Nonlinear Digital Filter to Delay Audible Audio Pre-Responses
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Solution Overview
Problem
Existing audio signal processing techniques 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
Employing a digital non-minimum-phase filter with zeroes outside the unit circle to introduce group delay at frequencies with pre-response energy, transforming pre-responses into less audible post-responses by delaying the time-advance of the impulse response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If linear phase filtering is used to downsample or process audio signals, then frequency response flatness is improved, but pre-responses with time-advance are introduced that are audible and undesirable
Solution Approach 1:
The patent changes the phase parameter of the filter from linear phase to non-linear phase (specifically minimum phase or near-minimum phase). This parameter change allows the filter to maintain frequency response flatness while eliminating the time-advance characteristic that causes audible pre-responses. The non-linear phase filter introduces group delay that delays the pre-response energy into the main impulse response, making it inaudible.
2Duration of action of moving object
If apodising is used to reduce pre-responses in high sample rate systems, then pre-response duration is reduced, but the transition band becomes wider and may affect frequency response
Solution Approach 1:
The patent changes the filter phase parameter to minimum phase, which inherently provides compact impulse response without requiring wide transition bands. This allows steep filtering at high frequencies while maintaining narrow transition bands and flat frequency response in the audible range, avoiding the trade-off present in apodising approaches.
3Measurement precision
If sharp cutoff filtering is applied at Nyquist frequency to maintain frequency response, then frequency precision is improved, but pre-responses are intensified
Solution Approach 1:
The patent changes the filter type from linear phase to minimum phase, which allows sharp cutoff at Nyquist frequency while eliminating the time-advance effect. The minimum phase filter concentrates the impulse response energy at later times, preventing pre-responses even with steep filtering.
4Loss of time
If non-minimum-phase filter with zeroes outside unit circle is used, then group delay at pre-response frequency is increased, but filter complexity increases
Solution Approach 1:
The patent places zeroes of the filter transfer function outside the unit circle in the z-plane, which creates non-minimum-phase behavior. This parameter change in zero location introduces the necessary group delay at pre-response frequencies to eliminate time-advance effects, while the filter order can be kept relatively low.
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.