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JP2007180922

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DESCRIPTION JP2007180922
The present invention provides a headphone which enables highly accurate and practically
sufficient noise cancellation while processing a small amount of information with a smallcapacity digital device. SOLUTION: A microphone for converting ambient noise into an electric
signal, a prediction means 20 for predicting a noise signal to be generated in the future from the
noise signal converted by the microphone, and a phase of the predicted noise signal by inverting
Cancel noise generation means 24 for generating, adder 26 for adding an audio signal and
cancellation noise and inputting the result to a signal acoustic conversion element, and a low
pass filter 30 for passing only a noise signal of a bass region from a noise signal converted by a
microphone And the cancellation noise generation means 24 generates cancellation noise only
for the noise signal in the bass region. [Selected figure] Figure 1
ノイズキャンセルヘッドホン
[0001]
The present invention relates to digital noise cancellation headphones using a digital signal
processor (hereinafter referred to as "DSP"), and in particular to be able to effectively eliminate
external noise using a prediction method. The present invention relates to a noise cancellation
headphone which can further enhance the noise cancellation effect by improving the noise
prediction accuracy.
[0002]
For example, portable music players such as tape players, CD players, and MD players have
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1
become widespread, and recently, small-sized, large-capacity portable music players such as hard
disk types and flash memory types are rapidly becoming widespread.
With the spread of portable music players, headphones with higher performance are also
required. Furthermore, when trying to listen to music in the city or in a vehicle, the surrounding
noise can not be heard, only the music Noise-canceling headphones that are made to be audible
have also become desirable. This is because if the ambient noise mixed with the reproduced
sound of the music enters the ear, even the reproduced sound with high sound quality is wiped
out by the noise and the music can not be enjoyed with high sound quality. In addition, when you
try to listen to music in the noise, you tend to listen to the volume just by raising the volume,
causing the sound to leak out of the headphones by raising the volume, which is unpleasant for
people around the vehicle etc. There is also the problem of becoming a noise source. With such a
background, noise cancellation headphones are beginning to spread.
[0003]
Most of the noise cancellation headphones currently marketed are noise cancellation headphones
of an analog system. This is a microphone built in to the headphones to capture the ambient
sound (hereinafter referred to as "noise" because it is noise to the playback sound), reverse the
phase of the captured noise and add it to the playback signal from the player Method. Noise
entering the inside of the headphone from the outside is canceled by the signal whose phase is
reversed, and only the reproduction signal from the player is in the ear of the user. The outline is
shown in FIG. In FIG. 6, noise N indicated by reference numeral 42 is ambient noise captured by
the microphone attached to the headphone, and its phase is inverted by 180 degrees by the
phase inverter 44 provided in the analog circuit 40, and a signal of -N 'is obtained. It becomes.
The inverted signal -N 'of the noise signal and the reproduction signal S reproduced by the player
are added by the adder 46, and the added signal drives an acoustic transducer or speaker. Noise
N wraps around inside the headphones. This looped noise N is combined with the inverted signal
-N 'to become an N-N' signal as indicated by the reference numeral 48, and the majority of the
noise N is canceled by the inverted signal -N 'and the user can substantially reproduce Only the
sound S can be heard. Also, in place of the adder 46, a signal acoustic conversion element
(speaker) for generating a sound of the opposite phase of the noise N may be provided to cancel
the noise N.
[0004]
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2
Recently, digital noise cancellation headphones have also been proposed. An example is shown in
FIG. In FIG. 7, reference numeral 50 denotes a processor (hereinafter referred to as “DSP”)
specialized in digital signal processing which is a main body of digital noise cancellation
headphones. The DSP 50 cancels the frequency from a fast Fourier transformer (hereinafter
referred to as "FFT") 54 that analyzes the frequency of the noise signal N captured by the
microphone incorporated in the headphone and a frequency characteristic 56 obtained by
analyzing the frequency by the FFT 54. A selection unit 58 for selecting a band, an inverse
Fourier transformer (IFFT) 60 for performing inverse Fourier transform on the signal of the
selected frequency band to generate a frequency-selected cancellation sound signal -N ′ ′, and
a reproduction signal S by the player And an adder 64 for adding the above cancellation sound
signal -N "and outputting the result. The noise N coming around into the headphone is combined
with the above cancellation sound -N "to become an N-N" signal 66, the majority of the noise N is
canceled, and the user listens to almost only the reproduction sound S Can.
[0005]
As a prior art of an analog system noise cancellation headphone, there exist an invention of
patent document 1, the patent document 2, etc., for example. There is an invention described in
Patent Document 3 as an example in which a signal-to-sound conversion element for generating
a sound of the opposite phase of noise N is provided. As a prior art of digital noise cancellation
headphones, for example, there is an invention described in Patent Document 4.
[0006]
JP-A-11-308685 JP-A-11-237889 JP-A-2000-59876 JP-A-09-93684
[0007]
According to the analog noise cancellation headphone as shown in FIG. 6, since the circuit for
inverting the phase is configured using a coil (inductance), a capacitor (capacitance) and a
resistor, (1) when the phase is inverted There is a problem that the power spectrum can not be
correctly inverted, and (2) when the phase is inverted, the delay can not be accurately realized,
that is, the antiphase signal is delayed with respect to noise, resulting in , Can not get enough
cancellation effect.
[0008]
According to the conventional digital noise cancellation headphone as shown in FIG. 7, (1) time is
required for calculation in FFT and IFFT, and the phase of the calculation result does not
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correspond to the phase change of noise, so it is sufficient (2) It takes a long time to calculate the
cancellation sound for each frequency of noise, and if the frequency bandwidth is not narrowed
to save the calculation time, it is necessary to obtain the cancellation effect As a result, sufficient
cancellation effects can not be obtained.
[0009]
Therefore, the present applicant has a microphone for converting ambient noise into an electrical
signal, a prediction means for predicting a noise signal to be generated in the future from a noise
signal converted by the microphone, and a cancellation noise by inverting the phase of the
predicted noise signal. Patent application for a digital noise cancellation headphone including a
cancellation noise generation means for generating a signal and an adder for adding an audio
signal and a cancellation noise and inputting the result to a signal acoustic conversion element
(Japanese Patent Application No. 2005- See 145868).
[0010]
According to the noise cancellation headphone concerning the above application, the noise signal
to be generated in the future is predicted from the past noise signal converted by the
microphone, and the phase of the predicted noise signal is inverted to generate the cancellation
noise. Since the configuration is configured to cancel ambient noise, noise can be canceled in
response to a phase change of noise that actually occurs even if there is a calculation delay or
operation delay due to cancellation noise generation means or the like.
[0011]
The noise cancellation headphone according to the above-mentioned patent application by the
applicant of the present invention detects the noise and cancels the noise, so that it is difficult to
predict the noise signal with high accuracy since the sound frequency target is the entire voice
frequency band. .
The reason will be described with reference to FIG.
The waveform of the noise present around the headphones and detected by the canceling
microphone is a complex waveform including from the low range to the high range as shown in
FIG.
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In order to digitally process and predict a waveform going to the future with a certain point of
this waveform as the present time, the next signal level is predicted while sampling with a cycle
according to a predetermined sampling frequency.
For example, the sampling frequency of a standard CD is 44.1 KHz, and the next waveform is
predicted for each period according to this frequency.
FIG. 8 is an image of predicting a noise waveform in the future by performing sampling at a
certain past time T from the present time.
[0012]
Here, an example of the prediction method will be described with reference to FIG. FIG. 9 shows
an example of a noise waveform, and each dot added to this waveform indicates a sampling time.
The example shown in FIG. 9 predicts the signal level at the sampling point one sample ahead
from the current point by statistical processing such as the Berg method, and predicts the next
signal level based on this prediction data. , It makes repeated predictions. In FIG. 9, a plurality of
circles attached from a certain sampling time point to a suitable number of sampling time points
indicate that the prediction is repeatedly performed as described above, and the arrows attached
along the plurality of samplings are The example of the inclination of the waveform by the
prediction data obtained by sampling is shown. As can be seen from this arrow, the accumulation
of the prediction errors causes a large deviation from the noise waveform that actually appears,
resulting in the disadvantage that sufficient noise cancellation accuracy can not be obtained.
[0013]
As in the example shown in FIG. 9, in the case of predicting the waveform one ahead, it is not
possible to accurately predict the future waveform. If it is possible to predict up to a waveform
obtained by sampling a plurality of aheads, for example, 10 aheads, it is desirable. Therefore, as
shown in FIG. 10, a method of extending the measurement time by statistical processing such as
the Berg method is considered. According to this method, there is no problem that the prediction
error is accumulated, but since it is not possible to follow the data that fluctuate with time, it is
not possible to generate a precise cancellation signal, and again, sufficient noise cancellation
accuracy There is a drawback that you can not get
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[0014]
In any case, the conventional noise cancellation headphones are targeted for noise cancellation
over the entire audio frequency band. However, if it is desired to perform digital processing with
predetermined accuracy for noise cancellation over the entire audio frequency band, it is
necessary to perform signal processing including high frequency range components, so the
amount of information to be processed becomes large and high speed processing Needs largecapacity digital devices. In addition, in order to accurately predict future waveforms, it is
necessary to make predictions based on a large amount of past data. Furthermore, performing
noise cancellation including the high-pitched range also requires high-speed processing, so highprecision noise cancellation becomes increasingly difficult.
[0015]
If it is only necessary to cancel only the low sound region (low frequency region) of noise, the
amount of information to be processed can be reduced, a small-capacity digital device is
sufficient, and the processing speed can be increased. Should be able to improve the noise
cancellation accuracy. As a result of the present inventor's verification, in the state where
headphones are worn, the housing of the headphone has a sound insulation effect in the high
sound area (high frequency area), and the noise transmitted around the headphone housing to
the ear is the bass area. I understood it. Therefore, if noise cancellation is performed on the bass
region, it sufficiently functions as an noise cancellation headphone.
[0016]
The present invention has been made based on the above technical background, and by
performing noise cancellation in the bass region, a small amount of information can be processed
with a small-capacity digital device while the accuracy is sufficient for practical use. It is an
object of the present invention to provide a headphone capable of performing various noise
cancellations.
[0017]
According to the present invention, a microphone that converts ambient noise into an electrical
signal, a prediction unit that predicts a noise signal to be generated in the future from a noise
signal converted by the microphone, and a phase of the predicted noise signal are inverted to
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generate cancel noise. Noise cancellation headphone provided with cancel noise generation
means for adding, an audio signal and the cancel noise and adding to the signal acoustic
conversion element, only noise signals in the bass region from noise signals converted by the
microphone The above-mentioned cancellation noise generating means is characterized in that
the cancellation noise is generated only in the noise signal of the bass region.
[0018]
The sampling circuit has a sampling circuit for sampling the noise signal converted by the
microphone at a constant period, and the prediction means is configured to predict the noise
signal to be generated in the future by a plurality of sampling data from the present to the past
obtained by the sampling circuit. Good.
[0019]
The noise signal converted by the microphone is cut through the low pass filter to cut the high
tone region, and only the low tone region is input to the cancellation noise generating means, and
the cancellation noise is generated only for the low tone region.
The generated cancellation noise cancels the noise in the bass region.
It is known that when the user wears the headphones, the high frequency range of the noise
entering the user's ear around the headphone casing is shielded, and only the low sound area of
the noise reaches the user's ear, as described above. Even if the noise in the bass region is only
canceled, a sufficient noise cancellation effect can be obtained.
Further, since it is sufficient to generate cancellation noise only in the bass region, the amount of
information processing by devices such as the prediction means and the cancellation noise
generation means may be much smaller than the amount of information processing in the
conventional noise cancellation headphone. Therefore, the capacity of the device may be small,
the time required for information processing may be short, rapid processing may be possible, and
noise cancellation with high accuracy and sufficient for practical use may be performed.
[0020]
A sampling circuit may be provided to sample the noise signal converted by the microphone at a
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constant cycle, and the prediction means may be configured to predict the noise signal to be
generated in the future by a plurality of sampling data from the present to the past obtained by
the sampling circuit, The prediction accuracy of the noise signal can be further enhanced, and a
noise cancellation head with a good noise cancellation effect can be obtained.
[0021]
Hereinafter, embodiments of the noise cancellation headphone according to the present
invention will be described with reference to FIGS. 1 to 5.
In FIG. 1, reference numeral 16 denotes a DSP that is the main component of information
processing in the digital noise cancellation headphone. A noise signal (noise) 17 is input to the
DSP 16 from a microphone incorporated in headphones and converting ambient noise into an
electrical signal, but an analog low pass filter 30 is interposed between the microphone and the
DP 16 When the noise signal is input to the DSP 16, the low pass filter 30 is passed through. The
low pass filter 30 is also a high cut filter, and cuts the high tone area from the noise signal 17,
and inputs only the low tone area N of the noise signal to the DSP 16. The DSP 16 has a sampling
circuit 18 that samples the bass region N of the noise signal with a predetermined number of bits
and converts it into digital data, and has prediction means 20 to which sampling data obtained
by the sampling circuit 18 is input There is. The prediction means 20 predicts a noise signal to
be generated in the future from past sampling data of the bass region N of the noise signal. For
example, an interpolation method such as a polynomial can be used. The prediction means 20
generates a generated prediction noise N ′ ′ as indicated by a reference numeral 22. The
generated prediction noise N ′ ′ is 180 ° out of phase by the cancellation noise generation
means 24 included in the DSP 16, and the cancellation noise −N ′ ′ Are configured to be
generated. Between the prediction means 20 and the cancellation noise generation means 24,
there is an analog-to-digital converter, and the cancellation noise -N "is converted into an analog
signal.
[0022]
The cancel noise -N "is input to the adder 26 included in the DSP 16, and the adder 26 adds the
audio signal S reproduced by an appropriate player or the like and the cancel noise -N" and
outputs the result. It is done. The speaker is driven by the addition output S-N '', and a sound
corresponding to S-N '' is emitted from the speaker. The surrounding noises wrap around the
headphones and get into the user's ear. Assuming that the noise entering the user's ear is N ', this
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noise N' is synthesized with the voice corresponding to the above-mentioned cancellation noise N "whose phase is reversed, and N-N" as shown by the reference numeral 28. It becomes noise
that is muted as. In other words, the sound that enters the user's ear is S + N'-N ". Since the
portion of N'-N "is canceled as described above, only the audio signal S will be in the user's ear.
[0023]
FIG. 2 schematically shows the embodiment of the noise cancellation headphone according to the
present invention described above. In FIG. 2, reference numeral 12 indicates a headphone unit.
The headphone unit 12 includes an enclosure covering the user's ear, a microphone 14
incorporated in the enclosure to convert ambient noise into an electrical signal, and an audio
signal S reproduced by the external player 10 as an acoustic signal. A speaker as a signal-tosound conversion element to be converted and the above-mentioned DSP 16 are provided. The
microphone 14 converts the noise around the headphone unit 12 into an electrical signal, and
the noise signal is input to the DSP 16 through the low pass filter 30 as described above and
processed as described above. The DSP 16 adds the cancellation noise -N "obtained by inverting
the generated prediction noise N" generated based on the bass region N of the noise signal, and
the audio signal S to output an S-N "signal. The speaker is driven by the signal S-N ''.
[0024]
On the other hand, the noise N around the headphone unit 12 goes around the headphone unit
12 and reaches the user's ear. The noise N changes the sound pressure level for each frequency
band by going around the headphone unit 12 and enters the user's ear as noise of N ‘.
Therefore, the sound that enters the user's ear is S-N '' + N '. Since the cancellation noise -N "is in
opposite phase to the noise N 'entering the user's ear, and the cancellation noise -N" and the
noise N' both originate from the noise N, the noise N 'is Cancel noise-substantially canceled by N
", and almost only the audio signal S enters the user's ear. The noise that wraps around the
housing of the headphone unit 12 and enters the user's ear, as described above, is cut off in the
high-pitched area and becomes only the low-pitched area. Since the cancellation noise N ′ ′
generated in the DSP 16 is a cancellation noise corresponding to the bass region of the noise
signal, the noise entering the user's ear is canceled by the cancellation noise N ′ ′, and the
cancellation is sufficient for practical use. It is possible to obtain noise cancellation headphones
that can obtain an effect.
[0025]
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FIG. 3 shows an example of the frequency characteristic of noise around the headphones, where
the horizontal axis is the frequency and the vertical axis is the gain. In this frequency
characteristic, a high-pitched area Fh indicated by hatching is an area blocked by the low-pass
filter 30 and an area blocked by going around the housing of the headphone unit. The lower limit
frequency of the above-mentioned treble range Fh to be cut off, in other words, the upper limit
frequency of the bass range F1 passing through the low pass filter 30 is the upper limit
frequency of the noise to be canceled. The frequency is called "maximum target frequency".
Furthermore, in other words, the analog low pass filter 30 is set to the maximum target
frequency.
[0026]
The relationship between the target frequency of the low pass filter 30 and sampling is shown in
FIG. In the waveform shown in FIG. 4, the high-pitched sound region is cut by passing the lowpass filter 30, and the maximum target frequency is a waveform of a noise signal of 2 KHz, for
example. It is assumed that N samplings are performed during one noise cycle of this maximum
target frequency. This sampling frequency is assumed to be 40 KHz (ie, an integer multiple of the
maximum frequency of interest for noise passing through the low pass filter). Therefore, the
noise frequency of 2 KHz is sampled at a sampling frequency of 40 KHz, and the sampling
number N in one noise cycle is 20, ie, 20 times of sampling. In other words, noise signal data of 2
KHz can be acquired for one cycle in 20 samplings. In this way, by cutting the treble range of
noise and keeping the maximum value of the target frequency low, more sampling can be
performed during one cycle of the noise signal, and prediction of the future noise waveform
based on sampling can be performed. Can be performed with high accuracy.
[0027]
FIG. 5 shows an example of noise signal prediction obtained by a plurality of earlier samplings
from the noise signal obtained by sampling as described above and this noise signal by an
interpolation method such as a polynomial. In FIG. 5, a waveform indicated by a thin line is an
example of a noise signal waveform that is actually obtained, and dots attached at certain time
intervals above this waveform indicate sampling positions. The waveforms indicated by thick
lines indicate examples of noise waveforms predicted based on the sampling results. The noise
signal waveform obtained by sampling is approximated by a second-to-fourth polynomial, and
coefficients obtained thereby are used to predict a waveform obtained by (preceding) sampling
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ahead of time.
[0028]
As described above, the sampling circuit 18 samples the noise signal converted by the
microphone at a constant cycle, and the prediction unit 20 predicts the noise signal to be
generated in the future by a plurality of sampling data from the present to the past obtained by
the sampling circuit 18 Configured to The noise signal waveform obtained by sampling is
approximated by a polynomial, and coefficients obtained thereby are used to predict a waveform
obtained by sampling at a plurality of points ahead of the current time. This has the following
advantages. (1) Since the number of inflection points of the signal waveform is known in
advance, highly accurate waveform prediction can be performed, and highly accurate noise
cancellation can be realized. (2) Polynomial approximation has a small amount of calculation
unlike statistical processing such as the Berg method, so that the load on a processing unit such
as a CPU is light and noise cancellation by a low-cost device can be realized.
[0029]
The model diagram shown in FIG. 2 describes one headphone unit 12. Usually, the headphones
are configured as stereo headphones comprising the left and right headphone units 12. In this
stereo headphone, how to arrange the microphone 14 for detecting ambient noise and the circuit
for noise cancellation accompanying it, that is, to provide them in one headphone unit 12 or both
headphones One of the units 12 will be selected.
[0030]
The prediction means 20 predicts and cancels a noise signal to be generated in the future based
on a signal obtained by removing a treble area from a noise signal converted by the microphone
14 with a microphone 14 in one of the left and right headphone units 12. The noise generation
means 24 may generate cancel noise, and the left and right headphone units 12 may be provided
with an adder 26 for adding the respective audio signals and the cancel noise. Although such a
configuration is a relatively simple configuration, a sufficient noise cancellation effect can be
obtained.
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[0031]
If larger noise cancellation effects can be obtained by the left and right headphone units 12, the
microphones 14, the prediction unit 20, the cancel noise generation unit 24, and the adder 26
are provided for each of the left and right headphone units 12. A noise signal to be generated in
the future may be predicted to generate cancellation noise, and the audio signal S and the
cancellation noise -N "may be added to each of the left and right headphone units 12. Of course,
low-pass filters are connected after the left and right microphones, and noise signals are
predicted on the left and right, respectively, to generate cancellation noise, based on the noise
signal obtained by removing the high-tone area from the noise signal. The left and right
headphone units 12 should be able to hear the surrounding noise differently, so by separately
detecting the noise separately and performing noise cancellation processing, the noise
cancellation effect is further enhanced. be able to.
[0032]
According to the present invention, it is also possible to cancel all ambient noise and make it
possible for the player to hear only the audio signal reproduced. However, in normal situations,
the situation where noise can not be heard at all is rather rare, and there may be a sense of
discomfort rather than human sense, so the user can adjust the amount of noise cancellation or
the rate of cancellation according to preference. It is good to do it. The adjustment of the noise
cancellation amount or the cancellation rate can be achieved by, for example, adjusting the gain
of the cancellation noise generating means 26 or the like.
[0033]
The noise-canceling headphone according to the present invention is used mainly with a player
that plays music, but as another application, for example, in order to exert concentration in noise,
without inputting an audio signal You can use the noise cancellation function to create a
situation as if you were in silence.
[0034]
It is a block diagram which shows the Example of the noise cancellation headphone concerning
this invention.
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It is a model figure which shows the outline | summary of the Example of the noise cancellation
headphone concerning this invention. It is a wave form diagram which shows the function of the
low pass filter in the said Example, and the maximum object frequency of a low pass filter. It is a
wave form diagram which shows the example of the sampling with respect to the object
frequency in the said Example. It is a wave form diagram which shows another example of the
sampling by the noise cancellation headphone concerning this invention. It is a block diagram
which shows the example of the conventional analog system noise cancellation headphone. It is a
block diagram which shows the example of the conventional digital system noise cancellation
headphone. It is a wave form diagram which shows the appearance of the sampling of the noise
signal in the conventional digital system noise cancellation headphone. It is a wave form diagram
which shows an example of prediction of the noise signal in the conventional digital system noise
cancellation headphone. It is a wave form diagram which shows another prediction example of
the noise signal in the conventional digital system noise cancellation headphone.
Explanation of sign
[0035]
DESCRIPTION OF SYMBOLS 10 player 12 headphone unit 14 microphone 16 DSP 18 sampling
circuit 20 prediction means 24 cancellation noise generation means 26 adder 30 low pass filter
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