Complex Modulated Filterbank for Shorter Subband Audio Filters
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Solution Overview
Problem
Existing filter banks in audio signal processing, particularly for HRTF applications, face challenges in achieving sufficient frequency resolution and efficient manipulation of time domain signals, with current methods often resulting in poor audio quality due to insufficient phase characteristics and computational inefficiencies.
Innovation Solution
A filter converter system that converts a given filter's impulse response into subband domain filters using a complex modulated filter bank, allowing for efficient filtering and manipulation of audio signals by downsampling and upsampling processes, reducing computational complexity and improving audio quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a complex-exponential modulated filter bank is used for spectral envelope adjustment, then frequency resolution is enhanced, but computational complexity increases
Solution Approach 1:
The audio signal is divided into multiple frequency subbands using a complex-exponential modulated filter bank. Each subband is processed independently with shorter filters, achieving high frequency resolution while reducing the computational burden on each individual filter operation.
Solution Approach 2:
The patent transforms the filtering operation from the time domain to the frequency domain using a complex modulated filter bank. This dimensional transformation allows for more efficient processing by operating on frequency coefficients rather than time-domain samples, reducing overall computational complexity.
2Productivity
If simple gain modifications are applied in each subband, then processing efficiency is improved, but audio quality deteriorates due to insufficient phase characteristics
Solution Approach 1:
The patent applies different processing strategies to different parts of the frequency spectrum. In subbands where phase characteristics are critical for audio quality, full complex filtering is applied rather than simple gain modification. This local adaptation ensures high audio quality in critical regions while maintaining processing efficiency in less critical regions.
Solution Approach 2:
The patent modifies the filtering approach by changing from simple real-valued gain modifications to complex-valued filtering operations in specific subbands. This parameter change introduces proper phase characteristics where needed, improving audio quality while maintaining the efficiency benefits of subband processing.
3Device complexity
If HRTF filtering is performed in the subband domain, then computational complexity is reduced, but filter conversion complexity increases
Solution Approach 1:
The patent pre-computes and stores the filter conversion matrices that map from time-domain HRTF filters to subband-domain filters. This preliminary action allows the complex conversion operation to be performed once offline, rather than in real-time during audio processing, reducing the computational burden during actual operation.
Solution Approach 2:
The patent creates a copied representation of the HRTF filters in the subband domain through a systematic transformation process. Instead of repeatedly performing complex convolutions in the time domain, the filters are copied and transformed into the subband domain once, enabling efficient repeated use during audio rendering.
Data Source
Figure 1a~1c
Figure 1d~1e
Figure 2~4
AI summary
A filter converter (104) for converting an impulse response signal indicative of an amplitude/frequency filter characteristic in a time domain into filter definition signal, the converter comprising a complex modulated filter bank (301) for filtering the impulse response signal to obtain a plurality of complex valued subband signals forming the filter definition signal, wherein each complex valued subband signal corresponds to an impulse response for a subband filter (102). At least one complex subband signal comprises at least two different non-vanishing values and each complex valued subband signal is shorter than the impulse response signal. The impulse response signal is a time domain FIR (Finite Impulse Response) filter. The complex modulated filter bank (301) is adapted to output L complex valued subband signals and the complex modulated filter bank (301) is adapted for providing the L complex valued subband signals downsampled by a factor L.