Non-Minimum-Phase Audio Filter for Pre-Response Delay
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Solution Overview
Problem
Existing audio signal processing technologies fail to effectively minimize the audible effects of pre-responses, particularly at lower sampling rates like 44.1 kHz, where linear phase filtering introduces undesirable ringing and time-advance issues.
Innovation Solution
Implementing a digital non-minimum-phase filter with zeros outside the unit circle to introduce group delay at pre-response frequencies, reducing the audibility of pre-responses by delaying them relative to the main impulse response, potentially transforming pre-responses into less audible post-responses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If linear phase filtering is used to process audio signals, then frequency response accuracy is improved, but pre-response artifacts are introduced that degrade sound quality
Solution Approach 1:
The patent changes the phase response parameter from linear phase to minimum phase while maintaining the magnitude response. This is achieved by applying a minimum-phase transformation to the filter coefficients, which redistributes the energy in the impulse response to eliminate pre-response artifacts while preserving the frequency response accuracy.
2Speed
If sampling rate is increased to 96 kHz or 192 kHz to extend audio bandwidth, then potential frequency range is improved, but pre-response issues persist and may become more pronounced
Solution Approach 1:
The patent applies minimum-phase filtering at high sampling rates (96 kHz, 192 kHz) to transform the impulse response characteristics. This maintains the extended bandwidth benefits while eliminating the pre-response artifacts that typically accompany high-rate sampling and steep filtering.
3Reliability
If antialias filtering with sharp cutoff is used at 44.1 kHz sampling rate, then aliasing is reduced, but pre-response ringing is introduced in the audible range
Solution Approach 1:
The patent transforms the antialias filter from linear phase to minimum phase, which concentrates the filter's energy in the post-response region rather than creating pre-response ringing. This maintains effective aliasing rejection while eliminating audible pre-response artifacts in the 20 Hz to 20 kHz range.
4Object-generated harmful factors
If apodising is used to reduce pre-response at high sample rates, then transition band is widened, but filter complexity and processing requirements increase
Solution Approach 1:
The patent replaces the complex apodising approach with a minimum-phase transformation of existing filter coefficients. This substitution achieves pre-response elimination through a mathematical transformation rather than requiring a completely different filter design, thereby reducing complexity while maintaining effectiveness.
Data Source
AI summary
Methods and devices are described for reducing the audible effect of pre-responses in an audio signal. The pre-responses are effectively delayed by employing a digital non-minimum-phase filter, which includes a zero lying outside the unit circle in its z-transform response. This zero is not paired with another zero at a reciprocal position inside the unit circle, as this would linearise the phase modification. The filtering can introduce a greater group delay at the pre-response frequency than at a low frequency, such as 500 Hz or even 0 Hz. The technique can be used to reduce pre-responses in an existing audio signal and also to pre-empt pre-responses that would be introduced to the audio signal by subsequent processing.