Public Address Howling Suppression via Adaptive Filter
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Solution Overview
Problem
The propagation characteristic in public address devices varies over time, making it challenging to accurately estimate and update the filter coefficient for feedback cancellers, which can lead to unstable suppression of howling.
Innovation Solution
A public address device configuration that includes a first adaptive filter to estimate the propagation characteristic from the loudspeaker to the microphone, and a time-varying processing unit to generate the loudspeaker driving signal, ensuring that the filter coefficient is updated to match the varying propagation characteristic.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a feedback canceller is used to suppress echo sound, then howling suppression is achieved, but the filter coefficient becomes inaccurate when propagation characteristic varies over time
Solution Approach 1:
The patent applies dynamics by making the filter coefficient adaptive rather than static. The filter coefficient is continuously updated based on the varying propagation characteristic between loudspeaker and microphone, allowing the feedback canceller to maintain accuracy despite environmental changes over time.
Solution Approach 2:
The patent uses feedback mechanisms where the microphone signal is processed to estimate the propagation characteristic, which then feeds back to update the filter coefficient. This closed-loop approach ensures the filter coefficient remains accurate by continuously adapting to changes in the acoustic environment.
2Reliability
If the filter coefficient is updated frequently to track propagation characteristic changes, then howling suppression stability improves, but computational complexity increases
Solution Approach 1:
The patent applies partial action by updating the filter coefficient only when necessary based on detected changes in propagation characteristic, rather than continuously at maximum frequency. This approach maintains stability while avoiding unnecessary computational overhead.
3Reliability
If a notch filter is used to block echo sound in narrow frequency band, then howling is suppressed at specific frequencies, but gain of public address device drops when blocking bandwidth is widened
Solution Approach 1:
The patent extracts only the harmful echo sound component from the microphone signal using adaptive filtering, rather than blocking entire frequency bands with a notch filter. This selective removal approach suppresses howling while preserving the gain and quality of desired audio frequencies.
Data Source
AI summary
An object is to provide a public address device that effectively prevents occurrence of howling without a drop of the gain. A public address device 100 includes: a loudspeaker 1 that generates a reproduced sound on the basis of a loudspeaker driving signal u(n); a microphone 2 that collects the reproduced sound and an input sound v(n) to generate a microphone-collected-sound signal y(n); a first filter 301 that generates, on the basis of the loudspeaker driving signal u(n), a pseudo echo signal e(n); an echo-cancelling unit 302 that obtains a difference between the microphone-collected-sound signal y(n) and the pseudo echo signal e(n) to generate an echo-cancelled signal d(n); a second filter 311 that whitens the input sound v(n) included in the loudspeaker driving signal u(n); a third filter 312 that whitens the input sound v(n) included in the microphone-collected-sound signal y(n); a first adaptive filter 313 that uses, as a reference signal, an output signal output from the second filter 311, and uses, as a desired signal, an output signal output from the third filter 311, and estimates a propagation characteristic Wo from the loudspeaker 1 to the microphone 2; a unit that repeatedly updates a filter coefficient W of the first filter 301 on the basis of a filter coefficient W identified by the first adaptive filter 313; and a frequency shifting unit 32 that performs a frequency shift on the echo-cancelled signal d(n) to generate the loudspeaker driving signal u(n).