Hearing Loop Gain Limiter for Fast Feedback Howl Control
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Solution Overview
Problem
Feedback control systems in hearing devices face challenges with extremely high loop gains, leading to rapid feedback buildup and instability, as they are typically designed for maximum loop gains within a limited range, and existing solutions are too slow to react to abrupt changes, such as moving a phone to the ear.
Innovation Solution
A loop gain limiter is introduced, which estimates and reduces the current loop gain quickly, within 1-2 feedback loops, using solely signal level information, to keep the loop gain below a predefined value, ensuring the system operates within its designed range by dynamically controlling the forward path gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional correlation-based loop gain measures are used, then measurement precision is improved, but response speed deteriorates (cannot provide estimate within 1-2 feedback loops)
Solution Approach 1:
The patent extracts only the essential signal level information from the complete correlation-based measurement process. By taking out just the level comparison aspect and removing the complex correlation computation, the system achieves fast response within 1-2 feedback loops while still providing sufficient loop gain measurement precision for control purposes.
Solution Approach 2:
The patent uses a simple, computationally inexpensive level-based measurement approach instead of expensive correlation-based methods. This disposable-style measurement provides sufficient information for immediate control action without the computational overhead that would slow down the response.
2Stability of the object's composition
If feedback control system is designed for maximum loop gain within limited range, then system stability is improved, but adaptability deteriorates (cannot handle extreme situations like moving phone to ear)
Solution Approach 1:
The patent implements dynamic loop gain control by continuously monitoring signal levels and adjusting the forward path gain in real-time. This dynamic adaptation allows the system to maintain stability during normal operation while automatically adjusting to handle extreme situations such as moving a phone to the ear, where loop gain may abruptly increase.
Solution Approach 2:
The patent employs a feedback mechanism where the estimated loop gain is fed back to control the forward path gain. This closed-loop control ensures the system remains stable under varying conditions by continuously adjusting parameters based on actual loop gain measurements, enabling adaptation to extreme situations while maintaining designed stability margins.
3Speed
If loop gain is allowed to be very high (e.g., +20 dB), then feedback build-up speed is improved (ultra-fast), but feedback control effectiveness deteriorates (very challenging to reduce feedback artifacts)
Solution Approach 1:
The patent applies preliminary action by proactively limiting the loop gain before feedback artifacts can develop. By continuously monitoring signal levels and preemptively adjusting the forward path gain to maintain loop gain within the designed range, the system prevents feedback howl from building up rather than attempting to correct it after occurrence.
Data Source
AI summary
A hearing device comprises an input transducer providing an input gain GI, a 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, and an output transducer for providing output stimuli perceivable as sound for the user based on a 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 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, and 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). A resulting loop gain, LG′, is determined as a sum of the resulting forward path gain G′ and a 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.