Method and apparatus for noise filtering
View Patent ↗Disclosed is an apparatus for and a method of filtering noise from a mixed sound signal to obtained a filtered target signal, comprising the steps of inputting the mixed signal through a pair of microphones into a first channel and a second channel, separately Fourier transforming each said mixed signal into the frequency domain, computing a signal short-time spectral amplitude |Ŝ| from said transformed signals, computing a signal short-time spectral complex exponential e i arg(S) from said transformed signals, where arg(S) is the phase of the target signal in the frequency domain, computing said target signal S in the frequency domain from said spectral amplitude and said complex exponential.
1. A method of filtering noise from a mixed sound signal to obtained a filtered target signal, comprising the steps of:
inputting the mixed signal through a pair of microphones into a first channel and a second channel;
separately Fourier transforming each said mixed signal into the frequency domain;
computing a signal short-time spectral amplitude |Ŝ| from said transformed signals;
computing a signal short-time spectral complex exponential e i arg(S) from said transformed signals, where arg(S) is the phase of the target signal in the frequency domain;
computing said target signal S in the frequency domain from said spectral amplitude and said complex exponential, further comprising the step of computing a spectral power matrix and using said spectral power matrix to compute said spectral amplitude and said spectral complex exponential.
2. The method of claim 1 wherein said target signal S in the frequency domain is inverse Fourier transformed to produce a filtered target signal s in the time domain.
3. The method of claim 1 wherein said spectral power matrix is computed by spectral channel subtraction.
4. The method of claim 1 wherein said signal short-time spectral amplitude is computed by the estimation equation
S
^
=
E
[
S
|
X
1
,
X
2
]
=
π
2
1
C
1
exp
(
-
C
2
2
8
C
1
)
[
1
+
C
2
2
4
C
1
I
0
(
C
2
2
8
C
1
)
+
C
2
2
4
C
1
I
1
(
C
2
2
8
C
2
)
]
where
I
0
(
z
)
=
1
2
π
∫
0
2
π
exp
(
z
cos
β
)
ⅆ
β
,
I
n
(
1
)
=
1
2
π
∫
0
2
π
cos
(
β
)
exp
(
z
cos
β
)
ⅆ
β
,
C
1
=
1
ρ
s
+
1
det
R
n
(
R
22
+
R
11
K
2
-
K
R
12
-
K
_
R
21
)
,
C
2
=
2
det
R
n
X
_
1
R
22
+
X
_
2
K
R
11
-
X
2
R
12
-
X
1
K
_
R
21
,
X 1 and X 2 are the Fourier transformed first and second signals respectively, R nm are elements of said spectral power matrix, and K is a constant.
5. The method of claim 1 wherein said signal short-time spectral complex exponential is computed by the estimation equation
z
≡
ⅇ
ⅈ
ar
g
^
(
S
)
=
R
22
X
1
+
R
11
K
_
X
2
-
R
21
K
_
X
1
-
R
12
X
2
R
22
X
1
+
R
11
K
_
X
2
-
R
21
K
_
X
1
-
R
12
X
2
.
6. The method of claim 1 wherein said signal short-time spectral complex exponential is computed by the estimation equation
z
≡
ⅇ
ⅈ
ar
g
^
(
S
)
=
R
22
X
1
+
R
11
K
_
X
2
-
R
21
K
_
X
1
-
R
12
X
2
R
22
X
1
+
R
11
K
_
X
2
-
R
21
K
_
X
1
-
R
12
X
2
.
7. The method of claim 6 wherein said target signal S in the frequency domain is computed by the equation
S=zA.
8. The method of claim 1 wherein said target signal is computed by multiplying said signal short-time spectral amplitude by said signal short-time spectral complex exponential.
9. The method of claim 1 further comprising the step of calibrating a function K(ω), said function equal to a ratio of one said Fourier transformed signal to the other, by the estimation equation
K
(
ω
)
=
∑
t
=
1
F
X
2
c
(
l
,
ω
)
X
1
c
(
l
·
ω
)
_
∑
t
=
1
F
X
1
c
(
l
,
ω
)
2
where X 1 c (l,ω), X 2 c (l,ω) represents the discrete windowed Fourier transform at frequency ω, and time-frame index l of the transformed signals x 1 c , x 2 c within time frame c.
10. An apparatus for filtering noise from a mixed sound signal to obtained a filtered target signal, comprising:
a pair of input channels for receiving mixed signals from a pair of microphones;
a pair of Fourier transformers, each receiving a mixed signal from one of said channels and Fourier transforming said mixed signal into a transformed signal in the frequency domain;
a filter, said filter receiving said transformed signals and computing a signal short-time spectral amplitude |Ŝ| and a signal short-time spectral complex exponential e i arg(S) from said transformed signals, where arg(S) is the phase of the target signal in the frequency domain;
Wherein said filter computes said target signal S in the frequency domain from said spectral amplitude and said complex exponential and further comprising a spectral power matrix updater, said updater receiving said transformed signals and computing therefrom a spectral power matrix, and outputting said spectral power matrix to said filter.
11. The apparatus of claim 10 further comprising an inverse Fourier transformer receiving said target signal S in the frequency domain and inverse Fourier transforming said target signal into a filtered target signal s in the time domain.
12. A program storage device readable by machine, tangibly embodying a program of instructions executable by machine to perform method steps for filtering noise from a mixed sound signal to obtained a filtered target signal, said method steps comprising:
inputting the mixed signal through a pair of microphones into a first channel and a second channel;
separately Fourier transforming each said mixed signal into the frequency domain;
computing a signal short-time spectral amplitude |Ŝ| from said transformed signals;
computing a signal short-time spectral complex exponential e i arg(S) from said transformed signals, where arg(S) is the phase of the target signal in the frequency domain;
computing said target signal S in the frequency domain from said spectral amplitude and said complex exponential, further comprising the step of computing a spectral power matrix and using said spectral power matrix to compute said spectral amplitude and said spectral complex exponential.
13. The device of claim 12 wherein said target signal S in the frequency domain is inverse Fourier transformed to produce a filtered target signal s in the time domain.
14. The device of claim 12 wherein said spectral power matrix is computed by spectral channel subtraction.
15. The device of claim 12 wherein said signal short-time spectral amplitude is computed by the estimation equation
S
^
=
E
[
S
|
X
1
,
X
2
]
=
π
2
1
C
1
exp
(
-
C
2
2
8
C
1
)
[
1
+
C
2
2
4
C
1
I
0
(
C
2
2
8
C
1
)
+
C
2
2
4
C
1
I
1
(
C
2
2
8
C
2
)
]
where
I
0
(
z
)
=
1
2
π
∫
0
2
π
exp
(
z
cos
β
)
ⅆ
β
,
I
n
(
1
)
=
1
2
π
∫
0
2
π
cos
(
β
)
exp
(
z
cos
β
)
ⅆ
β
,
C
1
=
1
ρ
s
+
1
det
R
n
(
R
22
+
R
11
K
2
-
K
R
12
-
K
_
R
21
)
,
C
2
=
2
det
R
n
X
_
1
R
22
+
X
_
2
K
R
11
-
X
2
R
12
-
X
1
K
_
R
21
,
X 1 and X 2 are the Fourier transformed first and second signals respectively, R nm are elements of said spectral power matrix, and K is a constant.
16. The device of claim 12 wherein said signal short-time spectral complex exponential is computed by the estimation equation
z
≡
ⅇ
ⅈ
ar
g
^
(
S
)
=
R
22
X
1
+
R
11
K
_
X
2
-
R
21
K
_
X
1
-
R
12
X
2
R
22
X
1
+
R
11
K
_
X
2
-
R
21
K
_
X
1
-
R
12
X
2
.
17. The device of claim 12 wherein said signal short-time spectral complex exponential is computed by the estimation equation
z
≡
ⅇ
ⅈ
ar
g
^
(
S
)
=
R
22
X
1
+
R
11
K
_
X
2
-
R
21
K
_
X
1
-
R
12
X
2
R
22
X
1
+
R
11
K
_
X
2
-
R
21
K
_
X
1
-
R
12
X
2
.
18. The device of claim 17 wherein said target signal S in the frequency domain is computed by the equation
S=zA.
19. The device of claim 12 wherein said target signal is computed by multiplying said signal short-time spectral amplitude by said signal short-time spectral complex exponential.
20. A program storage device readable by machine, tangibly embodying a program of instructions executable by machine to perform method steps for filtering noise from a mixed sound signal to obtained a filtered target signal, said method steps comprising:
inputting the mixed signal through a pair of microphones into a first channel and a second channel;
separately Fourier transforming each said mixed signal into the frequency domain;
computing a signal short-time spectral amplitude |Ŝ| from said transformed signals;
computing a signal short-time spectral complex exponential e i arg(S) from said transformed signals, where arg(S) is the phase of the target signal in the frequency domain;
computing said target signal S in the frequency domain from said spectral amplitude and said complex exponential, further comprising the step of calibrating a function K(ω), said function equal to a ratio of one said Fourier transformed signal to the other, by the estimation equation
K
(
ω
)
=
∑
t
=
1
F
X
2
c
(
l
,
ω
)
X
1
c
(
l
·
ω
)
_
∑
t
=
1
F
X
1
c
(
l
,
ω
)
2
where X 1 c (l,ω), X 2 c (l,ω) represents the discrete windowed Fourier transform at frequency ω, and time-frame index l of the transformed signals x 1 c , x 2 c within time frame c.
21. A program storage device readable by machine, tangibly embodying a program of instructions executable by machine to perform method steps for filtering noise from a mixed sound signal to obtained a filtered target signal, said method steps comprising:
inputting the mixed signal through a pair of microphones into a first channel and a second channel;
separately Fourier transforming each said mixed signal into the frequency domain;
computing a signal short-time spectral amplitude |Ŝ| from said transformed signals;
computing a signal short-time spectral complex exponential e i arg(S) from said transformed signals, where arg(S) is the phase of the target signal in the frequency domain;
computing said target signal S in the frequency domain from said spectral amplitude and said complex exponential, further comprising the step of updating a function K(ω), said function equal to a ratio of one said Fourier transformed signal to the other, said updating effected by using a linear combination between a previous value for K(ω) at a time t−1 and a current value for K(ω) at a time t according to the equation
K t (ω)=(1−α) K t−1 (ω)+α K (ω)
where α is an adaptation rate.