Asymmetric Modulated Filter Bank for Low-Delay Alias Suppression
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Solution Overview
Problem
Existing digital filter banks face challenges in minimizing aliasing artifacts and maintaining low system delay while ensuring near-perfect reconstruction, especially when subband signals are modified, due to limitations in current design methods for cosine and complex-exponential modulated filter banks.
Innovation Solution
The development of an improved alias term minimization (IATM) method for optimizing asymmetric prototype filters in complex-exponential modulated filter banks, which involves determining coefficients to reduce aliasing and pass-band errors, and constructing an M-channel filter bank with a specific design for a 64-channel filter bank with a prototype filter length of 640 coefficients and a system delay of 319 samples.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional cosine or complex-exponential modulated filter banks are used, then the system can process subband signals, but aliasing artifacts appear when subband signals are modified
Solution Approach 1:
The patent employs asymmetric prototype filters instead of symmetric ones. The asymmetric filter design allows for optimized control of aliasing terms while maintaining near-perfect reconstruction properties. The filter coefficients are specifically designed to minimize aliasing artifacts in the passband region, achieving both low aliasing and high reconstruction accuracy simultaneously.
Solution Approach 2:
The patent changes the modulation approach from conventional cosine or complex-exponential modulation to a modified modulation scheme using asymmetric prototypes. The key parameter change is in the filter bank structure itself, using asymmetric filters with specific coefficient designs that inherently suppress aliasing while preserving reconstruction fidelity, even when subband signals are modified.
2Object-affected harmful factors
If filter length is increased to reduce aliasing, then aliasing suppression improves, but system delay increases
Solution Approach 1:
The asymmetric prototype filter design enables more efficient aliasing suppression compared to symmetric filters of the same length. By optimizing the asymmetric filter coefficients specifically for aliasing minimization, the patent achieves superior aliasing suppression with shorter filter lengths, thereby reducing the system delay while maintaining effective aliasing control.
Solution Approach 2:
The patent optimizes the filter bank parameters including the asymmetric prototype filter coefficients, modulation frequencies, and decimation factors to achieve the best trade-off between aliasing suppression and system delay. The specific design parameters are tuned to minimize delay while maintaining effective aliasing rejection, avoiding the need for excessively long filters.
3Reliability
If near-perfect reconstruction is achieved, then reconstruction accuracy improves, but sensitivity to subband modifications increases
Solution Approach 1:
The asymmetric prototype filter design creates a filter bank system that is less sensitive to subband modifications while maintaining near-perfect reconstruction. The asymmetric structure provides better control over the aliasing terms, making the reconstruction process more robust to changes in subband signals such as quantization or modification operations.
Solution Approach 2:
The patent modifies the filter bank parameters and structure to reduce sensitivity to subband changes. By using asymmetric prototypes with optimized coefficients and a specific modulation scheme, the system achieves near-perfect reconstruction that is inherently more robust to subband modifications, reducing the harmful effects of quantization and other processing operations on subband signals.
Data Source
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AI summary
The document relates to modulated sub-sampled digital filter banks, as well as to methods and systems for the design of such filter banks. In particular, the present document proposes a method and apparatus for the improvement of low delay modulated digital filter banks. The method employs modulation of an asymmetric low-pass prototype filter and a new method for optimizing the coefficients of this filter. Further, a specific design for a (64) channel filter bank using a prototype filter length of (640) coefficients and a system delay of (319) samples is given. The method substantially reduces artifacts due to aliasing emerging from independent modifications of subband signals, for example when using a filter bank as a spectral equalizer. The method is preferably implemented in software, running on a standard PC or a digital signal processor (DSP), but can also be hardcoded on a custom chip. The method offers improvements for various types of digital equalizers, adaptive filters, multiband companders and spectral envelope adjusting filterbanks used in high frequency reconstruction (HFR) or parametric stereo systems.