Multiband Compressor Threshold Smoothing for Audio Scene Switches
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Solution Overview
Problem
Audio playback systems, especially those in mobile devices, face distortion issues due to limited output capabilities, where multi-band compression often requires content-dependent gain adjustments that are inadequate for broadband signals, leading to unnatural volume changes during scene switches.
Innovation Solution
A scene switch analyzer is integrated into a multiband compressor to dynamically adjust thresholds using a distortion audibility model, applying fast or slow smoothing based on scene changes to maintain a natural listening experience by modifying the attack and release time constants of the compressor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If multi-band compression with fixed thresholds is applied to reduce distortion, then distortion is reduced for narrowband signals, but playback level is unnecessarily limited for broadband signals due to over-compression
Solution Approach 1:
The patent implements dynamic threshold adjustment where the distortion threshold is no longer fixed but varies over time based on the distortion audibility measure. The threshold at any given time is calculated as a smoothed version of the instantaneous threshold, allowing the system to adapt thresholds dynamically to match the actual audibility of distortion in the current audio context, thereby resolving the contradiction between distortion reduction and playback level maintenance
Solution Approach 2:
The patent changes the parameter of distortion threshold from a static fixed value to a dynamic time-varying value. By introducing the distortion audibility model that computes a measure based on current signal characteristics and using smoothing to generate time-varying thresholds, the system adjusts the compression parameters adaptively, allowing higher playback levels for broadband signals while maintaining distortion reduction for narrowband signals
2Power
If distortion thresholds are dynamically adjusted based on distortion audibility model, then playback level is maximized with minimal perceived distortion, but rapid threshold changes during scene switches cause unnatural listening experience
Solution Approach 1:
The patent introduces dynamic control of the smoothing process itself. The smoothing factor or time constant is adjusted based on scene switch detection: during normal operation, strong smoothing is applied to maintain threshold stability, but when scene switches are detected, the smoothing is reduced or temporarily disabled to allow rapid threshold adjustments, thus maintaining both stability and adaptability
Solution Approach 2:
The system performs preliminary scene switch detection by analyzing audio signal characteristics (such as spectral content, energy distribution, or specific scene switch indicators) before making threshold adjustments. This preliminary detection allows the system to prepare for upcoming threshold changes by adjusting smoothing parameters in advance, preventing unnatural listening experiences during scene transitions
3Object-affected harmful factors
If content-dependent gain adjustments are applied to eliminate perceived distortion for narrowband signals, then distortion is reduced, but broadband signals receive excessive compression since they mask their own distortion
Solution Approach 1:
The patent changes the approach from content-dependent gain adjustment to content-dependent threshold adjustment. Instead of modifying gains based on signal type, the system computes a distortion audibility measure that inherently accounts for signal bandwidth and masking effects, then uses this measure to set appropriate thresholds. This unified threshold-based approach simplifies the control logic while achieving the same goal of reducing perceived distortion only when necessary
Solution Approach 2:
The patent introduces the distortion audibility measure as an intermediary parameter that mediates between the audio signal characteristics and the compression threshold. This measure computes the perceived audibility of distortion based on current signal properties (bandwidth, spectral content, masking effects), serving as an intelligent intermediary that automatically determines the appropriate compression level without requiring complex content-dependent gain adjustment logic
Data Source
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AI summary
The present application relates to a method, system, and computer program product of dynamically adjusting thresholds of a compressor responsive to an input audio signal. A scene switch analyzer receives an input audio signal having a plurality of frequency band components. The scene switch analyzer determines whether a scene switch has occurred in the input audio signal. The frequency band components of the input audio signal are processed. In response to determine that scene switch has not occurred, a distortion audibility system applies slow smoothing to compressor thresholds of the frequency band components. In response to determine that scene switch has occurred, the distortion audibility system applies fast smoothing or no smoothing to the compressor thresholds of the frequency band components.