Pre-shaping Filter for Active Noise Cancellation Bandwidth
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Solution Overview
Problem
Active noise cancellation systems in portable audio devices face challenges in effectively reducing noise in low frequency bands due to insufficient frequency precision of adaptive filters and roll-off in earpiece speaker responses below 250Hz, leading to inadequate noise reduction in hostile acoustic environments.
Innovation Solution
Incorporating a non-adaptive digital pre-shaping filter configured to provide additional gain in low frequency bands, implemented as a minimum phase filter with specific gain and phase response characteristics, to enhance the adaptive filter's ability to generate effective anti-noise signals across a broader frequency range without introducing significant delay.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the adaptive filter W uses a limited number of taps (128 taps) to meet device constraints, then device complexity is reduced, but frequency precision in low frequency bands below 400Hz deteriorates
Solution Approach 1:
The filter system is divided into two independent parts: a non-adaptive pre-shaping filter T that handles low-frequency gain enhancement, and an adaptive filter W that handles adaptive noise cancellation. This segmentation allows each filter to be optimized for its specific function, with the pre-shaping filter compensating for the limited precision of the constrained adaptive filter in low frequency bands.
2Productivity
If the adaptive filter W is constrained to 128 taps, then productivity is improved through faster processing, but noise reduction effectiveness in low frequency bands below 250Hz deteriorates
Solution Approach 1:
The pre-shaping filter T performs preliminary action by pre-enhancing low-frequency components of the reference signal before it enters the adaptive filter W. This preliminary gain enhancement ensures that when the adaptive filter processes the signal, the low-frequency noise components are already amplified, allowing the constrained adaptive filter to effectively cancel them despite its limited tap count and processing speed constraints.
3Reliability
If the earpiece speaker operates below 250Hz, then noise cancellation in low frequency bands is attempted, but speaker roll-off reduces the effectiveness of anti-noise signal generation
Solution Approach 1:
The pre-shaping filter T acts as a counterweight to the speaker's roll-off characteristic by providing inverse gain compensation. The filter is designed with a frequency response that boosts low frequencies, effectively counteracting the speaker's natural roll-off below 250Hz. This ensures that the anti-noise signal generated by the adaptive filter maintains sufficient energy and effectiveness in the low-frequency band despite the speaker's limited response.
Data Source
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AI summary
A feed forward active noise cancellation (ANC) system for use in a portable audio device has an adaptive digital filter and a reference microphone. A non-adaptive pre-shaping digital filter has an input coupled to the reference microphone and is in series with, and in front of, the adaptive filter. The pre-shaping filter is minimum phase and presents at least 2 dB more gain over a low audio frequency band than over a high audio frequency band. This may help compensate for low frequency band difficulties, and may thereby extend ANC bandwidth at the low end without a worsening impact on the high end. Other embodiments are also described and claimed.