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JP2008116782

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DESCRIPTION JP2008116782
A wide band enabling noise cancellation and a stable noise reduction effect can be obtained.
SOLUTION: A noise canceling system portion of a feedback system constituted by a microphone
and a microphone amplifier unit 11, an FB filter circuit 12, a first amplifying means, and a first
sound emitting means, and a second voice A noise canceling system portion of a feedforward
system constituted by the sound collecting means, the second signal processing means, the
second amplifying means and the second sound emitting means is made to function at the same
time. The noise canceling system part of the invention is intended to reduce the noise at the same
cancellation point. [Selected figure] Figure 8
Noise canceling system and noise canceling method
[0001]
The present invention relates to, for example, a noise canceling system applied to a headphone
for listening to reproduced music or the like, a headset for reducing noise, and a noise canceling
method.
[0002]
Conventionally, there is an active noise canceling system (noise reduction system) mounted on
headphones.
The noise canceling systems currently put into practical use are all configured with analog
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circuits, and the current systems can be roughly classified into two systems: a feedback system
and a feedforward system.
[0003]
For example, in Patent Document 1 (Japanese Patent Application Laid-Open No. 3-214892)
described later, the noise inside the acoustic tube collected by the microphone unit 6 provided in
the acoustic tube 1 mounted on the user's ear is phase inverted. An invention is disclosed that
reduces external noise by emitting sound from an earphone unit 3 provided in the vicinity of the
microphone unit 6.
[0004]
In addition, in Japanese Patent Application Laid-Open No. 3-96199, which will be described later,
the output of the second microphone 3 positioned between the headphone 1 and the user's ear
canal is used at the time of mounting. By identifying the transfer characteristic from the first
microphone 2 for picking up external noise provided near the ear to the headphone 1 as the
transfer characteristic until the external noise reaches the ear canal, it is possible to wear the
headphones Regardless, the invention is disclosed for a noise reduction headphone that allows
external noise to be reduced.
[0005]
In addition, said patent document 1 and patent document 2 are as follows.
JP-A-3-214892 JP-A-3-96199
[0006]
By the way, in general, a noise canceling system of a feedback type is characterized in that
although a band capable of canceling noise (band capable of reducing noise) is narrow, relatively
large reduction is possible.
On the other hand, in the feedforward noise canceling system, although the noise cancelable
band is wide and stable, noise does not coincide with the transfer function assumed due to the
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positional relationship with the noise source. It is thought that there is a possibility that it will
increase.
[0007]
For this reason, when using a stable feedforward noise canceling system with a wide noise
cancelable band, noise in a specific narrow band becomes noticeable even if the noise reduced
band is wide. In such a case, it may be considered that the listener (user) may not feel the
reduction effect.
[0008]
In view of the above, it is an object of the present invention to widen a band in which noise can
be canceled and stably obtain a large noise reduction effect.
[0009]
In order to solve the above-mentioned subject, the noise canceling system of the invention
according to claim 1 is provided in an inside of a case attached to a user's ear part, and the 1st
which leaks in an inside of the case concerned. A first voice pickup means for picking up a noise
signal; a first noise reduction means for reducing noise at a predetermined cancellation point
from the first noise signal picked up by the first voice pickup means First signal processing
means for forming a noise reduction signal; first amplification means for amplifying the first
noise reduction signal formed by the first signal processing means; and A first sound emitting
means for emitting the first noise reduction signal into the housing; and a second sound source
provided from an outside of the housing mounted on the user's ear. Second sound pickup to pick
up the noise signal of A second signal processing means for forming a second noise reduction
signal for reducing noise at the cancel point from the second noise signal collected by the second
sound collection means; A second amplification means for amplifying the second noise reduction
signal formed in the second signal processing means; and releasing the second noise reduction
signal from the second amplification means into the casing. And a second sound emitting means
for making sound.
[0010]
According to the noise canceling system of the first aspect of the present invention, the first
sound collecting means, the first signal processing means, the first amplifying means, and the
first sound emitting means Feedforward system comprising a noise canceling system portion of a
feedback system, a second sound collecting means, a second signal processing means, a second
amplifying means, and a second sound emitting means The noise canceling system part of the
invention is made to function at the same time, and both noise canceling system parts are made
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to reduce the noise at the same cancellation point.
[0011]
This allows noise cancellation to be performed over a wide band at a high level by attenuating
noise components by the noise cancellation portion of the feed forward method and adding the
characteristics of the noise cancellation system portion of the feedback method, and stronger
noise The reduction effect can be obtained.
[0012]
According to the present invention, in addition to the feedforward noise canceling system, by
simultaneously operating the feedback noise canceling system, the generated noise is internally
attenuated by the feedforward noise canceling system. And, by adding the characteristics of the
feedback type noise canceling system alone, a stronger noise reduction effect can be obtained.
[0013]
Hereinafter, an embodiment of the present invention will be described with reference to the
drawings.
[0014]
[Regarding Noise Canceling System] At present, a system that actively reduces external noise for
headphones and earphones, a so-called noise canceling system, is beginning to spread.
Most of products that have been commercialized are composed of analog circuits, and noise
canceling methods are roughly classified into feedback methods and feed forward methods.
[0015]
First, prior to specific description of an embodiment of the present invention, an example of the
configuration and operation principle of a noise canceling system of a feedback type and noise of
a feed forward type with reference to FIGS. 1 to 5. A configuration example and an operation
principle of the canceling system will be described.
[0016]
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FIG. 1 is a diagram for explaining a noise canceling system of a feedback type, and FIG. 2 is a
diagram for explaining a noise canceling system of a feed forward type.
Further, FIG. 3 is a diagram for explaining a calculation equation showing the characteristics of
the noise canceling system of the feedback method shown in FIG. 1, and FIG. 4 is a phase margin
and a gain margin in the noise canceling system of the feedback method. FIG. 6 is a Bode
diagram for describing FIG.
FIG. 5 is a diagram for explaining a calculation formula indicating the characteristics of the noise
canceling system of the feedforward method shown in FIG.
[0017]
[Regarding Feedback Type Noise Canceling System] First, a feedback type noise canceling system
will be described.
FIG. 1A shows the configuration on the right channel side when the headphone system to which
the noise canceling system of the feedback method is applied is attached to the user head (head
of the user (listener)) HD. FIG. 1B shows the overall configuration of the noise canceling system
of the feedback method.
[0018]
The feedback method is generally referred to as a microphone 111 (hereinafter referred to as a
microphone) inside the headphone housing (housing part) HP as shown in FIG. 1 (A).
, And reverse-phase components (noise reduction signals) of the signals (noise signals) collected
by the microphone 111 are returned and servo-controlled to attenuate noise that has entered the
headphone housing HP from the outside. .
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In this case, since the position of the microphone 111 is a cancellation point (control point) CP
corresponding to the listener's ear position, a position close to the listener's ear, that is, the
diaphragm of the driver 16 is generally taken into consideration. The microphone 111 is often
placed on the front.
[0019]
Specifically, referring to the block diagram of FIG. 1B, the noise canceling system of the feedback
type will be described.
The noise canceling system of the feedback system shown in FIG. 1B is a microphone and
microphone amplifier unit 11 including a microphone 111 and a microphone amplifier 112, and
a filter circuit designed for feedback control (hereinafter referred to as an FB filter circuit).
12), a combining unit 13, a power amplifier 14, a driver 15 including a drive circuit 151 and a
speaker 152, and an equalizer 16.
[0020]
In FIG. 1B, characters A, D, M, and -β described in each block are transfer functions of the power
amplifier 14, the driver 15, the microphone and microphone amplifier unit 11, and the FB filter
circuit 12. Similarly, in FIG. 1B, the letter E in the block of the equalizer 16 is the transfer
function of the equalizer 16 applied to the signal S to be listened to, and the block placed
between the driver 15 and the cancellation point CP The letter H is the transfer function (transfer
function between driver and cancellation point) of the space from the driver 15 to the
microphone 111. Each of these transfer functions is assumed to be expressed in a complex
manner.
[0021]
Further, in FIGS. 1A and 1B, the letter N is noise that has entered from the external noise source
(noise source) NS near the microphone position in the headphone casing HP, and the letter P is a
listener It represents the sound pressure (output sound) that reaches the ear of The noise N may
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be transmitted to the inside of the headphone case HP as, for example, the sound pressure
leaking from the gap of the ear pad portion of the headphone case HP, or the result of the
headphone case HP vibrating due to the sound pressure. It is conceivable that sound may be
transmitted to the inside of the case.
[0022]
At this time, in FIG. 1 (B), the sound pressure P reaching the listener's ear can be expressed as
shown in equation (1) of FIG. In the equation (1) of FIG. 3, if attention is paid to the noise N, it is
understood that the noise N is attenuated to 1 / (1 + ADHMβ). However, in order for the system
of equation (1) of FIG. 3 to operate stably as a noise canceling mechanism in the noise reduction
target band, equation (2) of FIG. 3 needs to be established.
[0023]
In general, the absolute value of the product of each transfer function in the feedback noise
canceling system is 1 or more (1 << ¦ ADHMβ ¦), and together with the stability determination of
Nyquist in classical control theory, The stability of the system involved in equation (2) in FIG. 3
can be interpreted as follows.
[0024]
In FIG. 1B, consider an open loop
portion related to noise N.
of (−ADHMβ) formed by cutting one portion of a loop
For example, in FIG. 1B, if a cut portion is provided between the microphone and the microphone
amplifier unit 11 and the FB filter circuit 12, an "open loop" can be formed. This open loop has,
for example, a characteristic represented by a Bode diagram as shown in FIG.
[0025]
When this open loop is targeted, according to the stability judgment of Nyquist, (1) Phase 0 deg.
When passing the point (0 degrees), the gain must be less than 0 dB (0 decibels). (2) When the
gain is 0 dB or more, the phase 0 deg. Do not include the points of It is necessary to satisfy the
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two conditions of (1) and (2).
[0026]
If the above conditions (1) and (2) are not satisfied, the loop is subjected to a positive feedback
and causes oscillation (howling). In FIG. 4, symbols Pa and Pb represent phase margins, and
symbols Ga and Gb represent gain margins. If these margins are small, various individuals of
listeners who use headphones to which the noise canceling system is applied. The risk of
oscillation will increase due to differences and variations in the wearing of the headphones.
[0027]
That is, in FIG. 4, the horizontal axis is frequency. And in the vertical axis, the lower half is the
gain and the upper half is the phase. And phase 0 deg. As shown by gain margins Ga and Gb in
FIG. 4, when passing through the point of, if the gain is not smaller than 0 dB, the loop is
subjected to positive feedback to cause oscillation, and when the gain is 0 dB or more, FIG. As
phase margins Pa and Pb indicate that phase 0 deg. If the loop does not contain a positive
feedback, it causes an oscillation.
[0028]
Next, in the feedback type noise canceling system shown in FIG. 1 (B), a case where necessary
sound is reproduced from headphones in addition to the above-described noise reduction
function will be described. The input sound S in FIG. 1B is, for example, a sound of a microphone
outside the housing (when used as a hearing aid function) or an audio signal through
communication such as telephone communication, in addition to the music signal from the music
playback device When used as a headset), etc., it is a generic term for audio signals to be
reproduced by the headphone driver.
[0029]
Focusing on the input speech S in the equation (1) of FIG. 3, the transfer function E of the
equalizer 16 can be expressed as the equation (3) of FIG. Then, considering also the transfer
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function E of the equalizer 16 of the equation (3) of FIG. 3, the output speech P of the noise
canceling system of FIG. 1 (B) is expressed as the equation (4) of FIG. Can.
[0030]
Assuming that the position of the microphone 111 is very close to the ear position, the letter H is
the transfer function from the driver 115 to the microphone 111 (ear), and the letter A and the
letter D are the transfer functions of the power amplifier 114 and the driver 115, respectively. It
can be seen that the same characteristics as the normal headphone without the noise reduction
function can be obtained. At this time, the transfer characteristic E of the equalizer 16 is
substantially the same as the open loop characteristic viewed on the frequency axis.
[0031]
[Regarding a Feedforward Noise Canceling System] Next, a feedforward noise canceling system
will be described. FIG. 2A shows the configuration on the right channel side when the headphone
system to which the noise canceling system of the feedforward method is applied is attached to
the user head (head of the user (listener)) HD. FIG. 2 (B) shows the overall configuration of the
feedforward noise canceling system.
[0032]
In the feed forward method, basically, as shown in FIG. 2A, the microphone 211 is installed
outside the headphone case HP, and appropriate filtering processing is performed on noise
collected by the microphone 211. This is a method intended to reproduce this by the driver 25
inside the headphone case HP and to cancel this noise near the ear.
[0033]
Specifically, referring to the block diagram of FIG. 2 (B), the noise canceling system of the
feedforward method will be described.
The noise canceling system of the feedforward method shown in FIG. 2B includes a microphone
and microphone amplifier unit 21 including a microphone 211 and a microphone amplifier 212,
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and a filter circuit designed for feedforward control (hereinafter referred to as an FF filter circuit)
It is said. 22), a combining unit 23, a power amplifier 24, and a driver 25 including a drive circuit
251 and a speaker 252.
[0034]
Also in the noise canceling system of the feedforward method shown in FIG. 2B, characters A, D
and M described in each block are transmitted from power amplifier 24, driver 25, microphone
and microphone amplifier unit 21. It is a function. Further, in FIG. 2, the letter N indicates an
external noise source (noise source). The main reason that noise according to the noise source N
intrudes into the headphone casing HP is as described in the feedback type noise canceling
system.
[0035]
Further, in FIG. 2B, the transfer function from the position of the external noise source N to the
ear position CP (the transfer function between the noise source and the cancellation point) is
represented by the letter F, and the noise source N to the microphone 211 The transfer function
(the transfer function between the noise source and the microphone) up to the point is denoted
by the letter F ', and the transfer function (the transfer function between the driver and the
cancel point) from the driver 25 to the cancel point (ear position) CP It is represented by the
letter H.
[0036]
Then, assuming that the transfer function of the FF filter circuit 22 that is the core of the
feedforward noise canceling system is -α, the sound pressure P (output voice) that reaches the
listener's ear is as shown in FIG. And can be expressed as shown in equation (1) of FIG.
[0037]
Here, considering an ideal state, the transfer function F between the noise source and the
cancellation point can be expressed as shown in equation (2) of FIG.
Then, if the equation (2) in FIG. 5 is substituted into the equation (1) in FIG. 5, the first term and
the second term will be offset, and as a result, the feedforward method shown in FIG. 2 (B) In the
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noise canceling system of FIG. 5, the output speech P can be expressed as shown in equation (3)
of FIG. 5, and the noise is cancelled, leaving only the music signal (or the speech signal etc. for
the purpose of listening) It can be seen that the same sound as the headphone operation can be
heard.
[0038]
However, in practice, it is difficult to form a complete filter having a transfer function such that
equation (2) shown in FIG. 5 is completely satisfied.
In particular, with regard to the mid-high range, the shape of the ear differs depending on the
person, and the wearing condition of the headphones is also different, and the characteristics
vary due to large individual differences and noise positions and microphone positions. Therefore,
usually, the active noise reduction process is not performed for the middle and high regions, and
passive sound insulation is often performed by the headphone case. Although equation (2) in FIG.
5 is obvious from the equation, it means that the transfer function from the noise source to the
ear position is imitated by an electric circuit including the transfer function α.
[0039]
The cancellation point CP in the feedforward noise canceling system shown in FIG. 2 is different
from the noise canceling system in the feedback method of FIG. 1A as shown in FIG. It can be set
at any ear position. However, in the normal case, the transfer function α is fixed, and in the
design stage, it is decided to target some target characteristics, and the shape of the ear differs
depending on the listener, so a sufficient noise cancellation effect is obtained. Or noise
components are added in a non-reversed phase, and phenomena such as noise may occur.
[0040]
From these facts, in general, the feedforward system has low possibility of oscillation and high
stability, but it is difficult to obtain a sufficient amount of attenuation, while the feedback system
can expect a large amount of attenuation instead. , Care must be taken on the stability of the
system. Each of the feedback method and the feed forward method has features.
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[0041]
In addition, noise reduction headphones using an adaptive signal processing method have been
separately proposed. In the case of noise reduction headphones using this adaptive signal
processing method, microphones are usually installed both inside and outside the headphone
case. The microphone inside analyzes the error signal that tried to cancel with the filter
processing component, and is used when generating / updating a new adaptive filter, but
basically the noise outside the headphone case is digitally filtered Because it is played back by
the driver, it takes the form of a feedforward system as a large framework.
[0042]
[About the Noise Canceling System According to the Present Invention] The present invention
has the advantages of both the feedback method and the feed forward method described above.
Also in the noise canceling system according to the embodiment to which the present invention
described below is applied, the same reference numerals are assigned to parts configured in the
same manner as the noise canceling system described with reference to FIGS. 1 and 2. And their
detailed description will be omitted.
[0043]
In the embodiment described below, the FF filter circuit in the feedforward noise canceling
system (also referred to as an α circuit in some cases after the transfer function −α of the
circuit) may be used. 22), and an FB filter circuit in a feedback type noise canceling system (also
called a β circuit after the transfer function −β of the circuit). And 12) will be described as
having a configuration of a digital filter.
[0044]
FIG. 6 is a block diagram for describing a configuration example in the case where the FF filter
circuit 22 and the FB filter circuit 12 are configured as digital filters. As shown in FIG. 6A, the FF
filter circuit 22 in the feedforward noise canceling system shown in FIG. 2 is provided between
the microphone amplifier 212 and the power amplifier 24. Further, as shown in FIG. 6B, the FB
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filter circuit 12 of the noise canceling system of the feedback type shown in FIG. 1 is provided
between the microphone amplifier 112 and the power amplifier 14.
[0045]
Then, when the FF filter circuit 22 and the FB filter circuit 12 are configured as digital filters, as
shown in FIG. 6C, the analog noise signal picked up by the microphone is converted to a digital
noise signal. The digital noise reduction signal from an ADC (Analog Digital Converter), a DSP
(Digital Signal Processor) / CPU (Central Processing Unit) that forms a noise reduction signal that
reduces noise from digital noise signals, and a DSP / CPU It can comprise by DAC (Digital Analog
Converter) which converts into an analog noise reduction signal. Note that in FIG. 6C, the
description DSP / CPU means that either a DSP or a CPU is used.
[0046]
Thus, by configuring the FF filter circuit 12 and the FB filter circuit 12 as a digital filter, (1) it
becomes possible to configure a system in which a plurality of modes can be selected
automatically or manually by the user. The use performance from the viewpoint of is enhanced.
(2) By performing digital filtering that can be finely controlled, it is possible to obtain highly
accurate control quality with less variation, and as a result, the amount of noise reduction and
the reduction band can be expanded.
[0047]
In addition, (3) the filter shape can be changed by changing software for an arithmetic
processing unit (DSP (Digital Signal Processor) / CPU (Central Processing Unit)) without changing
the number of parts. , It is easy to make changes accompanying system design and device
characteristic change. (4) By sharing the same ADC / DAC and DSP / CPU with external inputs
such as music playback and telephone calls, high precision digital equalization is applied to these
external input signals as well. Sound quality playback can be expected.
[0048]
As described above, by digitizing the FF filter circuit 22 and the FB filter circuit 12, it is possible
to expect at least the effects described in (1) to (4) described above, and it is flexible for various
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cases. This makes it possible to configure a system capable of canceling noise with high quality
regardless of the listener to use.
[0049]
[Problem of Feed-forward Noise Canceling System] As described above, the feed-forward method
is characterized by high stability.
However, one problem is inherent. FIG. 7 is a diagram for explaining the problems of the
feedforward method, and the headphone system to which the noise canceling system of the
feedforward method is applied is attached to the user head (head of the user (listener)) HD Is a
diagram showing the configuration of the right channel side in the case where
[0050]
In FIG. 7A, the transfer function from the noise source N1 to the cancellation point (target point
for noise cancellation) CP set to be provided in the vicinity of the ear canal inside the headphone
case is F1 and, similarly, noise The transfer function from the source N1 to the microphone 211
provided outside the housing of the headphone is F1 '.
[0051]
At this time, using the sound collected by the microphone 211 installed outside the headphone
case, the filter of the FF filter circuit (α circuit) 22 is adjusted, and as shown in (3) of FIG. The
transfer function F1 up to is simulated by (F1 ′ ADHMα), and finally it is subtracted in the
acoustic space inside the headphone, leading to noise reduction.
Here, equation (3) in FIG. 5 is usually applied only to the low band, and the phase is not clear in
the high band, so that it is usual not to take the gain of the FF filter circuit 22 (do not cancel). .
[0052]
Now, here, the filter of the FF filter circuit 22 is fixed, and the characteristic of the transfer
characteristic α is optimized at the time of the noise positional relationship as shown in FIG.
Considering that the position is not changed and the number is also one, it is not preferable when
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a noise source is present on the opposite side of the microphone 211 as shown by the noise
source N2 in FIG. 7B. .
[0053]
That is, in the case of the example shown in FIG. 7 (B), the sound wave of the noise emitted from
the noise source N2 first leaks from the gap between the headphone and the head and becomes
unpleasant noise in the headphone case.
After that, it reaches the outside of the headphone, is collected by the microphone 211, receives
specific filtering (−α) by the FF filter circuit 12, and is reproduced by the driver.
[0054]
As can be seen by comparing FIG. 7 (B) and FIG. 7 (A), the noise in FIG. Since the arrival zone and
the band in which both are in antiphase relation are wide, a constant noise reduction effect can
be obtained. However, in the case of FIG. 7B, noise that leaks into the headphone case and noise
that reaches the microphone 211 are present, and as a result, a signal with an unexpected time
difference is added. In particular, rather than the phase becoming opposite in phase in the middle
and high frequencies, the band added as the positive phase increases.
[0055]
Therefore, in the state shown in FIG. 7B, as a result, although the purpose is noise attenuation,
noise increases with respect to the frequency at which the phases coincide. At this time, even if a
large attenuation can be realized in a wide band, human auditory sense is not practical because it
feels uncomfortable for noise generation even in a narrow band.
[0056]
Naturally, the more you move to the high region where the phase rotation is fast, the easier it is
to create that situation. Therefore, this is a cause of narrowing the effective effect band (band
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where the gain of the α characteristic is present) of the noise cancellation in the FF filter circuit
22 of the feedforward type noise canceling system.
[0057]
[About the noise canceling system to which the embodiment of the present invention is applied]
Therefore, the noise canceling system of this embodiment includes the noise canceling system of
the feedback type and the noise canceling system of the feed forward type. One noise canceling
system is configured on the basis of the superimposed ones.
[0058]
That is, when the noise canceling system according to the embodiment described below is in the
state as shown in FIG. 7A, it is possible to stably perform noise canceling over a wide band by the
noise canceling system of the feedforward method. While being in the state as shown in FIG. 7
(B), the noise canceling system of the feedback type can effectively cancel the noise that leaks
into the headphone case as well. It is to be done.
[0059]
[Regarding Noise Canceling System of First Example] FIG. 8 is a block diagram for explaining a
first example of the noise canceling system of this embodiment.
9 is a block diagram for explaining the FF filter circuit 22 and the FB filter circuit 12 shown in
FIG.
As shown in FIG. 8, this first noise canceling system is composed of a feedback noise canceling
system formed on the right side and a feed forward noise canceling system formed on the left
side. It will be
[0060]
That is, in FIG. 8, the part including the microphone and microphone amplifier unit 21 including
the microphone 211 and the microphone amplifier 212, the FF filter circuit (α circuit) 22, the
power amplifier 24, and the driver 25 is of the feedforward type. It is a noise canceling system
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part. Here, as shown in FIG. 9A, the FF filter circuit 22 has a configuration of a digital filter
including an ADC 221, a DSP / CPU unit 222, and a DAC 223.
[0061]
Also, in this example, the ADC 27 receives input audio, which is an analog signal from, for
example, an external music reproduction device or a microphone of a hearing aid, converts it into
a digital signal, and supplies it to the DSP / CPU unit 222 It is. As a result, in the DSP / CPU unit
222, a noise reduction signal for reducing noise can be added to the input voice supplied from
the outside.
[0062]
In the noise canceling system part of the feedforward method shown in FIG. 8, the transfer
function of the microphone and the microphone amplifier unit 21 is "M1", the transfer function
of the FF filter circuit 22 is "-.alpha." The transfer function of the amplifier 24 is "A1", and the
transfer function of the driver 25 is "D1". In addition, in the noise canceling system portion of the
feedforward method, a transfer function "H1" between the driver and the cancellation point, a
transfer function "F" between the noise source and the cancellation point, and a transfer function
between the noise source and the microphone It is possible to consider "F '".
[0063]
Further, in FIG. 8, a microphone and microphone amplifier unit 11 consisting of a microphone
111 and a microphone amplifier 112, an FB filter circuit (β circuit) 12, a power amplifier 14,
and a driver 15 consisting of a drive circuit 151 and a speaker 152. The part which consists of is
a noise canceling system of a feedback system. Here, as shown in FIG. 9B, the FB filter circuit 12
has a configuration of a digital filter including an ADC 121, a DSP / CPU unit 122, and a DAC
123.
[0064]
In the noise canceling system portion of the feedback method shown in FIG. 8, the transfer
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function of the microphone and the microphone amplifier unit 11 is M2 , the transfer
function of the FB filter circuit 12 is −β , and the power amplifier The transfer function of
14 is A2 , and the transfer function of the driver 15 is D2 . In addition, in the noise
canceling system portion of the feedback method, the transfer function "H2" between the noise
source and the cancellation point can be considered.
[0065]
In the case of the noise canceling system having the configuration shown in FIG. 8, the noise
noise from the outside is first taken in and canceled by the noise canceling system portion of the
feedforward system. However, while the noise reduction band can be obtained inside the
headphone case practically as described above depending on the noise sound source and the
property of the sound wave (for example, spherical wave and plane wave behavior). The noise
can not be canceled effectively, and as a result, a band in which the noise remains may occur. The
same problem occurs in the wearing condition of the headphones and the shape of the
individual's ear.
[0066]
However, in the case of the noise canceling system having the configuration shown in FIG. 8,
noise components remaining in the feedforward noise canceling system and noise components
embedded in the headphone case Is effectively canceled by the operation of the feedback type
noise canceling system part. That is, by simultaneously functioning the noise canceling system
portion of the feed forward method and the noise canceling system portion of the feedback
method, it is possible to obtain the noise canceling effect (noise reducing effect) more than when
used alone. It is like that.
[0067]
Thus, in the case of the noise canceling system shown in FIG. 8, the noise leaking into the inside
of the headphone case is canceled by the noise canceling system portion of the feedback method
shown on the right in FIG. The noise can be appropriately canceled in the CP, and the noise from
the noise source N outside the headphone case can be properly corrected at the cancellation
point CP by the noise cancellation system portion of the feedforward method shown on the left
side in FIG. It is possible to cancel the noise.
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[0068]
Note that the noise canceling system shown in FIG. 8 includes a microphone and an amplifier, a
power amplifier, and a driver separately for the noise canceling system part of the feed-forward
method and the noise canceling system part of the feedback method. It is
[0069]
FIG. 10 is a diagram for explaining a general difference between attenuation characteristics of
the feedback noise canceling system and the feedforward noise canceling system.
In FIG. 10, the horizontal axis is frequency, and the vertical axis is attenuation.
And, as described above and also shown in FIG. 10, the attenuation characteristic of the feedback
type noise canceling system is narrow band and high level, whereas the attenuation
characteristic of the feedback type noise canceling system is Is broadband, low level.
[0070]
However, in the case of the noise canceling system shown in FIG. 8, it is a so-called twin type
noise canceling system including a feedforward noise canceling system part and a feedback noise
canceling system part, In the case of this twin system, the characteristics of the noise canceling
system of the feedback system shown in FIG. 10 and the characteristics of the noise canceling
system of the feedforward system have a combination of attenuation characteristics.
[0071]
FIG. 11 shows measured values of attenuation characteristics when using a twin noise canceling
system having the configuration shown in FIG. 8 and measured values of attenuation
characteristics when using a feedback noise canceling system; It is a figure which shows the
measured value of the attenuation characteristic at the time of using the noise canceling system
of a feedforward system.
[0072]
In FIG. 11, the horizontal axis is frequency, and the vertical axis is attenuation.
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Then, in FIG. 11, a graph indicated by a coarse dotted line and marked with the word Feed
Back indicates the attenuation characteristics of the feedback type noise canceling system,
indicated by a fine dotted line, Feed The graph to which the word "Forward" is attached shows
the attenuation characteristics of the feedforward noise canceling system, and the graph
indicated by a solid line and indicated by the word "Twin" is a graph of FIG. FIG. 8 is a graph
showing the attenuation characteristics of the twin noise canceling system having the
configuration shown in FIG.
[0073]
As can be seen from FIG. 11, in the case of the feedback type noise canceling system, the narrow
band, high level attenuation characteristics are provided, and in the case of the feed forward type
noise canceling system, the wide band, low level It can be seen that it has the attenuation
characteristics of
And in the case of the twin system, it is understood that high level attenuation characteristics are
realized over a wide band.
[0074]
As described above, in the case of the so-called twin noise canceling system having the
configuration shown in FIG. 8, it is possible to realize a wide band and high level attenuation
characteristics by combining the attenuation characteristics of the feedback method and the
feedforward method. It is a thing.
[0075]
[Regarding Noise Canceling System of Second Example] FIG. 12 is a block diagram for describing
a second example of the noise canceling system of this embodiment.
In the case of the second example of the noise canceling system shown in FIG. 12, an FF filter
circuit consisting of a microphone amplifier unit 21 including a microphone 211 and a
microphone amplifier 212, an ADC 321, a DSP / CPU unit 322, and a DAC 323. A noise canceling
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20
system portion of a feedforward system is formed by the driver 22 including the power amplifier
33 and the drive circuit 341 and the speaker 342.
[0076]
Furthermore, in the case of the second example of the noise canceling system shown in FIG. 12,
the microphone including the microphone 111 and the microphone amplifier 112 and the
microphone amplifier unit 11 and the FB including the ADC 324, the DSP / CPU unit 322, and
the DAC 323 The filter circuit 12, the power amplifier 33, and the driver 34 including the drive
circuit 341 and the speaker 342 form a noise canceling system portion of the feedback system.
[0077]
That is, in the case of the first example of the noise canceling system shown in FIG. 8, a
configuration in which the noise canceling system part of the feedback type and the noise
canceling system part of the feed forward type are formed separately and connected separately
In the case of the second example of the noise canceling system shown in FIG. 12, the DSP / CPU
322, the DAC 323, the power amplifier 33, and the driver 34 have a feedback method and a feed
forward method. In common with each other.
[0078]
Further, in the case of the second example of the noise canceling system shown in FIG. 12, the
transfer function of the microphone and the microphone amplifier unit 21 is M1 , the
transfer function of the FF filter circuit 22 is −α , The transfer function of the power
amplifier 33 is "A", and the transfer function of the driver 34 is "D".
The transfer function of the microphone and the microphone amplifier unit 11 is
the transfer function of the FB filter circuit 22 is −β .
M2
, and
[0079]
Also in the second example of the noise canceling system shown in FIG. 12, the transfer function
"H" between the driver and the cancellation point, the transfer function "F" between the noise
source and the cancellation point, and the noise source- It is possible to consider the transfer
function "F '" between microphones.
08-05-2019
21
[0080]
Also in the case of the second example shown in FIG. 12, the input voice is supplied to the DSP /
CPU unit 322 via the ADC 35 so that it can be added to the noise reduction signal.
[0081]
Therefore, in the noise canceling system of the second example shown in FIG. 12, the DSP / CPU
322 forms a noise reduction signal based on the sound collected by the microphone 211 outside
the headphone case, and the inside of the headphone case It is possible to form a noise reduction
signal based on the voice collected by the microphone 111 and to combine these signals.
Further, in the case of the example shown in FIG. 12, the DSP / CPU 322 also realizes a function
of receiving and adjusting the input voice received through the ADC 35 and synthesizing it into
the noise reduction signal.
That is, the DSP / CPU 322 can also realize a function as an input circuit (equalizer) for input
voice.
[0082]
Thus, in the case of the noise canceling system of the second example shown in FIG. 12, a portion
common to the noise canceling system portion of the feedback type and the noise canceling
system portion of the feed forward type is provided. Thus, the number of parts can be reduced
and the configuration can be simplified.
[0083]
However, as described above, the noise canceling system portion of the feedforward method,
which includes the microphone and microphone amplifier unit 21, the FF filter circuit 22, the
power amplifier 33 and the driver 34, and the microphone and microphone amplifier unit 11. , A
twin type noise canceling system which realizes a wide band and high level attenuation
characteristics by simultaneously functioning the feedback type noise canceling system portion
including the FB filter circuit 12, the power amplifier 33 and the driver 34 Can be configured.
[0084]
08-05-2019
22
[About the noise canceling system of the third example] By the way, in the twin type noise
canceling system shown in FIG. 8 and FIG. 12, as the input voice S shows, the music signal from
the music reproduction device or the microphone of the hearing aid When listening to an
external source such as an audio signal collected at step S5, the amount of reduction of noise
may not be so large because audio and music are heard.
On the other hand, although it is not necessary to listen to an external source, there are cases
where it is desired to create a high-quality silent state by reducing noise.
For example, when it is necessary to work in a loud noise, there is a high demand to reduce the
noise to high quality.
[0085]
Therefore, this third example is a twin noise canceling system having a noise canceling system
part of the feedback method and a noise canceling system part of the feed forward method, but
when listening to an external source It is not necessary to listen to an external source, to operate
only one of the noise canceling system part of the feedback system and the noise canceling
system part of the feed forward system, and to provide high-quality silence (as silent as possible
In the case where the near state is formed, both the noise canceling system part of the feedback
type and the noise canceling system part of the feed forward type can be made to function.
[0086]
FIGS. 13 and 14 are block diagrams for explaining a third example of the noise canceling system
of this embodiment.
The configuration of the noise canceling system of the third example shown in FIGS. 13 and 14 is
basically the same as that of the noise canceling system of the second example shown in FIG.
For this reason, in FIGS. 13 and 14, parts that are the same as in the noise canceling system of
the second example shown in FIG. 12 are given the same reference numerals, and detailed
descriptions thereof will be omitted. I assume.
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23
[0087]
The noise canceling system of the third example shown in FIG. 13 is provided with the switch
circuit 36 between the microphone and the microphone amplifier unit 11 and the ADC 324 in
the noise canceling system of the second example shown in FIG. It is possible to switch whether
to supply the audio signal from the microphone and the microphone amplifier unit 11 to the ADC
324 or to supply the input audio S as an external source supplied from the outside to the ADC
324.
[0088]
Therefore, in the case of the noise canceling system of the third example shown in FIG. 13, when
the switch circuit 36 is switched to the input end a side, the input voice S is not supplied, and the
FB filter circuit 12 and As the FF filter circuit 22 functions, the noise canceling system part of the
feedback method and the noise canceling system part of the feed forward function together so
that a high-quality silent state can be formed. Be done.
[0089]
Also, when the switch circuit 36 is switched to the input end b side, the audio from the
microphone and the microphone amplifier unit 11 is not supplied, and the ADC 324 and the DSP
/ CPU unit 322 input the input audio S (equalizer Be made to function as
Then, in this case, the FF filter circuit 22 functions so that only the noise canceling system
portion of the feedforward system functions to listen to the input sound S while canceling the
noise. .
[0090]
Therefore, in this case, the ADC 321, the DSP / CPU 322, and the DAC 323 realize the function of
the FF filter circuit 22, and the ADC 324, the DSP / CPU 322, and the DAC 323 realize the
function of the equalizer for the input sound S.
That is, the DSP / CPU 322 and the DAC 323 have both the function of the FF filter circuit and
the function as an equalizer for processing the input sound S.
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24
[0091]
Further, in the noise canceling system of the third example shown in FIG. 14, the switch circuit
37 is provided between the microphone and the microphone amplifier unit 21 and the ADC 321
in the noise canceling system of the second example shown in FIG. 12. It is possible to switch
whether to supply the audio signal from the microphone and the microphone amplifier unit 21 to
the ADC 321 or to supply the input audio S as an external source supplied from the outside to
the ADC 321.
[0092]
Therefore, in the case of the noise canceling system of the third example shown in FIG. 13, when
the switch circuit 37 is switched to the input end a side, the input voice S is not supplied, and the
FF filter circuit 22 and As a result of the function of the FB filter circuit 12 and the noise
canceling system part of the feed forward method and the noise canceling system part of the
feedback function functioning together, a high-quality silent state can be formed. Be done.
[0093]
Also, when the switch circuit 37 is switched to the input end b side, the audio from the
microphone and the microphone amplifier unit 21 is not supplied, and the ADC 321 and the DSP
/ CPU unit 322 input the input audio S (equalizer Will function as
Then, in this case, the function of the FB filter circuit 12 allows only the noise canceling system
portion of the feedback method to function, thereby making it possible to listen to the input
sound S while canceling the noise.
[0094]
Therefore, in this case, the ADC 324, the DSP / CPU 322, and the DAC 323 realize the function of
the FB filter circuit 12, and the ADC 321, the DSP / CPU 322, and the DAC 323 realize the
function of the equalizer for the input sound S.
That is, the DSP / CPU 322 and the DAC 323 have both the function of the FB filter circuit and
08-05-2019
25
the function as an equalizer for processing the input sound S.
[0095]
As described above, in the case of the noise canceling system of the third example described with
reference to FIGS. 13 and 14, the noise canceling system of the feed-forward method when
listening to the input speech S which is an external source. Only one of the part and the feedback
type noise canceling system part is made to function so that the input speech can be listened well
while canceling the noise (while reducing the noise).
[0096]
Furthermore, in situations where the listener wants to hear silence, noise from the outside world
and phase mismatch can be achieved using both the feedforward noise canceling system part and
the feedback noise canceling system part. Therefore, both of the self-generated noises are
cancelled, so that a high-quality silent state can be formed, and a large noise reduction effect can
be experienced.
[0097]
The noise canceling system of the third example shown in FIG. 13 is configured such that only
the noise canceling system of the feedforward system is made to function when reproducing the
input speech S. The noise canceling system of the third example shown in FIG. 8 is configured to
operate only the feedback type noise canceling system when reproducing the input speech S.
However, the present invention is not limited to this, and it is also possible for the listener to be
able to switch whether the noise cancellation system part of the feed forward system is
functional or the noise cancellation system part of the feedback system is functional. .
[0098]
That is, the noise canceling system of the third example shown in FIG. 13 and FIG. 14 is
incorporated, and both of the switch circuit 36 and the switch circuit 37 are provided.
Further, a switch circuit 38 is provided to switch whether to supply the input sound S to the
08-05-2019
26
switch circuit 36 or to the switch circuit 37.
[0099]
Then, when the switch circuit 38 provided anew is switched to supply the input sound S to the
switch circuit 36, the switch circuit 36 is switched to the input end b side and the switch circuit
37 is set to the input end a side. By switching, it is possible to listen to the input speech S after
functioning only the noise canceling system portion of the feedforward system.
[0100]
Conversely, when the switch circuit 38 provided anew is switched to supply the input sound S to
the switch circuit 37, the switch circuit 37 switches to the input end b side and the switch circuit
36 switches to the input end a side. In this case, it is possible to listen to the input speech S after
functioning only the noise canceling system portion of the feedback system.
[0101]
Of course, also in this case, when it is desired to form a high-quality silent state, the switch circuit
36 and the switch circuit 3 are both switched to the input end a side, whereby the noise
canceling system part of the feedforward method and the feedback method Both the noise
canceling system part of the system can be made to function to create high quality silence.
[0102]
Each of the switch circuits 36, 37, 38 described above can be configured as a mechanical switch
or can be configured as an electrical switch.
[0103]
Also, although the noise canceling system shown in FIG. 8, FIG. 12, FIG. 13, and FIG. 14 has been
described as being able to receive the supply of the input speech S which is an external source in
any case, It is not limited.
It is of course possible to realize as a noise canceling system for mere noise reduction without
having an input end for receiving an input voice S from the outside.
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27
[0104]
[Regarding Specific Examples of Digitization of the FB Filter Circuit 12 and the FF Filter Circuit
22] In the case of digitizing the FB filter circuit 12 and the FF filter circuit 22 in two, the ADC, the
DSP / CPU unit, and the DAC are used. And the configuration thereof are as described above with
reference to FIGS. 6C and 9.
In this case, it is possible to generate a noise reduction signal at an appropriate timing and realize
noise reduction by using, for example, an ADC and a DAC that are capable of high-speed
conversion of a sequential conversion type.
[0105]
However, successive conversion type high-speed conversion ADCs and DACs are expensive, and
the cost of the FB filter circuit 12 and the FF filter circuit 22 is increased.
Therefore, even when using so-called sigma-delta (Σ · Δ) ADCs and DACs that are conventionally
used, noise reduction signals can be generated at appropriate timings without causing large
delays. The technology to make
In the following, for the sake of simplicity, the case where the technique is provided to the FB
filter circuit 12 will be described as an example, but the same can be applied to the FF filter
circuit 22 as well.
[0106]
FIG. 15 is a block diagram for describing the configuration of the FB filter circuit 12, in
particular, the configurations of the ADC 121 and the DAC 123. As shown also in FIG. 6C and
also shown in FIG. 15A, the FB filter circuit 12 is composed of an ADC 121, a DSP / CPU unit
122, and a DAC 123. As shown in FIG. 15B, the ADC 121 includes a non-aliasing filter 1211, a
sigma-delta (Σ · Δ) ADC unit 1212, and a decimation filter 1213. The DAC 123 includes an
interpolation filter 1231, A sigma-delta (Σ · Δ) DAC unit 1232 and a low pass filter 1233 are
provided.
08-05-2019
28
[0107]
In general, both the ADC 121 and the DAC 123 often use an oversampling method and sigma
delta modulation using a 1-bit (bit) signal. For example, as shown in FIG. 15 (B), when digital
signal processing is performed on the analog input by the DSP / CPU unit 122, it is converted to
1 Fs / Multi bit (mostly 16 bits to 24 bits). In the method, usually, the sampling frequency Fs [Hz]
is taken up to M times MFs [Hz] and oversampling processing is often performed.
[0108]
As shown in FIG. 15B, the anti-aliasing filter 1211 installed at the inlet of the ADC 121 and the
low-pass filter 1233 installed at the outlet of the DAC 123, Signals in a band exceeding one half
(one half) of each sampling frequency Fs are not input / output. However, in fact, it is difficult to
obtain steep attenuation characteristics around Fs / 2 (Fs of 2 minutes) because they are both
analog.
[0109]
That is, in FIG. 15B, a decimation filter 1213 is included on the ADC side, an interpolation filter
1231 is included on the DAC side, and these filters are used to perform decimation processing or
interpolation processing (interpolation). At the same time, a high-order, steep digital filter is
applied and band limiting (LPF) is applied at the same time, and a non-aliasing filter 1211 for
receiving analog signals and a low output of analog signals are simultaneously performed. The
burden on the low pass filter 1233 is reduced.
[0110]
Now, most of the delays occurring in the ADC 121 and the DAC 123 occur in the high-order
digital filter in the decimating filter 1213 and the interpolating filter 1231.
That is, in order to obtain steep characteristics in the vicinity of Fs / 2, a filter with a high order
in a region having a sampling frequency of MFs [Hz] (in the case of a FIR (Finite Impulse
Response) filter, a filter with a long tap number) As a result, group delay will occur.
08-05-2019
29
[0111]
In this digital filter unit, an FIR filter having a linear phase characteristic is used to avoid the
adverse effect of deterioration of the time waveform due to phase distortion, and a moving
average that can realize the interpolation characteristic by the SINC function (sin (x) / x) among
others Those based on filters tend to be preferred. In the case of a linear phase filter, half of the
filter length is approximately the delay amount.
[0112]
As a matter of course, the FIR filter is steeper as the order (the number of taps) is larger, and can
express characteristics with a large attenuation effect. Short-order filters are generally not used
because they have poor attenuation (more leakage) and higher aliasing effects. However, in the
case of using this feedback type noise canceling system, the use of the FIR filter under the
conditions as described later becomes possible, and as a result, the delay time can be shortened.
[0113]
The shorter the delay time, the smaller the phase rotation. As a result, when designing the FB
filter circuit 12 and creating the overall open loop characteristic as described with reference to
FIG. The bandwidth can be expanded, and in the noise canceling mechanism, the bandwidth and
its attenuation characteristics have a great effect. In addition, it can be easily assumed that the
degree of freedom in creating the filter will also increase.
[0114]
Therefore, in FIG. 15B, with regard to the FIR filter which forms the decimation filter 1213 and
the interpolation filter 1231 which are digital filters, (1) the sampling frequency is Fs and the
frequency is approximately (Fs-4 kHz) to (Fs + 4 kHz) It is sufficient to use one in which
attenuation of −60 dB or more with respect to the band is secured.
[0115]
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30
In this case, (2) using a sampling frequency Fs twice or more (approximately 40 kHz) or more of
the audible band, and (3) using a sigma-delta (Σ · Δ) system as a conversion system.
(4) The group delay of the digital filter, which occurs in the processing mechanism inside the
conversion processing device, is suppressed to 1 ms or less by recognizing the aliasing leakage
component for other bands than the band shown in the condition (1). It is sufficient to use
something.
[0116]
An FIR filter satisfying the above conditions (1) and (4) is used as the decimation filter 1213 and
the interpolation filter 1231, and the sampling frequency Fs satisfies the condition (2), and the
conversion method is (3). By satisfying the conditions, it is possible to configure the FB filter
circuit 12 digitized using a conventional ・ · Δ ADC or DAC without increasing the cost.
[0117]
As to the detailed basis of the ability to form a digital filter that does not generate a large delay
by satisfying the above-described conditions (1) to (4), Japanese Patent Application No. 2006,
which is another application by the inventor of this application. -301211 is described in detail.
[0118]
[まとめ] (1).
As with the noise canceling system described with reference to FIG. 8, each of the inner and outer
sides of the headphone case has one or more microphone mechanisms, and the signals collected
by the microphones installed outside are specified The noise generated inside the headphone is
reduced by reproducing it with the driver inside the headphone case through the filter, and at the
same time the signal collected by the inner microphone is reproduced by the inner driver
through a specific filter. It is possible to configure a system that performs wide-band noise
reduction with a large attenuation effect.
[0119]
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31
(2).
As in the noise canceling system described with reference to FIG. 12, with regard to the above
(1), the filtered signal of the inner microphone and the filtered signal of the outer microphone
are mixed by analog or digital means, respectively. The driver can be only one.
[0120]
(3). As described with reference to FIGS. 6C, 9, and 15, in order to perform digital filtering
on the part of the filter realized as the FB filter circuit or the FF filter circuit with the arithmetic
device including the DSP or CPU, By having at least one or more ADCs and one or more DACs in
the system, a digital filter can be configured.
[0121]
(4). Like the noise canceling system described with reference to FIGS. 13 and 14, a noise
reduction system is configured in which both the microphones inside the headphone case and
the output signals from the outside microphone enter the ADC and are digitally processed. In the
first mode, one input of either the outer or inner microphone signal is switched to an external
signal (music signal or call signal) and connected to the same ADC, and at the same time the
noise reduction program for the DSP / CPU section And a second mode instructing to become an
equalizer program.
[0122]
In this case, when the first mode is used, high-quality silence can be formed, and when the
second mode is used, the noise canceling system portion of the feedback system and the feed
forward system Only one of the noise canceling system parts can function to reproduce and
listen to the input sound that is an external source while reducing noise. Further, the number of
ADCs can be reduced by providing the first mode and the second mode.
[0123]
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32
[About the Method According to the Present Invention] Also, as described with reference to FIG.
8, the portion for realizing the noise canceling system of the feedback type and the portion for
realizing the noise canceling system of the feed forward type are operated simultaneously, The
first method according to the present invention can be realized by simultaneously performing
noise cancellation in both the feedback method and the feed forward method.
[0124]
Further, as described with reference to FIG. 12, the DSP / CPU 322 and the DAC 323 are
commonly used in the FB filter circuit 12 and the FF filter circuit 22, and respective noise
reduction signals are formed in the DSP / CPU 322. By combining the formed noise reduction
signals, it is possible to realize the second method according to the present invention, in which
one power amplifier 33 and one driver 34 are used to effectively reduce noise.
[0125]
Also, by enabling the FB filter circuit 12 and the FF filter circuit 22 to be processed by an ADC, a
DSP / CPU, and a DAC, such as analog / digital conversion → noise reduction signal generation
processing → digital / analog conversion. The third method according to the present invention
can be realized.
[0126]
Further, as shown in FIG. 12, by sharing the DSP / CPU 322 and the DAC 323 by the FB filter
circuit 12 and the FF filter circuit 22, noise reduction of the feedback method in the DSP / CPU
322 is achieved. The fourth method according to the present invention can be realized by
forming a signal and also forming a feedforward noise reduction signal so that it can be
synthesized.
[0127]
Further, as shown in FIG. 13 and FIG. 14, the fifth method according to the present invention can
be implemented by switching which one of the sound collected by the microphone and the input
sound S is to be processed. It can be realized.
[0128]
[Others] In the embodiment described above, mainly, the microphone 111 implements the
function as the first sound collection means, and the FB filter circuit 12 realizes the function as
the first signal processing means, The power amplifier 14 realizes the function as the first
amplification means, and the driver 15 including the speaker 152 realizes the function as the
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33
first sound emission means to constitute a noise canceling system portion of the feedback
system. There is.
[0129]
In addition, the microphone 211 mainly realizes a function as a second sound collecting unit, the
FF filter circuit 22 realizes a function as a second signal processing unit, and the power amplifier
24 functions as a second amplifying unit. By realizing the function and the driver 25 including
the speaker 252 realizing the function as the second sound emitting means, the noise canceling
system portion of the feedforward system is configured.
[0130]
Further, the FB filter circuit 12 and the FF filter circuit 22 realize a function as a combining
means.
In a pseudo manner, as shown in FIG. 12, the function of the DSP / CPU which is the shared part
of the FB filter circuit 12 and the FF filter circuit 22 to form noise reduction signals of the
feedback system and the feedforward system. And also realize the function of combining the
formed noise reduction signals.
[0131]
Then, in FIG. 12, the power amplifier 33 realizes a function as one amplifying means for
amplifying one signal synthesized by the synthesizing means, and the driver 34 corresponds to a
voice corresponding to the signal amplified by the one amplifying means. To realize a function as
one sound emitting means for emitting sound.
Further, each of the switch circuit 36 in FIG. 13 and the switch circuit 37 in FIG. 14 realizes a
function as a switching unit that switches the output signal.
[0132]
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34
Further, in the above-described embodiment, although the case where the FB filter circuit 12 and
the FF filter circuit 22 are both configured as digital filters has been described as an example, the
present invention is not limited to this.
Even when the FB filter circuit 12 and the FF filter circuit 22 are configured as analog filters,
similar effects can be obtained.
[0133]
Further, in the above-described embodiment, although it has been described that the input
speech S can be received as the external source, it does not necessarily have to have the function
of receiving the external source.
That is, it is not necessary to listen to an external source such as music, and it can be configured
as a noise reduction system that can reduce only noise.
[0134]
Moreover, in the embodiment described above, although the case where the present invention is
applied to a headphone system is described as an example for simplification of the description,
not all the systems need to be implemented in the headphone main body.
For example, the processing mechanism such as the FB filter circuit, the FF filter circuit, and the
power amplifier may be divided as a box outside, or may be configured in combination with other
devices.
Here, the other device may be, for example, a portable audio player, a telephone device, a
network voice communication device, or various other hardware capable of reproducing audio
and music signals.
[0135]
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35
In particular, by applying the present invention to a mobile phone terminal and a headset
connected thereto, for example, noise can be reduced and good telephone conversation can be
performed even in an environment with a lot of noise on the road. It will be possible to
In this case, by providing the FF filter circuit, the FB filter circuit, the drive circuit and the like on
the mobile phone terminal side, the configuration on the headset side can be simplified. Of
course, all the configurations can be provided on the headset side so as to be able to receive the
audio supply from the mobile phone terminal.
[0136]
It is a figure for demonstrating the noise canceling system of a feedback system. It is a figure for
demonstrating the noise canceling system of a feedforward system. It is a figure for
demonstrating the formula which shows the characteristic of the noise canceling system of the
feedback system shown in FIG. FIG. 7 is a Bode diagram for explaining phase margin and gain
margin in a noise canceling system of a feedback type. It is a figure for demonstrating the
formula which shows the characteristic of the noise canceling system of the feedforward system
shown in FIG. It is a block diagram for demonstrating the structural example at the time of
setting the FF filter circuit 22 and the FB filter circuit 12 as a structure of a digital filter. It is a
figure for demonstrating the problem of a feedforward system. It is a block diagram for
demonstrating the 1st example of a noise canceling system. It is a block diagram for
demonstrating the FF filter circuit 22 and the FB filter circuit 12 which were shown in FIG. It is a
figure for demonstrating the general difference of the attenuation characteristics of the noise
canceling system of each with a feedback system and a feedforward system. It is a figure for
demonstrating the attenuation characteristic of the noise canceling system of the twin system
which has the structure shown in FIG. It is a block diagram for demonstrating the 2nd example of
a noise canceling system. It is a block diagram for demonstrating the 3rd example of a noise
canceling system. It is a block diagram for demonstrating the 3rd example of a noise canceling
system. FIG. 6 is a block diagram for describing a configuration of the FB filter circuit 12, in
particular, a configuration of an ADC 121 and a DAC 123.
Explanation of sign
[0137]
DESCRIPTION OF SYMBOLS 11 ... Microphone and microphone amplifier part, 111 ...
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Microphone, 112 ... Microphone amplifier, 12 ... FB filter circuit, 121 ... ADC, 122 ... DSP / CPU,
123 ... DAC, 13 ... Synthesis part, 14 ... Power amplifier, 15 ... Driver , 151: drive circuit, 152:
speaker, 16: equalizer, CP: cancel point, S: input voice, P: output voice, 21: microphone and
microphone amplifier unit, 211: microphone, 212: microphone amplifier, 22: FF filter Circuits
221 ADCs 222 DSP / CPU 223 DACs 23 synthesizers 24 power amplifiers 25 drivers 251 drive
circuits 252 speakers
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