Audio Signal Limiting for Echo Cancellation

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Solution Overview

Problem

Echo cancellation systems in communication systems face challenges when loudspeaker output exceeds the microphone's capabilities, leading to clipping and non-linearities that prevent effective echo removal, especially in systems with the loudspeaker near the microphone or in confined spaces.

Innovation Solution

A system that selectively limits the received audio signal using a soft limiter and a low-order infinite impulse response (IIR) filter to maintain the loudspeaker output within a linear range, preventing clipping and allowing traditional echo cancellation methods to effectively remove acoustic echo.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If the loudspeaker output is increased to improve audio signal quality, then the audio signal quality is improved, but the microphone picks up excessive volume causing clipping and non-linearities

Engineering Contradiction:

Improveaudio signal qualityVSAvoidclipping and non-linearities

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The system performs preliminary action by estimating the loudspeaker output signal before it is actually played, using the Line-Out signal as a reference. This estimation allows the system to predict potential clipping conditions and take preventive measures before the harmful effects occur.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system uses feedback by continuously monitoring the estimated loudspeaker output signal and comparing it against clipping thresholds. The echo cancellation system adjusts based on this feedback, modifying the reference signal or cancellation parameters to prevent clipping while maintaining audio quality.

Inventive Principle:

Principle #23Feedback

2Volume of moving object

If the loudspeaker is placed near the microphone to save space, then the device compactness is improved, but the microphone picks up loudspeaker output causing echo

Engineering Contradiction:

Improvedevice compactnessVSAvoidacoustic echo

Core Design Contradiction:

Volume of moving objectVSObject-generated harmful factors

Solution Approach 1:

The system introduces an intermediary approach by using the Line-Out signal as a reference to model and predict the loudspeaker output. This intermediary reference signal allows the system to identify and cancel echo components without requiring physical separation between the loudspeaker and microphone.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The system replaces the mechanical solution of physically separating the loudspeaker and microphone with an electronic/digital solution. Instead of increasing physical distance, the system uses signal processing and echo cancellation algorithms to eliminate echo, substituting mechanical separation with electronic intervention.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Device complexity

If traditional echo cancellation is used without output limiting, then the echo cancellation complexity is reduced, but the echo cancellation accuracy deteriorates due to non-linearities

Engineering Contradiction:

Improveecho cancellation complexityVSAvoidecho cancellation accuracy

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The system changes parameters by dynamically adjusting the reference signal used for echo cancellation. When clipping is detected or predicted, the system modifies the reference signal parameters to remain within the linear operating range, thereby maintaining cancellation accuracy without requiring complex non-linear processing.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS7894598B2System for limiting receive audio

Publication Date: 2011.02.22 CERENCE OPERATING CO

AI summary

A system for limiting a received audio signal in a communication system is provided. The receive audio signal is limited prior to being played over a loudspeaker to insure that the loudspeaker output will not be clipped when picked up by a nearby microphone associated with the communication system. By preventing clipping of the loudspeaker output at the microphone, the transfer function of the loudspeaker-enclosure-microphone system remains linear, facilitating accurate echo cancellation in the communication system.