JP2007025527A - Active noise reduction apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an active noise reduction apparatus which is low in cost and is high in practicality as an active noise reduction apparatus for reducing random noises like load noises. <P>SOLUTION: Active noise reduction apparatus includes a sinusoidal wave generating means 102 for generating a sinusoidal wave of a prescribed frequency, a cosine wave generating means 103 for generating the cosine wave of the same frequency as that of the sinusoidal wave generating means 102, a processing circuit 101 having two coefficient updating means 106, 107 to update the respective coefficients of two 1-tap digital filters 104, 105 by determining the outputs obtained by adding the outputs from the 1-tap digital filters 104, 105 and a control signal of the phase opposite to the phase of the original noise is generated by an adjusting circuit 108 for adjusting the phase, amplitude from the processing circuit 101. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an active noise reduction apparatus for reducing noise by generating and interfering with an interference wave having an opposite phase and an equal amplitude to an unpleasant noise so-called road noise generated in a passenger compartment of a vehicle or the like. .

  As a conventional active noise reduction device, a microphone is installed in a place where noise is to be reduced as a traditional method, and the signal of the microphone is processed by a phase and amplitude adjustment circuit so as to be in opposite phase to the original noise. A so-called feedback method has been known for a long time to output it as an interference wave from an electroacoustic conversion means such as a speaker to reduce noise at the microphone position.

  Also, as a so-called feed-forward method, a signal highly correlated with noise is input, and the input signal is processed by an N-tap adaptive digital filter that adapts so that the signal of a microphone installed in a place where noise is desired to be reduced is reduced. A method of reducing the noise at the microphone position by outputting it as an interference wave from electroacoustic conversion means such as a speaker is known.

As prior art document information related to the invention of this application, for example, Patent Document 1 is known.
JP-A-3-203792

  The problem of the traditional feedback method described above lies in a phase amplitude adjustment circuit that generates a signal having the same amplitude and opposite phase as the original noise. This is generally configured using capacitors, resistors, operational amplifiers, etc., which are analog elements, but there are tolerances in these capacitors, resistors, which are analog elements, and ideal design values in actual mass production An error from occurs. In addition, a large number of analog elements are required to realize steep characteristics and complex characteristics, and there is a drawback that the cost and size are increased.

  Similarly, the problem of the feedforward method described above is an N-tap adaptive digital filter that generates a signal having the same phase and amplitude as the original noise. To realize this, high-speed arithmetic processing is performed. A digital signal processor to do must be used. Since this high-speed digital signal processor is very expensive, it has been a major obstacle to cost.

  As described above, the conventional active noise reduction apparatus that reduces random noise such as road noise has problems in terms of error, size from the design value due to tolerance, in addition to the problem of cost.

  In order to solve the above-described problems, the present invention is an active noise reduction device that actively reduces random noise such as road noise, and has a specific frequency as a method for generating a control signal having a phase opposite to that of noise. A cosine wave generating means for generating a cosine wave having the same frequency as the sine wave generating means for generating a sine wave, two one-tap digital filters for processing outputs from the sine wave generating means and the cosine wave generating means, The sum of outputs from two 1-tap digital filters is used as an output, and the sum of this output and a signal from a conversion means such as a microphone placed at a position where noise is desired to be reduced, and the sine wave generating means, A processing circuit having two coefficient updating means for updating respective coefficients of the two one-tap digital filters with an output from the cosine wave generating means; The output of the phase, and constitutes the active noise reducing device by generating the adjustment circuit for adjusting the amplitude and emits this signal as interference noise by the conversion means such as a speaker.

  With this configuration, when an analog element is used, there is no influence of an error or the like due to a tolerance unique to the analog element. In addition, the arithmetic processing of the two adaptive one-tap digital filters is very small, and there is no need for a high-speed digital signal processor such as a feedforward type, and an active noise reduction device can be realized with an inexpensive microcomputer. it can.

  As described above, according to the active noise reduction apparatus of the present invention, a control signal having a phase opposite to that of the original noise in the feedback method can be generated digitally with a simple configuration, and an analog element is used. In comparison, problems such as errors due to tolerances of analog elements and an increase in the number of elements can be avoided, so that it is possible to realize an excellent active noise reduction apparatus that is inexpensive and highly practical.

  Hereinafter, an embodiment of an active noise reduction apparatus according to the present invention will be described. FIG. 1 is a block diagram showing a configuration of an active noise reduction apparatus according to an embodiment of the present invention.

  The processing circuit 101 outputs sine wave generation means 102 for generating a sine wave of a specific frequency, cosine wave generation means 103 for generating a cosine wave of the same frequency, sine wave generation means 102 and cosine wave generation means 103. Two coefficient updating units 106 and 107 to which two one-tap digital filters 104 and 105 to be processed, inputs and outputs from a sine wave generation unit 102 and a cosine wave generation unit 103 are input. , 107 sequentially update the coefficients of the 1-tap digital filters 104, 105 corresponding to each of them. The output of the processing circuit 101 is adjusted in amplitude and phase by the adjustment circuit 108 and is applied to the first conversion means 109 such as a speaker. The output of the second conversion means 110 such as a microphone is added to the input of the processing circuit 101 to constitute a feedback type active noise reduction apparatus.

  Here, in order to explain the noise reduction mechanism of the active noise reduction apparatus in the embodiment of the present invention shown in FIG. 1, the input / output characteristics of the processing circuit 101 in FIG. 1 will be described.

  FIG. 2 is a block diagram of the processing circuit 101 in FIG. 1 in which the connection from the output to the input is removed, and the input / output characteristics of the processing circuit 101 will be described using the notation described here. Here, the input signal of the processing circuit 101 is Cos (ωt + α), and the generated signals of the sine wave generating means 102 and the cosine wave generating means 103 are Sinωot and Cosωot, respectively. The coefficient updating means 106 and 107 generally update the coefficients of the 1-tap digital filters 104 and 105 corresponding to the LMS algorithm, respectively, but the coefficient update formula is that of the two 1-tap digital filters 104 and 105. The current coefficients are expressed as Bn and An, respectively.

  Here, μ is a small coefficient value called a convergence coefficient.

  First, changes ΔAn and ΔBn of the coefficients An and Bn of the adaptive filter are expressed by the following equations.

Here, An and Bn of the results updated sequentially are obtained by integrating the above equations.

Also, if the integration constant of A and B is 0 and ωx >> ωy, the term of ωx is ignored.

It becomes.

  Since the signals from the sine wave generating means and the cosine wave generating means 102 and 103 are added to this, the outputs Ea and Eb from the two 1-tap digital filters 105 and 104, respectively.

Therefore, the output Et is

  That is, Expression (1) is a signal output with Cos (ωt + α) added as an input. When ω <ω0, the phase is delayed by 90 ° from the input signal, and when ω = ω0, 180 ° ω> ω0 and 90 °. move on. It can also be seen that the amplitude becomes infinite when ω0 = ω, and decreases in inverse proportion to | ω0−ω | as ω moves away from ω0.

  FIG. 3 is a characteristic example of the transfer characteristic Et / Ei of the block diagram of FIG. 2 calculated as described above.

  Next, transfer characteristics of the processing circuit 101 will be described.

  FIG. 4 is a block diagram showing a state in which the output of the block diagram of FIG. 2 is fed back to the input so that the block diagram of FIG. Assume that the transfer function of the portion 111 corresponding to the processing circuit shown in FIG. 2 is represented by F (s) and has the characteristics shown in FIG.

  At this time, the transfer characteristic of the block diagram of FIG.

It is represented by

  FIG. 5 shows an example of the transfer characteristic represented by Expression (2). From FIG. 5, it can be seen that the processing circuit 101 has the characteristics of a bandpass filter centered on ω0. It can also be seen that the phase is 180 ° at ω0.

  Since ω0 is the generated frequency of the sine wave generating means 102 and cosine wave generating means 103, the center frequency of this bandpass characteristic can be easily changed by varying the generated frequencies of the sine wave generating means 102 and cosine wave generating means 103. Can be changed. Further, the bandwidth of this bandpass characteristic can be easily varied by varying μ according to the equation (1). FIG. 6 shows an example of transfer characteristics when μ of the processing circuit 101 is changed.

  Next, the overall silencing mechanism will be described.

  FIG. 7 is a block diagram for explaining the operation of the active noise reduction apparatus according to the embodiment of the present invention by simplifying it in terms of an electric circuit.

Reference numeral 112 denotes a portion corresponding to the processing circuit 101, the transfer characteristic of which is represented by F ₁ (S), and 113, a portion corresponding to the adjustment circuit 108, whose transfer characteristic is represented by F ₂ (S). . Reference numeral 114 denotes a portion representing the total transfer characteristics including the first transfer means 109, the second conversion means 110, and the spatial transfer characteristics between them, and the transfer characteristics are represented by F ₃ (S). Shall be. The input Vn corresponds to the original noise, and Ve is the noise after control.

  Here is the relationship between Vn and Ve

It becomes.

This formula shows that when the absolute value of 1-F ₁ (S), F ₂ (S), F ₃ (S) is greater than 1, the post-control noise Ve is smaller than the original noise Vn. It shows that it becomes. That is, if F ₁ (S) · F ₂ (S) · F ₃ (S) is expressed in terms of frequency characteristics, it can be said that the phase effect is 180 ° and the greater the gain, the greater the control effect.

In the case of the present invention, since F ₁ (S) has the characteristics as shown in FIG. 5, F ₂ (S) · F ₃ (S) is selected so that the phase is 0 ° at ω 0. In general, F ₃ (S) is a transfer function including the first transfer means 109, the second conversion means 110, and the spatial transfer characteristics between them, and thus cannot be arbitrarily set. F ₂ (S), that is, adjustment is performed by the adjustment circuit 108. Setting in the adjustment circuit 108 adjusts the F ₂ (S) As has been mentioned above becomes phase of 0 ° at F ₂ (S) · F ₃ a (S) ω0.

  Although the adjustment circuit 108 can be configured by an analog circuit, a circuit example configured by a digital circuit is shown in FIG. 115 simplifies the processing circuit 101 of FIG. 1, and shows the situation when the coefficients of the two one-tap digital filters 104 and 105 are A and B, respectively, and 116 is a one-tap digital having the coefficients described, respectively. It is a filter.

  When Vout1 and Vout2 described here are calculated

You can see that it ’s only progressing. Thus, by appropriately selecting the coefficients Sa and Sb of the 1-tap digital filter 116, the amplitude and phase can be adjusted, and errors due to tolerances such as analog circuits do not occur.

  FIG. 9 is a block diagram showing a configuration of an active noise reduction apparatus according to another embodiment of the present invention in which a plurality of processing circuits 101 having different frequencies are connected in parallel.

  Reference numeral 117 denotes a block processing unit in which a plurality of processing circuits having different frequencies are connected in parallel. An example of the transfer characteristic of the block processing unit 117 is shown in FIG. Comparing this with FIG. 5, it can be seen that the passband of the bandpass characteristic is widened, and as can be seen from the above description, noise in a wide band can be reduced.

  The active noise reduction device of the present invention can generate a control signal having a phase opposite to that of the original noise in a digital and simple configuration, and can realize an active noise reduction device with low cost and high practicality. It is useful for application to active noise reduction devices.

The block diagram of the active noise reduction apparatus in embodiment of this invention Block diagram for calculating the transfer characteristics of a processing circuit The figure which shows the example of a transfer characteristic in the block diagram of FIG. Block diagram for determining transfer characteristics of processing circuit Diagram showing transfer characteristic example of processing circuit Diagram showing an example of transfer characteristics when μ of the processing circuit is changed The block diagram for demonstrating the silencing operation | movement of the active noise reduction apparatus in embodiment of this invention Block diagram showing a configuration example of the adjustment circuit The block diagram of the active noise reduction apparatus in other embodiment of this invention The figure which shows the example of the transfer characteristic of the processing circuit of the active noise reduction apparatus in other embodiment of this invention.

Explanation of symbols

Reference Signs List 101 processing circuit 102 sine wave generating means 103 cosine wave generating means 104 1 tap digital filter 105 1 tap digital filter 106 coefficient updating means 107 coefficient updating means 108 adjustment circuit 109 first conversion means (speaker)
110 Second conversion means (microphone)
Reference numeral 111: part corresponding to processing circuit 112: transfer function of processing circuit 113: transfer function of adjustment circuit 114: transfer function including first transfer means, second conversion means, and spatial transfer characteristics between them 115: simplification of processing circuit 116 1-tap digital filter 117 Block processing unit

Claims (2)

Cosine wave generating means for generating a cosine wave having the same frequency as the sine wave generating means for generating a sine wave having a specific frequency, and two one-tap digital processing for processing outputs from the sine wave generating means and the cosine wave generating means An output obtained by adding the outputs from the filter and the two one-tap digital filters is used as an output, and the output from the sine wave generating means and the cosine wave generating means is obtained by adding the output and the input. A processing circuit having two coefficient updating means for updating each coefficient of the tap digital filter, an adjustment circuit for adjusting the phase and amplitude of the output from the processing circuit, and an output from the adjustment circuit as a sound wave, or Active noise composed of first conversion means for converting to vibration, and second conversion means for converting sound waves or vibration connected to the input of the processing circuit into electrical signals. Reduction device. 2. The active noise reduction apparatus according to claim 1, wherein processing circuits for generating sine waves and cosine waves having different specific frequencies are connected in parallel.

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