Adaptive notch filter with variable bandwidth, and method and apparatus for canceling howling by using the adaptive notch filter with variable bandwidth
A variable-bandwidth adaptive notch filter which cancels howling from an input signal with a bandwidth varying according to a howling frequency to generate an output signal.
1. A method of canceling howling, the method comprising canceling howling from an input signal with a bandwidth varying according to a howling frequency to generate an output signal, wherein the cancelling of howling comprises cancelling howling from the input signal with a variable-bandwidth adaptive notch filter,
wherein the variable-bandwidth adaptive notch filter operates in a narrow bandwidth when the howling frequency occurs in a low frequency spectrum, and operates in a broad bandwidth when the howling frequency occurs in a high frequency spectrum.
2. The method of claim 1 , wherein the canceling of howling comprises:
generating a feed-forward signal by using the input signal;
generating a feed-back signal by using the output signal; and
generating the output signal by adding the feed-forward signal and the feed-back signal,
wherein the generating of the feed-forward signal comprises:
applying a frequency warping filter on the input signal from once up to N times, in which N is a natural number greater than 2;
multiplying the input signal and signals to which the frequency warping filter is applied from once up to N times by first through (N+1)th coefficients, respectively; and
adding the coefficient-applied signals to generate the feed-forward signal, and
wherein the generating of the feed-back signal comprises:
applying the frequency warping filter on the output signal from once up to N times;
multiplying the signals to which the frequency warping filter is applied from once up to N times by 1 st through N'th coefficients, respectively; and
adding the coefficient-applied signals to generate the feed-back signal.
3. The method of claim 2 , wherein a transfer function D(z) of the frequency warping filter is given by:
D
(
z
)
=
z
-
1
-
λ
1
-
λ
z
-
1
,
where λ is an adjustment parameter for warping a frequency response and is a positive real number less than or equal to 1.
4. The method of claim 3 , wherein if N is equal to 2,
the multiplying by the first through (N+1) th coefficients comprises multiplying the input signal and the signals to which the frequency warping filter is applied from once up to N times by 1, −a(n), and 1, respectively,
the multiplying by the 1 st through N′ th coefficients comprises multiplying the signals to which the frequency warping filter is applied from once up to N times by a(n)ρ and ρ 2 , respectively, and
a(n) is a coefficient for determining a howling frequency and ρ is a positive real number less than or equal to 1 and is a parameter for determining a howling cancellation frequency bandwidth.
5. The method of claim 4 , further comprising:
generating a signal w 1 and signals w 1 through w m obtained by applying a frequency warping filter to the signal w 0 from once up to M times as the input signal, in which M is a natural number greater than 2; and
obtaining the coefficient a(n) by using an equation as follows:
a
(
n
+
1
)
=
a
(
n
)
+
μ
z
(
n
)
(
x
(
n
-
1
)
-
ρ
y
(
n
-
1
)
)
(
x
(
n
-
1
)
-
ρ
y
(
n
-
1
)
)
2
,
where x(n) indicates the input signal, z(n) indicates the feed-forward signal, and y(n) indicates the output signal.
6. The method of claim 1 , further comprising, before canceling howling from the input signal with a bandwidth varying according to a howling frequency, filtering a signal in a first frequency band from a signal from which howling is to be canceled to generate the input signal.
7. The method of claim 6 , further comprising adding signals remaining in the signal from which howling is to be canceled except for the signal in the first frequency band to the output signal.
8. A method of canceling howling, the method comprising canceling howling from an input signal with a bandwidth varying according to a howling frequency to generate an output signal,
wherein the canceling of howling from the input signal with a bandwidth varying according to a howling frequency comprises:
performing frequency warping on the input signal;
canceling howling in a howling frequency of the frequency-warped signal with the same bandwidth; and
performing frequency dewarping on the howling-canceled signal, and
wherein the canceling of howling with the same bandwidth comprises:
estimating the howling frequency in the frequency-warped signal;
canceling howling in the estimated howling frequency with the same bandwidth;
calculating a difference between an energy of a signal prior to cancellation of howling in the estimated howling frequency and an energy of the howling-canceled signal; and
outputting the howling-canceled signal if the difference exceeds a threshold value.
9. A variable-bandwidth adaptive notch filter which cancels howling from an input signal with a bandwidth varying according to a howling frequency to generate an output signal, wherein the variable-bandwidth adaptive notch filter comprises:
a feed-forward part and
a feed-back part,
wherein the cancelling of howling comprises cancelling howling from the input with a variable-bandwidth adaptive notch filter, and
wherein the variable-bandwidth adaptive notch filter operates in a narrow bandwidth when the howling frequency occurs in a low frequency spectrum, and operates in a broad bandwidth when the howling frequency occurs in a high frequency spectrum.
10. The variable-bandwidth adaptive notch filter of claim 9 , wherein the feed-forward part comprising N frequency warping filters, (N+1) multipliers which apply a variable coefficient to the input signal and N signals which have been frequency-warped from once up to N times, and N adders which add the coefficient-applied signals; and the feed-back part comprising N frequency warping filters, N multipliers which apply a variable coefficient to N signals which have been frequency-warped from once up to the N times, and N adders which add the coefficient-applied signals and the feed-forward signal generated by the feed-forward part, thus generating an output signal.
11. The variable-bandwidth adaptive notch filter of claim 10 , wherein a transfer function D(z) of the frequency warping filter is given by:
D
(
z
)
=
z
-
1
-
λ
1
-
λ
z
-
1
,
where λ is an adjustment parameter for warping a frequency response and is a positive real number less than or equal to 1.
12. The variable-bandwidth adaptive notch filter of claim 11 , wherein if N is equal to 2,
three multipliers included in the feed-forward part apply a coefficient of 1 to the input signal, a coefficient of −a(n) to a signal obtained by applying a frequency warping filter to the input signal once, and a coefficient of 1 to a signal obtained by applying the frequency warping filter to the input signal twice,
two multipliers included in the feed-back part apply a coefficient of a(n)ρ to a signal obtained by applying the frequency warping filter to the output signal once and a coefficient of ρ 2 to a signal obtained by applying the frequency warping filter to the output signal twice, and
a(n) is a coefficient for determining a howling frequency and ρ is a positive real number less than or equal to 1 and is a parameter for determining a howling cancellation frequency bandwidth of the variable-bandwidth adaptive notch filter.
13. The variable-bandwidth adaptive notch filter of claim 12 , wherein a(n) is given by:
a
(
n
+
1
)
=
a
(
n
)
+
μ
z
(
n
)
(
x
(
n
-
1
)
-
ρ
y
(
n
-
1
)
)
(
x
(
n
-
1
)
-
ρ
y
(
n
-
1
)
)
2
,
wherein when a signal w 0 and signals w 1 through w m obtained by applying the frequency warping filter to the signal w 0 from once up to M times are sequentially input to the variable-bandwidth adaptive notch filter as an input signal x(n), z(n) indicates an output signal of the feed-forward part and y(n) indicates an output signal of the variable-bandwidth adaptive notch filter in which M is a natural number greater than 2.
14. An apparatus for canceling howling, the apparatus comprising a variable-bandwidth adaptive notch filter which cancels howling from an input signal with a bandwidth varying according to a howling frequency to generate an output signal, wherein the variable-bandwidth adaptive notch filter comprises:
a feed-forward part; and
a feed-back part, wherein the variable-bandwidth adaptive notch filter cancels howling from the input signal by operating in a narrow bandwidth when the howling frequency occurs in a low frequency spectrum, and operating in a broad bandwidth when the howling frequency occurs in a high frequency spectrum.
15. The apparatus of claim 14 , wherein the feed-forward part comprises N frequency warping filters, (N+1) multipliers which apply a variable coefficient to the input signal and N signals which have been frequency-warped from once up to N times, and N adders which add the coefficient-applied signals, and the feed-back part comprising N frequency warping filters, N multipliers which apply a variable coefficient to N signals which have been frequency-warped from once up to N times, and N adders which add the coefficient-applied signals and the feed-forward signal generated by the feed-forward part, thus generating an output signal.
16. The apparatus of claim 15 , wherein a transfer function D(z) of the frequency warping filter is given by:
D
(
z
)
=
z
-
1
-
λ
1
-
λ
z
-
1
,
where λ is an adjustment parameter for warping a frequency response and is a positive real number less than or equal to 1.
17. The apparatus of claim 16 ,
wherein if N is equal to 2,
three multipliers included in the feed-forward part apply a coefficient of 1 to the input signal, a coefficient of −a(n) to a signal obtained by applying a frequency warping filter to the input signal once, and a coefficient of 1 to a signal obtained by applying the frequency warping filter to the input signal twice,
two multipliers included in the feed-back part apply a coefficient of a(n)ρ to a signal obtained by applying the frequency warping filter to the output signal once and a coefficient of ρ 2 to a signal obtained by applying the frequency warping filter to the output signal twice, and
a(n) is a coefficient for determining a howling frequency and ρ is a positive real number less than or equal to 1 and is a parameter for determining a howling cancellation frequency bandwidth of the variable-bandwidth adaptive notch filter.
18. The apparatus of claim 17 , wherein a(n) is given by:
a
(
n
+
1
)
=
a
(
n
)
+
μ
z
(
n
)
(
x
(
n
-
1
)
-
ρ
y
(
n
-
1
)
)
(
x
(
n
-
1
)
-
ρ
y
(
n
-
1
)
)
2
,
wherein when a signal w 0 and signals w 1 through w m obtained by applying the frequency warping filter to the signal w o from once up to M times are sequentially input to the variable-bandwidth adaptive notch filter as an input signal x(n), z(n) indicates an output signal of the feed-forward part and y(n) indicates an output signal of the variable-bandwidth adaptive notch filter in which M is a natural number greater than 2.
19. The apparatus of claim 17 , further comprising a howling determining unit for determining whether howling is generated,
wherein the variable-bandwidth adaptive notch filtering unit estimates the howling frequency in the frequency-warped signal and cancels howling in the estimated howling frequency with the same bandwidth, and
the howling determining unit calculates a difference between an energy of a signal prior to howling cancellation in the estimated howling frequency and an energy of the howling-canceled signal, controls the variable-bandwidth adaptive notch filtering unit to output the howling-canceled signal if the difference exceeds a threshold value, and controls the variable-bandwidth adaptive notch filtering unit to output the signal prior to howling cancellation if the difference does not exceed the threshold value.
20. The apparatus of claim 14 , further comprising a howling frequency range setting unit which filters a signal in a first frequency band from a signal from which howling is to be canceled to generate the input signal.
21. The apparatus of claim 20 , further comprising an adder which adds signals remaining in the signal from which howling is to be canceled, except for the signal in the first frequency band to the output signal.
22. The apparatus of claim 14 , wherein the variable-bandwidth adaptive notch filtering unit performs frequency warping on the input signal, cancels howling in a howling frequency of the frequency-warped signal with the same bandwidth, and performs frequency dewarping on the howling-canceled signal.
23. A non-transitory computer-readable recording medium having recorded thereon a program for executing a method of canceling howling, the method comprising canceling howling from an input signal with a bandwidth varying according to a howling frequency to generate an output signal, wherein the cancelling of howling comprises cancelling howling from the input signal with a variable-bandwidth adaptive notch filter,
wherein the variable-bandwidth adaptive notch filter operates in a narrow bandwidth when the howling frequency occurs in a low frequency spectrum, and operates in a broad bandwidth when the howling frequency occurs in a high frequency spectrum.