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JP2009260452

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DESCRIPTION JP2009260452
An object of the present invention is to prevent an increase in noise due to a phase change of a
filter to which a signal collected by a microphone is supplied. An output signal of a microphone is
phase-inverted and supplied to a band pass filter. The components of the band near both ends of
the pass band of the filter 3 are extracted by the band pass filters 7 and 8. The output signals of
the filters 3, 7 and 8 are added by the mixing circuit 6. An output signal of the mixing circuit 6 is
supplied to the speaker 2 via the amplifier 2. Since the sound wave of the phase opposite to the
noise is emitted from the speaker 2, the sound wave of the speaker 2 and the noise at the
position of the microphone 1 cancel each other. The filters 7 and 8 can prevent an increase in
noise due to the phase change of the filter 3. [Selected figure] Figure 1
Noise canceling device
[0001]
The present invention relates to a noise canceling apparatus applied to, for example, reducing
noise in a car.
[0002]
With the spread of portable audio playback devices, the opportunity to listen to audio signals
outdoors has increased.
For example, the audio signal is reproduced in a vehicle such as a bus or a train. In vehicles, there
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are many noises and often the level of the reproduced audio signal is increased, and as a result,
the sound leakage is increased, which may cause inconvenience to the surrounding people. This
problem can be avoided when using noise canceling headphones.
[0003]
A feedback method is known as noise canceling headphones. In this method, a microphone is
installed inside the headphone, and the signal of the reverse phase obtained by mainly inverting
the phase of the low frequency component among the signals collected by the microphone is
supplied to the driver unit of the headphone, noise component (environmental noise) To cancel.
[0004]
An on-vehicle noise canceling device feeds back an in-vehicle noise signal detected by a
microphone to an amplifier for driving a speaker, generates a sound wave of opposite phase to
the noise by the speaker, and reduces noise in the vicinity of the microphone . For example,
Patent Document 1 below describes a noise canceling device that reduces noise in a car.
[0005]
JP 10-333687 A
[0006]
FIG. 7 shows a conventional noise canceling apparatus, in which noise is collected by the
microphone 1 and an output signal of the microphone 1 is supplied to the microphone amplifier
2.
The microphone amplifier 2 amplifies the output signal of the microphone 1 and reverses the
phase. An output signal of the microphone amplifier 2 is supplied to a band pass (band pass)
filter 3. The band pass filter 3 mainly separates signals in a low band, for example, a band of 80
Hz to 800 Hz. An output signal of the band pass filter 3 is supplied to the speaker 5 via the
amplifier 4. The speaker 5 emits a sound having the same phase and amplitude as the noise, and
the noise is canceled near the microphone 1.
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[0007]
FIG. 8 shows the effect of the noise canceling device when the positions of the microphone, the
speaker and the listener's ear are substantially the same. In FIG. 8, reference numeral 101 for the
dotted line indicates the frequency characteristic of noise observed when noise canceling is not
performed. Noise is mainly distributed in the low region. When noise canceling is performed,
noise components distributed in the low range are reduced as indicated by the reference numeral
102 for the solid line. The vertical axis in FIG. 8 indicates the sound pressure level (dBspl (sound
pressure level)).
[0008]
However, as shown by reference numerals 103 and 104 in FIG. 8, the noise level of the bandpass
filter 3 near the lower cutoff frequency and the higher cutoff frequency is lower than that before
noise canceling is performed. Increase. Such increased levels of noise can be annoying to the
listener. In particular, on the high frequency side, an increase in wind noise and an audio echo
are produced to give an unpleasant feeling. The problem that the level of noise increases is due
to the phase characteristic of the band pass filter 3. By the band pass filter 3, a phase change
occurs, the relationship that the output of the speaker with respect to noise does not have an
antiphase relationship does not hold, and the problem of noise increase as a result of noise
canceling arises.
[0009]
Therefore, an object of the present invention is to provide a noise canceling apparatus capable of
solving the above-mentioned conventional problems.
[0010]
In order to solve the problems described above, according to the present invention, there is
provided a microphone that collects ambient sound as noise, a noise cancellation signal
generation unit that receives a signal from the microphone and generates a noise cancellation
signal, and the noise cancellation. A speaker driven by a signal, the noise cancellation signal
generation unit comprising a first and second filter, and a combining unit for combining the
output signals of the first and second filters; The filter separates the signal of the frequency band
which cancels noise out of the signal from the microphone, and the second filter takes out the
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component of the frequency at which the noise increases due to the phase change occurring in
the first filter. It is a noise canceling device.
[0011]
Preferably, the high frequency side of the noise cancellation signal is reduced or eliminated.
On the high frequency side, the effect of the phase change of the first filter is prevented by not
performing the noise cancellation control.
[0012]
Preferably, the first filter is a first band pass filter, and the second filter is a second band pass
filter for passing the low-pass side cutoff frequency component of the first band pass filter; It is
comprised by the 3rd band pass filter which passes the component of the cutoff frequency of the
high band side of a 1st band pass filter.
[0013]
When the first filter is a low pass filter, the second filter is a band pass filter that passes the
component of the cutoff frequency of the low pass filter.
[0014]
When the first filter is a band pass filter, the second filter is a band pass filter that passes the
component of the cutoff frequency on the high band side of the band pass filter.
[0015]
Preferably, when the first filter is a band pass filter, the second filter is for attenuating high
frequency components of the noise cancellation signal.
[0016]
When the first filter is a low pass filter, the second filter is to attenuate high frequency
components of the noise cancellation signal.
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[0017]
According to the present invention, the frequency component whose noise increases due to the
phase change of the first filter for generating the noise cancellation signal is separated by the
second filter and added to the noise cancellation signal, thereby preventing the noise increase. it
can.
[0018]
Hereinafter, embodiments of the present invention will be described with reference to the
drawings.
The embodiments described below are preferable specific examples of the present invention, and
various technically preferable limitations are given. However, the scope of the present invention
is particularly suitable for the present invention in the following description. As long as there is
no statement to the effect of limitation, it shall not be limited to an embodiment.
[0019]
A first embodiment of the present invention will be described.
The present invention has a microphone 1 for collecting ambient sound as noise.
Furthermore, a microphone amplifier 2 as an inverting circuit, a band pass filter 3 as a first filter,
band pass filters 7 and 8 as a second filter, and a combining unit for combining the output
signals of these filters A noise cancellation signal generation unit is configured by the mixing
circuit 6.
The noise cancellation signal generation unit generates a noise cancellation signal, and the
speaker 5 is driven by the noise cancellation signal.
Among the signals collected by the microphone 1, the band pass filter 3 separates the signal of
the frequency band that cancels the noise, and the band pass filters 7 and 8 increase the noise
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due to the phase change occurring in the band pass filter 3. Component of the frequency to be
[0020]
As shown in FIG. 1, the noise collected by the microphone 1 is supplied to the band pass filter 3
as a first filter through the microphone amplifier 2.
The microphone amplifier 2 amplifies the output signal of the microphone 1 and reverses the
phase.
At the output of the microphone amplifier 2, a signal in reverse phase to the noise is obtained. An
output signal of the band pass filter 3 is supplied to the mixing circuit 6. The band pass filter 3 is
for separating the component of the frequency of noise to be canceled, and mainly takes out a
low frequency component, for example, a component of 80 Hz to 800 Hz.
[0021]
An output signal of the microphone amplifier 2 is supplied to the band pass filter 7 and the band
pass filter 8. The band pass filters 7 and 8 separate the components of the frequency band where
noise increases as a result of noise canceling. For example, the band pass filter 7 extracts a
component of 60 Hz to 80 Hz near the cutoff frequency of the low pass band of the band pass
filter 3, and the bandpass filter 8 receives 800 Hz near the cutoff frequency of high band of the
band pass filter 3. Take out the 1 kHz component. The output signals of the band pass filters 7
and 8 are supplied to the mixing circuit 6.
[0022]
The noise cancellation signal output from the mixing circuit 6 is supplied to the speaker 5 via the
amplifier 4. Since the sound wave having the same amplitude and the opposite phase as the noise
is emitted from the speaker 5, the noise at the position of the microphone 1 can be canceled.
Furthermore, since the signals around 60 Hz and around 1 kHz extracted by the band pass filters
7 and 8 are added by the mixing circuit 6, noise can be prevented from increasing at these
frequencies.
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[0023]
FIG. 2 shows the frequency characteristics of noise after noise canceling when the positions of
the microphone, the speaker, and the listener's ear are substantially the same. In FIG. 2, reference
numeral 101 indicates the frequency characteristics of noise observed when noise cancellation is
not performed. Noise is mainly distributed in the low region. When noise canceling is performed,
noise components distributed in the low range are canceled as in the characteristic indicated by
the reference numeral 102, and noise is reduced. The vertical axis in FIG. 2 indicates the sound
pressure level (dBspl). As indicated by reference numerals 103 and 104, an increase in noise
level can be prevented in the vicinity of the cutoff frequency of the band pass filter 3 which has
conventionally been a problem.
[0024]
The band pass filter 3 may be a low pass filter. In this case, the band pass filter 7 may be omitted
and only the band pass filter 8 may be provided.
[0025]
A second embodiment of the present invention will be described with reference to FIG.
The output signal of the microphone amplifier 2 is supplied to a band pass filter 3 having a pass
band of (80 to 800 Hz), a band pass filter 7 having a pass band of (60 to 80 Hz), and a band pass
filter 8 having a pass band of (800 Hz to 1 kHz) The output signals of these band pass filters are
added by the mixing circuit 6.
[0026]
An output signal of the mixing circuit 6 is supplied to an amplifier 4 via a low pass (low pass)
filter 9 having a cutoff frequency Fc = 2 kHz. The output signal of the amplifier 4 is supplied to
the speaker 5. The adverse effect due to the phase shift in the band pass filter 3 is significant at
high frequencies where the wavelength is shorter than the low frequency. Therefore, in the
second embodiment, the low pass filter 9 is inserted so as not to control the noise canceling at a
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high frequency of, for example, 2 kHz or more. It is preferable that the characteristics of the low
pass filter 9 be as steep as possible. For example, -48 dB / oct.
[0027]
The cutoff frequency of the low pass filter 9 may be a frequency lower than 2 kHz. However, it is
higher than the cutoff frequency (1 kHz) of the band pass filter 8.
[0028]
As shown by reference numeral 105 in FIG. 4, according to the second embodiment, the
influence of the phase shift can be reduced by not controlling the high frequency difficult to
control. That is, the disturbance of the frequency characteristic 102 after noise reduction can be
reduced compared to the conventional case (see FIG. 2).
[0029]
A third embodiment of the present invention will be described with reference to FIG. The output
signal of the microphone amplifier 2 is supplied to a band pass filter 3 whose passband is (80 to
800 Hz). An output signal of the band pass filter 3 is supplied to the mixing circuit 6 and is also
supplied to the high pass (high pass) filter 11 having a cutoff frequency Fc = 800 Hz through the
inverting circuit 10. An output signal of the high pass filter 11 is supplied to the mixing circuit 6.
[0030]
An output signal of the mixing circuit 6 is supplied to the amplifier 4 through a low pass filter 9
having a cutoff frequency Fc = 2 kHz. The output signal of the amplifier 4 is supplied to the
speaker 5. As described in the second embodiment, the low pass filter 9 is inserted, for example,
at a high frequency of 2 kHz or higher for the purpose of not performing noise cancellation
control. The characteristic of the low pass filter 9 is, for example, -48 dB / oct.
[0031]
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The inverting circuit 10 and the high pass filter 11 replace the band pass filters 7 and 8 in the
second embodiment, and the circuit configuration can be simplified. That is, the phase of the
noise cancellation signal obtained from the band pass filter 3 is inverted by the inverting circuit
10, and the high pass filter 11 extracts high frequency components above the band where noise
increases. By supplying the output signal of the high pass filter 11 to the mixing circuit 6, the
signal component in the band of 800 Hz or more is reduced. Therefore, as a result of supplying
the output signal of the mixing circuit 6 to the speaker 5 through the low pass filter 9 and the
amplifier 4, it is possible to prevent an increase in noise in the high frequency band.
[0032]
In the third embodiment, the increase in noise in the high frequency band (800 Hz-1 kHz) can be
prevented, but the increase in noise in the low frequency band (60-80 Hz) can not be improved.
However, since noise in a high frequency band which is aurally unpleasant compared to noise in
a low frequency band can be reduced, a noise reduction effect can actually be obtained.
[0033]
As indicated by reference numeral 106 in FIG. 6, according to the third embodiment of the
present invention, as in the second embodiment, by controlling the high frequency difficult to
control, the phase shift can be reduced. The impact can be reduced. That is, the disturbance of
the frequency characteristic 102 after noise reduction can be reduced compared to the
conventional case (see FIG. 2).
[0034]
The present invention is not limited to the above-described embodiment, and various
modifications based on the technical idea of the present invention are possible. For example,
even in the case where the band pass filter 3 is provided, the band pass filter 7 in FIG. 1 may be
omitted to suppress only the increase in noise in the high frequency band that is easy to be heard
by the ear. Also, instead of the band pass filter 3, a low pass filter may be used. Furthermore, in
the configuration of FIG. 5, instead of the inverting circuit 10 and the high pass filter 11, a circuit
for removing frequency components lower than 800 Hz may be provided on the output side of
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the band pass filter 3.
[0035]
It is a block diagram of a 1st embodiment of this invention. It is a graph which shows the
frequency characteristic of the noise after the noise canceling in the 1st Embodiment of this
invention. It is a block diagram of 2nd Embodiment of this invention. It is a graph which shows
the frequency characteristic of the noise after the noise canceling in 2nd Embodiment of this
invention. It is a block diagram of 3rd Embodiment of this invention. It is a graph which shows
the frequency characteristic of the noise after the noise canceling in 3rd Embodiment of this
invention. It is a block diagram of an example of the conventional noise canceling apparatus. It is
a graph which shows the frequency characteristic of the noise after the conventional noise
canceling.
Explanation of sign
[0036]
DESCRIPTION OF SYMBOLS 1 ... Microphone 2 ... Microphone amplifier 3, 7, 8 ... Band pass filter
5 ... Speaker 6 ... Mixing circuit 9 ... Low pass filter 11 ... High pass filter
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