Multi-band noise reduction system and methodology for digital audio signals

A hearing instrument according to claim1, 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 ξk0(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 ζk(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)=10P(x1/P−1)/log 10, where P>1 and is a positive real number.

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 Yk(n);
• b) determining respective sub-band noise estimates {circumflex over (σ)}k2(n) the plurality of sub-band signals Yk(n);
• c) determining respective first signal-to-noise ratio estimates ξk0(n) of the plurality of sub-band signals based on the respective sub-band noise estimation signals and the respective sub-band signals Yk(n);
• d) filtering the plurality of first signal-to-noise ratio estimates ξk0(n) of the plurality of sub-band signals Yk(n) with respective time-varying low-pass filters to produce respective second signal-to-noise ratio estimates ζk(n) of the plurality of sub-band signals Yk(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 Gk(n) to the plurality of sub-band signals Yk(n) based on the respective second signal-to-noise ratio estimates ζk(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.

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.