Controllable Allpass Filter for Audio Signal Decorrelation
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Solution Overview
Problem
In multichannel adaptive systems, especially when using upmixing algorithms like Logic7 or Dolby Pro Logic, the generated multichannel signals often lack decorrelation, leading to ambiguities in estimating room impulse responses (RIRs), making it difficult to automatically compensate for room changes and maintain a consistent sound impression.
Innovation Solution
A controllable allpass filter arrangement with time-variable filter quality and cut-off frequency is used to decorrelate input signals, employing lattice ladder filters and a filter controller to adjust parameters dynamically, ensuring the signals are sufficiently decorrelated without introducing excessive group delay or acoustic artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If upmixing algorithms are used to generate multichannel signals from stereo input, then the multichannel signal capability is improved, but the decorrelation between channels deteriorates
Solution Approach 1:
An allpass filter arrangement is introduced as an intermediary component between the upmixing algorithm and the multichannel output. This filter arrangement processes the generated multichannel signals to introduce additional phase shifts and temporal modifications, thereby enhancing decorrelation between channels while preserving the multichannel capability provided by the upmixing algorithm.
2Measurement precision
If reference signals are made statistically independent through decorrelation, then the accuracy of room impulse response estimation is improved, but the system complexity increases
Solution Approach 1:
The allpass filter arrangement modifies signal parameters (phase, group delay) rather than fundamentally changing the signal content or requiring complex processing architectures. By adjusting filter coefficients and cutoff frequencies, the system achieves better decorrelation for RIR estimation while maintaining relatively simple system structure compared to alternative approaches.
3Reliability
If allpass filter parameters are made time-variable to enhance decorrelation, then the decorrelation effectiveness is improved, but the computational overhead increases
Solution Approach 1:
The filter arrangement employs time-variable parameters (cutoff frequency, quality factor) that can adapt to changing signal characteristics. This dynamic approach enhances decorrelation effectiveness by adjusting filter behavior over time, while the implementation uses efficient filter structures that minimize computational overhead compared to more complex adaptive filtering methods.
Data Source
AI summary
Decorrelating an input signal includes allpass filtering to phase shift the first input signal by a phase shift, the allpass filtering comprising filtering with one or more subsequent controllable allpass filter stages, each controllable allpass filter stage having a filter quality and a cut-off frequency. Decorrelating further includes controlling at least one of the filter quality and the cut-off frequency of the controllable allpass filter stages to change over time.