Hearing Aid Loop Gain Limiter for Fast Feedback Suppression
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Solution Overview
Problem
Hearing devices face challenges in managing extremely high loop gains that occur abruptly, leading to rapid feedback buildup and instability, which conventional feedback control systems struggle to address effectively.
Innovation Solution
A loop gain limiter is introduced that estimates the current loop gain based solely on signal level, allowing for rapid reduction of loop gain to maintain it below a predefined limit, even in extreme situations, using a loop gain estimator and controller to dynamically adjust the forward path gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a very high loop gain is used to achieve ultra-fast feedback build-up, then the feedback control speed is improved, but the feedback control system becomes unstable and produces feedback howl
Solution Approach 1:
The patent applies dynamics by making the loop gain adjustable and time-varying. The system transitions from a static high loop gain configuration to a dynamic configuration where the loop gain is initially high for fast feedback detection, then automatically reduced to a lower stable value once feedback is detected. This dynamic adjustment resolves the contradiction by allowing high speed initially, then maintaining stability subsequently.
Solution Approach 2:
The patent applies preliminary action by pre-configuring the system with a high loop gain value before feedback occurs, enabling ultra-fast feedback build-up for rapid detection. Once feedback is detected through the feedback path, the system then takes action to reduce the loop gain to a stable value. This preliminary high-gain configuration allows the system to quickly detect feedback issues before they become problematic.
2Measurement precision
If traditional correlation-based loop gain measures are used to estimate and reduce loop gain, then measurement precision is improved, but the response time becomes too slow to prevent feedback buildup
Solution Approach 1:
The patent applies partial action by using only the necessary portion of loop gain estimation - specifically monitoring the feedback path signal presence rather than performing complete correlation-based measurements. This partial monitoring approach provides sufficient information to detect feedback buildup while maintaining extremely fast response times, avoiding the computational overhead of traditional correlation methods.
Solution Approach 2:
The patent substitutes complex correlation-based measurement mechanisms with a simpler signal level monitoring mechanism. Instead of using computationally intensive correlation algorithms to estimate loop gain, the system monitors the presence and level of feedback signals directly, achieving both speed and sufficient measurement precision for feedback control.
Data Source
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AI summary
A hearing device, e.g. a hearing aid, comprises a) a forward path comprising al) an input transducer for providing an electric input signal representing sound in the environment of the hearing device, the input transducer providing an input gain Gi, a2) a signal processor for processing said electric input signal or a signal based thereon and providing a processed signal, the signal processor comprising a compressor for determining a frequency and level dependent desired compressor gain GP to compensate for a hearing impairment of the user, and to provide a resulting compressor gain G'P, a3) an output transducer for providing output stimuli perceivable as sound for the user based on said processed signal, the output transducer providing an output gain, Go. A resulting forward path gain G' is defined in a logarithmic representation as GI+G'P+GO. The hearing device further comprises b1) a loop gain estimator for continuously estimating a current loop gain ΔL(n), configured to provide a loop gain estimate within a predefined number of feedback loop delays after a feedback buildup has started, wherein the loop gain estimate is calculated as the current level of a signal of the forward path at time index n minus the level of the same signal one feedback loop earlier, and b2) a loop gain controller for dynamically controlling said resulting forward path gain G' in dependence of said estimate of said current loop gain ΔL(n). The feedback loop is represented by the electric forward path of the hearing device from the input transducer to the output transducer and an acoustic feedback path from the output transducer to the input transducer, the feedback path exhibiting a feedback gain H. A resulting loop gain, LG', is determined as a sum of the resulting forward path gain G' and the feedback gain H when given in a logarithmic representation. The loop gain controller is configured to provide that the resulting loop gain is limited to stay below a predefined value. A method of operating a hearing device is further disclosed.