted from the terminal 34 through the switch circuit 22 to the adder 23. Then, it is added to a control signal output from an AFC circuit including a frequency divider 25, a phase detector 26, and a loop filter 27. The output signal of the adder 23 is input to an oscillator 24 (in this embodiment, an emitter-coupled multivibrator is used), and the FM
undergoes modulation. Here, the operation of the AFC circuit is to divide the output signal of the oscillator 24 into 1/46725 by the frequency divider 25 and the reference signal input from the terminal 28 (a 30Hz rectangular wave in this example). Detector 2
6 performs phase detection, a loop filter 27 removes the pulsation component to obtain an oscillation control signal for the oscillator 24, and the oscillator 24 is controlled through the adder 23.In this example, the oscillation frequency is 89H (H represents the frequency of the horizontal synchronizing signal).

Furthermore, since the oscillation frequency of the oscillator 24 changes depending on the input audio signal, the AFC circuit needs to be unaffected by fluctuations in this oscillation frequency, and the frequency for phase detection must be lower than the audio signal band. Must be a low frequency. Therefore, the reference signal input from the input terminal 28 must have a low frequency, such as the 30 Hz rectangular wave used in this embodiment. Therefore, the oscillation frequency control signal that is the output of the AFC circuit input to the adder 23 has the pulsation component further removed by the loop filter 27, so that it contains almost only a DC component. In addition to this example, the reference signal may be a horizontal synchronizing signal or a chromaticity signal carrier wave divided to a low frequency. Here, a capacitor 46, resistors 47, 48 and a variable resistor 49 are connected to terminals 35, 36.
is for adjusting the oscillation frequency of the oscillator 24, and the resistor connected to the terminal 38 is a current source resistor for the oscillator 24 using the emitter-coupled multivibrator. At terminal 39
A capacitor 51 forming a loop filter 27 of the AFC circuit is connected.

Next, the output signal of the oscillator 24 is input to the LPF 5 through the terminal 37, and after unnecessary band components are removed, the recording level is adjusted by the level adjuster 6. Thereafter, the adder 18 adds the FM video signal input from the input terminal 19 to the magnetic head 7.
The data is recorded on the magnetic tape 8 at the same time. Here, as the monitor signal during recording, since the AFC control signal applied to the adder 23 is almost only a DC component, the output signal of the adder 23, that is, the oscillator 2
The input signal to 4 is connected to the resistor 52 at the terminal 40.
After passing through a de-emphasis circuit 16 consisting of a capacitor 53 and a capacitor 53, the signal is outputted from an output terminal 17.

As explained above, the circuit including the AFC circuit and the oscillator 24 (the part surrounded by the dashed line in the figure) constitutes the FM modulator 4 with a stable FM carrier frequency.

During playback, the BPF 13 extracts only the FM audio signal from the FM signal reproduced from the magnetic tape 8 by the magnetic head 7, and the FM audio signal is input to the terminal 45. After passing through the limiter circuit 9, the FM audio signal input to the terminal 45 passes through the phase detection circuit 29,
Switch circuit 22, loop filter 30 and the above
FM modulator 4 consisting of AFC and oscillator 24
FM demodulation is performed by an FM demodulator configured with a PLL detection circuit. The FM demodulated audio signal is an output signal of the adder 23, that is, an input signal to the oscillator 24, and is outputted from the output terminal 17 after passing through the de-emphasis circuit 16 connected to the terminal 40. Here, a capacitor 54 constituting the loop filter 30 is connected to the terminal 44. Further, the limiter circuit 9 and the phase detection circuit 29 are connected to the terminal 4.
It is configured to operate only during reproduction according to the recording/reproduction switching signal inputted from 3, and consists of a phase detection circuit 26, a frequency divider 25, and a loop filter 27.
FM modulator 4 consisting of AFC circuit and oscillator 24
is constantly operating with the power supply _Vcc input from the input terminal 41.

In addition, in this embodiment, the monitor signal and
Although the output signal of the adder 23, that is, the input signal to the oscillator 24 was used as the FM demodulated audio signal, the output signal of the switch circuit 22 may also be used. In this way, the FM modulator 4 composed of the AFC circuit and the oscillator 24 is
Since it is used for recording and reproduction as part of the PLL detection circuit, the input signal level to the FM modulator 4 during recording is equal to the input signal level to the FM modulator 4 during reproduction. In other words, there is no need to adjust the output level between the monitor signal and the reproduced audio signal during recording, and the input signal level vs. frequency conversion characteristics of the FM modulator 4 is compensated during recording and reproduction, so the distortion rate can be improved. . In addition, FM modulation circuit 4
Since it is shared as part of the FM demodulation circuit during playback, the circuit scale can be reduced.

Furthermore, when integrated into an IC, this example uses quadrature detection of an FM modulator using an emitter-coupled multivibrator shown in Figure 2.
Compared to an FM modulation/demodulation circuit that is simply combined with an FM demodulator, the number of lead-out terminals can be reduced by two terminals for the demodulation system, and compared to FM modulation/demodulation circuits using other combinations, the number of lead-out terminals can be reduced by one to three terminals.

Next, a description will be given of the switch circuit 22 which allows the FM modulator 4, which is composed of an AFC circuit and an oscillator 24, to be used both during recording and reproduction. As mentioned above, the FM modulator 4 in FIG.
It operates constantly with the power supply _Vcc input from 1.
The limiter circuit 9 and the phase detection circuit 29 are connected to the terminal 4.
The switch circuit 22 is configured to operate only during reproduction according to the recording/reproducing switching signal inputted from the switch circuit 22, and the switch circuit 22 is controlled to switch during recording/reproducing according to the recording/reproducing switching signal. FIG. 4 shows an example of a specific configuration of the switch circuit 22 and the adder 23 for adding the control signals of the AFC circuit. The base of transistor Q2 is connected to resistor 55~5.
A potential is applied through resistors 58 and 59 from a bias circuit consisting of diodes D1 to D3 and a transistor Q1, and a transistor Q4, which operates only during reproduction, is connected to an input terminal 34 for recording audio signals. Furthermore, the base of the transistor Q3 is connected to the phase detection circuit 2 from the input terminal 64 during reproduction.
9 (Figure 3) and loop filter 30 (Figure 3)
A signal and potential are applied through the . The emitters of transistors Q2 and Q3 are coupled, a resistor 60 and a transistor Q5 are connected in series to the coupled emitters, and a resistor 61 is connected to the emitter of transistor Q5. This transistor Q5
The phase detection circuit 2 is connected to the base of the input terminal 63.
6 (FIG. 3) and an AFC control signal that has passed through a loop filter 27 (FIG. 3) is applied. Therefore, during recording, transistors Q1, Q2, and Q5 are on, and transistors Q3 and Q4 are off, so that
The audio signal input from the input terminal 34 passes through the emitter of the transistor Q2 and the resistor 60, and is output to the collector of the transistor Q5. Here, a potential corresponding to the AFC control voltage applied to the base of the transistor Q5 appears at the collector of the transistor Q5, and a DC potential corresponding to this AFC control voltage is added to the audio signal, and the output terminals 65, 6
6. The sum signal of the audio signal output to the output terminal 65 and the AAFC control signal is generated by the oscillator 24.
The above-mentioned addition signal inputted to the de-emphasis circuit 1, subjected to FM modulation, and outputted to the output terminal 40 is
6 and becomes a monitor signal during recording. During reproduction, transistors Q1, Q3, Q4, and Q5 are turned on and transistor Q2 is turned off, so that the FM demodulated signal that has passed through the phase detection circuit 29 (Fig. 3) and the loop filter 30 (Fig. 3) is transmitted to the transistors. It is output to the collector of transistor Q5 via the emitter of Q3 and resistor 60. here,
Since a DC potential corresponding to the AFC control voltage is output to the collector of the transistor Q5, it is added to the FM demodulated signal and output to the output terminals 40 and 65. The addition signal output to the output terminal 65 is input to the oscillator 24, and the addition signal output to the output terminal 40 is input to the de-emphasis circuit 16 to become a reproduced audio signal. Therefore, by the function of this switch circuit, the FM modulator 4 consisting of the AFC circuit and the oscillator 24 can be used in both recording and reproduction. In the switch circuit shown in FIG. 4, the recording monitor signal and the FM demodulation signal are taken out from the collector of the transistor Q5, but they may be taken out from the combined emitter section of the transistors Q2 and Q3. Furthermore, various configurations other than those described in the above-mentioned embodiments may be employed as specific configurations of the switch circuit that performs the above operations.

By the way, in general, in VTRs, in order to obtain a tracking margin or to perform so-called variable speed playback of slow motion, still images, etc., the head width H is set to the video tracking width T, so that T≦H≦ It is set to about 2T. Therefore,
Receives crosstalk interference from adjacent tracks.
In particular, in the audio FM multiplexing system, crosstalk noise caused by this crosstalk interference occurs in proportion to the difference frequency with the crosstalk signal.
It is very unpleasant to hear along with the reproduced audio signal. One way to prevent this crosstalk interference is to use different FM carrier frequencies for each video track. FIG. 5 shows an embodiment in which the present invention is applied to a method for preventing the above-mentioned crosstalk interference. During recording, the audio signal input from input terminal 1 is sent to AGC circuit 2 and bass assist circuit 3.
Pre-emphasis is carried out through . Here, 6
6 is a detection circuit for controlling the AGC circuit.
The pre-emphasized audio signal is divided into two parts, one of which is debiased by a level adjuster 21, passed through a switch circuit 22 and an adder 23, and is input to an oscillator 24, where it is subjected to FM modulation. The other signal is similarly subjected to debiasing adjustment by a level adjuster 67, and then input to an oscillator 76 via a switch circuit 68 and an adder 69, where it is subjected to FM modulation. Here, in the adders 23 and 69, the control signals of the AFC circuits composed of frequency dividers 25 and 71, phase detectors 26 and 72, and loop filters 27 and 73 are added to the pre-emphasized audio signal. . In addition, the oscillators 24, 76 and each of the above
The AFC circuit and the FM modulator 4, 85 (the part surrounded by the dotted line in the figure) are configured. The oscillation frequency of FM modulator 4 is 89H, and the oscillation frequency of FM modulator 85 is 91H.
It is. The signals FM modulated by the oscillators 24 and 76 have unnecessary band components removed by the LPFs 5 and 74, respectively, and then the recording level is adjusted by the level adjusters 6 and 75.
input terminal 19 by adders 18 and 76.
These signals are added to the input FM video signal. Each added signal is recorded on the magnetic tape 8 by the rotary heads 7 and 77, respectively. Here, switch circuit 2
The pre-emphasized audio signal which is the output of 2 and the output signal of the switch circuit 68 are each supplied to the switch circuit 84 without cutting off the DC potential, but the output signal of the switch circuit 68 is is adjusted by the DC potential adjuster 82 and the signal level adjuster 83 so that it becomes the same as the output signal of the switch circuit 22. The recording monitor signal is an output signal of the switch circuit 84 which is switched by the track switching signal inputted from the input terminal 86, and is outputted from the output terminal 17 through the de-emphasis circuit 16.

During playback, the magnetic head 7,
From the burst-shaped FM signal played on 77
The BPF 13 and 78 each extract the FM audio signal. The extracted FM audio signals pass through limiter circuits 9 and 79, respectively, and then pass through a phase detection circuit 29, a switch circuit 22, a loop filter 30, and the AFC described above.
FM demodulation is performed by an FM demodulator consisting of a position detection circuit 86, a switch circuit 68, a loop filter 81, and the FM modulator 85 similar to the FM demodulator 4 consisting of an oscillator 24, and an FM modulator 85. be done. The FM demodulated audio signal is outputted to the output terminal 17 as an output signal of the switch circuits 22 and 68 by being switched by the switch circuit 84 along the same route as the recording monitor signal. However, the FM modulator/demodulator according to the present invention has the advantage that the oscillation frequency is stable and the signal level of the recording monitor signal and the reproduced audio signal is the same.
There is no need to adjust the DC potential regulator 82 and signal level regulator 83 again. Furthermore, the present invention
When an FM modulator/demodulator is used, AFC is used even in so-called compatible playback, in which a magnetic tape recorded on another VTR is played back on another VTR, so the oscillation frequency is extremely stable, and the FM modulator can also be played back. Because they are sometimes shared, there is an advantage that there is almost no direct current potential deviation or signal level deviation during playback, and that noise and distortion are less likely to occur at the time of track switching.

In the embodiments of the present invention detailed above, the case where the audio is monaural has been illustrated, but it goes without saying that the present invention can also be applied to the case where the audio is stereophonic. As described above, by using the present invention, a method for preventing crosstalk interference from adjacent tracks by varying the FM carrier frequency for each video track can be realized with a simple circuit and simple adjustment. In particular, by fixedly adjusting the direct current potential and signal level of the burst-like reproduced audio signal, it is possible to make it into a continuous signal with almost no distortion in the track switching cycle, which is effective in terms of sound quality.

In the embodiment shown in FIG. 5, the output is switched by the monitor signal track switching signal during recording, but the output may be output while the switch circuit 84 is switched to either one. Furthermore, in all the embodiments described above, the oscillator 24,
Although an example in which an emitter-coupled microvibrator is used as the oscillator 76 has been described, it is clear that other oscillators may be used. Furthermore, in this example, the AFC circuit is
Although the PLL circuit configuration has been described, other configurations may be used. Furthermore, in this embodiment, the oscillator is controlled by the sum signal of the AFC control signal and the pre-emphasized audio signal.
It is clear that even in a configuration in which the oscillator is controlled separately by the AFC control signal and the audio signal, this is equivalent to controlling the oscillator using the addition signal.

As explained above, if the present invention is used, (1) the output level of the recording monitor signal and the reproduced audio signal are equal, and no adjustment is necessary.

(2) The distortion rate of the reproduced audio signal is good.

(3) When implemented as an IC, there are fewer lead-out terminals, fewer peripheral components, and a smaller circuit scale.

(4) The oscillation frequency of the oscillator is stable.

(5) With the above advantages, each video track
The means for preventing crosstalk interference from adjacent tracks by varying the FM carrier frequency can also be implemented with a small circuit scale and fewer adjustment points.

The effects of the present invention are extremely large.

[Brief explanation of drawings]

Figure 1 is a diagram showing an example of a recorded signal spectrum in the audio FM multiplex recording system, Figure 2 is a block diagram showing a conventional example of an FM modulation/demodulation circuit in the audio FM multiplex recording system, and Figure 3 is an FM to which the present invention is applied. FIG. 4 is a block diagram showing a specific example of the switch circuit shown in FIG. 3; FIG. 5 is a block diagram showing an embodiment of the modem; and FIG. FIG. 2 is a block diagram showing one embodiment of the present invention. 22, 68... switch circuit, 23, 69...
Adder, 25, 71... Frequency divider, 26, 29, 7
2, 80...phase detector, 27, 30, 73, 8
1... Loop filter, 24, 70... Oscillator.

Claims (1)

[Scope of Claims] 1. A variable oscillator having an input terminal and an output terminal, and generating an output signal at the output terminal at a frequency corresponding to a signal supplied to the input terminal; and an automatic frequency control circuit that detects the frequency deviation amount between the output signal of the variable oscillator and the reference signal, converts the detected frequency deviation amount into direct current, and feeds it back to the input terminal of the variable oscillator to reduce the frequency deviation amount. a recording means for supplying an audio signal to be recorded to an input terminal of the variable oscillator for frequency modulation and supplying this frequency modulated signal to a magnetic head; and a recording means for supplying a reproduction signal obtained from the magnetic head. A phase detector having a first input terminal and a second input terminal to which an output signal from the variable oscillator is supplied, and generating a phase detection output according to a phase difference between the signals supplied to the first and second input terminals. a loop filter for smoothing the phase detection output, and supplying the smoothed phase detection output output from the loop filter to the input terminal of the variable oscillator, so that the variable oscillator, the phase detector, and the loop filter An audio signal frequency modulator/demodulator comprising demodulating means for forming a loop to PLL demodulate the frequency modulated signal and outputting the smoothed phase detection output to the outside as a reproduced signal.