Hearing Aid Filter Bank With Linear Phase and Pure Delay
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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, especially in hearing aids where phase distortion and amplitude distortion are prevalent.
Innovation Solution
A filter bank design with analysis and synthesis filters modulated from linear phase prototype filters, using specific modulation sequences and delays to ensure pure delay transfer functions and minimize aliasing, implemented in a poly-phase DFT-modulated structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional filter banks are used in hearing devices, then computational complexity can be reduced, but phase distortion and amplitude distortion occur
Solution Approach 1:
The patent changes the parameter of filter symmetry by using asymmetric prototype filters instead of symmetric ones. This parameter change enables the filter bank to achieve linear phase response and constant group delay while maintaining low computational complexity through the poly-phase DFT-modulated structure.
Solution Approach 2:
The patent applies asymmetry by designing asymmetric prototype filters where the impulse response is not symmetric around its center. This asymmetric structure, when combined with specific modulation sequences, produces filter bank channels with truly linear phase and constant group delay, resolving the contradiction between simplicity and phase accuracy.
2Manufacturing precision
If filter banks with linear phase are designed, then phase distortion is eliminated, but computational complexity increases
Solution Approach 1:
The patent segments the filter bank into poly-phase components, where each phase corresponds to a specific frequency band. This segmentation allows the use of efficient DFT-modulated structures that achieve linear phase response without requiring computationally intensive operations, thus maintaining low complexity while ensuring phase linearity.
Solution Approach 2:
The patent substitutes traditional time-domain filter design with a frequency-domain approach using DFT modulation. This substitution replaces complex time-domain computations with efficient frequency-domain operations, achieving linear phase characteristics through the mathematical properties of the DFT rather than through complex mechanical or computational structures.
3Measurement precision
If filter length is increased to improve frequency resolution, then band separation improves, but group delay variability increases
Solution Approach 1:
The patent changes the parameter of filter symmetry to asymmetric design, which fundamentally alters the group delay characteristics. This parameter change enables the filters to achieve constant group delay across the passband regardless of filter length, thus improving frequency resolution without introducing group delay variability.
Solution Approach 2:
By employing asymmetric prototype filters with specific modulation sequences, the patent achieves truly linear phase response where the group delay is constant and equal to the filter length parameter. This asymmetric structure decouples the relationship between filter length and group delay variability, allowing independent optimization of frequency resolution and time delay characteristics.
Data Source
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AI summary
The application relates to a filter bank for an audio processing device, e.g. a hearing aid. The filter bank comprises • an analysis filter bank comprising a plurality of M first filters hm(n), where m=0, 1, ..., M-1 is a frequency band index, and whose impulse responses are modulated from a first linear phase prototype filter h(n) with a first predetermined modulation sequence ms1, n being a time index, the first prototype filter h(n) having a first filter length of Lh; • a synthesis filter bank comprising a plurality of M second filters gm(n), m=0, 1, ..., M-1, whose impulse responses are modulated from a second linear phase prototype filter g(n) with a second predetermined modulation sequence ms2, the second prototype filter g(n) having a second filter length of Lg; • the plurality of first and second filters being arranged in pairs, each pair forming a frequency channel. • the first modulation sequence is a complex or real function of time n, frequency band index m, and a first prototype filter delay τh, • the second modulation sequence is a complex or real function of time n, frequency band index m, and a second prototype filter delay τg, • the first filter length Lh and the second filter length Lg are both uneven, and • the first prototype filter delay τh is equal to (Lh-1)/2 and second prototype filter delay τg, is equal to (Lg-1)/2, and the first and second prototype filter delay τh and τg, are constants of the analysis filter bank and the synthesis filter bank, respectively.