Forward-Backward Filtering for Precise Non-Zero Phase Shift
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Solution Overview
Problem
Existing audio and video signal processing techniques for achieving a desired non-zero phase shift are limited by computational intensity and accuracy, particularly in multichannel audio matrix coding, where filtering methods either shift front channels' phase or approximate Hilbert transforms with limited precision.
Innovation Solution
A filtering technique involving cascaded forward and backward filters with phase responses functions of frequency, combined with signal segmentation and look-ahead intervals, to achieve precise non-zero phase shifts with reduced computational overhead and minimal phase shift of front channels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If FIR filters are used to approximate the Hilbert transform, then the desired phase shift can be obtained, but the computational complexity increases and accuracy is limited due to truncation
Solution Approach 1:
The patent segments the infinite impulse response into overlapping segments that are processed separately. Each segment is filtered, reversed, and combined with adjacent segments to approximate the Hilbert transform. This segmentation reduces computational complexity by avoiding processing of the entire infinite sequence while maintaining accuracy through overlapping and combining techniques.
Solution Approach 2:
The patent applies filtering in both forward and backward directions. The signal is filtered forward, then the filtered signal is reversed and filtered again in the backward direction. This bidirectional filtering approach allows the system to achieve the desired phase shift with reduced computational requirements compared to traditional unidirectional FIR filters, while maintaining accuracy.
2Measurement precision
If filtering techniques are applied to both surround and front channels, then the relative phase shift of ninety degrees is achieved, but the phase of the front channels is also shifted
Solution Approach 1:
The patent extracts and applies filtering only to the surround channels while leaving the front channels unfiltered. By selectively applying the filtering operation only where needed (to surround channels for matrix coding), the system achieves the desired relative phase shift without inadvertently shifting the phase of the front channels, thus preserving front channel phase integrity.
Solution Approach 2:
The filtering operation is applied locally only to the surround channels rather than uniformly to all channels. This localized application of filtering ensures that the phase shift is introduced only where required for matrix coding purposes, maintaining the original phase characteristics of the front channels while achieving the necessary relative phase relationship.
3Productivity
If the infinite impulse response is truncated, then computational efficiency is improved, but the accuracy of the phase shift approximation deteriorates
Solution Approach 1:
The patent performs preliminary filtering in the forward direction before reversing and filtering in the backward direction. This preliminary action allows the system to capture the essential characteristics of the infinite impulse response in a truncated form, then refine the approximation through the backward filtering process, achieving both computational efficiency and acceptable accuracy.
Solution Approach 2:
The patent uses overlapping segments where portions of the signal are processed multiple times in different contexts. The feedback from adjacent overlapping segments compensates for the truncation effects, allowing the system to achieve higher accuracy without proportionally increasing computational complexity, as the same computational resources are utilized more efficiently across overlapping regions.
Data Source
AI summary
Forward and backward filters in cascade establish a specified phase shift in audio or video signals. The backward filter applies its filtering in a backward direction to impart a phase shift to its backward-filtered output that is a function of frequency. The forward filter applies its filtering in a forward direction to impart a phase shift to its forward-filtered output that has the specified phase shift relative the phase shift of the backward filter. Preferably, the two filters are recursive and are applied to signals that represent overlapping segments of the audio or video information. The overlap interval is used for filter initialization.