Adaptive Noise Canceling Secondary Path Estimation
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Solution Overview
Problem
Personal audio devices, such as wireless telephones, face challenges in maintaining effective noise cancellation when source audio is absent or low in amplitude, as the secondary path estimate cannot typically be updated in these conditions.
Innovation Solution
Incorporating an adaptive noise-canceling (ANC) circuit that injects noise at a level below the source audio, allowing continuous adaptation of the secondary path estimate, even when source audio is absent or low, using a reference microphone to measure ambient noise and an error microphone to control the adaptation of the anti-noise signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If source audio is absent or low in amplitude, then power consumption is reduced and listening comfort is improved, but the secondary path estimate cannot be updated and noise cancellation effectiveness deteriorates
Solution Approach 1:
The system performs preliminary action by injecting test noise signals into the transducer even when source audio is absent or low. This allows the secondary path estimate to be updated in advance, ensuring the adaptive filter remains accurate and ready for when source audio returns, thus maintaining noise cancellation effectiveness without requiring continuous source audio
Solution Approach 2:
An intermediary test noise signal is introduced between the available components (signal generator, transducer, error microphone) to enable secondary path estimation. This intermediary signal serves as a mediator that allows the system to measure and adapt the secondary path without relying on source audio, resolving the contradiction between maintaining adaptation capability and preserving low-power operation
2Measurement precision
If injected noise level is increased to improve secondary path estimation, then adaptation accuracy improves, but the injected noise becomes noticeable and listening comfort deteriorates
Solution Approach 1:
The system applies partial action by injecting noise at a level that is sufficient for secondary path estimation but deliberately kept below the threshold of human perception. This partial injection provides enough signal energy for the adaptive filter to converge and maintain accurate estimation without exceeding the acceptable noise floor that would degrade listening comfort
Solution Approach 2:
The system dynamically adjusts the injected noise parameters (amplitude, frequency content, temporal distribution) to optimize the trade-off between estimation accuracy and perceptibility. By changing these parameters, the system achieves sufficient measurement precision for secondary path adaptation while keeping the injected noise unnoticeable to the listener
Data Source
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AI summary
A personal audio device, such as a wireless telephone, includes an adaptive noise canceling (ANC) circuit that adaptively generates an anti-noise signal from a reference microphone signal and injects the anti-noise signal into the speaker or other transducer output to cause cancellation of ambient audio sounds. An error microphone is also provided proximate the speaker to provide an error signal indicative of the effectiveness of the noise cancellation. A secondary path estimating adaptive filter is used to estimate the electro- acoustical path from the noise canceling circuit through the transducer so that source audio can be removed from the error signal. Noise is injected either continuously and inaudibly below the source audio, or in response to detection that the source audio is low in amplitude, so that the adaptation of the secondary path estimating adaptive filter can be maintained, irrespective of the presence and amplitude of the source audio.