Multi-band noise reduction system and methodology for digital audio signals
The present invention relates to a multi-band noise reduction system for digital audio signals producing a noise reduced digital audio output signal from a digital audio signal. The digital audio signal comprises a target signal and a noise signal, i.e. a noisy digital audio signal. The multi-band noise reduction system operates on a plurality of sub-band signals derived from the digital audio signal and comprises a second or adaptive signal-to-noise ratio estimator which is configured for filtering a plurality of first signal-to-noise ratio estimates of the plurality of sub-band signals with respective time-varying low-pass filters to produce respective second signal-to-noise ratio estimates of the plurality of sub-band signals. A low-pass cut-off frequency of each of the time-varying low-pass filters is adaptable in accordance with a first signal-to-noise ratio estimate determined by a first signal-to-noise ratio estimator and/or the second signal-to-noise ratio estimate of the sub-band signal.
1. A hearing instrument comprising:
a microphone arrangement for picking-up acoustic signals from the surrounding environment and generating one or more microphone signals in response; and
a multi-band noise reduction system for digital audio signals comprising:
a signal input for receipt of a digital audio input signal originating from the one or more microphone signals, an analysis filter bank configured for dividing the digital audio input signal into a plurality of sub-band signals Y k (n),
a noise estimator configured for determining respective sub-band noise estimates {circumflex over (σ)} k 2 (n) of the plurality of sub-band signals Y k (n),
a first signal-to-noise ratio estimator configured for determining respective first signal-to-noise ratio estimates ξ k 0 (n) of the plurality of sub-band signals based on the respective sub-band noise estimation signals and the respective sub-band signals Y k (n),
a second signal-to-noise ratio estimator configured for filtering the plurality of first signal-to-noise ratio estimates ξ k 0 (n) of the plurality of sub-band signals Y k (n) with respective time-varying low-pass filters to produce respective second signal-to-noise ratio estimates ζ k (n) of the plurality of sub-band signals Y k (n) wherein a low-pass cut-off frequency of each of the time-varying low-pass filters is adaptable in accordance with the first signal-to-noise ratio estimate of the sub-band signal or the second signal-to-noise ratio estimate of the sub-band signal,
a gain calculator configured for applying respective time-varying gains G k (n) to the plurality of sub-band signals Y k (n) based on the respective second signal-to-noise ratio estimates ζ k (n) and respective sub-band gain laws to produce a plurality of noise compensated sub-band signals, and
a synthesis filter bank configured to combine the plurality of noise compensated sub-band signals into a noise reduced digital audio output signal at a signal output.
2. A hearing instrument according to claim 1 , wherein the microphone arrangement is configured to perform a beamforming operation on the two or more microphone signals to supply a directional microphone signal.
3. A hearing instrument according to claim 1 , wherein the second signal-to-noise ratio estimator of the multi-band noise reduction system is configured to, for each of the plurality of sub-band signals Y k (n), increase the low-pass cut-off frequency of the time-varying low-pass filter with increasing values of the first and/or second signal-to-noise ratio estimates of the sub-band signal.
4. A hearing instrument according to claim 3 , wherein the low-pass cut-off frequency of the time-varying low-pass filter is larger than 50 Hz if the second signal-to-noise ratio estimate of the sub-band signal is larger than 5 dB.
5. A hearing instrument according to claim 3 , wherein the low-pass cut-off frequency of the time-varying low-pass filter is larger than 200 Hz if the second signal-to-noise ratio estimate of the sub-band signal is larger than 8 dB.
6. A hearing instrument according to claim 3 , wherein the low-pass cut-off frequency of the time-varying low-pass filter is smaller than 1 Hz at negative values of the second signal-to-noise ratio estimate of the sub-band signal.
7. A hearing instrument according to claim 3 , wherein the low-pass cut-off frequency of the time-varying low-pass filter is smaller than 5 Hz, or 2 Hz, at signal-to-noise ratio estimates of the sub-band signal smaller than minus 5 dB.
8. A hearing instrument according to claim 1 , wherein each of the plurality of time-varying low-pass filters of the multi-band noise reduction system comprises an IIR filter structure wherein an input of the IIR filter structure receives the first signal-to-noise ratio estimate and an output of the IIR filter structure in response supplies the second signal-to-noise ratio estimate.
9. A hearing instrument according to claim 8 , wherein the IIR filter structure comprises:
a first input summing node ( 205 ) configured for receipt of the first signal-to-noise ratio estimate;
an output node supplying the second signal-to-noise ratio estimate;
a unit delay function coupled to the output node and configured to supply a delayed second signal-to-noise ratio estimate to the first input summing node, the input summing node configured to combine an output signal of the first input summing node and the delayed second signal-to-noise ratio estimate to generate a first intermediate signal;
a multiplication function configured to multiply the first intermediate signal and a limited delayed second signal-to-noise ratio estimate to generate a second intermediate signal;
a first intermediate summing node configured to combine the second intermediate signal and the delayed second signal-to-noise ratio estimate; and
a maximum operator configured to:
at a first input, receive the delayed second signal-to-noise ratio estimate and at a second input, receive the first signal to noise-ratio estimate or a look-ahead estimate of the first signal to noise-ratio estimate, and
generate a maximum signal-to-noise ratio estimate from the first and second inputs; and
a first feedback path configured to couple a first time-varying portion of the maximum signal-to-noise ratio estimate to the multiplication function by a time-varying transfer coefficient of a first monotonic function in accordance with the first signal-to-noise ratio estimate of the sub-band signal.
10. A hearing instrument according to claim 9 , wherein the first monotonic function of the IIR filter structure comprises a logistic function:
f
(
x
)
=
f
0
+
1
-
f
0
1
+
exp
(
-
4
a
(
x
-
x
f
,
0
)
)
;
wherein
f 0 =offset constant,
α=maximum slope parameter.
11. A hearing instrument according to claim 10 , wherein the second signal-to-noise ratio estimator further comprises a sound environment adjustment value e k (n) which is added to the maximum signal-to-noise ratio estimate; and
said sound environment adjustment value indicating speech modulation in the digital audio input signal.
12. A hearing instrument according to claim 1 , wherein the multi-band noise reduction system comprises:
a monotonic compressive function C(x) arranged in front of the second signal-to-noise ratio estimator and configured for mapping a numerical range of each of the plurality of first signal-to-noise ratio estimates ξ k 0 (n) into a smaller output numerical range before application to the second signal-to-noise ratio estimator; and
a monotonic expansive function C −1 (x), possessing an inverse transfer characteristic of the monotonic compressive function, arranged after the second signal-to-noise ratio estimator and configured for mapping a numerical range of each of the plurality of second signal-to-noise ratio estimates ζ k (n) into a larger output numerical range before application to the gain calculator, wherein said monotonic compressive function C(x) comprises a non-logarithmic function such as:
C ( x )=10 P ( x 1/P −1)/log 10, where P >1 and is a positive real number.
13. A hearing instrument according to claim 1 , wherein the gain calculator of the multi-band noise reduction system is configured for computing the respective time-varying gains G k (n) of the plurality of sub-band signals Y k (n) according to:
G
k
(
n
)
=
max
(
G
m
i
n
,
ξ
k
(
n
)
ξ
k
(
n
)
+
1
)
;
wherein
G min is a predetermined minimum gain value.
14. A hearing instrument according to claim 13 , wherein G min lies between 0.01 and 0.1.
15. A hearing instrument according to claim 1 , wherein the first signal-to-noise ratio estimator of the multi-band noise reduction system comprises a bounded maximum likelihood estimate of the power ratio between target speech signal and a noise signal:
ξ
k
ML
(
n
)
=
max
(
ξ
m
i
n
ML
,
Y
k
(
n
)
2
σ
^
k
2
(
n
)
-
1
)
(
1
)
where the function max(a,b) selects the larger one of the numbers a and b, and ξ min ML is a positive lower bound such as a value between 0.01 and 0.05.
16. A hearing instrument according to claim 1 , wherein the multi-band noise reduction system comprises and look-ahead function for supplying a look-ahead signal-to-noise ratio estimate l k (n) to the second signal-to-noise ratio estimator.
17. A hearing instrument according to claim 16 , wherein the look-ahead function comprises a look-ahead processor and tapped delay line of unit delay elements;
wherein the tapped delay line comprises a plurality intermediate signal nodes between each pair of neighbouring unit delay elements; and
wherein said look-ahead processor is configured to compare inputs values from the plurality intermediate signal nodes and select a maximum of the input values as output.
18. A hearing instrument according to claim 1 , wherein the analysis filter bank of the multi-band noise reduction system comprises a block-based FFT algorithm or Discrete Fourier Transform (DFT).
19. A hearing instrument according to claim 1 , wherein of the analysis filter bank of the multi-band noise reduction system comprises a time domain filter bank including a ⅓ octave filter bank or a Bark scale filter bank.
20. A hearing instrument according to claim 1 , wherein of the analysis filter bank of the multi-band noise reduction system comprises between 16 and 128 frequency bands.
21. A method of reducing noise of a digital audio signal originating from one or more microphone signals of a hearing instrument, said method comprising steps of:
a) dividing or splitting the digital audio input signal into a plurality of sub-band signals Y k (n);
b) determining respective sub-band noise estimates {circumflex over (σ)} k 2 (n) the plurality of sub-band signals Y k (n);
c) determining respective first signal-to-noise ratio estimates ξ k 0 (n) of the plurality of sub-band signals based on the respective sub-band noise estimation signals and the respective sub-band signals Y k (n);
d) filtering the plurality of first signal-to-noise ratio estimates ξ k 0 (n) of the plurality of sub-band signals Y k (n) with respective time-varying low-pass filters to produce respective second signal-to-noise ratio estimates ζ k (n) of the plurality of sub-band signals Y k (n) wherein a low-pass cut-off frequency of each of the time-varying filters is adapted in accordance with the first signal-to-noise ratio estimate of the sub-band signal;
e) applying respective time-varying gains G k (n) to the plurality of sub-band signals Y k (n) based on the respective second signal-to-noise ratio estimates ζ k (n) and respective sub-band gain laws to produce a plurality of noise compensated sub-band signals; and
f) combining the plurality of noise compensated sub-band signals into a noise reduced digital audio output signal at a signal output.
22. A method of reducing noise of a digital audio input signal according to claim 21 , comprising further steps of:
before step d) mapping a numerical range of each of the plurality of first signal-to-noise ratio estimates ξ k 0 (n) into a smaller output numerical range in accordance with a monotonic compressive function; and
before step e) mapping a numerical range of each of the plurality of second signal-to-noise ratio estimates ζ k (n) into a larger output numerical range in accordance with a monotonic expansive function possessing an inverse transfer characteristic of the monotonic compressive function.
23. A method of reducing noise of a digital audio input signal according to claim 22 wherein said monotonic compressive function C(x) comprises a non-logarithmic function such as:
C ( x )=10 P ( x 1/P −1)/log 10, where P >1 and is a positive real number.
24. A multi-band noise reduction system for noisy digital audio signals, comprising:
an analysis filter bank configured for dividing the noisy digital audio input signal into a plurality of sub-band signals;
a noise estimator configured for determining respective sub-band noise estimates of the plurality of sub-band signals;
a first signal-to-noise ratio estimator configured for determining respective first signal-to-noise ratio estimates of the plurality of sub-band signals; and
a second signal-to-noise ratio estimator configured for filtering the plurality of first signal-to-noise ratio estimates by respective time-varying lowpass filters to produce respective second signal-to-noise ratio estimates of the plurality of sub-band signals, wherein a lowpass cut-off frequency of each lowpass filter of the plurality of time-varying lowpass filters is adaptable in accordance with the second signal-to-noise ratio estimate of the corresponding sub-band signal by increasing the cut-off frequency of the lowpass filter for increasing values of the second signal-to-noise ratio estimate of the sub-band signal.