JPS61187105A - Noise reducing circuit - Google Patents

Abstract

PURPOSE:To reduce a transient sound without extending a muting period by supplying a DC voltage to a resistance with its one end connected to a joint between a holding capacitor and the buffer input terminal of a buffer amplifier. CONSTITUTION:A positive DC voltage VCC is supplied to the resistance R1 whose one end is connected to the joint of the holding capacitor C and the input terminal of the buffer amplifier 28 and one fixed terminal of a variable resistance R2 whose sliding body is connected to the other end of the resistance R1 and whose other fixed terminal is grounded. Here, the resistance value of the resistance R1 is set far smaller than that of the resistance R2, and the sliding body of the resistance R2 takes a DC voltage equal to the central current potential of a terminal voltage when the capacitor C is not holding. A time constant equal to the product of values of the resistance R1 and the capacitor C is selected to a value larger by one digit than the sufficient holding time to remove an impulsive noise in a reproduction sound signal at the time of head switching and to a value smaller by one digit than a time constant when the DC potential of the terminal voltage of the capacitor is changed due to the leakage current of the amplifier 28.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a noise reduction circuit, and particularly to a helical scan V
In a system that records a frequency-modulated audio signal obtained by frequency-modulating a carrier wave with an audio signal on a magnetic tape using a rotating TR head and plays it back, noise in the reproduced audio signal is reduced by holding the previous value. Related to noise reduction circuits.

Conventional Technology Conventionally, in order to record and reproduce audio signals with high quality in a helical scan VTR, a frequency modulated audio signal (hereinafter referred to as F) obtained by frequency modulating a carrier wave with an audio signal has been used.
It is known to record audio signals (referred to as M audio signals) on magnetic tape and to reproduce them. FIG. 7 shows a block diagram of an example of an audio signal recording/reproducing system of such a VTR. In addition,
For convenience of explanation, the audio signal is shown to be recorded and played back in one channel in FIG. 7, but in reality, in order to record and play back a stereo sound signal, the FM sound signal passes through a two-channel recording and playback system. Recorded and played back.

In FIG. 7, the audio signal to be recorded that has entered the input terminal 1 is encoded by the noise reduction circuit 2 to reduce noise during playback, and then is converted into high-frequency noise by the reproduction emphasis circuit 3. The high frequency components are emphasized for the purpose of reduction and then supplied to the frequency modulator 4. The FM audio signal taken out from the frequency modulator 4 is supplied to a recording amplifier 5, where it is amplified and then sent to rotary heads 8 and 9, respectively, through switches 6 and 7 connected to contact R during recording. Supplied.

The rotating heads 8 and 9 are mounted on the rotating drum 10 facing each other at 180°,
FM is performed by forming an inclined track on a magnetic tape 11 that is run while being wound diagonally over a wide angular range.
Record the audio signal. In addition, when the rotary heads 8 and 9 are audio-only heads, two video-only heads are further attached to the rotary drum 10, and the audio recorded deep into the magnetic layer of the magnetic tape 11 by the rotary heads 8 and 9 is attached to the rotary drum 10. A video-dedicated head scans the track and records video signals, and if the rotary head 8.9 is a rotary head that is shared for recording and reproducing video and audio signals, the rotary head 8.9 also has FM audio. It is well known that a video signal having a different band from the signal is supplied, and that the FMR audio signal and the video signal are recorded simultaneously on the same track.

Next, to explain the operation during playback, the rotating head 8.9
As a result, the recorded edge FM audio signal of the recording track of the magnetic tape 11 is reproduced, and the switch 6 connected to the contact P is
．． 7 to the terminals 12a and 12b of the switch circuit 12. The switch circuit 12 is switched by a drum pulse supplied via an input terminal 13 to selectively output the output reproduction signal of the rotary head of the rotary head 8.9 that is currently scanning the magnetic tape 11. The reproduced FM audio signal is continuously extracted from the switch circuit 12 and supplied to the FM demodulator 14 and the envelope detector 15, respectively. The reproduced audio signal taken out from the FM demodulator 14 is supplied to a hold circuit 16, where it is held for a period during which a hold signal is supplied from an adder circuit 19 that adds the rainfall power signals of the hold signal generating circuits 17 and 18.

By the way, the reproduced audio signal output from the FM demodulator 14 generates a large amount of noise in the following cases. ■ Signal connections at the time of switching of the rotary heads 8 and 9, ■ When the envelope level of the reproduced FM audio signal decreases due to dropouts due to scratches or dust on the magnetic tape 11, ■ When the envelope level of the reproduced FM audio signal decreases due to tracking deviation, etc. When the envelope level drops, ■When playing a magnetic tape on which FM audio signals are not recorded. this house,
① indicates that the continuity of the waveform is disrupted at the connection part of the reproduced FM audio signal during head switching due to factors such as differences in tape tension during recording and reproduction, as shown by al in Figure 9 (A) in the demodulated reproduced audio signal. This is due to the generation of pulse noise. Also, ■, ■, and ■ are essentially FM
This is noise due to disappearance of the audio signal, and as shown by a2 in FIG. 10 (A>), the pulsed noise a
It occurs over a long period of time compared to No. 1 (the period of noise generation is generally ■〈■ku■).

These noises are reduced by the hold circuit 16 and muting circuit 22 shown in FIG. The hold circuit 16 conventionally has a configuration as shown in FIG. In the figure, the reproduced audio signal from the FM demodulator 14 that has entered the input terminal 25 is applied to the switch circuit 26, where the hold signal is supplied from the adder circuit 19 via the input terminal 27 during the non-incoming period. (The low level period of the hold signal) is passed through and applied to the hold capacitor C, charging it. The terminal voltage of the hold capacitor C (the hold voltage of the input reproduced audio signal) The signals are respectively supplied to a de-emphasis circuit 20 shown in FIG.

Now, the input playback audio signal of the input terminal 25 is as shown in FIG. 9(A).
Pulse noise a during head switching as shown in
Assuming that the input terminal 27 has a short period T+ ( For example, a high level hold signal is applied for about 10 μs. Therefore, period T1
During this period, the switch circuit 26 is turned off, so the playback audio signal level immediately before the period T1 is held in the hold capacitor C. As a result, the output signal of the hold circuit 16 becomes as shown in FIG.
I disappears.

The output signal of the hold circuit 16 is sent to a de-emphasis circuit 20 which attenuates the high frequency components emphasized by the pre-emphasis circuit 3, and then to a noise reduction circuit 21.
The signal is supplied to the muting circuit 22 after being given a level expansion characteristic complementary to that of the noise reduction circuit 2 . The muting signal generating circuit 23 becomes high level when the output detection signal of the envelope detector 15 continues to be lower than a certain level for a certain period or more, and after returning to a level higher than the certain level, a predetermined period of time elapses. A muting signal that maintains a high level until the time elapses is generated and output to the muting circuit 22. The muting circuit 22 performs the muting operation only while the muting signal is at a high level. This results in
When the signal shown in FIG. 9(C) is taken out from the hold circuit 16, the output signal of the noise reduction circuit 21 becomes as shown in FIG. 9(D), and when the head is switched, the envelope of the reproduced FM signal Since the level is higher than the constant value, the output signal of the muting signal generating circuit 23 remains at a low level, as shown in FIG. Therefore, since the muting circuit 22 does not perform a muting operation, the reproduced audio signal waveform output to the output terminal 24 is as shown in FIG. 9(F) and as shown in FIG. 9(C).

The signal waveform is the same as that shown in (D). As described above, the conventional circuit has a large reduction effect on the pulse noise a1.

On the other hand, due to the reduction in the envelope level of the reproduced FM audio signal in the process of adjusting the relatively long dropout and tracking to the best condition, the reproduced audio signal from the FM demodulator 14 has a ), if noise occurs for a long time as shown in a2, the hold signal generation circuit 1
8, the hold signal shown in FIG. 10(B) is supplied to the hold circuit 16 at a high level for a period T2, which is generated based on the output detection signal of the envelope detector 15.
The output signal waveform of the bold circuit 16 is as shown in FIG. That is, in the actual hold circuit 16, for example, the terminal voltage of the hold capacitor C changes over time due to the influence of leakage current of the transistors constituting the output buffer circuit.
2, the DC potential of the terminal voltage changes significantly, and at the end of the hold period T2, the terminal voltage is returned to the DC potential of the normal input reproduced audio signal.

In this way, the reproduced audio signal having the waveform shown in FIG. Since the DC component is blocked when passing through the DC voltage several times, the output signal waveform of the noise reduction circuit 21 is as shown in FIG. Most of the DC potential changes are absorbed by the coupling capacitor, and the waveform becomes such that the center potential of the signal is shifted at the end of the hold period T2. Since this shift in the center potential gradually becomes smaller with the passage of time, the muting signal generation circuit 23 outputs the muting signal shown in FIG. The DC potential deviation at the time of generating and outputting the muting is to be small.As a result, the muting circuit 22 performs the muting operation during the high level period T3 of the muting signal, and
A reproduced audio signal having a waveform as shown in F) is output to the output terminal 24.

Problems to be Solved by the Invention However, in the above-mentioned conventional circuit, as shown in FIG. 10(F), when muting is released immediately after the muting operation is completed, the DC potential of the reproduced audio signal is still at the center potential. There was a problem in that a transient sound that sounded like a ``pop'' was generated at that moment. This means that, for example, in the process of adjusting tracking in search of a better tracking position for the knee amount, muting works well when a bad tracking position is reached, but when the center position is output, the above-mentioned transient sound occurs. This is a problem.

In order to prevent the occurrence of this transient sound, the muting time can be made longer, but this results in problems such as slower response and difficulty in tracking adjustment.
At the time of dropout, a small scratch causes muting to take longer than necessary, so it is not possible to make the muting period too long.

Furthermore, the above problem can basically be solved by adding a muting circuit that immediately mutes the output signal of the hold circuit 16 without coupling it with a capacitor. The muting circuit at the final output section after the output is also essential for eliminating noise that occurs when the power is turned on and off.
This increases the number of muting circuits, which is disadvantageous in terms of cost.

Therefore, the present invention aims to provide a noise reduction circuit that can reduce the above transient sound without extending the muting period by connecting one end of the hold capacitor to a DC voltage source via a resistor network. purpose.

Means for Solving the Problems The noise reduction circuit according to the present invention includes a resistor whose one end is connected to the connection point between the hold capacitor and the input end of the buffer amplifier, and a DC voltage is supplied to the other end of this resistor. Consists of a DC voltage source. The above resistor has a hold time that is sufficiently longer than the hold time required to reduce pulse noise, and the hold time during the bold period caused by the leakage current of the buffer amplifier.
A second time constant having a value sufficiently shorter than the first time constant of the DC potential change of the terminal voltage of the capacitor is formed together with the hold capacitor. In addition, the terminal voltage of the hold capacitor when the DC voltage supplied by the DC voltage source stabilizes after being held for a sufficiently longer time than the first time constant, and the hold capacitor when no signal is being held and the hold capacitor is stable.
The terminal voltage of the capacitor becomes approximately equal.

When performing a hold operation for a long time, the hold capacitor has a second time constant that is faster than the DC potential change of the terminal voltage of the hold capacitor during the hold period caused by the leakage current of the buffer amplifier, and the DC current during no signal is Therefore, the direct current potential of the output signal of the hold circuit immediately after the end of the hold period substantially matches the center potential of the signal. On the other hand, for pulse noise, a complete hold operation similar to the conventional method can be obtained.

Each embodiment of the present invention will be described below with reference to FIGS. 1 to 6.

Embodiment FIG. 1 shows a circuit system diagram of an embodiment of the circuit of the present invention. In the figure, the same components as in FIGS. 7 and 8 are denoted by the same reference numerals, and the explanation thereof will be omitted as appropriate.

This embodiment includes a resistor R1, one end of which is connected to the connection point between the hold capacitor C and the input end of the buffer amplifier 28, a slider connected to the other end of this resistor R1, and two fixed terminals. A power supply voltage light supplying a positive DC power supply voltage Vcc to a variable resistor R2, one of which is grounded, and the other fixed terminal of the variable resistor R2.

It is characterized by the provision of a circuit section consisting of a main circuit (not shown). The power supply voltage generation circuit and the variable resistor R2 constitute a DC voltage source for applying a predetermined DC voltage to the hold capacitor C via the resistor R+. Here, R
1)) The resistance value is selected to be R2. In addition, by dividing the DC power supply voltage VCC through resistance, a DC voltage approximately equal to the center DC potential of the terminal voltage when the hold capacitor C is not holding is applied to the slider of the variable resistor R2. taken out.

Further, a second time constant approximately equal to the product of the total value of resistor R1 and hold capacitor C is shown in FIG. 2(A).

The hold time required to remove the pulse noise a1 in the reproduced audio signal during head switching shown in FIGS. 5(A) and 9(A) (FIGS. 2(B) and 5) (B
) and FIG. 9(B), respectively, and are indicated by T+, for example, about 10μ.
The first time constant of the DC potential change of the terminal voltage of the hold capacitor C during the hold period caused by the leakage current of the buffer amplifier 28 (about several tens of microseconds or more) ) is selected to be one or more orders of magnitude smaller. - As an example, the capacitance value of the hold capacitor C is 5eoopF and the value of the resistor R1 is 390
The second time constant is approximately 2.2ms.
becomes. This value 2,2a+s is the hold time 10μ
Discharge the hold capacitor C toward the DC potential level when there is no signal with a current that is sufficiently longer than s~several tens of microseconds and sufficiently shorter than the first time constant, and that is more than an order of magnitude larger than the leakage current. It turns out.

In addition, the DC voltage set by the variable resistor R2 is more precisely the terminal voltage of the hold capacitor C when it stabilizes after a sufficiently long time compared to the first time constant after the start of the hold. Adjust so that the terminal voltage when there is no signal is the same as when the terminal is not connected.

As a result, the input terminal 25 shown in FIG. 1 is connected to the input terminal 25 shown in FIG.
As shown, the pulse noise al during head switching
When a reproduced audio signal containing a signal is input, the reproduced audio signal passes through a low impedance switch circuit 26, is amplified by a buffer amplifier 28, and then is de-emphasized until this pulse noise a1 is input. circuit 20
．． The signal is outputted to the output terminal 24 via the noise reduction circuit 21 and the muting circuit 22. At this time, a hold capacitor C whose one end is connected between the output end of the switch circuit 26 and the input end of the buffer amplifier 28
does not impose a large load, and the input reproduced 8-voice signal is output virtually unchanged.

However, when the pulse noise a1 enters, the hold signal shown in FIG. Set to off state. As a result, no signal current flows into the hold capacitor C from the switch circuit 26, and since the input impedance of the buffer amplifier 28 is set sufficiently high, the terminal voltage of the bold capacitor C is as shown in Fig. 2 (B). The voltage at the time when the hold signal rises from the low level to the high level shown in is held for a high level period T1 of the hold signal. As a result, as shown in FIG. 2(C), the buffer amplifier 28 outputs pulsed noise a.
Since the 1 part is held at the signal voltage immediately before the noise input, the noise a1 is not transmitted to the subsequent circuit, and noise can be reduced. Therefore, the muting circuit 22
The input signal waveform, the muting signal waveform from the input terminal 29, and the reproduced audio signal waveform output to the output terminal 24 are as shown in FIGS. 2(D), (E), and (F).

That is, the same operation as the conventional circuit shown in FIGS. 7 and 8 is performed for a reproduced audio signal containing pulse noise a1.

On the other hand, the input terminal 25 shown in FIG.
As shown in FIG. 3, when a reproduced audio signal containing noise a2 is input for a relatively long period T2, the switch circuit 26 uses the hold signal from the terminal 27 as shown in FIG. It is turned off for a period T2.

Therefore, the hold capacitor C performs a hold operation for this period T2, but as described above, the second time constant determined by the hold capacitor C and the resistor R+ is the hold operation caused by the leakage current of the buffer amplifier 28. Since the time constant of the DC potential change of the terminal voltage of the hold capacitor C during the period is sufficiently shorter than the first time constant of the DC potential change of the terminal voltage of the hold capacitor C, the charging charge of the hold capacitor C is transferred through the resistors R+ and R2 at a faster rate than the charging due to the leakage current. It will be discharged.

Therefore, the output signal waveform of the buffer amplifier 28 is as shown in FIG.
As shown in C), at the end of the hold period, the waveform becomes approximately equal to the terminal voltage of the hold capacitor C when no hold operation is performed during the no-signal period, and the first
A large step in the DC potential as shown in FIG. 0 (C) does not occur.

As a result, the input signal waveform of the muting circuit 22 becomes as shown in FIG. 3 (D), and the muting signal waveform from the input terminal 29 becomes as shown in FIG. A signal waveform that causes muting to be performed during the period T,+ is suitable. Therefore, as shown in FIG. 3(F), a reproduced audio signal with a short muting period and reduced noise is outputted to the output terminal 24.

Next, a second embodiment of the circuit of the present invention will be explained with reference to FIGS. 4 to 6. This embodiment is characterized in that a feedback loop including an interpolation circuit 30 is provided from the output end of the buffer amplifier 28 to the input end, and the other features are the same as the first embodiment. As the interpolation circuit 30, the signal interpolation circuit previously proposed by the present applicant in Japanese Patent Application No. 133128/1983 or the signal interpolation circuit previously proposed by the present applicant in Japanese Patent Application No. 155668/1988 is used. can do.

Of these, the latter signal interpolation circuit has a time constant that makes the gain approximately constant at low frequencies in the first-order differentiator circuit in the slope prediction circuit, and also allows the output voltage of the slope prediction circuit to It has a configuration in which the gain is set to be approximately the same as the terminal voltage, and is a voltage-to-current converter that converts the output voltage of the slope prediction circuit and outputs the current for ideal interpolation to the hold capacitor. It has the advantage that the circuit can be configured with only a simple impedance element such as a single resistor.

On the other hand, the former signal interpolation circuit has a one-round loop configuration in which the output signal voltage of the hold circuit is supplied to the hold capacitor through a slope prediction circuit and a voltage-to-current conversion circuit, respectively. The signal can be interpolated based on . By using these signal interpolation circuits proposed by the present applicant as the interpolation circuit 30, the reproduced audio having the pulse noise al during head switching shown in FIG. If a signal comes in, the signal from input terminal 27 (Figure 5 (B))
Even if the same triangular error as in the conventional method occurs particularly at high frequencies in the output signal waveform of the switch circuit 26, as shown by 01 in FIG. 9(C), the hold signal shown in FIG. By passing a charging/discharging current through the hold capacitor C, a reproduced audio signal having a waveform as shown in FIG. I can do it.

As a result, the input signal waveform of the muting circuit 22,
The muting signal waveform from the input terminal 29 and the output reproduction audio signal waveform from the output terminal 24 are shown in FIGS. 5(D) and (E).
and (F), respectively.

Furthermore, when a reproduced audio signal having long-term noise a2 as shown in FIG. 6(A) enters the input terminal 25 shown in FIG. 4, the hold signal waveform from the input terminal 27 is changed to As shown in B), although it is the same as the conventional circuit and the first embodiment, signal interpolation is performed by the operation of the interpolation circuit 30 at the beginning of the hold period T2, but after that, the buffer amplifier is operated by the same operation as the first embodiment. The output signal waveform of 28 and the input signal waveform of muting circuit 22 are as shown in FIGS. 6(C) and 6(D), respectively. Furthermore, the muting signal has the same waveform as the muting signal shown in FIG. 3(E), as shown in FIG. 6(E), and the output terminal 24 has noise with a waveform as shown in FIG. A reduced playback audio signal is retrieved.

As described above, although the present embodiment includes the interpolation circuit 30, the interpolation circuit 30 does not have a large effect on the noise reduction operation when a reproduced audio signal having long-term noise is input. The interpolation circuit 3o is not significantly affected by this embodiment, and a more suitable noise reduction operation can be performed when a reproduced audio signal having short-time noise is input.

Effects of the Invention As described above, according to the present invention, it is possible to substantially prevent the generation of transient sounds that conventionally occur when inputting a reproduced audio signal that contains noise for a long period of time, and also The required muting period can also be shorter than before, which makes the response faster than before, makes tracking adjustment easier, and eliminates unnecessary muting. Since it can be configured simply by adding two resistors without any additional elements, it can be configured easily and inexpensively.Additionally, by adding an interpolation circuit, a triangular It has many features such as being able to obtain a reproduced audio signal with significantly reduced shape errors.

[Brief explanation of drawings]

FIG. 1 is a circuit system diagram showing a first embodiment of the circuit of the present invention, FIGS. 2 and 3 are signal waveform diagrams for explaining the operation of the circuit system shown in FIG. 5 and 6 are signal waveform diagrams for explaining the operation of the circuit system shown in FIG. 4, respectively. FIG. 7 is a block diagram showing an example of an audio signal recording and reproducing system of a VTR. , FIG. 8 is a circuit system diagram showing an example of a main part of a conventional circuit, and FIGS. 9 and 10 are signal waveform diagrams for explaining the operation of FIGS. 7 and 8, respectively. 1... Audio signal input terminal, 4... Frequency modulator, 8
゜9... Rotating head, 11... Magnetic tape, 14.
...FM demodulator, 15...Envelope detector, 16...
Hold circuit, 17.18... Hold signal generation circuit, 22... Muting circuit, 23... Muting signal generation circuit, 24.29... Playback audio signal output terminal, 25... Playback audio signal Input terminal, 26...switch times. 27... Hold signal input terminal, 29... Muting signal input terminal, 30... Interpolation circuit, C...
・Hold capacitor, R1...Resistor, R2...
Variable resistance.

Claims (1)

[Claims] A reproduced audio signal is supplied from a demodulator that demodulates a frequency-modulated audio signal obtained by frequency-modulating a carrier wave with an audio signal, which is reproduced from a recording medium by a rotating head, and the noise contained in the reproduced audio signal is In a noise reduction circuit that holds the signal level immediately before the input of the noise during the input period of the noise and outputs the reproduced audio signal with the noise reduced to the output terminal via the buffer amplifier, a hold capacitor and the buffer amplifier are connected to each other. One end is connected to the connection point with the input end, and the hold capacitor is sufficiently longer than the hold period necessary to reduce pulse noise, and is used during the hold period caused by leakage current of the buffer amplifier. a resistor that, together with the hold capacitor, provides a second time constant selected to be sufficiently shorter than the first time constant of the DC potential change of the terminal voltage of and a sufficiently longer time compared to the first time constant; A DC voltage that supplies the other end of the resistor with a DC voltage that makes the terminal voltage of the hold capacitor when the hold is stable and the terminal voltage of the hold capacitor when there is no signal when the hold is not held substantially equal. A noise reduction circuit characterized by consisting of a voltage source.