Negative Group Delay Audio Filters for Low-Latency Noise Reduction
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Solution Overview
Problem
Existing audio filtering systems introduce undesirable delay when attempting to reduce out-of-band noise, which is detrimental to low-latency and high dynamic range requirements in personal audio devices.
Innovation Solution
The implementation of a system with a filter having a negative group delay within human-audible frequencies, combined with modulators and detectors to dynamically adjust filter modes based on signal conditions, allowing for noise filtering without added latency through pre-emphasis and de-emphasis techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If out-of-band filtering is applied to reduce out-of-band noise, then power dissipation is reduced and dynamic range is improved, but latency increases due to filter delay
Solution Approach 1:
The patent applies pre-emphasis filtering before the digital interface to boost out-of-band noise components, and then applies de-emphasis filtering after the digital interface to remove them. This preliminary action of boosting noise before the interface allows the subsequent de-emphasis filter to effectively remove out-of-band noise without introducing significant latency, as the critical filtering occurs after the interface where latency is already incurred.
Solution Approach 2:
The digital interface acts as an intermediary that introduces latency. By placing pre-emphasis before the interface and de-emphasis after the interface, the patent uses the interface as a mediator to separate the noise boosting function from the noise removal function, allowing the critical de-emphasis filtering to occur after the latency-inducing interface without compounding the delay.
2Object-generated harmful factors
If aggressive out-of-band filtering is applied to eliminate ultrasonic noise, then spectral folding into audible band is reduced, but filter delay increases
Solution Approach 1:
The pre-emphasis filter applies a negative group delay to boost out-of-band noise components before they reach the digital interface. This preliminary action prepares the signal so that the subsequent de-emphasis filter can aggressively remove out-of-band noise without introducing additional delay, as the negative group delay from pre-emphasis compensates for the delay that would otherwise be introduced by aggressive filtering.
Solution Approach 2:
The patent changes the group delay parameter of the filter by using a pre-emphasis filter with negative group delay followed by a de-emphasis filter with positive group delay. This parameter change allows the overall system to achieve aggressive out-of-band noise removal while maintaining low or zero net delay, as the negative group delay compensates for the positive group delay of the aggressive de-emphasis filter.
3Object-generated harmful factors
If filter order is increased to improve out-of-band rejection, then noise filtering performance is improved, but latency and complexity increase
Solution Approach 1:
The patent segments the filtering function into two separate filters: a pre-emphasis filter with negative group delay and a de-emphasis filter with positive group delay. This segmentation allows each filter to be of lower order while achieving the same overall out-of-band rejection as a single high-order filter, reducing both latency and complexity while maintaining effective noise filtering performance.
Data Source
AI summary
A system may include an input configured to receive an audio signal, a filter having a negative group delay within a range of frequencies which are human-audible, the filter configured to receive the audio signal and filter the audio signal to generate a filtered audio signal, and a modulator configured to receive the filtered audio signal and modulate the filtered audio signal to generate a modulated filtered audio signal for communication over a digital interface.