Digital Noise-Reduction Circuit With Low Group Delay Filtering
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Solution Overview
Problem
Existing ambient noise-reduction systems for earphones face limitations due to phase lags and group delays in feedback and feed-forward methods, which restrict the frequency range and effectiveness of noise cancellation, and are impractical with analogue filters due to high costs and complexity.
Innovation Solution
A digital circuit arrangement that converts analogue signals into N-bit digital signals, using a digital delta-sigma modulator and adjustable filtering to minimize group delay, allowing for effective noise reduction across a wider frequency range without the limitations of analogue filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If feedback method is used for ambient noise reduction, then noise cancellation is achieved, but phase lags and group delays limit the frequency range to about 1 kHz or below
Solution Approach 1:
The patent replaces the traditional feedback method with a feed-forward method that uses a microphone to detect ambient noise before it enters the ear, processes the signal through a filter, and generates an anti-phase signal to cancel the noise. This substitution eliminates the phase lag problems inherent in feedback systems, enabling effective noise cancellation across a broader frequency range including high frequencies.
2Reliability
If feed-forward method is used with analogue filters, then noise reduction is achieved, but high costs and complexity make it impractical
Solution Approach 1:
The patent substitutes complex analogue filters with a digital signal processing approach using a microprocessor or digital signal processor. The system converts the analog microphone signal to digital, applies digital filtering algorithms to generate the anti-phase noise cancellation signal, and converts it back to analog for speaker output. This digital implementation reduces hardware complexity and cost while maintaining or improving noise reduction performance.
3Stability of the object's composition
If high-frequency filtering is incorporated into feedback loop, then oscillation instability is reduced, but upper frequency operation is severely restricted
Solution Approach 1:
The patent inverts the traditional feedback approach by using feed-forward processing. Instead of detecting noise after it enters the ear and attempting to cancel it (which causes phase lag and stability issues at high frequencies), the system detects noise before it enters, processes it through a filter designed to produce the correct phase relationship, and generates the cancellation signal in advance. This inversion of the signal flow direction eliminates the oscillation instability problem while maintaining high-frequency performance.
Data Source
AI summary
A digital circuit arrangement for an ambient noise-reduction system affording a higher degree of noise reduction than has hitherto been possible. The arrangement converts the analog signals into N-bit digital signals at sample rate f0, and then subjects the converted signals to digital filtering. The value of N in some embodiments is 1 but, in any event, is no greater than 8, and f0 may be 64 times the Nyquist sampling rate but, in any event, is substantially greater than the Nyquist sampling rate. This permits digital processing to be used without incurring group delay problems that rule out the use of conventional digital processing in this context. Furthermore, adjustment of the group delay can readily be achieved, in units of a fraction of a micro-second, providing the ability to fine tune the group delay for feed forward applications.