JPH03245606A - Frequency modulator-demodulator circuit - Google Patents

Abstract

PURPOSE:To eliminate the need for the level adjustment for recording and reproduction by providing a voltage controlled oscillator(VCO), a phase detector, a loop filter and a switch circuit and controlling the VCO so that the same frequency deviation of a signal FM-modulated by the VCO at the recording as that at the reproduction is obtained. CONSTITUTION:A sound signal inputted from an input terminal 1 at recording is outputted from a terminal 20 after passing through an AGC circuit 2 and a pre-emphasis circuit 3, and the level for deviation adjustment is implemented by a variable resistor 32. Then an undesired band component is eliminated by an LPE 5, the recording level is adjusted by a level adjustment device 6 and recorded on a magnetic tape 8 with a magnetic head 7. An FM wave reproduced by the magnetic head 7 from the magnetic tape 8 is inputted to a limiter circuit 9 from a terminal 25 and a PLL detection circuit demodulates the FM wave. Since an input signal voltage to the VCO 4 at the recording is equal to an input signal voltage to the VCO 4 at the reproduction, the output level adjustment for the monitor signal at recording and the reproduced sound signal is not required.

Description

[Detailed description of the invention]

[0001]

[Industrial application field]

The present invention relates to a frequency modulation/demodulation circuit suitable for use in, for example, an audio recording and reproducing apparatus that records and reproduces an audio signal by subjecting it to frequency modulation (FM modulation). [0002]

[Conventional technology]

Traditionally, as one of the methods for recording and reproducing signals,
A method of recording with M modulation is known, and its application range is wide-ranging. [0003]Here, an example will be described in which the present invention is applied to the recording of audio signals in a video magnetic recording/reproducing apparatus (hereinafter simply referred to as a VTR). [0004] The improvement in recording density of VTRs has been remarkable in recent years, reaching more than 17 times that of about 10 years ago. Further, the miniaturization of VTRs is also being promoted. Along with these developments, the running speed of magnetic tape and the recording track width are decreasing. As a result, the method of recording audio signals with a fixed head using the high frequency bias method results in deterioration of reproduced sound quality such as a reduction in frequency bandwidth, deterioration of reproduction S/N, and deterioration of wow/flutter characteristics. One way to prevent these deteriorations in playback quality is to FM modulate the audio signal.
A method of frequency-multiplexing video signals and recording them using a rotating head (hereinafter simply referred to as audio FM multiplex recording method) is known. The features of this audio 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. [0005] (2) The reproduction frequency band does not depend on the tape running speed, and a wide band is possible. [0006] etc. [0007] 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. As shown in A of FIG. 1(a), there is a method of frequency multiplexing between the FM modulated luminance signal Y and the low frequency converted chromaticity signal C, and a method of frequency multiplexing the FM modulated luminance signal Y and the low frequency converted chromaticity signal C as shown in FIG. 1(b). A method such as frequency multiplexing on the lower side of the converted chromaticity signal C can be considered. In this way, the audio FM multiplex recording system requires an FM modulation circuit and an FM demodulation circuit due to its configuration, but conventionally, an FM modulation and demodulation circuit was configured by simply combining an FM modulation circuit and an FM demodulation circuit. . [0008] FIG. 2 shows a configuration example of a conventional FM modulation/demodulation circuit in the audio FM recording system. During recording, an audio signal input from an input terminal 1 passes through an automatic gain control circuit (AGC circuit) 2 and a pre-emphasis circuit 3, and then is input to an FM modulator 4 where it is FM modulated. The output signal of the FM modulator 4 is filtered by a low pass filter (LPF) 5 to remove unnecessary band components, and after the recording level is adjusted by a level adjuster 6, it is recorded on a magnetic tape 8 by a magnetic head 7. be done. The output signal of the pre-emphasis circuit 3 is level-adjusted by a level adjuster 13, passes through a switch circuit 15 and a de-emphasis circuit 16, and is outputted from an output terminal 17 as a recording monitor signal. [0009] During reproduction, an FM wave reproduced from the magnetic tape 8 by the magnetic head 7 passes through the limiter circuit 9, and then is FM demodulated by the quadrature detection circuit 12 composed of a multiplier 10 and a phase shifter 11. be done. F in the quadrature detection circuit 12 above.
The M-demodulated signal 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 adjusters 13 and 14 are 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 switch during recording and playback. . [0010] As the FM modulator 4, a voltage-controlled tokenizer (vC○) using a multivibrator, a varicap, etc.
Further, as the FM demodulation circuit 12, in addition to the above-mentioned quadrature detection circuit, a phase-locked loop detection circuit, a pulse count detection circuit, etc. are often used. [0011]

[Problem to be solved by the invention]

However, although the above-mentioned FM modulation/demodulation circuit simply combining an FM modulator and an FM demodulator has no problems with respect to electrical characteristics, (1) the circuit scale is large. [0012] (2) When considering IC implementation, a large number of terminals are required. For example, if an emitter-coupled valve-stabilized multivibrator is used as the FM modulator and a quadrature detection circuit is used as the FM demodulator, a total of 6 terminals will be required: 3 terminals for the FM modulator and 3 terminals for the FM demodulator. be. Furthermore, there are many peripheral circuit components such as coils and capacitors. [0013] (3) Level adjustment is required between the monitor signal level during recording and the audio output level during playback. [0014] There were drawbacks such as. [0015]An object of the present invention is to eliminate the drawbacks of the prior art described above, and to provide an FM modulation/demodulation circuit which is small in circuit scale and requires a small number of external lead-out terminals when implemented as an IC. [0016]

[Means to solve the problem]

In the present invention, an FM modulation/demodulation circuit is configured with a voltage-controlled keepake (VCO), a phase detector, a loop filter, and a switch circuit.
Performs modulation, and at the time of demodulation, the vCO1 phase detection circuit,
The feature is that FM demodulation is performed using PLL detection using a loop filter and a switch circuit. By doing this, the circuit size can be reduced, the number of lead-out terminals can be reduced when integrated into an IC, and there is no need to adjust the output level of the monitor signal and demodulated audio signal during recording. . [0017]

[Effect]

The frequency modulation/demodulation circuit of the present invention has a VC during recording during playback.
■C○4 is controlled so that the frequency shift is the same as the frequency shift of the FM modulated signal at O4. In other words, when registering VC
Since the input signal voltage to O4 and the input signal voltage to vC○4 during playback are equal, there is no need to adjust the output level of the monitor signal and playback audio signal during recording, and the voltage-frequency conversion of VCO4 is performed by recording and playback. Since the characteristics are compensated, the distortion rate can be improved. [0018]

【Example】

Hereinafter, the present invention will be explained in detail with reference to Examples. [0019] FIG. 3 shows an FM audio FM multiplex recording system to which the present invention is applied.
An example of a modulation/demodulation circuit is shown, and in this example, the portion surrounded by a broken line has an IC configuration. At the time of recording, the audio signal inputted from the input terminal 1 passes through the AGC circuit 2 and the pre-emphasis circuit 3, and then is outputted from the terminal 20, and the level is adjusted by the variable resistor 32 for deviation adjustment. Here, resistors and capacitors are connected to terminals 19 and 20 as shown in the figure to form a pre-emphasis circuit, and terminal 18 is connected to an AGC circuit control signal obtained by detecting the pre-emphasized signal with a diode. is applied. The level-adjusted signal is input from a terminal 21 through a switch circuit 28 to a VCO 4 using an emitter-coupled multivibrator, and is subjected to FM modulation. F
After the M modulated signal is output from the terminal 24, the L
Unnecessary band components are removed by the PF5, the recording level is adjusted by the level adjuster 6, and the magnetic head 7 records the magnetic tape 8.
recorded above. Here, a capacitor 38, resistors 36, 37, and a variable resistor 39 are connected to the terminals 22, 23 as shown, and the oscillation frequency of the VCO 4 is adjusted. The terminal 27 is connected to a current source resistor of CO4 using the emitter-coupled multivibrator. In addition, as a monitor signal during recording, an output signal of the switch circuit 28,
That is, the pre-emphasized signal passes through a de-emphasis circuit 16 comprising a resistor 34 and a capacitor 35 connected to a terminal 33, and then is outputted from an output terminal 17. [0020] During reproduction, the FM wave reproduced from the magnetic tape 8 by the magnetic head 7 is input to the limiter circuit 9 from the terminal 25. The FM wave that has passed through the limiter circuit 9 is transmitted to a phase detection circuit 10.
, a switch circuit 28, a VCO 4, and a loop filter 11. F
The M-demodulated audio signal passes through the de-emphasis circuit 16 connected to the terminal 33, and then is output from the output terminal 17. Here, a capacitor 40 forming the loop filter 11 is connected to the terminal 26. [0021] In this way, since the recording VCO 4 is also used as a PLL detection circuit, the above ■C○4
is controlled. In other words, since the input signal voltage to VCO4 during recording and the input signal voltage to vC04 during playback are equal, there is no need to adjust the output level of the monitor signal and playback audio signal during recording, and the voltage of VCO4 - Since the frequency conversion characteristics are compensated, the distortion rate can be improved. Furthermore, since the circuit is shared, the circuit scale can be reduced. [0022] Furthermore, when integrated into an IC, in this example, the conventional VCO
Compared to an FM modulation/demodulation circuit that simply combines an FM modulator based on FM modulator and an FM demodulator based on quadrature detection, the number of lead-out terminals can be reduced by two terminals.
Compared to an M modem, the number of terminals can be reduced by 1 to 3. [0023] Next, the switch circuit 28 that allows the VCO to be shared during recording and reproduction will be described. In this embodiment shown in FIG. 3, an AGC circuit 2, a pre-emphasis circuit 3
, VCO4 is always operated with power supply ALL9 input from terminal 30, and limiter circuit 9 and phase detection circuit 10 are configured to operate only during reproduction with power supply PB9 input from terminal 31. The switch circuit 28 is controlled by these two power sources to switch during recording and reproduction. FIG. 4 shows an example of a specific configuration of this switch circuit 28. [0024] The base of the transistor Q2 is supplied with a potential from a bias circuit consisting of resistors 41 to 43, diodes D1 to D3, and a transistor Q1, and is connected to the input terminal 21 of the recording audio signal and a transistor Q4 that operates only during reproduction. ing. In addition, the base 45 of the transistor Q3 has
During reproduction, a signal that has passed through the phase detection circuit 10 (FIG. 3) and the loop filter 11 (FIG. 3) is applied, and at the same time, the same potential as the base potential of the transistor Q2 is applied. Transistor Q2. The emitters of Q3 are coupled and a resistor 44 is connected to the coupled emitters. [0025] Therefore, during recording, transistors Ql and Q2 are on and transistors Q3 and Q4 are off, so the audio signal input from the input terminal 21 passes through the emitter follower circuit composed of the transistor Q2 and the resistor 44. Through,
The signal is outputted from the terminal 47 to vCO4 and subjected to FM modulation, and is outputted from the terminal 33 to the de-emphasis circuit 16 and becomes a monitor signal during recording. [0026] During reproduction, transistor Q1. Q3. Since Q4 is turned on and transistor Q2 is turned off, the FM demodulated signal that has passed through the phase detection circuit 10 and loop filter 11 is
Through an emitter follower circuit consisting of a transistor Q3 and a resistor 44, the signal is output from the terminal 47 to vCO4, and the signal from the terminal 33 is output to the de-emphasis circuit 16. The output signal of the de-emphasis circuit becomes a reproduced audio signal. [0027] Therefore, by the function of this switch circuit, vCO4 can be shared by the FM modulator and the FM demodulator. Note that various configurations other than the above-described embodiments may be employed as specific configurations of the switch circuit that performs the above operations. Also, vCO
As an example of No. 4, an example using an emitter-coupled valve-stabilized multivibrator has been described, but it is clear that other vCOs may be used. [0028]

【Effect of the invention】

As described above, in the present invention, the same vC○4 is commonly used as a frequency modulator during recording and as a variable oscillator of a PLL type frequency demodulation circuit during reproduction. The PLL type frequency demodulation circuit operates so that the oscillation frequency of the vC○4 matches the instantaneous frequency of the input signal (playback FM signal), so the change in the oscillation frequency of the VCO4 is exactly the same during recording and playback. Become. In other words, VC○ at the time of recording
Since the input signal of VC4 and the input signal of VC4 during playback are the same, there is no need to adjust the level of the monitor signal during recording and the playback audio signal. Also, the voltage of VCO4 -
Even if the linearity of the frequency conversion characteristic is poor, since a variable oscillator with the same characteristic is used, it will not be affected by the conversion characteristic, and therefore distortion rate deterioration that occurs between modulation and demodulation can be prevented. Furthermore, since the variable oscillator, which is the main element of the FM modulator and FM demodulator, is shared, the number of lead-out terminals can be reduced and the circuit size can be reduced compared to when the FM modulator and demodulator are provided separately. Therefore, the present invention ■

[Brief explanation of drawings]

[Figure 1] Recording signal frequency spectrum diagram of audio FM multiplex recording method, Also suitable for C conversion. Spectrum showing drum example

[Fig. 2] 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 modulation/demodulation circuit to which the present invention is applied;

4 is a circuit diagram showing a specific example of the switch circuit shown in FIG. 3. FIG.

[Explanation of symbols]

4...■C0 10... Phase detection circuit 11...Loop filter 28...Switch circuit

[Document basis]

drawing

[Figure 1] 7 (a) (4)

[Figure 2] L, -/--, a & a+, J 2

[Figure 3]

[Figure 4]

Claims (1)

[Claims] A variable oscillator having an input terminal and an output terminal, the output terminal generating an output signal with an oscillation frequency according to a control signal supplied to the input terminal, and a variable oscillator connected to the input terminal of the variable oscillator. an output terminal, a first input terminal to which an audio signal to be recorded is supplied, and a second input terminal.The first input terminal and the output terminal are connected during recording, and the second input terminal is connected during playback. a switch circuit to which the terminal and the output terminal are connected; a magnetic head connected to the output terminal of the variable oscillator;
It has a first input terminal to which a reproduced signal obtained from the magnetic head during reproduction is supplied, and a second input terminal to which an output signal from the variable oscillator is supplied. a phase detector that generates a phase detection output according to the phase difference; a loop filter that smoothes the phase detection output and supplies the smoothed phase detection output to a second input terminal of the switch circuit; A frequency modulation/demodulation circuit comprising an audio output means for outputting an audio signal to the outside from an output terminal, and characterized in that the audio signal output from the audio output means has the same level during recording and during playback.