Patent Description:
A noise cancelling system that provides a satisfactory music playback environment for a listener (user) by reducing (cancelling) ambient noise (noise) in the external environment when the listener listens to music or the like through earphones, headphones, or the like is known. In one example, Patent Literature <NUM> discloses a twin-type ambient noise cancellation device in which a feedback-based noise cancelling technique using a microphone installed in the inside of a casing and a feedforward-based noise cancelling technique using a microphone installed on the outside of the casing are integrated.

The technical problem can be seen in selecting and reducing frequencies unnecessary for the listener.

In view of this, the present disclosure proposes a novel and improved sound processing device, sound processing method, and computer program, capable of effectively reducing ambient noise over an extended frequency band.

According to the present disclosure as described above, it is possible to provide a novel and improved sound processing device, sound processing method, and computer program, capable of effectively reducing ambient noise at low cost.

Moreover, the description is given in the following order.

An overview of an embodiment of the present disclosure is described and then embodiments of the present disclosure are described in detail. It is noted that the embodiments with regard to <FIG>, and <FIG> are not claimed and are published herein for information and illustration purposes only.

A noise cancelling system that provides a satisfactory music playback environment for a listener (user) by reducing (cancelling) ambient noise (noise) in the external environment when the listener listens to music or the like through earphones, headphones, or the like is known. Portable music players are especially widely used nowadays, and many users listen to music using headphones in music trial listening environments in the outside of the home in many cases. Thus, there is a growing demand for a noise cancellation function capable of listening to music in a condition similar to a quiet environment by reducing surrounding ambient noise even under noisy conditions.

The noise cancellation processing is typically known to use a feedback system and a feedforward system. In addition, a technique for performing twin-type noise cancellation processing using a combination of feedback system and feedforward system is also proposed, as described above. An overview of feedback-based noise cancellation processing is now described.

An ambient noise reduction device that performs the feedback-based noise cancellation processing is often designed on the basis of classical control theory. In the following description, a feedback-based noise cancellation method based on classical control theory is referred to as CCT method, taking the acronym for Classical Control Theory.

<FIG> is a diagram illustrated to describe an exemplary configuration of an ambient noise reduction device <NUM> that performs the feedback-based noise cancellation processing using CCT method. As illustrated in <FIG>, the ambient noise reduction device <NUM> includes a microphone <NUM>, a filter circuit <NUM>, and a speaker <NUM>.

The microphone <NUM> is provided at a position considered to be close to the user's ear, and collects sound at a position close to the user's ear. The microphone <NUM> thus collects external ambient noise reaching the ear. The microphone <NUM> sets the collected sound as a noise signal d and outputs it to the filter circuit <NUM>. The sound collected by the microphone <NUM> is collected again by the microphone <NUM> via the filter circuit <NUM> and a transfer function F between the speaker <NUM> and the microphone <NUM>. Thus, the microphone <NUM>, the filter circuit <NUM>, and the speaker <NUM> form what is called a closed loop.

The filter circuit <NUM> performs predetermined filtering processing on the noise signal that is output from the microphone <NUM> to generate a noise cancellation signal used to cancel external ambient noise reaching the user's ear. The filter circuit <NUM> performs the operation of gain, phase, and amplitude characteristics using a parameter β<NUM> for the noise signal output from the microphone <NUM>. The filter circuit <NUM> can be implemented as, in one example, a finite impulse response (FIR) filter or an infinite impulse response (IIR) filter.

The speaker <NUM> outputs sound by vibrating a diaphragm (not shown) on the basis of the noise cancellation signal output from the filter circuit <NUM>. The sound output from the speaker <NUM> is collected by the microphone <NUM> together with external ambient noise. Thus, the microphone <NUM> outputs a residual signal y corresponding to the noise that fails to be cancelled from the sound that is output on the basis of the noise cancellation signal. Moreover, the microphone <NUM> and the speaker <NUM> are provided inside a housing (or casing), which is not shown.

The residual signal y at the position of the microphone <NUM> in the feedback-based noise cancellation processing using CCT method is calculated in relation with the noise signal d, as expressed in Formula <NUM> below. <NUM>] <MAT>.

Here, in Formula <NUM>, <NUM>/(<NUM>+β<NUM>) is called a sensitivity function. It can be said that as the sensitivity function approaches zero, the noise signal d at the position of the microphone <NUM> decreases and the residual signal y approaches zero. In other words, it can be said that the feedback-based noise cancellation processing using CCT method can consequently reduce the noise signal d at the position of the microphone <NUM> by making the gain of β<NUM> of the filter circuit <NUM> large to increase the denominator of the sensitivity function.

The technique relating to the twin-type ambient noise reduction device that further reduces noise by combining the feedback-based noise cancellation processing with feedforward-based noise cancellation processing is disclosed, as described above. However, the twin-type ambient noise reduction device necessitates microphones installed on both the inside and the outside of the housing, which leads to an increase in cost and the size of device.

In view of the above-mentioned points, those who conceived the present disclosure have conducted intensive studies on the technology capable of improving the quality of noise reduction without increasing the cost or the size of device. As a result, those who conceived the present disclosure have devised the technology capable of improving the quality of noise reduction without increasing cost or the size of device, as described below.

An exemplary configuration of an ambient noise reduction device that performs feedback-based noise cancellation processing using an internal model control method is now described. In the following description, the internal model control method is also referred to as IMC method, taking the acronym for Internal Model Control.

<FIG> is a diagram illustrated to describe an exemplary configuration of an ambient noise reduction device <NUM> that performs the feedback-based noise cancellation processing using IMC method. As illustrated in <FIG>, the ambient noise reduction device <NUM> includes a microphone <NUM>, a characteristic applying unit <NUM>, a subtractor <NUM>, a filter circuit <NUM>, and a speaker <NUM>. In the ambient noise reduction device <NUM> illustrated in <FIG>, the characteristic applying unit <NUM> and the subtractor <NUM> are further included, as compared to the ambient noise reduction device <NUM> that performs the feedback-based noise cancellation processing using CCT method illustrated in <FIG>.

The microphone <NUM> is provided at a position considered to be close to the user's ear, and collects sound at a position close to the user's ears. Thus, the microphone <NUM> collects external ambient noise reaching the ear. The microphone <NUM> sets the collected sound as a noise signal d and outputs it to the subtractor <NUM>. The sound collected by the microphone <NUM> is collected again by the microphone <NUM> via the subtractor <NUM>, the filter circuit <NUM>, and a transfer function F between the speaker <NUM> and the microphone <NUM>. Thus, the microphone <NUM>, the subtractor <NUM>, the filter circuit <NUM>, and the speaker <NUM> form what is called a closed loop.

The characteristic applying unit <NUM> is a circuit that applies a predetermined characteristic F' to the output of the filter circuit <NUM> and outputs it. This characteristic F' is a characteristic obtained by simulating the transfer function F between the speaker <NUM> and the microphone <NUM>, and is designed as plant simulation characteristics of the transfer function F. The characteristic applying unit <NUM> outputs a result obtained by applying the predetermined characteristic F' to the output of the filter circuit <NUM> to the subtractor <NUM>.

The subtractor <NUM> subtracts the output of the characteristic applying unit <NUM> from the noise signal that is output from the microphone <NUM>. The subtractor <NUM> outputs the signal obtained by subtraction to the filter circuit <NUM>.

The filter circuit <NUM> performs predetermined filtering processing on the signal that is output from the subtractor <NUM> to generate a noise cancellation signal used to cancel the external ambient noise reaching the user's ear. The filter circuit <NUM> performs the operation of gain, phase, and amplitude characteristics using a parameter β<NUM> for the signal that is output from the subtractor <NUM>. The filter circuit <NUM> can be implemented as, in one example, an FIR filter or an IIR filter.

The speaker <NUM> outputs sound by vibrating a diaphragm (not shown) on the basis of the noise cancellation signal that is output from the filter circuit <NUM>. The sound that is output from the speaker <NUM> is collected by the microphone <NUM> together with external ambient noise. Thus, the microphone <NUM> outputs a residual signal y corresponding to the noise that fails to be cancelled from the sound that is output on the basis of the noise cancellation signal. Moreover, the microphone <NUM> and the speaker <NUM> are provided inside a housing (or casing), which is not shown.

The IMC method is a control method mainly used to control a system including dead time. As illustrated in <FIG>, the IMC method has a feature that the internal model is included in a loop. In other words, the characteristic applying unit <NUM> that applies the characteristic F' corresponds to the internal model.

Similarly to CCT method, the residual signal y at the position of the microphone <NUM> in the feedback-based noise cancellation processing using IMC method is calculated in relation with the noise signal d, as expressed in Formula <NUM> below. <NUM>] <MAT>.

Here, in Formula <NUM>, the transfer function between d and y is called a sensitivity function. In the IMC method, the internal model F' is designed to approximate the plant F. Thus, if F' = F is approximately established, it can be said that the IMC method preferably design a filter used to minimize "(<NUM>+β<NUM>F')" that is a term of the numerator in the sensitivity function.

To summarize the CCT method and the IMC method, the CCT method can also be a method of making the denominator of the sensitivity function larger to reduce ambient noise by division. In addition, the IMC method can also be a method of reducing ambient noise by subtracting the numerator of the sensitivity function.

It can be said that the IMC method can be similar to the feedforward system. The reasons are as follows.

<FIG> is a diagram illustrated to describe blocks for signal processing in the feedforward-based noise cancellation processing.

In the feedforward system, a characteristic G is assumed to represent the transfer function from a noise source N to a reference microphone <NUM>, and a characteristic G' is assumed to represent the transfer function from the noise source N to an error microphone <NUM>. In addition, the transfer function between the speaker <NUM> and the error microphone <NUM> is set to F. In addition, in the feedforward system, the gain of an ambient noise reduction filter circuit <NUM> is set to α.

The gain α of the ambient noise reduction filter circuit <NUM> for minimizing the residual signal at the position of the error microphone <NUM> in the feedforward system can be expressed as Formula <NUM> below. <NUM>] <MAT>.

On the other hand, in the feedback-based noise cancellation processing using IMC method illustrated in <FIG>, if the internal model F' coincides with the transfer function F between the speaker <NUM> and the microphone <NUM>, i.e., F' = F is established, the gain β<NUM> of the filter circuit <NUM> that minimizes the residual signal at the position of the microphone <NUM> can be expressed as Formula <NUM> below. <NUM>] <MAT>.

When comparing Formula <NUM> with Formula <NUM>, the feedback-based noise cancellation processing using IMC method can be expressed to be equivalent to the feedforward-based noise cancellation processing in the case where it is considered that the reference microphone is the same as the error microphone. In other words, the feedback-based noise cancellation processing using IMC method achieves the effect equivalent to that of the feedforward-based noise cancellation processing.

If the feedback-based noise cancellation processing using IMC method can achieve the effect equivalent to that of the feedforward-based noise cancellation processing, the combination of the feedback-based noise cancellation processing using CCT method with feedback-based noise cancellation processing using IMC method should make it possible to achieve the effect equivalent to that of the above-described twin-type noise cancellation processing with only one microphone.

<FIG> is a diagram illustrated to describe an exemplary configuration of an ambient noise reduction device <NUM> that performs the feedback-based noise cancellation processing using a combination of the CCT method and the IMC method according to the present disclosure. The feedback-based noise cancellation processing using the combination of the CCT method and the IMC method is also called a double feedback-based noise cancellation processing.

As illustrated in <FIG>, the ambient noise reduction device <NUM> includes a microphone <NUM>, filter circuits <NUM> and <NUM>, a characteristic applying unit <NUM>, a subtractor <NUM>, an adder <NUM>, and a speaker <NUM>.

The microphone <NUM> is provided at a position considered to be close to the user's ear and collects sound at a position close to the user's ears. Thus, the microphone <NUM> collects external ambient noise reaching the ear. The microphone <NUM> sets the collected sound as a noise signal d and outputs it to the subtractor <NUM>.

The filter circuit <NUM> performs predetermined filtering processing on the signal that is output from the microphone <NUM> to generate a noise cancellation signal used to cancel external ambient noise reaching the user's ear. The filter circuit <NUM> performs the operation of gain, phase, and amplitude characteristics using a parameter β1 for the signal that is output from the subtractor <NUM>. The filter circuit <NUM> can be implemented as, in one example, an FIR filter or an IIR filter.

The filter circuit <NUM> performs predetermined filtering processing on the signal that is output from the subtractor <NUM> to generate a noise cancellation signal used to cancel external ambient noise reaching the user's ear. The filter circuit <NUM> performs the operation of gain, phase, and amplitude characteristics using parameter β2 for the signal output from the subtractor <NUM>. The filter circuit <NUM> can be implemented as, in one example, an FIR filter or an IIR filter.

The characteristic applying unit <NUM> is a circuit that applies a predetermined characteristic F' to the output of the adder <NUM> and outputs it. This characteristic F' is a characteristic obtained by simulating the transfer function F between the speaker <NUM> and the microphone <NUM>, and is designed as a plant simulation characteristic of the transfer function F. The characteristic applying unit <NUM> outputs a value, which is obtained by applying a predetermined characteristic F' to the output of the adder <NUM>, to the subtractor <NUM>.

The subtractor <NUM> subtracts the output of the characteristic applying unit <NUM> from the noise signal that is output from the microphone <NUM>. The subtractor <NUM> outputs the signal obtained by subtraction to the filter circuits <NUM> and <NUM>.

The adder <NUM> adds the noise cancellation signal generated by the filter circuit <NUM> and the noise cancellation signal generated by the filter circuit <NUM>. The adder <NUM> outputs the noise cancellation signal obtained by addition to the speaker <NUM>.

The speaker <NUM> outputs sound by vibrating a diaphragm (not shown) on the basis of the noise cancellation signal that is output from the adder <NUM>. The sound that is output from the speaker <NUM> is collected by the microphone <NUM> together with external ambient noise. Thus, the microphone <NUM> outputs a residual signal y corresponding to the noise that fails to be cancelled from the sound that is output on the basis of the noise cancellation signal. Moreover, the microphone <NUM> and the speaker <NUM> are provided inside a housing (casing), which is not shown.

The sensitivity function between the noise signal d and the residual signal y in the ambient noise reduction device <NUM> is calculated as expressed in Formula <NUM> below. <NUM>] <MAT>.

Considering the sensitivity function in Formula <NUM>, in the double feedback system, as the gain of the filter circuit <NUM> using CCT method increases and the gain of the filter circuit <NUM> using IMC method approaches the inverse characteristic of F', the ambient noise is reduced and the residual signal y approaches zero. In other words, the double feedback system can be a system intended to reduce the ambient noise from both terms of denominator and numerator in the sensitivity function in Formula <NUM>.

The feedback-based noise cancellation processing using IMC method can obtain the effect equivalent to that of the feedforward-based noise cancellation processing. Thus, the ambient noise reduction device <NUM> illustrated in <FIG> uses the combination of the feedback-based noise cancellation processing using CCT method and the feedback-based noise cancellation processing using IMC method, thereby achieving the effect equivalent to that of the above-described twin-type noise cancellation processing. In addition, the ambient noise reduction device <NUM> illustrated in <FIG> can achieve the effect equivalent to that of the above-described twin-type noise cancellation processing with only one microphone <NUM>.

The feedback-based noise cancellation processing using IMC method can be combined with the feedback-based noise cancellation processing using CCT method, but it also can be combined with the feedforward-based noise cancellation processing.

<FIG> is a diagram illustrated to describe an exemplary configuration of an ambient noise reduction device <NUM> that employs the combination of the feedback-based noise cancellation processing using IMC method and the feedforward-based noise cancellation processing according to the present disclosure.

As illustrated in <FIG>, the ambient noise reduction device <NUM> includes microphones <NUM> and <NUM>, filter circuits <NUM> and <NUM>, a characteristic applying unit <NUM>, a subtractor <NUM>, an adder <NUM>, and a speaker <NUM>. In <FIG>, the transfer function from a noise source N to the microphone <NUM> is defined as G, and the transfer function from the noise source N to the microphone <NUM> is defined as G'. In other words, the noise signal d in the drawings referred to in the above description can be regarded as d = NG'.

The microphone <NUM>, the characteristic applying unit <NUM>, the subtractor <NUM>, and the filter circuit <NUM> are equivalent to those of the ambient noise reduction device <NUM> that performs the feedback-based noise cancellation processing using IMC method illustrated in <FIG>.

The microphone <NUM> and the filter circuit <NUM> are intended to perform the feedforward-based noise cancellation processing. The ambient noise coming from the noise source N is collected by the microphone <NUM> and is output to the filter circuit <NUM> as a noise signal. The filter circuit <NUM> performs the feedforward-based noise cancellation processing on the basis of the noise signal and outputs the noise cancellation signal to the adder <NUM>. The adder <NUM> adds the noise cancellation signals that are output from the filter circuits <NUM> and <NUM> and outputs the resultant value to the speaker <NUM>. Moreover, the microphone <NUM> and the speaker <NUM> are provided inside a housing (casing) that is not shown, and the microphone <NUM> is provided outside the housing (casing).

The ambient noise reduction device <NUM> illustrated in <FIG> combines the feedback-based noise cancellation processing using IMC method and the feedforward-based noise cancellation processing, thereby achieving more advantageous noise reduction effect as compared to the case where each is used individually.

The combination of the feedforward-based noise cancellation processing and the double feedback-based noise cancellation processing makes it possible to achieve more advantageous noise reduction effect.

<FIG> is a diagram illustrated to describe an exemplary configuration of an ambient noise reduction device <NUM> that employs the combination of the feedforward-based noise cancellation processing and the double feedback-based noise cancellation processing according to the present disclosure.

As illustrated in <FIG>, the ambient noise reduction device <NUM> includes microphones <NUM>, <NUM>, filter circuits <NUM>, <NUM>, and <NUM>, a characteristic applying unit <NUM>, a subtractor <NUM>, adders <NUM> and <NUM>, a speaker <NUM>. In <FIG>, similarly, the transfer function from the noise source N to the microphone <NUM> is defined as G, and the transfer function from the noise source N to the microphone <NUM> is defined as G'.

The ambient noise reduction device <NUM> illustrated in <FIG> has a configuration in which the filter circuit <NUM> and the adder <NUM> are added to the ambient noise reduction device <NUM> illustrated in <FIG>. The microphone <NUM>, the characteristic applying unit <NUM>, the subtractor <NUM>, the filter circuits <NUM> and <NUM>, and the adder <NUM> are equivalent to those of the ambient noise reduction device <NUM> that performs the double feedback-based noise cancellation processing illustrated in <FIG>.

The sensitivity function between the ambient noise from the noise source N and the residual signal y in the ambient noise reduction device <NUM> illustrated in <FIG> is calculated as expressed in Formula <NUM>. <NUM>] <MAT>.

As is apparent from the sensitivity function in Formula <NUM>, the noise cancellation processing employing the combination of the feedforward system and the double feedback system can be regarded as the addition of the terms of the feedforward system to the double feedback system. Thus, the noise cancellation processing employing the combination of the feedforward system and the double feedback system makes it possible to reduce noise of the residual signal, which is reduced using the IMC method, by further using the feedforward system. In other words, the ambient noise reduction device <NUM> illustrated in <FIG> can achieve more advantageous noise reduction effect as compared to the noise cancellation processing using only the double feedback system.

Each of the above-described ambient noise reduction devices may have additional processing of analyzing digital signals of sound collected by the microphone and selecting an optimum one of the ambient noise reduction filters on the basis of the analysis result.

<FIG> is a diagram illustrated to describe an exemplary configuration of an ambient noise reduction device <NUM> that employs a combination of the feedback-based noise cancellation processing using the double feedback system and the feedforward-based noise cancellation processing, according to the present disclosure.

As illustrated in <FIG>, the ambient noise reduction device <NUM> includes microphones <NUM> and <NUM>, filter circuits <NUM>, <NUM>, and <NUM>, a characteristic applying unit <NUM>, a subtractor <NUM>, adders <NUM> and <NUM>, a speaker <NUM> , a noise analyzer <NUM>, an optimum filter coefficient evaluation unit <NUM>, a memory controller <NUM>, and a memory <NUM>. In <FIG>, similarly, the transfer function from the noise source N to the microphone <NUM> is defined as G, and the transfer function from the noise source N to the microphone <NUM> is defined as G'.

The noise analyzer <NUM> analyzes the digital noise signal that is collected and output by the microphone <NUM>. The analysis of the noise signal by the noise analyzer <NUM> makes it possible to perceive what extent of noise at what kind of frequency band in the noise signal.

<FIG> is a diagram illustrated to describe an example of patterns of noise. In <FIG>, three noise patterns N1, N2, and N3 are shown, but the noise pattern is not limited to such example, of course. In this way, even if it is simply referred to as noise, various patterns of noise exist. The noise cancellation processing is necessary to be performed in a frequency band where the energy of noise concentrates to achieve the effective reduction of noise. To this end, the noise analyzer <NUM> analyzes the noise signal.

The optimum filter coefficient evaluation unit <NUM> determines a filter coefficient that provides the most favorable noise cancellation effect on the basis of the result of analysis of the noise signal by the noise analyzer <NUM>. Then, the memory controller <NUM> reads filter coefficients for the filter circuits <NUM>, <NUM>, and <NUM>, which are stored in the memory <NUM>, on the basis of the determination result of the filter coefficient by the optimum filter coefficient evaluation unit <NUM>, and sets the read filter coefficient for each of the filter circuits <NUM>, <NUM>, and <NUM>. Moreover, the optimum filter coefficient evaluation unit <NUM> can determine filter coefficients that provide the most favorable noise cancellation effect for at least one of the filter circuits <NUM>, <NUM>, or <NUM>, not all of them.

In the example illustrated in <FIG>, although the noise signal collected by the microphone <NUM> to perform the feedforward-based noise cancellation processing is analyzed, the present disclosure is not limited to this example. In other words, the noise signal collected by the microphone <NUM> to perform the feedback-based noise cancellation processing can be analyzed.

<FIG> is a diagram illustrated to describe an exemplary configuration of an ambient noise reduction device <NUM> that employs the combination of the feedback-based noise cancellation processing using the double feedback system and the feedforward-based noise cancellation processing.

The ambient noise reduction device <NUM> illustrated in <FIG> is similar to the ambient noise reduction device <NUM> illustrated in <FIG> in that it includes the noise analyzer <NUM>, the optimum filter coefficient evaluation unit <NUM>, the memory controller <NUM>, and the memory <NUM>. However, the noise analyzer <NUM> receives output from the subtractor <NUM> as an input, which is different from the configuration of the ambient noise reduction device <NUM> illustrated in <FIG>.

The reason why the noise analyzer <NUM> receives the output from the subtractor <NUM> rather than the output from the microphone <NUM> as an input is that a component close to the original noise signal can be taken out by using the difference from the path of the IMC system.

When the filter coefficients of the filter circuits <NUM>, <NUM>, and <NUM> are changed, it is undesirable to make a sudden change. Sudden changes can cause abnormal sound at the time of switching, and this abnormal sound may cause discomfort to the listener.

Thus, the filter circuits <NUM>, <NUM>, and <NUM> can have several filter regions in parallel. <FIG> is a diagram illustrated to describe an exemplary configuration of the filter circuit <NUM>. The filter circuit <NUM> illustrated in <FIG> has two filter regions 304a and 304b. In addition, volume faders 311a and 311b and an adder <NUM> are provided at a stage following the filter regions 304a and 304b. The adder <NUM> adds the outputs of the volume faders 311a and 311b.

In one example, when the filter region 304a is switched into the filter region 304b, the switching is performed smoothly by adjusting the volume faders 311a and 311b without abrupt switching from the filter region 304a to the filter region 304b. This smooth switching performed by adjusting the volume faders 311a and 311b makes it possible to prevent the occurrence of abnormal sound in switching from the filter region 304a to the filter region 304b, thereby preventing the listener from feeling uncomfortable.

The switching between the filter circuits <NUM> using IMC method in the double feedback-based noise cancellation processing may be performed by switching filters using the volume faders 311a and 311b. <FIG> is a diagram illustrated to describe an example of characteristics of the volume faders 311a and 311b. <FIG> illustrates an output F1 of the volume fader 311a and an output F2 of the volume fader 311b. In the example illustrated in <FIG>, the output of the volume fader 311a is gradually lowered from <NUM> times to finally become <NUM>, and conversely the output of the volume fader 311b is gradually increased from <NUM> times to finally become <NUM>. The characteristics of the volume faders 311a and 311b are certainly not limited to such an example.

The multiplexing of the feedback-based noise cancellation processing using IMC method is now described. <FIG> is a diagram illustrated to describe an exemplary configuration of an ambient noise reduction device <NUM> that performs the feedback-based noise cancellation processing using the multiplexed IMC method. In <FIG>, a double-IMC method in which two IMC methods are combined is exemplified, but the multiplexing may be performed for the feedback-based noise cancellation processing using three or more IMC methods.

As illustrated in <FIG>, the ambient noise reduction device <NUM> includes a microphone <NUM>, characteristic applying units <NUM> and <NUM>, subtractors <NUM> and <NUM>, filter circuits <NUM> and <NUM>, an adder <NUM>, and a speaker <NUM>.

The ambient noise reduction device <NUM> illustrated in <FIG> is configured by further adding a configuration for performing the feedback-based noise cancellation processing using IMC method to the ambient noise reduction device <NUM> that performs the feedback-based noise cancellation processing using IMC method illustrated in <FIG>. In other words, the configuration of the ambient noise reduction device <NUM> illustrated in <FIG> is obtained by adding the characteristic applying unit <NUM>, the subtractor <NUM>, and the filter circuit <NUM> to the ambient noise reduction device <NUM>.

Considering the IMC method from different perspectives, the IMC method is considered to be processing that can cancel the influence of its own hierarchy and execute signal processing on the restored signal using the internal model. In other words, in the ambient noise reduction device <NUM> illustrated in <FIG>, the purpose of the internal model F' applied by the characteristic applying unit <NUM> is to cancel the influence of the signal that is output from the driver (the speaker <NUM>) and to reproduce the noise signal d.

Referring back to <FIG>, the multiplexed IMC method has two feedback paths using the internal model F'. As described above, if the internal model control using the IMC method is used, the influence of its own hierarchy can be eliminated. In other words, at point1 in <FIG>, the influence of the output signal from the driver (the speaker <NUM>) is cancelled and the noise signal d is restored.

On the other hand, focusing on point2, the influence of the hierarchy (referred to as second hierarchy, for convenience) in the filter circuit <NUM> that applies the gain β<NUM> is excluded by using the internal model F'. Thus, only the residual signal cancelled by the hierarchy in the filter circuit <NUM> that applies the gain β<NUM> (referred to as first hierarchy, for convenience) is restored. In other words, the ambient noise reduction processing can be executed again in the second hierarchy on the residual signal that fails to be reduced in the first hierarchy. Thus, the configuration illustrated in <FIG> makes it possible to perform the multiplexing of the IMC method.

The sensitivity function between the noise signal d from the noise source N and the residual signal y in the ambient noise reduction device <NUM> illustrated in <FIG> is calculated as expressed in Formula <NUM> below. <NUM>] <MAT>.

Referring to Formula <NUM>, two terms in the numerator can be brought close to <NUM> using β<NUM> and β<NUM>, so the ambient noise reduction device <NUM> illustrated in <FIG> can multiplex the noise reduction effect using the IMC method.

Further, the multiplexing of the feedback-based noise cancellation processing using IMC method makes it possible to change the frequency band of a target for which ambient noise is to be reduced in each hierarchy. Even if the feedback-based noise cancellation processing using CCT method is multiplexed, although the noise reduction effect in the same frequency band can be enhanced, the frequency band of the target for which ambient noise is to be reduced is failed to be changed. On the other hand, the multiplexing of the feedback-based noise cancellation processing using IMC method makes it possible to change the frequency band of the target for which ambient noise is to be reduced by setting the parameters β<NUM> and β<NUM>, so the effect of reducing ambient noise in a wider range is achieved.

Moreover, <FIG> illustrates an exemplary configuration of the ambient noise reduction device <NUM> that performs the feedback-based noise cancellation processing using the multiplexed IMC method. However, it is also possible to add one or both of the configuration that performs the feedback-based noise cancellation processing using CCT method or the configuration that performs the feedforward-based noise cancellation processing to the feedback-based noise cancellation processing using the multiplexed IMC method.

A way of using by combining the IMC method and a monitor is now described.

It seems that it is highly demanded that the ambient noise is necessary to be reduced in sound unnecessary for the users who use an active headphone having a microphone while checking surrounding environmental sound. The use of the above-described double feedback system makes it possible to achieve monitoring by adding a signal in phase using a monitor signal processing filter to the IMC method while reducing ambient noise in a band undesirable for the user in the CCT method.

<FIG> is a diagram illustrated to describe an exemplary configuration of the ambient noise reduction device <NUM>. <FIG> illustrates blocks for signal processing in a case where a filter circuit <NUM> (filter coefficient y) in the loop of the IMC method is used for a monitor application, not for reduction of ambient noise. The filter circuit <NUM> is provided not for reducing noise but for adding signals in phase. Of course, the sound collected by the microphone <NUM> is a leakage sound in the headphone, so there is also a possibility that is not suitable for monitor sound.

Thus, in the case of combining the feedforward system and the double feedback system, the signal of the microphone arranged outside the casing is used as a monitor application, and the ambient noise in the unnecessary frequency band can be effectively reduced by using the double feedback system.

<FIG> is a diagram illustrated to describe an exemplary configuration of the ambient noise reduction device <NUM> according to the embodiment of the invention. <FIG> illustrates blocks for signal processing in a case where a filter circuit <NUM> (filter coefficient y) of the feedforward system is used as a monitor application. The filter circuit <NUM> is provided not for reducing noise but for adding signals in phase. Moreover, similarly to the feedforward system, the IMC method can tune the target frequency, and the ambient noise reduction device <NUM> illustrated in <FIG> can select and reduce a frequency unnecessary for the listener.

The ambient noise reduction device for performing the noise cancellation processing using IMC method has been described above. Then, an example of an application to a music canceller that cancels a music signal supplied from the outside of the sound processing device is described.

<FIG> is a diagram illustrated to describe an exemplary configuration of an ambient noise reduction device <NUM> according to the present disclosure. As illustrated in <FIG>, the ambient noise reduction device <NUM> includes a microphone <NUM>, a characteristic applying unit <NUM>, a subtractor <NUM>, a filter circuit <NUM>, an adder <NUM>, and a speaker <NUM>.

The microphone <NUM> is provided at a position considered to be close to the user's ear and collects sound at a position close to the user's ear. Thus, the microphone <NUM> collects external ambient noise reaching the ear. The microphone <NUM> sets the collected sound as a noise signal d and outputs it to the subtractor <NUM>.

The characteristic applying unit <NUM> is a circuit that applies a predetermined characteristic F<NUM>' to a music m and outputs it. This characteristic F<NUM>' is a characteristic obtained by simulating the transfer function F<NUM> between the speaker <NUM> and the microphone <NUM>, and is designed as a plant simulation characteristic of the transfer function F<NUM>. The characteristic applying unit <NUM> outputs a value, which is obtained by applying the predetermined characteristic F<NUM>' to the music m, to the subtractor <NUM>.

The filter circuit <NUM> performs predetermined filtering processing on the signal that is output from the subtractor <NUM> to generate a noise cancellation signal used to cancel the external ambient noise reaching the user's ear. The filter circuit <NUM> performs the operation of gain, phase, and amplitude characteristics using the parameter β on the signal that is output from the subtractor <NUM>. The filter circuit <NUM> can be implemented as, in one example, an FIR filter or an IIR filter.

The adder <NUM> adds the noise cancellation signal generated by the filter circuit <NUM> to the music m supplied from the outside of the sound processing device.

The speaker <NUM> outputs sound by vibrating a diaphragm (not shown) on the basis of the noise cancellation signal that is output from the adder <NUM>. The sound that is output from the speaker <NUM> is collected by the microphone <NUM> together with external ambient noise. Thus, the microphone <NUM> outputs the residual signal y corresponding to the noise that fails to be cancelled by the sound output on the basis of the noise cancellation signal. The microphone <NUM> and the speaker <NUM> are provided inside a housing (casing) that is not shown.

The sensitivity function between the noise signal d, the music m, and the residual signal y in the ambient noise reduction device <NUM> is calculated as expressed in Formula <NUM>. <NUM>] <MAT>.

The use of the music canceller allows a music component to be prevented from being mixed in a loop using the CCT method in the ambient noise reduction device <NUM>. Thus, the ambient noise reduction device <NUM> eliminates the necessity for an equalizer for music (or only minor adjustment is necessary).

In Formula <NUM>, β is excluded from the music component if F<NUM> and F<NUM>' are equivalent. Thus, it can be said that, from Formula <NUM>, the music canceller of the ambient noise reduction device <NUM> is useful.

Moreover, although <FIG> illustrates only the configuration for performing the feedback-based noise cancellation processing using CCT method, the configuration for performing the feedback-based noise cancellation processing using the IMC method instead of the CCT method can be used, or the configuration for performing the double feedback-based noise cancellation processing can be used.

The canceller of the feedforward loop is now described. <FIG> is a diagram illustrated to describe an exemplary configuration of an ambient noise reduction device <NUM> according to the present disclosure. As illustrated in <FIG>, the ambient noise reduction device <NUM> includes microphones <NUM> and <NUM>, filter circuits <NUM> and <NUM>, a characteristic applying unit <NUM>, a subtractor <NUM>, an adder <NUM>, and a speaker <NUM>.

The sensitivity function between the noise function N and the residual signal z in the ambient noise reduction device <NUM> is calculated as expressed in Formula <NUM>. <NUM>] <MAT>.

The characteristic F1' applied in the canceller of the feedforward loop is a characteristic obtained by simulating the transfer function F1 between the speaker <NUM> and the microphone <NUM>. The use of the canceller of the feedforward loop allows a feedforward component to be prevented from being mixed in the loop of the CCT method in the ambient noise reduction device <NUM>. Further, the use of the characteristic F1' makes it possible to exclude the component of F1 that is a cause of individual difference and mounting error. Moreover, the last equation in Formula <NUM> is arranged by replacing F1' of the immediately preceding equation with F1 on the assumption that F1' is equal to F1.

Moreover, although <FIG> illustrates only the configuration for performing the feedback-based noise cancellation processing using CCT method, the configuration for performing the feedback-based noise cancellation processing using IMC system instead of the CCT method can be used, or the configuration for performing the double feedback-based noise cancellation processing can be used.

It is also possible to combine the music canceller and a feedforward canceller. <FIG> is a diagram illustrated to describe an exemplary configuration of an ambient noise reduction device <NUM> according to the present disclosure. As illustrated in <FIG>, the ambient noise reduction device <NUM> includes microphones <NUM> and <NUM>, filter circuits <NUM> and <NUM>, a characteristic applying unit <NUM>, a subtractor <NUM>, adders <NUM> and <NUM>, and a speaker <NUM>. The configuration illustrated in <FIG> is a combination of the ambient noise reduction device <NUM> including the music canceller illustrated in <FIG> and the feedforward canceller illustrated in <FIG>.

The ambient noise reduction device <NUM> having the configuration illustrated in <FIG> has both functions of the music canceller and the feedforward canceller.

Moreover, although <FIG> illustrates only the configuration for performing the feedback-based noise cancellation processing using the CCT method, the configuration for performing the feedback-based noise cancellation processing using the IMC method instead of the CCT method can be used, or the configuration for performing the double feedback-based noise cancellation processing can be used.

In the noise cancellation processing using the IMC method described above, the noise cancellation signal is generated using the characteristic F' obtained by simulating the characteristic F. However, the characteristic F contains a variable element. Thus, if the error between the characteristic F and the characteristic F' is large, there is a possibility that the expected noise cancellation effect fails to be achieved.

Thus, the ambient noise reduction device that performs the noise cancellation processing using the IMC method may detect the state of the characteristic F' to lower the gain of the noise cancellation signal or to stop the noise cancellation processing depending on the detection result.

<FIG> is a diagram illustrated to describe an exemplary configuration of an ambient noise reduction device <NUM> according to the present disclosure. <FIG> illustrates an exemplary configuration of the ambient noise reduction device <NUM> in which a detection unit <NUM> and a fader <NUM> are added to the ambient noise reduction device <NUM> illustrated in <FIG>.

The detection unit <NUM> detects the state of the signal that is output by the subtractor <NUM> and is applied with the characteristic F'. Specifically, the detection unit <NUM> detects the state of the signal applied with the characteristic F', and detects the state of error between the characteristic F and the characteristic F'. The detection unit <NUM> can detect the state of the signal with respect to the output of the subtractor <NUM> by using, in one example, a time axis signal, a frequency axis signal, an envelope, a power value, or the like.

The fader <NUM> changes the gain of the noise cancellation signal that is output by the adder <NUM> on the basis of the detection result of the detection unit <NUM>. In one example, if the error between the characteristic F and the characteristic F' is within a predetermined range as the result of the detection by the detection unit <NUM>, the fader <NUM> does not change the gain of the noise cancellation signal that is output by the adder <NUM>. However, if the error between the characteristic F and the characteristic F' exceeds a predetermined range and becomes an abnormal state as the result of the detection by the detection unit <NUM>, the fader <NUM> reduces the gain of the noise cancellation signal that output by the adder <NUM> to less than <NUM> times. The fader <NUM> may change the reduction amount of the gain depending on the magnitude of the error between the characteristic F and the characteristic F'. In addition, the fader <NUM> can set the gain to <NUM> times, that is, not to output the noise cancellation signal output from the adder <NUM> when the error between the characteristic F and the characteristic F' further increases beyond a predetermined range.

<FIG> is a diagram illustrated to describe another exemplary configuration of the ambient noise reduction device <NUM> according to the present disclosure. The ambient noise reduction device <NUM> illustrated in <FIG> has the configuration similar to that of the ambient noise reduction device <NUM> illustrated in <FIG>, but the detection unit <NUM> receives a music signal M as an input in addition to the signal that is output from the subtractor <NUM>. Then, the detection unit <NUM> detects the state of the signal to which the characteristic F' is applied. In this event, in addition to the above-described time axis signal, frequency axis signal, envelope, power value, or the like, the detection unit <NUM> may use the correlation with the music signal M. Then, the fader <NUM> changes the gain applied to the noise cancellation signal that is output from the adder <NUM> depending on the detection result of the detection unit <NUM>.

<FIG> is a diagram illustrated to describe another exemplary configuration of the ambient noise reduction device <NUM> according to the present disclosure. The ambient noise reduction device <NUM> illustrated in <FIG> has the configuration similar to that of the ambient noise reduction device <NUM> illustrated in <FIG>, but the detection unit <NUM> receives the output from the microphone <NUM> as an input in addition to the signal that is output from the subtractor <NUM>. Then, the detection unit <NUM> detects the state of the signal to which the characteristic F' is applied. In this event, in addition to the above-described time axis signal, frequency axis signal, envelope, power value, or the like, the detection unit <NUM> may use the correlation with the output from the microphone <NUM>, the difference from the output from the microphone <NUM>, the ratio with the output from the microphone <NUM>, or the like. Then, the fader <NUM> changes the gain applied to the noise cancellation signal that is output from the adder <NUM> depending on the detection result obtained by the detection unit <NUM>.

In this way, the state of the signal to which the characteristic F' is applied can be detected and the gain to be applied to the noise cancellation signal can be changed depending on the detection result. This makes it possible for the ambient noise reduction device <NUM> to slightly weaken the noise cancellation effect or temporarily stop the noise cancellation processing in the case where the error between the characteristic F and the characteristic F' becomes large.

The ambient noise reduction device that performs the noise cancellation processing using the IMC method as described above is applicable to not only headphones but also other fields. Here, an example of cancelling the noise leaking into the interior of the vehicle by providing any one of the above-described ambient noise reduction devices on an automobile seat is described.

<FIG> is a diagram illustrated to describe an appearance example of an automobile seat <NUM> provided with any one of the above-described ambient noise reduction devices. In <FIG>, a headrest <NUM> of the automobile seat <NUM> is provided with speakers 802a and 802b and microphones 801a and 801b. The automobile seat <NUM> can be used as any of a driver's seat, a passenger's seat, or a rear seat.

The microphones 801a and 801b are provided at a position considered to be close to the user's ears and collect sound at a position close to the user's ears, which is similar to that of the ambient noise reduction device described above. Moreover, although two microphones 801a and 801b are illustrated in <FIG>, the present disclosure is not limited to this example, and the number of microphones provided in the automobile seat <NUM> can be one or can be three or more. The speakers 802a and 802b output the sound based on the noise cancellation signal used to cancel the sound collected by the microphones 801a and 801b.

The automobile seat <NUM> having such structure illustrated in <FIG> makes it possible to cancel the ambient noise leaking into the interior of the vehicle or being felt by the occupant of the automobile. Especially, the ambient noise reduction device that performs the feedback-based noise cancellation processing using the IMC method or the double feedback-based noise cancellation processing as described above makes it possible for the automobile seat <NUM> illustrated in <FIG> to provide passengers of automobiles with advantageous noise reduction characteristics at low cost.

According to the present disclosure as described above, there is provided an ambient noise reduction device that performs noise cancellation processing using the IMC method. The ambient noise reduction device that performs the noise cancellation processing using the IMC method can be provided with a microphone on the outside of the casing, thereby achieving an effect equivalent to that of the ambient noise reduction device for reducing the noise transmitted to the user's ear.

Further, according to the present disclosure, there is provided an ambient noise reduction device that performs the double feedback-based noise cancellation processing in which the noise cancellation processing using the CCT method employed in related art and the noise cancellation processing using the IMC method are combined. The ambient noise reduction device that performs the double feedback-based noise cancellation processing with one microphone has the effect equivalent to that of the twin-type noise cancellation processing employed in related art. Thus, the ambient noise reduction device that performs the double feedback-based noise cancellation processing eliminates the necessity for additional hardware, so the ambient noise can be effectively reduced at low cost.

Further, according to the present disclosure, there is provided an ambient noise reduction device in which the double feedback-based noise cancellation processing and the feedforward-based noise cancellation processing are combined. Such an ambient noise reduction device employing the combination of the double feedback-based noise cancellation processing and the feedforward-based noise cancellation processing allows further noise reduction effect to be achieved.

In the noise cancellation processing using the IMC method, the fine-tuning is possible for each frequency, which is similar to the feedforward-based noise cancellation processing. Thus, the ambient noise reduction device that performs the noise cancellation processing using the IMC method is capable of handling dynamically a plurality of modes by switching the filter characteristics depending on the feature of noise.

The noise cancellation processing using the IMC method is also the processing of removing the influence of the hierarchy of characteristics. Thus, the ambient noise reduction device that performs the noise cancellation processing using the IMC method is capable of multiplexing the noise cancellation processing using the IMC method by arranging the internal model in a plurality of layers and restoring the residual signal.

Steps in processes executed by the respective devices in this specification are not necessarily executed chronologically in the order described in the sequence chart or the flow chart. In one example, steps in processes executed by the respective devices may be executed in a different order from the order described in the flow chart or may be executed in parallel.

Further, it is also possible to produce a computer program for causing hardware such as a CPU, ROM, or RAM, incorporated in the respective devices, to execute a function equivalent to each configuration of the above-described respective devices. Furthermore, it is possible to provide a recording medium having the computer program recorded thereon. In addition, the respective functional blocks illustrated in the functional block diagram can be configured as hardware or hardware circuits, and thus a series of processing can be implemented using the hardware or hardware circuits.

Examples of the present disclosure has/have been described above with reference to the accompanying drawings, whilst the present disclosure is not limited to the above examples. A person skilled in the art may find various alterations and modifications, the present invention being defined by the appended claims.

Claim 1:
A sound processing device (<NUM>) comprising:
a first sound collector (<NUM>) configured to collect a first noise signal from a noise source of noise leaking into a casing mounted to a user's ear;
a first signal filter circuit (<NUM>) configured to receive as input the first noise signal and to form a first noise reduction signal used to reduce noise at a predetermined cancellation point;
a second signal filter circuit (<NUM>) configured to receive as input a first pseudo noise signal and to form a second noise reduction signal used to reduce noise at a predetermined cancellation point;
a second sound collector (<NUM>) being provided outside the casing and configured to collect a second noise signal from the noise source;
a third filter circuit (<NUM>) configured to receive as input the second noise signal collected by the second sound collector (<NUM>) and to form a signal in phase;
an adder (<NUM>, <NUM>) configured to add the first noise reduction signal, the second noise reduction signal and the signal in phase formed by the filter circuit (<NUM>);
a sound emitter (<NUM>) configured to emit an output of the adder (<NUM>, <NUM>) into the casing as sound;
a characteristic applying unit (<NUM>) configured to receive as input an output of the adder (<NUM>, <NUM>), and configured to simulate a transfer characteristic from the sound emitter (<NUM>) to the first sound collector (<NUM>), and
wherein the first pseudo noise signal is a signal obtained by subtracting an output of the characteristic applying unit (<NUM>) from an output of the first sound collector (<NUM>).