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 delay issues in feedback and feed-forward methods, leading to restricted frequency ranges and impracticality of analogue filters, which hinder effective noise cancellation.
Innovation Solution
A digital circuit arrangement that converts analogue signals into N-bit digital signals, utilizing a digital delta-sigma modulator and adjustable filtering to minimize group delay, allowing for efficient noise reduction across a wider frequency range without the limitations of conventional digital processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If feedback method is used for ambient noise reduction, then noise cancellation is achieved, but frequency range is limited due to phase lags and group delay
Solution Approach 1:
The patent replaces the conventional feedback method with a feed-forward method that uses a microphone to detect ambient noise before it enters the ear, converting the noise cancellation approach from reactive (feedback) to proactive (feed-forward). This substitution eliminates the phase lag problem inherent in feedback systems, allowing effective noise cancellation across a broader frequency range including higher frequencies.
Solution Approach 2:
The system performs preliminary detection of ambient noise using a microphone positioned to capture noise before it reaches the ear. The detected noise signal is processed and cancelled in advance, rather than reacting to noise after it has already entered the ear canal. This preliminary action enables effective cancellation of high-frequency noise that would otherwise cause phase lag issues in feedback systems.
2Object-affected harmful factors
If analogue filters are used in feedback loop, then noise reduction is achieved, but device complexity increases and manufacturing becomes impractical
Solution Approach 1:
The patent replaces complex analogue filter circuits with a digital signal processing approach. A digital signal processor implements filtering algorithms that achieve the same noise reduction function as analogue filters but with significantly reduced component count and simplified manufacturing. The digital implementation allows for precise filtering characteristics without requiring multiple precision analogue components.
Solution Approach 2:
The system changes the domain from analogue to digital, transforming continuous signal processing into discrete digital signal processing. This parameter change enables the use of software-based filtering algorithms that can be easily adjusted and optimized without changing physical hardware components, greatly simplifying manufacturing and allowing for flexible adaptation to different noise conditions.
3Object-affected harmful factors
If conventional digital processing is used, then noise reduction is achieved, but group delay increases limiting effectiveness
Solution Approach 1:
The feed-forward architecture performs noise detection and cancellation in advance, processing the noise signal before it reaches the ear. This preliminary action reduces the effective processing time required, minimizing group delay. The system has sufficient time to process the noise signal because it starts processing as soon as the noise is detected by the microphone, rather than waiting for the noise to enter the ear and then reacting to it.
Solution Approach 2:
The patent extracts the noise cancellation function from the feedback loop and implements it as a separate feed-forward path. By taking out the noise detection and processing function and placing it in a dedicated feed-forward path with its own microphone and processing chain, the system eliminates the circular delay inherent in feedback systems, achieving lower group delay and better 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.