ANC Secondary Path Adaptation Sequencing for Downlink Tone Stability
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Solution Overview
Problem
Existing personal audio devices, such as wireless telephones, face challenges in maintaining effective noise cancellation due to tones like downlink ringtones in the source audio signal, which cause improper adaptation of secondary path adaptive filters and instability in noise-canceling operations.
Innovation Solution
Incorporating a tone detection mechanism within the ANC processing circuit that sequences the adaptation of secondary path and anti-noise signal filters to prevent instability, ensuring stable operation by waiting for non-tone source audio to adapt the filters properly, and using a reference microphone to measure ambient noise and an error microphone to assess transducer output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the secondary path adaptive filter continuously adapts to the source audio signal, then the filter can track changes in the audio path, but tones in the source audio cause improper adaptation and instability
Solution Approach 1:
A tone detection mechanism is introduced as an intermediary between the source audio signal and the adaptive filter. This detector identifies tone components in the source audio and generates control signals to prevent the adaptive filter from adapting during tone periods, thereby mediating between the need for continuous adaptation and the need to avoid tone-induced instability
Solution Approach 2:
The system employs feedback through the error microphone to monitor the output of the transducer. The error signal is used to continuously update the secondary path adaptive filter, but this feedback loop is controlled by the tone detection mechanism to pause adaptation when tones are detected, preventing improper adaptation while maintaining stability
2Productivity
If the adaptive filter adapts during tone presence, then the filter can respond to signal changes, but the overall ANC operation becomes unstable
Solution Approach 1:
The adaptive filter's behavior is made dynamic by switching between adaptation and non-adaptation states based on tone detection. The filter continuously updates its coefficients when no tones are present, but freezes coefficient updates when tones are detected, creating a dynamic system that adapts its behavior to maintain stability while responding to signal changes
3Measurement precision
If the secondary path estimate is continuously updated, then the noise canceling effectiveness is improved, but tones cause improper operation of the adaptive filter
Solution Approach 1:
The tone detection mechanism performs preliminary analysis of the source audio signal to identify tone components before they can corrupt the adaptive filter adaptation process. By detecting tones in advance and preventing adaptation during their presence, the system protects the secondary path estimation from improper updates caused by tone interference
Data Source
AI summary
An adaptive noise canceling (ANC) circuit adaptively generates an anti-noise signal from a reference microphone signal that is injected into the speaker or other transducer output to cause cancellation of ambient audio sounds. An error microphone proximate the speaker provides an error signal. A secondary path estimating adaptive filter estimates the electro-acoustical path from the noise canceling circuit through the transducer so that source audio can be removed from the error signal. Tones in the source audio, such as remote ringtones, present in downlink audio during initiation of a telephone call, are detected by a tone detector using accumulated tone persistence and non-silence hangover counting, and adaptation of the secondary path estimating adaptive filter is halted to prevent adapting to the tones. Adaptation of the adaptive filters is then sequenced so any disruption of the secondary path adaptive filter response is removed before allowing the anti-noise generating filter to adapt.