JPH0452003B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a noise reduction circuit for removing noise during recording and playback of an audio tape recorder, for example, and is particularly suitable for use as a decoder. .

[Conventional technology]

FIG. 10 shows a circuit configuration used on the encoding side of a noise reduction system as a conventional example of such a noise reduction circuit. In FIG. 10, an input signal such as an audio signal supplied to an input terminal 1 is encoded by an encoder circuit 10, which is a noise reduction circuit, and sent to an output terminal 2, and this encoded output signal is transmitted to, for example, a tape recorder. sent to.
The encoder circuit 10 includes a high-pass filter 3 for pre-emphasis (high frequency enhancement) connected to the input terminal 10, a variable gain amplifier 4 inserted and connected between the high-pass filter 3 and the output terminal 2,
The control circuit 5 includes a control circuit 5 which sends a signal that detects the output of the variable gain amplifier 4 to a control terminal of the variable gain amplifier 4 as a control signal. Further, the variable gain amplifier 4 includes, for example, an operational amplifier 6, an input resistor 7, a negative feedback resistor 8, and a variable resistance element 9 connected in parallel to the negative feedback resistor 8. The resistance value of this variable resistance element 9 changes according to the control signal from the control circuit 5, and the amount of negative feedback changes, so that the amplifier 4
The gain of changes.

In this encoder circuit 10, an input signal such as an audio signal has its high frequency emphasized in a high pass filter 3, and is sent to a variable gain amplifier 4. The gain of the variable gain amplifier 4 is controlled by controlling the resistance value of the variable resistance element 9 in accordance with the control signal from the control circuit 5, and the higher the input level, the more the negative feedback circuit increases. The resistance value of variable resistance element 9 decreases, and the gain decreases. Therefore, the level of the output is compressed relative to the input. Negative feedback resistor 8 of this variable gain amplifier 4
acts to suppress the upper limit of the resistance value of the negative feedback circuit and controls the maximum gain of the variable gain amplifier 4 when the input level becomes extremely low and the resistance value of the variable resistance element 9 becomes extremely large.

[Problem to be solved by the invention]

However, in this encoder circuit 10, since the input of the variable gain amplifier 4 is the output of the high-pass filter 3 for pre-emphasis, the input level of the variable gain amplifier 4 has frequency characteristics.
As the level decreases in the lower range, the input/output characteristics of the encoder circuit 10, as shown in FIG. 11, have a characteristic curve when a low level input is input, which differs depending on the frequency. In other words, the variable gain amplifier 4 itself has a limited gain that remains unchanged below a certain input level at any frequency, and has no frequency characteristics, but the high-pass filter 3 for pre-emphasis has a gain (or This is because the input level of the input terminal 1 when the gain of the variable gain amplifier 4 remains unchanged differs depending on the frequency since the attenuation rate and transmission rate differ. Even when the high-pass filter 3 for pre-emphasis is connected to the output terminal 2 side, the input and output will have almost the same frequency characteristics. Furthermore, when level expansion and de-emphasis are performed on the decoder side, the input and output shown in FIG. 11 are reversed.

When using a noise reduction circuit with input/output characteristics having frequency characteristics as shown in Fig. 11, if the encoder circuit output level and decoder circuit input level are slightly different, for example, in a tape recorder, the tape sensitivity may change. When a discrepancy occurs between the recording input level and the reproduction output level due to variations, etc., level fluctuations occur in the decoder output depending on the frequency. For example, when encoding the input level at point a in Figure 11, a 10KHz signal corresponds to an input/output characteristic with a compression ratio of 2, and becomes the output level at point b, whereas a 100Hz signal corresponds to an input/output characteristic of 2. The compression ratio at which the gain of the variable gain amplifier 4 remains unchanged corresponds to the input/output characteristic of 1, resulting in an output level at point c. At this time, if, for example, a level drop in Δl occurs in a signal transmission system such as a tape recorder connected to the output terminal 2 of the encoder circuit 10,
Points b and c have the same level as points b' and c', respectively, and are sent to a decoder circuit (not shown). This decoder circuit has an input/output characteristic opposite to that of the encoder circuit 10, and has a characteristic in which the input level and output level shown in FIG. 11 are interchanged.
The level at point b' in FIG. 11 is decoded to output the level at point d, and the level at point c' at point e. For example, when the compression ratio is 2, the level difference between points ad is twice the level difference between points ae (in dB). Furthermore, when the level of the encoded input signal changes near point a, the level change of the decoded output signal is twice as different between 10KHz and 100Hz, making it impossible to perform good reproduction. Therefore, the range of encode input levels that can actually be used is the low frequency range shown in Figure 11 (for example,
The disadvantage is that the gain of the signal (100 Hz) exceeds the level l at which it remains unchanged, and the dynamic range becomes narrow.

The present invention has been made to eliminate these conventional drawbacks, and enables good decoding over the entire frequency range even at low level input, and allows the decoder circuit to be inserted and connected in the feedback circuit of a high gain amplifier. By doing so, it is possible to achieve good encoding, widen the dynamic range, and obtain a variable emphasis characteristic in which a large amount of emphasis is applied when a relatively large input is applied, and the amount of emphasis is slightly suppressed in other conditions. The purpose of the present invention is to provide a noise reduction circuit that can achieve low frequency noise modulation.

[Means to solve the problem]

That is, the features of the noise reduction circuit according to the present invention include a first filter for emphasis, which is connected in series to the first filter, and whose gain changes according to a control signal, and when the input signal level is small, the gain changes. a control circuit that detects the input signal level and uses it as a control signal; a second filter for emphasis connected in series to the variable gain circuit;
A signal is supplied from the input terminal of the series connection circuit with the filter, and is added to the output of the series connection circuit of the variable gain circuit and the second filter, and the signal is obtained when the input signal to this series connection circuit is at a small level. A flat path circuit having substantially flat frequency characteristics without any gain change in order to keep the gain constant;
When the input signal is at a small level, the first filter controls the overall frequency characteristics, and as the level of the input signal increases, a frequency characteristic is obtained in which the characteristics of the first filter and the characteristics of the second filter are added. This is how it was configured.

[Effect]

When the input signal is at a low level, the overall frequency characteristics are controlled by the first filter, making it possible to perform good decoding over the entire frequency range. A frequency characteristic is obtained by adding the characteristic of the second filter to the characteristic of the filter, and a large de-emphasis is applied to reduce noise modulation.

〔Example〕

Hereinafter, preferred embodiments of the present invention will be described.
This will be explained with reference to the drawings.

FIG. 1 is a block circuit diagram showing a noise reduction circuit according to a first embodiment of the present invention. For example, the present invention is applied to a decoder circuit provided on the playback output side of a noise reduction system of an audio tape recorder. This is an example.

The decoder circuit 30 shown in FIG. 1 performs signal processing such as high frequency attenuation and level expansion on a reproduced audio signal supplied from a reproduction output terminal of an audio tape recorder (not shown) to an input terminal 21. and sends it to the output terminal 22. For the above-mentioned day emphasis, the first,
The second low-pass filters 23 and 24 are used, and a variable gain circuit, for example, is used for level expansion.
A VCA (voltage controlled amplifier) 25 is used. Further, the second low-pass filter 24 and the VCA 25 are connected in series, and a flat-pass circuit 26 is connected in parallel to this series-connected circuit. This flat path circuit 26 has a flat frequency characteristic with no change in gain, is made up of, for example, only a resistor, and performs feed forward to the VCA 25. The output from the VCA 25 and the output from the flat pass circuit 26 are added by an adder 27 and sent to the first low pass filter 23. Furthermore, as a control signal to VCA25,
For example, a signal from the input terminal 21 is passed through a high-pass filter 29 for weighting to the control circuit 2.
8, and this control circuit 28 performs detection, smoothing, etc., and uses the signal.

In this first embodiment, a second low-pass filter 24, a VCA 25 which is a variable gain circuit, and an adder 2 are connected between the input terminal 21 and the output terminal 22.
7 and the first low-pass filter 23 are connected in series in this order to form the main path, but this order can be changed. That is, the second low-pass filter 24 and the VCA 25 can be converted to each other, and the series connection circuit and the first low-pass filter 23 can also be exchanged. Also VCA
The control signal to the VCA 25 can be obtained not only from the input side of the VCA 25 but also from the output side, or by using the sum or difference between a signal from the input side and a signal from the output side. However, since the VCA 25 performs a level expansion operation, it is necessary to satisfy conditions such as decreasing the gain when the level of the input signal is low, or increasing the gain when the level of the input signal is high. Additionally, the VCA
Various variable gain circuits other than 25 can be used.

In addition, the first and second low-pass filters 23, 2
4. For example, first and second high-pass filters 13 and 14 in the encoder circuit of FIG. 2 as described later.
It is sufficient to set the frequency characteristics to be opposite to the frequency characteristics of (see Figure 3 A and B), and the response on the reduction side is about 10 dB higher than on the good side, and both A first-order (response slope of up to 6 dB/
oct.) low-pass filter characteristics.

In the noise reduction decoder circuit 30 configured in this way, when the level of the input signal is low, the gain of the VCA 25 is small;
The characteristics of the flat path circuit 26 are strongly expressed. Therefore, the overall frequency characteristics of the decoder circuit 30 are almost determined by the first low-pass filter 23, and are first-order low-pass filter characteristics with a maximum response slope of about 6 dB/oct. The difference in response between the two sides is also about 10dB. Therefore, the difference in input/output characteristics between high and low frequencies when a small signal is input is reduced, and errors in frequency characteristics that conventionally occur due to tape sensitivity errors can be reduced.

On the other hand, as the level of the input signal increases, the gain of the VCA 25 increases and the second
Since the frequency characteristics of the low-pass filter 24 appear, the frequency characteristics of the decoder circuit 30 become
The characteristic approaches that of adding a second low-pass filter 24 to the low-pass filter 23 of FIG. Therefore, the maximum slope of the response is approximately 12 dB/oct., and the difference in response between the reduction and high frequencies is at most 20 dB.
As the frequency increases, a large amount of de-emphasis is applied, increasing the degree of separation between the mid-low and high frequencies, and reducing noise modulation.

Next, FIG. 2 shows an example of an encoder circuit that has a symmetrical configuration with respect to the noise reduction circuit (decoder circuit) of the first embodiment and is provided on the recording input side of an audio tape recorder. FIG. 3 is a block circuit diagram.

The encoder circuit 20 in FIG. 2 performs signal processing such as high frequency enhancement (pre-emphasis) and level compression on the audio signal supplied to the input terminal 11, and outputs the signal to an audio tape recorder (not shown) via the output terminal 12. ) to the recording input terminal. For the above-mentioned pre-emphasis, the first high-pass filter 13 and the second high-pass filter 1 are used.
4, and a variable gain circuit, such as a VCA (voltage controlled amplifier) 15, for level compression.
are used respectively. The VCA 15 and the second high-pass filter 14 are connected in series, and a flat-pass circuit 16 consisting of only a resistor, which has no gain change (is not subject to gain control) and has a flat frequency characteristic, is connected in parallel to this series-connected circuit. has been done. This flat path circuit 16 is
The VCA 15 and the second high-pass filter 14 form a negative feedback path that is sent as a subtraction signal to the adder 17 on the input side of the output of the series-connected circuit.
When the gain of VCA 15 becomes large, the amount of negative feedback is increased, and these VCA 15, second high pass filter 14, and flat pass circuit 16
The increase in gain of the negative feedback amplifier circuit consisting of is limited. That is, when the gain of the VCA 15 increases, the gain of the entire negative feedback amplifier circuit remains unchanged near the upper limit value. Next is the variable gain circuit.
A gain control signal for the VCA 15 is obtained from a control circuit 18 that includes a detector, a smoothing circuit, and the like. This control circuit 18 detects the input or output of the VCA 15 and uses it as a control signal, for example.
Second high-pass filter 1 on the output side of VCA15
The output from 4 is taken out and input to the control circuit 18 via a high-pass filter 19 for weighting.

In this encoder circuit, the input terminal 11
A first high-pass filter 13, an adder 17, and a VCA which is a variable gain circuit are connected between the output terminal 12 and the output terminal 12.
15 and the second high-pass filter 14 are connected in series in this order to form a main path, but this connection order is not fixed. That is, VCA15
and the second high-pass filter 14 can be replaced with each other, and the VCA1 is connected immediately before the output terminal 12.
5 may be memorized. Further, the negative feedback amplifier circuit made up of the VCA 15, the second high-pass filter 14, and the flat-pass circuit 16 and the first high-pass filter 13 may be replaced with each other.
A high-pass filter 13 can be placed immediately before the output terminal 12.

Further, as a signal to be supplied to the control circuit 18, a signal at an arbitrary point on the main path on the output side of the VAC 15 can be extracted and used, a signal at an arbitrary point on the main path on the input side of the VCA 15 can be extracted and used, Alternatively, the sum or difference signal of the signals from the output side and the input side can be used. However, since this first embodiment is used as an encoder circuit and the VCA 15 performs level compression operation, the gain of the VCA 15 is increased as the level of the input signal decreases, or the gain of the VCA 15 is increased as the level of the input signal decreases. The higher the value, the more VCA15
A control circuit 18 that reduces the gain of
It is necessary to have a configuration that satisfies the relationship of the VCA 15. Note that, of course, various variable gain circuits other than the VCA 15 can be used.

Next, the first high-pass filter 13 has a frequency characteristic as shown in FIG. 3A, for example. In other words, the response (or gain) on the low frequency side is approximately 10 dB lower than on the high frequency side, and 1K
It rises from Hz to 3.18KHz with a curve of approximately 6dB/octave. In addition, the second high-pass filter 14
has a frequency characteristic as shown in FIG. 3B, for example, and there is a curve response change of approximately 6 dB/oct. between 1.58 KHz and 5 KHz, and the maximum width of the response change is approximately 10 dB.

In the noise reduction encoder circuit 20 having the above configuration, the gain of the VCA 15 is large when the level of the input signal is low. Since this VCA 15 constitutes a negative feedback amplifier circuit with the flat path circuit 16 as a negative feedback path, when the gain of the VCA 15 is large, the gain of the entire negative feedback amplifier circuit has a value almost equal to the inverse characteristic of the flat path circuit 16. It remains unchanged in the vicinity of . Here, since the flat path circuit 166 is a simple resistor, the inverse characteristic of the resistance value R is 1/
R gives a flat frequency characteristic. Therefore, the frequency characteristics of the second high-pass filter 14 no longer appear outside the negative feedback amplifier circuit, and the overall frequency characteristics of the encoder circuit 20 are almost controlled by the first high-pass filter 13, for example, as shown in FIG. A frequency characteristic like A is obtained. The frequency characteristics of this first high-pass filter 13 are so-called first-order filter characteristics with a response slope of about 6 dB/octave, and the difference in response between high and low frequencies is about 10 dB.
The pre-emphasis effect is becoming smaller. Therefore, in the low level region of the input signal, the input/output characteristic curves for each frequency, for example, 10KHz, 1KHz, and 100Hz, are expressed close to each other as shown in FIG. 4, and the gain of the negative feedback amplifier circuit is The range of input levels up to level L at which the value remains unchanged can be wider than the conventional level l in FIG. 11.

Next, as the input signal level increases,
Since the gain of VCA15 decreases, this VCA1
The frequency characteristics of the second high-pass filter 14 appear in the negative feedback amplifier circuit including the filter 5. Therefore, the overall characteristics of the encoder circuit 20 approach the frequency characteristics shown in FIG. 3C, for example, where the second high-pass filter 14 is added to the first high-pass filter 13. The characteristics shown in Figure 3C are a combination of the characteristics shown in Figures 3A and B.
The maximum slope of the response is about 12 dB/oct., which is a so-called second-order filter characteristic, and the difference in response between low-mid frequencies and high frequencies can be as large as about 20 dB. Therefore, a large amount of pre-emphasis is applied, and the separation between the low-mid range and the high range is excellent, so that the noise modulation phenomenon can be further reduced by the mid-low range signal.

Next, FIG. 5 shows a decoder circuit 50 serving as a noise reduction circuit as a second embodiment of the present invention.
In the first embodiment shown in FIG. A transmission line 43 (not subject to gain control) is connected. That is, the input signal from the input terminal 41 of the decoder circuit 50 is sent to the above-mentioned decoder circuit 30 via the adder 44, the output signal from this decoder circuit 30 is sent to the output terminal 42, and the above-mentioned transmission line 43 is sent to the adder 44 as a subtraction signal. Here, the transmission line 43
It is preferable to have a low-pass type frequency characteristic or a flat-pass characteristic like that of a resistor. The configuration of the decoder circuit 30, which is a part of FIG. 5, is the same as that of FIG. 1 described above, so the same parts are given the same reference numerals and the explanation will be omitted.

In the case of this second embodiment, not only the same operation as in the first embodiment described above is performed, but also the influence of the transmission line 43 becomes apparent when the level of the input signal further increases. Since this transmission line 43 is a negative feedback path, the decoder circuit 30 is affected by the frequency characteristics of the high-pass type or flat-pass type, which are the opposite characteristics of the transmission line 43, and the amount of de-emphasis is reduced. Therefore, it is possible to reduce noise modulation in the middle and low frequencies caused by high-frequency signals when a large input is applied. Further, an anti-limiter circuit can be inserted and connected into the main path including the VCA 25.

Next, FIG. 6 shows an encoder circuit 40 having a circuit configuration symmetrical to that of the second embodiment.
The encoder circuit 40 shown in FIG. 6 is used in the encoder circuit 20 shown in FIG. It has a configuration in which a transmission line 33 with no gain change (not subject to gain control) is added to the above. This transmission line 33 is provided in parallel to the encoder circuit 20, and preferably has a low-pass characteristic or a flat-pass characteristic as a frequency characteristic. Then, the audio signal from the input terminal 31 is sent to the adder 34 via the transmission line 33, and the adder 34 adds it to the output from the encoder circuit 20 and outputs it to the output terminal 3.
I am sending it to 2. Since the other configurations are the same as those in FIG. 2, the same parts are given the same reference numerals and the explanation will be omitted.

Such an encoder circuit 40 not only performs the same operation as the encoder circuit 20 shown in FIG. 2, but also when the level of the input signal becomes even higher, the transmission line 33 becomes effective and the amount of pre-emphasis is reduced. , or the high range tends to drop slightly. By reducing the amount of pre-emphasis at the time of such a large input, it is possible to reduce noise modulation in the middle and low frequencies caused by high-frequency signals. Furthermore, by inserting and connecting a limiter circuit to the output side of the VCA 15 in the main path including the VCA 15, it is possible to prevent overshoot and the like when the input signal level increases rapidly. Since the large input signal at this time is sent to the output terminal 32 via the transmission line 33, no signal distortion occurs.

Next, FIG. 7 shows a third embodiment in which a practical compander type noise reduction circuit is constructed using the second embodiment shown in FIG. 5.

This noise reduction circuit 100 in FIG.
The encoder can arbitrarily select between the encoding operation and the decoding operation by switching the switch, and is suitable for use as a built-in noise reduction device in a general audio cassette deck or as an independent noise reduction device, for example. Here, a decoder circuit 150 that is almost the same as the decoder circuit 50 of the second embodiment is used as a circuit section that performs actual noise reduction, and during encoding operation, a negative feedback circuit of a high gain amplifier 103 such as an operational amplifier is used. By inserting and connecting the decoder circuit 150 therein, an encoding characteristic as an inverse characteristic of the decoding characteristic is obtained.

That is, the input terminal 101 of the noise reduction circuit 100 is supplied with an audio signal from a sound source such as a microphone or a tuner during an encoding operation, and is supplied with an audio signal obtained via a transmission medium during a decoding operation. In particular, a reproduction output signal from an audio tape recorder is supplied. This input terminal 101 is connected to a non-inverting input terminal of a high gain amplifier 103 such as an operational amplifier, and the output terminal of this high gain amplifier 103 is connected to an encode output terminal 102 and an input terminal 141 of a decoder circuit 150. This decoder circuit 15
The output terminal 0 is connected to the decoder output terminal 142 and the encode switching terminal e of the switching switch 104. Further, the output terminal of the high gain amplifier 103 is connected to the changeover switch 10 via a negative feedback resistor 105.
The fixed terminal (common terminal) of this switch 104 is connected to the inverting input terminal of the high gain amplifier 103.

The decoder circuit 150 in FIG. 7 is constructed almost the same as the decoder circuit 50 in FIG. 5, and corresponding parts are indicated by the same reference numerals with 100 added. However, the flat path circuit 26 uses the resistor 126 and the transmission line 43.
However, the resistor 143 is used. Moreover, the high pass filter 29 for weighting shown in FIG.
It is constructed by connecting two high-pass filters 130 and 129 in series, and a part of the output from the connection point of these high-pass filters 130 and 129 is sent to an adder 127 via an anti-limiter circuit 131.

Decoder circuit 15 having such a circuit configuration
0, the transfer function of the first low-pass filter 123 is F ₁ and the transfer function of the second low-pass filter 124 is F ₂ , and these F ₁ and F ₂ are F ₁ =g ₁・1+sT ₂ /1+sT ₁ ... F ₂ = g ₂・1 + sT ₄ /1 + sT ₃ ... However, when s = jω, for example, g ₁ = g ₂ = 10dB, T ₁ =
159 μs, T ₂ =50 μs, T ₃ =100 μs, and T ₄ =31.8 μs, the characteristics of each of the low-pass filters 123 and 124 will be opposite to those of FIGS. 3A and 3B described above. Furthermore, the gain of the VCA 125 is G, and the transfer function of the resistor 126, which is the flat path circuit described above, is
H ₁ is the transfer function of the resistor 143, which is the transmission path.
When _H2 , the transfer function H of the decoder circuit 150 can be expressed as H= _F2G + _H1 /1+ _{H2F1 ( F2G} + _H1 )... During the decoding operation, the changeover switch 104 is connected to the decode changeover terminal d, and the resistor 105 is connected to the negative feedback path of the high gain amplifier 103.
is connected, the audio signal from the input terminal 101 is simply amplified by the high gain amplifier 103, subjected to decoding operations such as de-emphasis and level expansion by the decoder circuit 150, and sent to the decode output terminal 142.

Next, during encoding operation, selector switch 1
01 is switched and connected to the encode switching terminal e, the decoder circuit 150 having the transfer function H is inserted and connected to the negative feedback circuit of the high gain amplifier 103. Here, when the bare gain of the high gain amplifier 103 is A, the input/output terminals 101, 1
The transfer function U between 02 and 02 can be expressed as U=A/1+AH... When A in this equation is very large (high gain), U≒1/H..., and a transfer function having an inverse characteristic of the transfer function H of the decoder circuit 150, that is, an encoding characteristic is obtained. become.

That is, FIGS. 8 and 9 show how the output signal from the encode output terminal 102 is changed in response to the input signal from the input terminal 101 during the encoding operation when the changeover switch 104 of the noise reduction circuit 100 is switched and connected to the encode changeover terminal e. This is a measured graph. Here, Figure 8 shows a frequency characteristic graph for two signal inputs, where curve A shows the state with no input, curve B shows the state with 400 Hz and 0 dB input, and curve C shows the state with 1 KHz and -10 dB input. Indicates the status of each. Next, FIG. 9 is a graph showing input and output levels for input signals of each frequency, for example, 100Hz, 1KHz, and 10KHz.

As is clear from the above explanation, a large noise reduction effect of approximately 30 dB can be obtained in the high range by using a VCA with a wide variable gain width, and a noise reduction effect of more than 90 dB can be obtained using a compact audio cassette tape recorder. You can get honey cleanse. In addition, various measures have been taken to prevent sound quality deterioration and audible discomfort.In particular, as a countermeasure against noise modulation and tape sensitivity error, as shown in Figures 8 and 9, we have taken measures to reduce The emphasis characteristic when inputting a relatively high level in the midrange is a second-order filter characteristic of about 12 dB/octave to obtain a large amount of emphasis. Emphasis is suppressed as a first-order filter characteristic using only one filter, and a so-called variable emphasis characteristic is obtained. In addition, in a wide input level range (+15dB to -50dB),
By providing a constant (approximately 2) compression ratio that does not depend much on frequency, and by making the input level dependence of the variable emphasis gentle, the amplitude and spectrum caused by variations in tape sensitivity can be reduced. It reduces errors.

Furthermore, in order to prevent tape saturation, waveform distortion, etc. when the input signal rises quickly, a limiter circuit (anti-limiter circuit 131 in FIG. 7) that operates only in high frequencies can be easily used. Also,
In order to prevent malfunctions with sources that have components in a wide frequency band, the frequency bands of the first and second filters (low-pass filters 123 and 124 in FIG. 7) and the weighting circuit are limited, and the operation is controlled. I try not to make the sensitivity too high.

Note that the present invention is not limited to the above-mentioned embodiments. For example, the first embodiment can also be inserted into the feedback circuit of a high-gain amplifier such as an operational amplifier to obtain the opposite characteristics. Alternatively, as in the third embodiment, a changeover switch may be used to switch between decoding and encoding operations. Various other changes can be made without departing from the gist of the present invention.

〔Effect of the invention〕

When the input level is low, the frequency characteristics of the first filter appear dominantly, so good decoding and encoding can be performed over the entire frequency band.
The dynamic range can be wider than before, and when the input level becomes large, the characteristics of the first filter and the second filter are combined, and a large degree of defense is applied, reducing the noise modulation. can reduce the amount of shock.

[Brief explanation of the drawing]

FIG. 1 is a block circuit diagram showing a decoder circuit as a first embodiment of the noise reduction circuit according to the present invention. 2 to 4 show an encoder circuit having a configuration symmetrical to that of the decoder circuit according to the first embodiment, FIG. 2 is a block circuit diagram, and FIGS. FIG. 4 is a graph showing the frequency characteristics of the first and second high-pass filters and their combined frequency characteristics, and FIG. 4 is a graph showing the input/output characteristics of the circuit shown in FIG. FIG. 5 is a block circuit diagram showing a decoder circuit as a second embodiment of the present invention. FIG. 6 is a block circuit diagram showing an encoder circuit having a configuration symmetrical to that of the decoder circuit of the second embodiment. 7 to 9 show a third embodiment of the present invention, in which FIG. 7 is a block circuit diagram, FIG. 8 is a graph showing frequency characteristics, and FIG. 9 is a graph showing input/output characteristics. Fig. 10 is a block circuit diagram showing a conventional example of a noise reduction circuit, and Fig. 11 is a block circuit diagram showing a conventional example of a noise reduction circuit.
FIG. 2 is a block circuit diagram showing the input/output characteristics of the circuit of FIG. 11, 21, 31, 41, 101... Input terminal, 12, 22, 32, 42... Output terminal, 1
3, 23, 123...first filter, 14, 2
4,124...second filter, 15,25,3
5, 45, 125... VCA, 16, 26... Flat path circuit, 17, 27, 34, 44... Adder, 18, 28, 128... Control circuit, 20,
40... Encoder circuit, 30, 50, 150...
...Decoder circuit, 33, 43...Transmission line, 100
... Noise reduction circuit, 103 ... High gain amplifier, 102 ... Encode output terminal, 142
...Decode output terminal.

Claims (1)

[Scope of Claims] 1. A first filter for emphasis; a variable gain circuit connected in series to the first filter, whose gain changes according to a control signal and whose gain decreases when the input signal level is small; A control circuit that detects the input signal level and uses it as a control signal, a second filter for emphasis connected in series to the variable gain circuit, and an input of a series connection circuit of these variable gain circuits and the second filter. A signal is supplied from the terminal and is added to the output of the series connection circuit of the variable gain circuit and the second filter, and the gain is changed to keep the gain unchanged when the input signal to the series connection circuit is at a small level. the first filter controls the overall frequency characteristic when the input signal is at a small level, and as the level of the input signal increases, the first filter A noise reduction circuit characterized in that it is configured to obtain frequency characteristics obtained by adding the characteristics of the second filter to the characteristics of the filter.