JP2021157046A - Analog ANC control system - Google Patents

Abstract

To provide an ANC system which can respond well to sudden changes in sound and is less likely to cause howling.SOLUTION: An analog ANC control system S comprises: a microphone 1 which detects sound and outputs the sound as a detection signal; an analog ANC circuit 2 which converts the detection signal with phase conversion and outputs the converted signal as an ANC control signal; and a speaker which generates control sound. The analog ANC circuit 2 inverts the phase of the detection signal and increases or decreases the amplitude. A plurality of gain and phase conversion elements 5 having different phase conversion characteristics between the microphone 1 and the analog ANC circuit 2 is provided in parallel. A phase difference ωτ between a phase of a sound wave Aeiωt detected by the microphone 1 and a phase of a control sound wave F=-A×Beiωt+φ/(1+Beiφ) emitted from the speaker 3 is adjusted to -90°<ωτ<-90°.SELECTED DRAWING: Figure 3

Description

The present invention includes a microphone that detects sound and outputs it as a detection signal, an analog ANC circuit that accepts the detection signal, converts the detection signal with phase conversion, and outputs it as an ANC control signal, and the ANC control. It is configured with a speaker that accepts signals and produces control sounds.
The present invention relates to an ANC control system that inverts the phase of the detection signal and increases / decreases the amplitude in the analog ANC circuit.

As ANC (Active Noise Control) control, a technique for digitally performing control (hereinafter, referred to as digital ANC) is known. The technique described in Patent Document 1 is an example of this.

On the other hand, the inventors have proposed in Patent Document 2 a vibration damping technique for the window glass G generated by noise. In this vibration suppression control, in the feedback circuit formed between the pickup 11, the analog circuit 12, the amplifier 13, the exciter 14, and the pickup 11, the vibration generated in the window glass to be controlled is adjusted by the feedback circuit. Focusing on the phase of the specific frequency to be suppressed.

If sound is regarded as a kind of vibration, in the technique disclosed in Patent Document 2, the pickup corresponds to a microphone and the exciter corresponds to a speaker. In this technique, analog circuits and amplifiers are responsible for phase inversion and amplitude increase / decrease functions.

^{Then, the phase difference ωτ between the phase of the vibration acceleration Ae iωt} measured by the pickup 11 and the phase of the adjustment signal input to the exciter 14 is set to −90 ° (π / 2) <ωτ <90 ° (π). A phase adjusting unit including an analog circuit 12 for adjusting within the range of / 2) is provided.

A known low-pass filter and a graphic equalizer dedicated to phase operation are adopted for this analog circuit (paragraph [0025]).

Japanese Unexamined Patent Publication No. 7-210175 JP-A-2018-22029

However, the digital ANC cannot cope with a sudden change in the sound to be muted (external sound, for example, noise). The reason is that since the main processing process is performed in the digital domain, A / D conversion and D / A conversion are indispensable in the system. As a result, a delay of about several tens of msec occurs in the control. In addition, when generating anti-phase sound by adaptive control such as Filtered-X LMS, it is necessary to update the Filter coefficient according to the change, but when trying to cope with a sudden change, the adjustment speed of the convergence speed and the estimation system is paralyzed. It is necessary to increase the step size parameter, which is a meter, and the robustness is reduced.

Further, in the technique disclosed in Patent Document 1, fail-safe control is performed to avoid a sound increase state at a predetermined frequency, control is stopped, and control is initialized, but fundamental sound increase avoidance is performed. It cannot be said that there is, and there is room for improvement.

In the technique described in Patent Document 2, it is described that a low-pass filter and a graphic equalizer are added in front of the amplifier to adjust the phase, but in this configuration, the phase and gain can be adjusted with respect to a specific frequency. However, amplification of noise in a frequency range other than a specific frequency is unavoidable, and in some cases howling may occur in the frequency range to be muted.

In view of this situation, a main problem of the present invention is to provide an ANC system that can cope with a sudden change in sound and further reduces howling.

The first characteristic configuration of the present invention is
A microphone that detects sound and outputs it as a detection signal, an analog ANC circuit that accepts the detection signal, converts the detection signal with phase conversion and outputs it as an ANC control signal, and accepts the ANC control signal. It is configured with a speaker that produces a control sound,
An analog ANC control system that inverts the phase of the detection signal and increases or decreases the amplitude in the analog ANC circuit.
A plurality of gain / phase conversion elements having different phase conversion characteristics are provided in parallel between the microphone and the analog ANC circuit.
In a predetermined frequency range, the phase difference ωτ between the phase of ^{the sound wave Ae iωt} detected by the microphone and the control sound wave F = −A × Be ^{iωt + b} / (1 + Be ^{ib) generated from the speaker is −90 ° <ωτ <90.} It is at the point where it is adjusted to °.

[Here, n is the number of gain / phase conversion elements, Σ is the sum of all gain / phase conversion elements, the gain of each gain / phase conversion element is Fn, the phase is θn, the speaker characteristics, and between the speaker and the microphone. ^{Be ib} = Σ (F _n e ^iθn ) × Se ^iφ, where S is the total gain of the transmission function and microphone characteristics and φ is the phase]

According to this feature configuration, instead of controlling using A / D conversion, D / A conversion, and DSP (Distal Signal Processor), a plurality of gain / phase conversion elements are provided between the microphone and the speaker, and they are provided. Since the output is sent to the analog ANC circuit for processing, this processing can be performed in analog space. As a result, it is possible to immediately generate a sound (control sound) having an opposite phase with respect to the target frequency band with a short delay time of about several tens of μsec. That is, it is possible to deal with sudden fluctuation noise (for example, a sudden change sound at the time of starting a rotating machine or the like, a low frequency sound such as a boiler whose frequency changes randomly, a sound having time deficiency such as snoring).

Further, in the feedback circuit, the phase difference ωτ between the phase of ^{the sound wave Ae iωt} detected by the microphone and the control sound wave F = −A × Be ^{iωt + b} / (1 + Be ^{ib) generated from the speaker with respect to a predetermined frequency range is −.} By adjusting 90 ° <ωτ <−90 °, it is possible to suppress sound increase and howling in a severe case in this frequency range.

As described above, the fact that the sound increase and howling can be suppressed by keeping the phase difference in the feedback circuit within a predetermined finite value range will be specifically described in a later embodiment.

Here, the difference from the technique described in Patent Document 2 will be described. According to the technique described in Patent Document 2, the gain of a specific frequency (particularly the frequency at which there is a high possibility of howling on the high frequency side) with a low-pass filter or the like. If an attempt is made to lower the frequency, the howling frequency shifts according to the amount of gain change, and the howling frequency may appear in a predetermined frequency range, making it difficult to adjust the phase as desired.

For example, when a low-pass filter as a kind of gain / phase control element is added for phase control, in the example shown in FIG. 5 described later, a problem occurs in the vicinity of 700 Hz to 1000 Hz due to the reduction on the high frequency side of 1000 Hz or higher. There is a region where the phase difference is extremely low. The examples shown in FIGS. 6 and 7 aim to reduce howling in the frequency band of 1000 Hz or less appearing in FIG. 5, but in the example shown in FIG. 6, the frequencies shown by arrows h2 and h3 (near 150 Hz, around 750 Hz). ) Shows that howling may occur at the frequencies (280 Hz, 420 Hz) indicated by the arrows h4 and h5 in addition to the frequencies indicated by the arrows h2 and h3.

However, in the configuration of the present invention, the howling frequency generated by a specific gain / phase conversion process is also obtained by adjusting the sound detected by the microphone by at least a plurality of gain / phase conversion elements arranged in parallel. -The phase conversion element can reduce the gain and phase adjustment, and finally howling can be avoided.

The number of gain / phase conversion elements used in the present invention is at least two, and by increasing the number of gain / phase conversion elements in series or in parallel with other elements as needed, muffling in a predetermined frequency range can be satisfactorily realized.

This kind of analog ANC control system
A microphone that detects sound and outputs it as a detection signal, an analog ANC circuit that accepts the detection signal, converts the detection signal with phase conversion and outputs it as an ANC control signal, and accepts the ANC control signal. It is configured with a speaker that produces a control sound,
The analog ANC circuit is an ANC control system that inverts the phase of the detection signal and increases / decreases the amplitude.
A plurality of gain / phase conversion elements having different gain / phase conversion characteristics are provided in parallel between the microphone and the analog ANC circuit.
In a predetermined frequency range, the phase difference ωτ between the phase of ^{the sound wave Ae iωt} detected by the microphone and the control sound wave F = −A × Be ^{i ((ωt + b))} / (1 + Be ^{ib) generated from the speaker is predetermined.} In the frequency range, the analog ANC control system is adjusted within a range smaller than the positive finite value (90 ° in the above example) and larger than the negative finite value (−90 ° in the above example).

The second characteristic configuration of the present invention is that the gain / phase conversion element is any one of a low-pass filter, a band-pass filter, a high-pass filter, a graphic equalizer, and a phase inversion element, or a combination thereof.

When a low-pass filter, a band-pass filter, a high-pass filter, a graphic equalizer, and a phase inversion element are incorporated into the feedback circuit described above, the feedback circuit affects the transmission characteristics, and as will be described later, at least this book. It is possible to keep the phase difference ωτ, which is a problem in the invention, within a suitable range.

Therefore, by appropriately combining a plurality of these gain / phase conversion elements, appropriate muffling can be realized in a predetermined frequency range.

The third characteristic configuration of the present invention is
A plurality of low-pass filters having different cutoff frequencies are provided as the gain / phase conversion element, and the gain at the howling frequency at which the phase difference is −180 ° in the frequency range higher than the predetermined frequency range is reduced. ..

By adopting this configuration, by using a low-pass filter having a specific cutoff frequency as a gain / phase conversion element, it is possible to limit the gain in a frequency band higher than the cutoff frequency of the gain / phase conversion element. A howling frequency in which the phase difference is inverted may be formed in the vicinity of the cutoff frequency, but the descending ring at the frequency thus formed can be adjusted by other gain / phase conversion elements in parallel.

The fourth characteristic configuration of the present invention is
At a frequency where the phase difference is smaller than −90 °, a bandpass filter is provided as the gain / phase conversion element, and the phase difference is adjusted to be larger than −90 °.

With this configuration, the bandpass filter can adjust the gain and phase difference in that band, and as a result, the phase difference characteristics of the system in a predetermined frequency range are within a predetermined finite value range (+ 90 ° in the above example). It is possible to realize analog ANC control without howling on the high frequency side by keeping it in a smaller range (within a range larger than −90 °).

The fifth characteristic configuration of the present invention is
A graphic equalizer is provided as the gain / phase conversion element, and the gain at the howling frequency remaining in the frequency range higher than the predetermined frequency range is reduced.

According to this feature configuration
The gain at the howling frequency remaining in the frequency range higher than the predetermined frequency range can be appropriately adjusted by the graphic equalizer.

As a result, in an analog ANC control system provided with an analog ANC circuit that inverts the phase of the detection signal and increases or decreases the amplitude, for example, howling occurs on the high frequency side, which is outside the frequency to be muted by the system. By adding a plurality of gain / phase control elements in parallel while reducing howling, control divergence unlike a digital ANC system is eliminated, and stable muffling becomes possible.

Diagram showing the basic configuration of an analog ANC control system The figure which shows the gain and the phase difference characteristic of the system shown in FIG. The figure which shows the structure of the analog ANC control system which concerns on this invention. The figure which shows the signal increase / decrease, gain and phase difference characteristic of the system shown in FIG. The figure which shows the signal increase / decrease, gain and phase difference characteristic of the system which provided LPF0. The figure which shows the signal increase / decrease, gain and phase difference characteristic of the system which provided LPF0 and LPF1. The figure which shows the signal increase / decrease, gain and phase difference characteristic of the system which provided LPF0 to LPF2. Diagram of another embodiment

An embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows the basic configuration of the analog ANC control system S0 according to the present invention, and FIG. 3 shows the configuration of the analog ANC control system S according to the present invention that performs phase adjustment.

In the following description, first, the basic configuration shown in FIG. 1 will be described.
As shown in FIG. 1, the analog ANC control system S0 receives a microphone 1 that detects sound and outputs it as a detection signal, and a detection signal detected by the microphone 1, and converts the detection signal with phase conversion. It is configured to include an analog ANC circuit 2 that outputs as an ANC control signal, and a speaker 3 that receives the ANC control signal and generates a control sound. In the figure, the analog ANC circuit 2 is abbreviated as Amp in the box.

In the analog ANC circuit 2, the phase of the detection signal is inverted and the amplitude is increased or decreased. As a result, the external sound and the control sound from the speaker 3 can be muted by overlapping each other.

Here, the sound pressure of the external sound is A, the sound pressure of the control sound is F, the gain of the transmission function of the feedback circuit formed via the microphone 1 → analog ANC circuit 2 → speaker 3 → microphone 1 is B, and the transmission function is concerned. When the phase and phi, the control wave ^{F = -A × be iωt + φ} / (1 + be iφ) next, -A × be in the description in the control wave (complex domain (Ae ^Aiomegati in the description of the complex region) outside waves ^{As a result of combining iωt + φ} / (1 + Be iφ)), the obtained attenuation amount ΔL is ΔL = 20 log (1 + Be ^iφ ).

By the way, in order to realize effective sound deadening, the inventors have a constant ^{phase of the external sound wave Ae iωt and} the phase of the control sound wave F = −A × Be ^{iωt + φ} / (1 + Be ^iφ ) in a predetermined frequency range. We believe that it is preferable to construct the analog ANC control system S so as to satisfy the relationship.

Therefore, a plurality of gain / phase conversion elements 5 having different gain / phase conversion characteristics are provided in parallel between the microphone 1 and the analog ANC circuit 2, and a sound wave Ae ^{iωt detected by the microphone 1 in a predetermined frequency range is provided.} , The phase difference ωτ between the control sound wave F = −A × Be ^{iωt + b} / (1 + ^{Be ib} ) generated from the speaker 3 is adjusted within a predetermined range (−90 ° <ωτ <90 °).
[Here, n is the number of gain / phase conversion elements 5, Σ is the sum of all gain / phase conversion elements 5, the gain of each gain / phase conversion element is Fn, the phase is θn, the speaker characteristics, and the speaker. ^{The gain obtained by combining the} inter-mic transmission function and the microphone characteristics is S, the phase is φ, and Be ^ib = Σ (F _n e iθn) × Se ^iφ ].
Naturally, the gain / phase conversion element 5 is also a circuit element that processes the signal in an analog manner.

The analog ANC control system S according to the present invention shown in FIG. 3 has three low-pass filters LPF (LPF0 to LPF2) and two bandpass filters BPF (BPF1, BPF2) as a plurality of gain / phase conversion elements 5. ) Is combined. As can be seen from the figure, an attenuator ATT4 is provided on the lower side (in front of the analog ANC circuit 2) of the low-pass filter LPF (LPF0 to LPF2) and the bandpass filter BPF (BPF1 and BPF2), respectively. The gain of the signal obtained through the above is adjusted.

The relationship between LPF0 and LPF1, 2, BPF1, and BPF2 is in series, and the relationship between LPF1, LPF2, BPF1, and BPF2 is in parallel.
This combination method will be described in detail later.

By using the plurality of gain / phase conversion elements 5 in combination in this way, the phase difference ωτ can be made approximately −90 ° <ωτ <90 ° with respect to the transfer function of the feedback circuit in a predetermined frequency range.
In the example shown in FIG. 3, the predetermined frequency range is 20 Hz or more and 1000 Hz or less, and the phase difference ωτ in this frequency range is within the range of approximately −90 ° <ωτ <90 °.

The method of selecting the gain / phase conversion element 5 will be described in order.
1. 1. Characteristics of Analog ANC Control System S0 with Basic Configuration FIG. 2 shows the gain and phase difference characteristics of this analog ANC control system S0.
In this figure, the frequency range is from 10 to 10000 Hz (same in FIG. 5).
As can be seen from the figure, the gain is below 0 over the entire frequency domain. However, it becomes a minimum at h1 near 1500 Hz, shows a maximum value in a frequency region higher than that, and decreases while repeating a decrease and increase toward a high frequency side.

On the other hand, with respect to the phase difference, a frequency that generally shows a decreasing tendency from 10 Hz to 1000 Hz and has a phase difference of −180 ° appears at a frequency exceeding 1000 Hz, which is the minimum value of the gain. Further, since the phase difference due to the delay increases toward a high frequency, the in-phase and the anti-phase repeatedly appear. This increase in phase difference becomes a howling factor on the high frequency side, which is a problem in the present invention.

In (b) showing the phase difference, the range where the phase difference exceeds 90 ° is described as "increasing sound", and the range where the phase difference is less than -90 ° is described as "increasing sound / howling". , The sound that is input to the microphone again through the analog ANC circuit is in phase, and in the present invention that cancels the noise by inverting ±, the phase difference is from -90 ° to -180 °, 90 ° to When it becomes 180 °, it becomes a range in which effective sound reduction cannot be performed in ANC control, and the frequency at which the phase difference becomes −180 ° is the frequency at which howling occurs in ANC control.

2. Analog ANC control system according to the present invention This analog ANC control system S is shown in FIG. The cutoff frequencies of the plurality of gain / phase conversion elements 5 provided in the analog ANC control system S are shown in Table 1 below.

Regarding the high-pass filter BPF, the lower and upper limits of the cutoff frequency are described as the low-pass filter LPF and the high-pass filter HPF.

FIG. 4 shows the signal increase / decrease, gain, and phase difference characteristics of the analog ANC control system S.
In this figure, the frequency range is shown from 10 to 1000 Hz (same in FIGS. 6 and 7).
In FIG. 4, the thick broken line shows the system (analog ANC control system S shown in FIG. 3) in which all the gain / phase conversion elements 5 are added to the analog ANC control system S0 having the basic configuration shown in FIG. It is a characteristic.

Looking at the characteristics of the analog ANC control system S, the gain is almost-in the entire frequency range (10 to 1000 Hz), and the phase difference ωτ is in the frequency range of interest (20 Hz or more and 1000 Hz). , -90 ° <ωτ <90 ° is almost satisfied. Furthermore, there is no frequency at which the phase difference ωτ that causes howling is −180 °. That is, the phase difference ωτ is smaller than + 90 ° and larger than −90 °, and the characteristic lines described by the horizontal axis frequency and the vertical axis phase difference are smoothly connected within a finite value.

Hereinafter, the combination method of the gain / phase conversion element 5 as described above will be described.

3. 3. Addition of LPF0 Only FIG. 5 shows the characteristics of a system in which only LPF0 is provided in the analog ANC control system S0 having a basic configuration.
In this system, since the cutoff frequency of LPF0 is selected to 500 Hz, the gain is sufficiently reduced on the higher frequency side than this cutoff frequency. However, the sound reduction effect below 100 Hz is small. Looking at the phase difference, frequencies that cause howling appear from around 700 Hz, and sequentially appear on the high frequency side.

4. Addition of LPF0 and LPF1 This analog ANC control system includes LPF0 and LPF1 in the analog ANC control system S0 having a basic configuration. The characteristics of the system are shown in FIG.
In this system, the cutoff frequency of LPF1 is selected to 100 Hz, which is lower than the cutoff frequency of LPF0 of 500 Hz. The thick solid line shows the result when only LPF0 is added to the analog ANC control system S0 having the basic configuration, and the dotted line shows the result when LPF1 is added in addition to LPF0.
With reference to the characteristics, it is possible to reduce the sound to around 60 Hz in the frequency range of 100 Hz or less, which is insufficient in the sound reduction in FIG. However, in this case, the howling frequency appears at h2 near 150 Hz and further at h3 near 750 Hz.

5. Addition of LPF0, LPF1 and LPF2 This analog ANC control system includes LPF0, LPF1 and HPF2 in the analog ANC control system S0 having a basic configuration. The characteristics of the system are shown in FIG.
The cutoff frequency of LPF2 is selected to be 300 Hz, which is lower than the cutoff frequency of LPF0 of 500 Hz and higher than the cutoff frequency of LPF1 of 100 Hz. The thick solid line and the dotted line are the same as those shown in FIG. Further, the long dashed line shows the result of adding LPF2 in addition to LPF0, and the alternate long and short dash line shows the result of adding all of LPF0, LPF1 and LPF2.

With reference to the characteristics, LPF2 having a higher cutoff frequency than LPF1 can offset howling remaining in the vicinity of h2 near 150 Hz, in the vicinity of 750 Hz h3, and further in the vicinity of h4 near 250 Hz. It is considered that the signal of LPF0 helps to offset the howling of LPF1.
However, howling of h5 around 425 Hz still remains.

6. Addition of LPF0 to 2, BPF1 and 2 This system is the analog ANC control system S according to the present invention, and is a system to which all the gain / phase conversion elements 5 shown in Table 1 are added.
Here, as can be seen from Table 1, BPF1 has a cutoff frequency of 200 Hz as an LPF and a cutoff frequency of 400 Hz as an HPF. On the other hand, BPF2 has a cutoff frequency of 700 Hz as an LPF and a cutoff frequency of 1000 Hz as an HPF.

In FIG. 4, the thick solid line is the result of adding all of LPF0 to 2 and BPF1 to the analog ANC control system S0 having the basic configuration, and the dotted line is the result of adding all of LPF0 to 2 and BPF2 to the analog ANC control system S0 having the basic configuration. The added results are shown respectively.
As shown above, the thick broken line is the characteristic of the analog ANC control system S according to the present invention shown in FIG.
In FIG. 7 (c), the remaining howling of h5 near 425 Hz is reduced.

Therefore, the analog ANC control system according to the present invention is constructed from the above description.

1. 1. A plurality of low-pass filters LPF0 to 2 having different cutoff frequencies are provided as the gain / phase conversion element 5, and the gain at the howling frequency at which the phase difference becomes −180 ° in the frequency range higher than the predetermined frequency range (20 to 400 Hz) is obtained. It will be reduced.

2. Bandpass filters BPF1 and 2 as a gain / phase conversion element 5 are provided at frequencies where the phase difference is smaller than −90 °, and the phase difference is adjusted to be larger than −100 °. In the explanation so far, the lower limit of the phase difference is set to 90 °, but depending on the sound environment, even if the phase difference is about -100 °, it may be acceptable, and howling itself is prevented. For the purpose, if the phase difference does not become -180 °, the phase difference is within a predetermined range (within the above range of -100 ° to + 100 °), and the phase difference in the frequency domain is smoothly connected. good. Further, even when the phase difference is −180 °, howling can be prevented by sufficiently lowering the gain.

[Another Embodiment]
(1) In the above embodiment, an example in which the gain / phase conversion element 5 is a low-pass filter LPF or a band-pass filter BPF is shown, but any of a graphic equalizer, a high-pass filter, and a phase inversion element can be adopted. can.

When a graphic equalizer is used as the gain / phase conversion element 5, the gain in the howling frequency remaining in the frequency range higher than the predetermined frequency range can be effectively reduced.

(2) In the above embodiment, an example in which a single speaker 3 is commonly used as a speaker is shown, but these gain / phase conversion elements are supported on the lower side of each of the gain / phase conversion elements 5. The speaker 83 may be provided. An example of this configuration is shown in FIG. An analog ANC control system S'can be constructed by providing an analog ANC circuit 82 on the lower side (each speaker side) of each gain / phase conversion element 85.

1 Microphone 2 Analog ANC circuit (phase reversal amplifier circuit / AMP)
3 Speaker 4 Attenuator 5 Gain / phase conversion element 82 Analog ANC circuit 83 Speaker 85 Gain / phase conversion element (phase reversal amplifier circuit / AMP)
LPF low-pass filter (gain / phase conversion element)
BPF band path filter (gain / phase conversion element)
S Analog ANC control system S'Analog ANC control system S0 Basic structure analog ANC control system

Claims (5)

A microphone that detects sound and outputs it as a detection signal, an analog ANC circuit that accepts the detection signal, converts the detection signal with phase conversion and outputs it as an ANC control signal, and accepts the ANC control signal. It is configured with a speaker that produces a control sound,
An analog ANC control system in which the analog ANC circuit inverts the phase of the detection signal and increases / decreases the amplitude.
A plurality of gain / phase conversion elements having different gain / phase conversion characteristics are provided in parallel between the microphone and the analog ANC circuit.
In a predetermined frequency range, the phase difference ωτ between the phase of ^{the sound wave Ae iωt} detected by the microphone and the control sound wave F = −A × Be ^{i (ωt + b)} / (1 + Be ^{ib) generated from the speaker is −90 ° <ωτ.} <Analog ANC control system adjusted to 90 °.
[Here, n is the number of gain / phase conversion elements, Σ is the sum of all gain / phase conversion elements, the gain of each gain / phase conversion element is Fn, the phase is θn, and the speaker characteristics and between the speaker and the microphone. The total gain of the transmission function and microphone characteristics means S, the phase means φ, and Be ^ib = Σ (F _n e ^iθn ) × Se ^{i φ} ]. The analog ANC control system according to claim 1, wherein the gain / phase conversion element is any one of a low-pass filter, a band-pass filter, a high-pass filter, a graphic equalizer, and a phase inversion element, or a combination thereof. Claim 1 in which a plurality of low-pass filters having different cutoff frequencies are provided as the gain / phase conversion element, and the gain at a howling frequency in which the phase difference is −180 ° in a frequency range higher than the predetermined frequency range is reduced. Or the analog ANC control system according to 2. Any one of claims 1 to 3 provided with a bandpass filter as the gain / phase conversion element at a frequency at which the phase difference is smaller than −90 °, and the phase difference is adjusted to be larger than −90 °. The analog ANC control system described. The analog according to any one of claims 1 to 4, wherein a graphic equalizer is provided as the gain / phase conversion element, and the gain at the howling frequency remaining in the frequency range higher than the predetermined frequency range is reduced. ANC control system.

Images