Hearing Aid Signal Processing with Post-Synthesis Level Limiting
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Solution Overview
Problem
Existing hearing aids face issues with excessively high signal levels during loud sound sources, leading to unpleasant output, high energy consumption, and potential malfunctions due to voltage dips.
Innovation Solution
A hearing aid design with a signal processing unit that includes a filter bank for frequency band division, a processing unit, and a synthesis unit, featuring a level detector upstream and a level limiter downstream, allowing precise determination and broadband signal limiting after initial processing steps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If signal level determination is performed before the filter bank divides the input signal into frequency bands, then the level detection can be performed early in the signal processing chain, but the signal level determination is imprecise because it cannot account for the frequency-specific processing that will occur later
Solution Approach 1:
The level detector is positioned to perform level detection on the broadband input signal before frequency-specific processing, providing early warning of high signal levels. This preliminary detection triggers preparatory actions (setting maximum gain limits) that prevent excessive output levels even though the precise frequency-band level distribution is not yet known.
Solution Approach 2:
The system dynamically adjusts the maximum gain for each frequency band based on the detected broadband signal level. When high signal levels are detected, the system reduces maximum gains across all bands to prevent distortion, and dynamically releases these restrictions as signals return to normal levels, allowing optimal processing under varying conditions.
2Adaptability or versatility
If channel-specific maximum gains are used in each frequency band, then each channel can be optimized independently, but distortion occurs when high signal levels in one channel cause excessive output levels in other channels
Solution Approach 1:
The level detector continuously monitors the broadband signal level and provides feedback to the processing unit. Based on this feedback, the system dynamically adjusts maximum gain restrictions across all frequency bands, ensuring that high signal levels in any channel trigger appropriate gain reductions to prevent distortion in the overall output.
Solution Approach 2:
The level detection mechanism operates on the broadband signal encompassing all frequency bands simultaneously, creating a universal control signal that applies to all channels. This single level detection approach serves all frequency bands, ensuring coordinated gain control across the entire audio spectrum rather than independent control that could lead to instability.
3Reliability
If maximum gains are restricted to prevent high output levels, then signal distortion is reduced, but the hearing aid cannot take full advantage of available dynamic range for normal signal levels
Solution Approach 1:
The system dynamically adjusts maximum gain restrictions based on real-time signal level detection. During normal signal conditions, channels can utilize their full dynamic range and maximum gains for optimal processing efficiency. When high signal levels are detected, maximum gains are temporarily restricted to prevent distortion, and these restrictions are dynamically released when signals return to normal levels.
Solution Approach 2:
The system changes the maximum gain parameters for each frequency band based on detected signal levels. Under normal conditions, maximum gains are set to allow full dynamic range utilization. When high signal levels are detected, the maximum gain parameters are reduced to prevent distortion, and these parameter changes are continuously adjusted to balance reliability and processing efficiency.
Data Source
Figure 1
AI summary
The hearing aid (2) has an input transducer (4), a signal processing unit (6), and an output transducer (8), wherein the signal processing unit (6) has a filter bank (14) for dividing an input signal (E, E') into a plurality of signal components (Sf) in different frequency bands, a processing unit (12) for processing the signal components (Sf), and a synthesis unit (18) for synthesizing the processed signal components (Sf) to form an output signal (A, A'). To limit the level of an output signal (A), a level detector (20) is arranged upstream of the synthesis unit (18), which is designed to determine a signal level (P) based on the signal components (Sf) and to output a level signal (Sp). A level limiter (24), to which the level signal (Sp) is transmitted, is arranged downstream of the synthesis unit (18) and is designed to limit the level of the output signal (A) as a function of the level signal (Sp).