Non-Minimum Phase Filter for Independent Amplitude and Phase Control
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Solution Overview
Problem
Conventional ANC filter structures struggle to accurately match damped amplitude responses with undamped phase responses, leading to suboptimal noise cancellation performance, especially at frequencies above 1 kHz where comb filter effects cause notches and phase damping issues.
Innovation Solution
A noise cancellation filter structure is implemented with a non-minimum phase filter, specifically a maximum phase all-pass filter, arranged in parallel with a conventional IIR filter, allowing independent adjustment of amplitude and phase responses to mimic the acoustic domain, thereby achieving a damped notch amplitude response with an undamped phase response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a conventional IIR filter is used to match notch amplitude responses, then amplitude damping is achieved, but phase response becomes damped as well, creating a linear relationship between amplitude and phase damping
Solution Approach 1:
The filter is divided into two independent stages: a minimum phase filter for amplitude response and a maximum phase filter for phase response. This segmentation allows each stage to be optimized independently, breaking the linear coupling between amplitude and phase damping that exists in conventional single-stage IIR filters.
Solution Approach 2:
The patent combines a minimum phase filter and a maximum phase filter into a single noise cancellation filter structure. The minimum phase filter handles amplitude damping while the maximum phase filter handles phase response, merging their functions to achieve independent control over both parameters.
2Reliability
If the notch is tuned to become increasingly damped to match acoustic transfer function, then amplitude response improves, but phase response becomes increasingly damped, degrading noise cancellation performance
Solution Approach 1:
By separating the filter into minimum phase and maximum phase components, the system can match the acoustic transfer function's amplitude response with the minimum phase filter while maintaining an undamped phase response through the maximum phase filter, preventing the degradation that occurs in conventional filters.
Solution Approach 2:
The patent changes the phase response parameter by introducing a maximum phase filter with undamped phase characteristics. This parameter change allows the phase response to remain undamped while the amplitude response is damped, reversing the conventional trade-off and improving overall noise cancellation performance.
3Speed
If conventional ANC filters are designed as minimum phase to minimize delays, then processing speed improves, but the filter cannot match damped amplitude responses with undamped phase responses occurring in acoustic domain
Solution Approach 1:
The filter structure is segmented into minimum phase and maximum phase filters, allowing the system to maintain minimum phase characteristics for fast processing while incorporating maximum phase characteristics to accurately model the acoustic domain's damped amplitude and undamped phase behavior.
Solution Approach 2:
The noise cancellation filter is constructed as a composite structure combining minimum phase and maximum phase filters. This composite approach enables the filter to simultaneously achieve fast processing speeds and accurate acoustic domain matching, overcoming the limitations of conventional minimum phase filters.
Data Source
AI summary
A noise cancellation filter structure for a noise cancellation enabled audio device, in particular headphone (HP), comprises a noise input for receiving a noise signal (N0) and a filter output for providing a filter output signal (OUT). A first noise filter (CF) produces a first filter signal by filtering the noise signal (N0) and a second noise filter (AF) produces a second filter signal by filtering the noise signal (N0). The second noise filter (AF) has a frequency response with a non-minimum-phase, in particular maximum-phase. A combiner (CMB) is configured to provide the filter output signal (OUT) based on a linear combination of the first filter signal and the second filter signal.