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JP2014160953

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DESCRIPTION JP2014160953
Abstract: To provide a sound field plane wave expansion technology that can expand plane waves
even if the microphone array has a cylindrical shape. A planar wave expanding unit 3 stores a
discretization angle (.theta., .Phi.) (D = 1,..., D), where D is a predetermined positive integer, in the
expansion angle storage unit 2. A plane wave expansion signal P corresponding to each
discretization angle (θ, φ) read from the expansion angle storage unit using the sound pressure
signal P (k) generated based on the signal collected by the microphone (k) is calculated taking
into account changes in the transfer path due to the rigid baffle. [Selected figure] Figure 1
Sound field plane wave expansion method, apparatus and program
[0001]
The present invention relates to a technology for collecting a sound signal with a microphone
installed in a certain sound field and expanding the sound signal into a plane wave.
[0002]
Plane wave expansion of the sound field is a technology that can be used for various applications
such as sound field analysis, sound field coding, sound field reproduction and the like.
In the sound field reproduction technology, assuming that a desired sound field is a
superposition of plane waves, a speaker array drive signal for reproducing each plane wave is
calculated, and the sum is output as a drive signal. Reproduction is possible. As a sound field
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reproduction technology, for example, the technology described in Non-Patent Document 1 is
known.
[0003]
Shoichi Koyama, 3 others, "Spatio-temporal frequency domain signal conversion method for
sound field collection and reproduction", Proceedings of the Acoustical Society of Japan,
September 2011, P. 635-636
[0004]
In the prior art, although it was possible to plane-wave expand a sound collection field using a
plane, a straight line, a sphere, and a circular array, plane-wave expansion was not possible when
the microphone array had a cylindrical shape.
[0005]
An object of the present invention is to provide a sound field plane wave expansion method,
apparatus and program that can expand plane waves even if the microphone array has a
cylindrical shape.
[0006]
In order to solve the above-mentioned problem, the sound field plane wave expansion method
according to one aspect of the present invention comprises two or more radii Rcyl having a
cylindrical rigid body centering on the axis of the rigid baffle and the circumferential direction of
the baffle being a circumferential direction. Assuming that at least two microphones are arranged
in each of the circles, let ω be the frequency, c be the speed of sound, k = ω / c, and in the
expansion angle storage unit, D be a predetermined positive integer, Assuming that the
discretization angles (θ dir, d, φ dir, d) (d = 1,..., D) are stored, sound pressure signals Pmic, ab
(k (k) generated based on the signals collected by the microphones Plane wave expansion signal
corresponding to each discretization angle (θ dir, d, φ dir, d) read from the expansion angle
storage section) by using the baffle of the rigid body It has a plane wave expansion step
calculated taking into consideration changes in the transmission path.
[0007]
Even if the microphone array has a cylindrical shape, plane wave development can be performed.
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[0008]
FIG. 1 is a functional block diagram showing an example of a sound field plane wave expansion
device according to a first embodiment.
The functional block diagram which shows the example of the sound field plane wave expansion
¦ deployment apparatus of 2nd embodiment.
FIG. 1 is a functional block diagram showing an example of a sound field plane wave expansion
device according to a first embodiment.
The figure for demonstrating the example of arrangement ¦ positioning of a microphone and a
speaker.
The figure for demonstrating the example of arrangement ¦ positioning of a microphone and a
speaker.
The flowchart which shows the example of the sound field plane wave expansion method. The
figure for demonstrating the example of how to determine a discretization angle.
[0009]
Hereinafter, embodiments of the present invention will be described with reference to the
drawings. In the following description, the symbol ˜ and the like used in the text should be
written directly above the previous character, but due to the restriction of the text notation, it will
be written immediately after the character. In the formula, these symbols are described at their
original positions. Moreover, the processing performed in each element unit of a vector or a
matrix is applied to all elements of the vector or the matrix unless otherwise noted.
[0010]
First Embodiment <Arrangement of Microphone Array> The sound field plane wave expansion
device and method is a microphone array composed of Na × Nb microphones arranged in a
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cylindrical shape of radius Rcyl shown in FIG. The sound field is developed into a plane wave.
[0011]
The microphone is disposed in a cylindrical shape of radius Rcyl by being fixed to the cylindrical
baffle B of radius Rb as illustrated in FIG. 5.
The microphone is supported by, for example, a thin rod-like member that protrudes
perpendicularly from the circumferential surface of the baffle B. In the example of FIG. 5, the
microphone is disposed at a position separated from the surface of the peripheral surface of the
baffle B by (Rcyl−Rb) as Rcyl ≧ Rb, but the accuracy is higher in the case of Rcyl = Rb.
[0012]
In other words, Na microphones are arranged at equal intervals in each of Nb circles whose
circumferential direction is the circumferential direction of the cylindrical rigid body baffle B
centered on the axis of the cylindrical rigid body baffle B Ru. Na and Nb are predetermined
integers of 2 or more. That is, by arranging the two microphones in each of two circles whose
circumferential direction is the circumferential direction of the baffle B, at least four microphones
are disposed at positions Rcyl away from the axis of the baffle B.
[0013]
The microphones may be spaced at any distance. That is, each of zc and θc, which is the
distance between the adjacent microphones, can take an arbitrary value. However, plane wave
expansion can be performed with high accuracy by arranging the microphones at equal intervals,
that is, setting the respective values of θc and φc, which are the intervals between adjacent
microphones, to the same value.
[0014]
The microphone is disposed outward of the circumferential surface of the cylindrical rigid baffle.
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[0015]
The position of the microphone Ma-b is expressed as (Rcyl, θmic, a, zmic, b) (a = 1, 2,..., Na, b =
1, 2,..., Nb).
[0016]
<Sound Field Plane Wave Expansion Device> As shown in FIG. 1, the sound field plane wave
expansion device includes, for example, a frequency conversion unit 1, an expansion angle
storage unit 2 and a plane wave expansion unit 3, and each step shown by a solid line in FIG.
Planar wave expansion is performed by performing the processing of
[0017]
The microphone arrays M1-1, M2-1,..., MNa-Nb pick up the sound emitted by the sound source S
and generate time domain signals.
The generated signal is sent to the frequency converter 1.
A signal of time t collected in time by the microphone Ma-b at the position (Rcyl, θmic, a, zmic,
b) is denoted as Pmic, ab (t).
[0018]
<Frequency Converter 1> The frequency converter 1 converts the signals Pmic, ab (t) collected
by the microphone Ma-b into frequency domain signals Pmic, ab (k) by Fourier transform (FIG. 6,
step S1). ).
The generated frequency domain signals Pmic, ab (k) are sent to the plane wave expansion unit
3. Let ω be the frequency, c be the speed of sound, and k = ω / c.
[0019]
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For example, frequency domain signals Pmic, ab (k) are generated by short time discrete Fourier
transform. Of course, frequency domain signals Pmic, ab (k) may be generated by other existing
methods. Alternatively, frequency domain signals Pmic and ab (k) may be generated using a
method such as overlap ad. When the input signal is long or when the signal is continuously
input as in real time processing, processing is performed every frame, for example, every 10 ms.
[0020]
The frequency domain signals Pmic, ab (k) are defined, for example, as follows. I in the argument
of the function exp is an imaginary unit.
[0021]
[0022]
<Development Angle Storage Unit 2> The development angle storage unit 2 stores the
discretization angles (θ dir, d, φ dir, d) (d = 1,..., D) with D as a predetermined positive integer. .
That is, in the expansion angle storage unit 2, D discretization angles (θ dir, 1, φ dir, 1), (θ dir,
2, φ dir, 2),..., (Θ dir, D, φ dir, D) are stored. ing.
[0023]
The discretization angle (θ dir, d, φ dir, d) is a pair of θ dir, d and φ dir, d. As shown in FIG. 4,
θdir, d is an elevation angle, and φdir, d is an azimuth angle.
[0024]
The setting of D discretization angles (θ dir, d, φ dir, d) (d = 1,..., D) may be performed in any
manner. For example, the discretization angles (θ dir, d, φ dir, d) are equally spaced.
Alternatively, the discretization angle (θ dir, d, φ dir, d) may be determined using the vertex
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direction of the regular polyhedron. That is, the directions of a plurality of vertices of a regular
polyhedron whose origin is the center of the regular polyhedron are made discrete angles (θ dir,
d, φ dir, d) (d = 1,..., D).
[0025]
FIG. 7 is a conceptual diagram in the case of determining the discretization angle (θ dir, d, φ dir,
d) using an icosahedron. For example, as shown in FIG. 7, assuming that an icosahedron is
disposed on the central axis of a cylindrical microphone array, a predetermined distance from the
central axis of the cylindrical microphone array at the apex of the icosahedron The directions of
the plurality of vertices excluding the vertex in are respectively set as discretization angles (θ
dir, d, φ dir, d) (d = 1,..., D).
[0026]
Setting a large value of D increases the computational cost but increases the resolution of the
sound field. The value of D is determined in consideration of the calculation cost and the
resolution of the sound field.
[0027]
<Planar Wave Expansion Unit 3> The plane wave expansion unit 3 uses the sound pressure
signals Pmic, ab (k) generated based on the signal collected by the microphone Ma-b to read each
from the expansion angle storage unit 2. A plane wave expansion signal Pdir, d (k) corresponding
to the discretization angle (θ dir, d, φ dir, d) is calculated in consideration of the change of the
transfer path by the baffle of the rigid body (step S2). The sound pressure signals Pmic, ab (k)
generated based on the signals collected by the microphone Ma-b are, in this example, the
frequency domain signals Pmic, ab (k) generated by the frequency conversion unit 1. It is.
[0028]
The plane wave expansion unit 3 calculates, for example, a plane wave expansion signal Pdir, d
(k) determined by the following equation.
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[0029]
[0030]
Let i be an imaginary unit, Hn <(1)> (·) be an n-th first kind Hankel function, and Hn <(1)> '(.) be
an n-th first kind Hankel function Hn <(1) W (k, θmic, a, zmic, b, θdir, d, φdir, d) is given as
follows as a derivative of> (·).
N is the resolution of the sound field as described above, and is a predetermined positive integer.
For example, N is a positive integer equal to or less than the value NMAX determined by the
discretization method. In many cases, N can be determined based on numerical calculation so as
to be numerically stable. The superscript + represents the operation of the pseudo inverse matrix.
[0031]
[0032]
The n-th first kind Hankel function Hn <(1)> (x) is defined as follows using the n-th first kind
Bessel function Jn (x) and the second kind Bessel function Yn (x) Ru.
[0033]
[0034]
Hn <(1)> '(ψ) which is a derivative of the n-th first kind Hankel function Hn <(1)> (·) is defined as
follows.
[0035]
[0036]
Thus, in order to handle the signal collected by the cylindrical microphone, the plane wave
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expansion unit 3 handles the Helmholtz equation in the helical wave spectral region as the
Helmholtz equation representing the wave equation in the frequency domain. The plane wave
expansion signal Pdir, d (k) is calculated using the filters W (k, θmic, a, zmic, b, θdir, d, φdir, d)
calculated by
[0037]
In addition, a plane wave and a plane wave expansion ¦ deployment signal are indices which
represent the strength of the signal for every direction.
For example, the plane wave expansion signal Pdir, d (k) represents the strength of the signal in
the direction of the discretization angle (θ dir, d, φ dir, d).
[0038]
As described above, according to the sound field plane wave expansion device and method of the
first embodiment, plane wave expansion can be performed even if the microphone array has a
cylindrical shape.
[0039]
Second Embodiment The sound field plane wave expansion device according to the second
embodiment differs from the first embodiment in the portion further including the discrete
spherical harmonic conversion unit 4 as shown in FIG. 2, and the other portions are the first
embodiment. Is the same as
[0040]
The following description will center on the discrete spherical harmonic conversion unit 4 which
is a part different from the first embodiment, and the description of the same parts as the first
embodiment will be omitted.
[0041]
The plane wave expansion signals Pdir, d (k) (d = 1,..., D) calculated by the plane wave expansion
unit 3 are transmitted to the discrete spherical harmonic conversion unit 4.
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[0042]
The discrete spherical harmonic transformation unit 4 transforms the plane wave expansion
signal Pdir, d (k) into a spherical harmonic spectrum signal B to p (k) by discrete spherical
harmonic transformation (step S3).
[0043]
The discrete spherical harmonic conversion unit 4 performs discrete spherical harmonic
conversion, for example, according to the following equation to generate a spherical harmonic
spectrum signal B to p (k).
[0044]
[0045]
As an index p = (n, m), Ap is a coefficient for normalization and is a predetermined real number.
For example, the Furse-Malham coefficient is used as Ap.
Here, n and m are 0 ≦ n ≦ N, −n ≦ m ≦ n and ¦ m ¦ ≦ M.
The maximum value of the magnitude of the index p = (n, m) is M <2> + 2N + 1.
[0046]
M is a predetermined positive integer less than or equal to N.
The magnitude of the value of M also corresponds to the resolution.
[0047]
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Yn <m> is, for example, a spherical harmonic function defined as follows.
n and m are the orders of the spherical harmonic spectrum.
In the following formula, j is an imaginary unit.
[0048]
[0049]
P <m> n (·) is a Legendre 陪 function and is defined as follows.
P n (·) represents a Legendre polynomial.
[0050]
[0051]
As described above, the plane wave expansion signal Pdir, d (k) may be converted into the
spherical harmonic spectrum signal B to p (k) which is a format that can be used for the
reproduction format such as ambisonics.
[0052]
Third Embodiment The sound field plane wave expansion device of the third embodiment differs
from the first embodiment in the portion further including the frequency inverse transformation
unit 5 as shown in FIG. 3, and the other portions are the first embodiment. Is the same as
[0053]
In the following, the frequency inverse transform unit 5 which is a part different from the first
embodiment is mainly described, and the description of the same part as the first embodiment is
omitted.
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[0054]
The plane wave expansion signal Pdir, d (k) (d = 1,..., D) calculated by the plane wave expansion
unit 3 is transmitted to the frequency inverse transform unit 5.
[0055]
The frequency inverse transform unit 5 transforms the plane wave expansion signal Pdir, d (k)
into a time domain signal Psp, d (t) by inverse Fourier transform, and converts the transformed
time domain signal Psp, d (t) into the speaker Sd. It outputs (step S4).
The time domain signals Psp, d (t) obtained for each frame by the inverse Fourier transform are
appropriately shifted and linearly summed to form a continuous time domain signal.
As the inverse Fourier transform, an existing method such as a short time discrete inverse
Fourier transform may be used.
[0056]
The speaker arrays S1, S2,..., SD are composed of D speakers S1, S2,.
Specifically, the speaker Sd is disposed in the direction of the discretization angle (θ dir, d, φ
dir, d) as d = 1,.
For example, as illustrated in FIG. 7, when the direction of each vertex of the regular polyhedron
is a discretization angle (θ dir, d, φ dir, d), the speaker is arranged at the position of each vertex
of the regular polyhedron.
[0057]
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The speaker arrays S1, S2,..., SD reproduce sounds based on the time domain signals Psp, 1 (t),
Psp, 2 (t),..., Psp, D (t).
Specifically, the speaker Sd reproduces a sound based on the time domain signal Psp, d (t) as d =
1,.
Thereby, the sound fields picked up by the microphone arrays M1-1, M2-1,..., MNa-Nb can be
reproduced by the speaker arrays S1, S2,.
[0058]
When the number of microphones is larger than the number of speakers, the reproduction signal
may be thinned.
On the other hand, when the number of microphones is smaller than the number of speakers,
interpolation may be performed by averaging the time domain signals Psp, d (t).
As a method of performing interpolation, for example, linear interpolation or sinc interpolation
can be applied.
[0059]
[Modifications, Etc.] As long as the sound field plane wave expansion device includes the plane
wave expansion unit 3, it may not include other units.
[0060]
The processing of the frequency conversion unit 1 and the processing of the plane wave
expansion unit 3 may be performed simultaneously.
The sound field plane wave expansion device can be realized by a computer.
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In this case, the processing content of each part of this apparatus is described by a program.
And each part in this apparatus is implement ¦ achieved on a computer by running this program
by computer.
[0061]
The program describing the processing content can be recorded in a computer readable
recording medium.
Further, in this embodiment, these devices are configured by executing a predetermined program
on a computer, but at least a part of the processing contents may be realized as hardware.
[0062]
The present invention is not limited to the above-described embodiment, and various
modifications can be made without departing from the spirit of the present invention.
[0063]
Reference Signs List 1 frequency conversion unit 2 development angle storage unit 3 plane wave
expansion unit 4 discrete spherical harmonic conversion unit 5 frequency inverse conversion
unit
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