FIR Filter Cross-Fading for Click-Free Multi-Channel Audio
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Solution Overview
Problem
Existing methods for switching FIR filter coefficients in audio systems cause transient effects like plops, clicks, or noises, leading to DSP overload and inefficient resource utilization, and require additional processing power or memory, which are not feasible in many DSP implementations.
Innovation Solution
A system comprising two side-specific mixing units, FIR filters, and morphing units that allow for seamless coefficient replacement by temporarily using a mixed output signal from one FIR filter while transitioning to new coefficients, ensuring no audible disruptions during the process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If FIR filter coefficients are changed by direct switching, then the coefficient update is simple and fast, but transient effects like plops, clicks, or noises occur at the output
Solution Approach 1:
The system performs preliminary fading out of the current FIR filter output before switching coefficients. This preliminary action prepares the audio signal by gradually reducing the current filter's contribution to zero, preventing abrupt transitions and the associated transient effects when new coefficients are applied.
Solution Approach 2:
A cross-fade mechanism acts as an intermediary between the old and new FIR filter coefficients. During the transition period, the system uses a combination of both filters with time-varying gain factors that smoothly transfer from the old filter to the new filter, eliminating direct switching artifacts.
2Object-affected harmful factors
If coefficients are changed smoothly using small steps, then transient effects become inaudible, but DSP processing power is overloaded during the morphing period
Solution Approach 1:
Instead of performing a complete smooth morphing of all coefficients, the system applies partial action by using a simplified cross-fade approach that achieves the essential goal of eliminating transients while requiring significantly less DSP processing power. The system updates coefficients more frequently than a full smooth morph would require, but with reduced computational complexity.
Solution Approach 2:
The system changes the gain parameter of the FIR filter output over time during the transition period, fading it out from 1.0 to 0.0 while simultaneously fading in the new filter. This parameter change approach allows coefficient switching with minimal transient effects without requiring complex smooth morphing algorithms that would overload the DSP.
3Productivity
If coefficients are updated in real-time tasks, then processing efficiency is maintained, but the entire DSP is overloaded for the duration of morphing
Solution Approach 1:
The coefficient update process is segmented into distinct phases: fading out the current filter, switching coefficients, and fading in the new filter. This segmentation allows the system to perform coefficient updates in a structured manner that maintains real-time processing efficiency while distributing the computational load, preventing DSP overload.
Solution Approach 2:
The system uses periodic coefficient updates with a fixed duration (e.g., 10-20 milliseconds) for the cross-fade transition. This periodic action allows the DSP to predict and prepare for coefficient changes, maintaining processing efficiency by knowing when additional processing will be required without causing unexpected overloads.
4Object-affected harmful factors
If additional memory is allocated for smooth coefficient changing, then transient effects are eliminated, but memory resources are insufficient for DSP implementations
Solution Approach 1:
The system uses the existing FIR filter structure and coefficients to serve the dual purpose of filtering and transition management. By fading out the current filter output and fading in the new filter output using the same filter objects, the system eliminates the need for additional memory buffers that would be required for traditional smooth morphing approaches.
Solution Approach 2:
The FIR filter structure is made universal by using it both for audio processing and for managing the coefficient transition process. The same filter object handles both the filtering function and the cross-fade transition, eliminating the need for separate memory resources dedicated to transition management.
Data Source
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AI summary
The invention relates to stereo or multi-channel audio system configured to process an audio signal including one or more paired two side specific audio channels, which involve FIR filters, for which the coefficients may need to be replaced during the normal operation with the aim of not introducing unpleasant audio effects, distortions or noises. Each side specific audio channel in the pair includes also two side specific mixing units, , and two morphing units. Each side specific mixing unit is configured to receive as input the two side specific audio channels of the pair and to output a mixed output signal mixed on the basis of the two side specific audio channels, each side specific FIR filter is configured to receive as input the mixed output signal of the side specific mixing unit of the corresponding side, and to output a side specific filtered signal. Each morphing unit is configured to receive as input the two side specific filtered signals of the two side specific FIR filters, and to output a side specific morphed output signal, wherein each of the morphing units is configured, during a replacement period, during which filter coefficients of one of the two side specific FIR filters are replaced, to output, as side specific morphed output signal, only the side specific filtered signal received as input from the other filter of the two side specific FIR filters.