ANC Secondary Path Adaptation Under Downlink Tone Detection
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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 ringtones in the downlink audio signal, which cause improper adaptation of adaptive filters and instability in noise-canceling operations.
Innovation Solution
The implementation of a personal audio device with an adaptive noise-canceling (ANC) circuit that includes a tone detector to sequence the adaptation of secondary path and anti-noise signal filters, ensuring stable operation by waiting for non-tone source audio to adapt properly, thereby preventing instability and maintaining effective noise cancellation.
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 electro-acoustic path, but tones in the source audio signal cause improper adaptation and instability in the ANC system
Solution Approach 1:
The tone detection mechanism proactively identifies tones in the source audio signal before they can corrupt the adaptive filter adaptation process. By detecting tones ahead of time and preventing adaptation during tone periods, the system avoids improper filter coefficient updates that would lead to instability, while still maintaining the ability to adapt during clean audio segments.
Solution Approach 2:
The patent introduces an intermediary tone detection and control mechanism that mediates between the source audio signal and the adaptive filter adaptation process. This intermediary layer analyzes the source audio for tones and controls when adaptation should occur, preventing direct adaptation during tone periods while allowing adaptation during clean segments, thus resolving the contradiction between adaptability and stability.
2Productivity
If the adaptive filter adapts during tone presence, then the filter can respond to signal changes, but the noise cancellation effectiveness deteriorates due to improper adaptation
Solution Approach 1:
The adaptive filter operates in periodic cycles, alternating between adaptation phases and hold phases based on tone detection. During clean audio segments, the filter adapts to track path changes; during detected tone segments, adaptation is suspended and the filter holds its previous coefficients. This periodic on/off adaptation pattern maintains noise cancellation effectiveness while still allowing the filter to update when conditions are favorable.
3Measurement precision
If the system allows continuous adaptation of the secondary path filter, then the model remains current with the transducer response, but tones cause the filter to adapt to narrowband characteristics instead of maintaining broadband response
Solution Approach 1:
The tone detection mechanism extracts and identifies tone components from the source audio signal spectrum. By detecting these narrowband tone components and separating them from the broadband audio content, the system can prevent adaptation during tone periods while allowing adaptation during broadband clean segments, thus maintaining the broadband characteristics of the secondary path model.
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.