Hearing Aid Filter Bank With Linear Phase and Low Distortion
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Solution Overview
Problem
Existing filter banks in hearing devices face challenges in achieving truly linear phase, constant group delay, and minimal distortion while maintaining low computational complexity, which affects the quality of signal processing in hearing aids.
Innovation Solution
A filter bank design with analysis and synthesis filters modulated by linear phase prototype filters, using specific modulation sequences and delays to ensure linear phase and minimal distortion, and employing a poly-phase DFT-modulated structure for efficient implementation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional filter banks are used in hearing devices, then signal processing can be performed in frequency bands, but phase distortion and non-linear group delay occur
Solution Approach 1:
The patent changes the fundamental parameters of the filter bank design by using modulated filter structures with specific modulation sequences (cosine or sine modulation) and carefully selected filter lengths. This transforms the phase characteristics to achieve linear phase response while maintaining frequency band separation capabilities essential for hearing device signal processing.
Solution Approach 2:
The filter bank is segmented into multiple frequency channels with analysis filters and synthesis filters for each channel. Each channel processes specific frequency bands independently, allowing the overall system to maintain linear phase characteristics while providing the frequency band processing capability needed for spatial filtering, noise reduction, and dynamic range compression.
2Measurement precision
If filter banks with high frequency resolution are designed, then band overlap and separation improve, but computational complexity increases
Solution Approach 1:
The patent applies preliminary modulation to the filter impulse responses using predetermined modulation sequences before the filtering operation. This pre-modulation approach allows the filter bank to achieve high frequency resolution with controlled computational complexity by embedding the frequency selection characteristics directly into the filter structure rather than requiring complex post-processing operations.
Solution Approach 2:
The modulated filter bank structure serves multiple functions simultaneously: it provides frequency band separation, achieves linear phase response, and maintains computational efficiency. The same filter bank structure supports various signal processing operations including spatial filtering, noise reduction, and dynamic range compression without requiring separate dedicated structures for each function.
3Manufacturing precision
If filter lengths are increased to reduce distortion, then amplitude distortion and aliasing decrease, but group delay variability increases
Solution Approach 1:
The patent employs asymmetric filter designs where the analysis filters and synthesis filters have different characteristics. The analysis filters use one modulation sequence while the synthesis filters use a complementary modulation sequence, creating an asymmetric but coordinated system that achieves both low distortion and constant group delay through the complementary nature of the asymmetric design.
Solution Approach 2:
The filter bank design incorporates implicit feedback through the synthesis stage that compensates for distortions introduced in the analysis stage. The synthesis filters are designed to reverse the effects of the analysis filters, creating a feedback loop that cancels out amplitude distortion and aliasing while maintaining constant group delay through the coordinated modulation sequences.
Data Source
AI summary
The application relates to a filter bank for an audio processing device, e.g. a hearing aid. The filter bank comprisesan analysis filter bank comprising a plurality of M first filters hm(n), where m=0, 1, . . . , M−1 is a frequency band index, n being a time index, the first filters hm(n) having a first filter length of Lhm;a synthesis filter bank comprising a plurality of M second filters gm(n), m=0, 1, . . . , M−1, the second filters gm(n) having second filter lengths of Lgm;the plurality of first and second filters being arranged in pairs, each pair forming a frequency channel.the first filters hm(n) exhibiting a first filter delay τh,the second filters gm(n) exhibiting a second filter delay τg,each of the first filter lengths Lhm and the second filter lengths Lgm is uneven, andwherein the first filters are subject to the constraint that the sum of the first filters hm(n) of the analysis filter bank is a delta function δ(n−τh).