Mobile Microphone AGC With Split Analog-Digital Gain Control

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Solution Overview

Problem

In mobile telephone systems, high background noise and echo can cause microphone saturation, leading to distorted speech and ineffective echo cancellation, with existing automatic gain control (AGC) systems experiencing issues with continuous gain changes affecting speech volume and echo cancellation performance.

Innovation Solution

An automatic gain control scheme that separates gain control into analog and digital components, using a first gain control circuit to decrease gain in larger increments when signal levels exceed a threshold and a second circuit to increase gain in smaller increments when signal levels fall below a lower threshold, with a 'dead-zone' to maintain stability, and an inverse AGC to reverse gain adjustments, ensuring stable echo cancellation and speech quality.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If continuous gain control is applied to the microphone signal, then the microphone can adapt to varying background noise levels, but speech volume variation occurs and echo cancellation performance deteriorates

Engineering Contradiction:

Improvemicrophone adaptability to background noiseVSAvoidecho cancellation performance

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The patent segments the gain control process into distinct phases: a first gain control phase that operates continuously to prevent saturation, and a second gain control phase that operates more slowly to maintain stability. This segmentation allows the system to adapt to noise while preserving echo cancellation performance by preventing continuous rapid gain changes.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent implements dynamic gain control with two different time constants: a first time constant for rapid gain adjustment when saturation is detected, and a second time constant for slower gain recovery. This dynamic approach allows the system to respond quickly to harmful conditions while maintaining stability during normal operation, resolving the contradiction between adaptability and reliability.

Inventive Principle:

Principle #15Dynamics

2Reliability

If gain is decreased rapidly to prevent saturation in high noise conditions, then microphone saturation is prevented, but speech quality deteriorates due to excessive gain reduction

Engineering Contradiction:

Improveprevention of microphone saturationVSAvoidspeech quality

Core Design Contradiction:

ReliabilityVSLoss of information

Solution Approach 1:

The patent changes the gain parameter dynamically based on signal level conditions. When the signal exceeds a first threshold indicating saturation risk, the gain is decreased rapidly by a first amount. When the signal falls below a second threshold indicating safe levels, the gain is increased slowly by a second amount. This asymmetric parameter adjustment prevents saturation while minimizing speech quality loss.

Inventive Principle:

Principle #35Parameter changes

3Loss of information

If gain is increased slowly when signal level decreases, then speech quality is maintained, but the system responds slowly to improving noise conditions

Engineering Contradiction:

Improvespeech qualityVSAvoidgain recovery speed

Core Design Contradiction:

Loss of informationVSSpeed

Solution Approach 1:

Inventive Principle:

Principle #15Dynamics

4Device complexity

If a single gain control threshold is used, then the control logic is simple, but the system cannot distinguish between different noise conditions and applies inappropriate gain adjustments

Engineering Contradiction:

Improvegain control logic complexityVSAvoidresponse to different noise conditions

Core Design Contradiction:

Device complexityVSAdaptability or versatility

Solution Approach 1:

The patent applies different gain control parameters to different signal level conditions. A first gain control parameter (decrease amount and time constant) is applied when the signal exceeds a first threshold, while a second gain control parameter (increase amount and time constant) is applied when the signal falls below a second threshold. This localized parameter adjustment allows the system to adapt to different noise conditions effectively.

Inventive Principle:

Principle #3Local quality

Data Source

PatentUS8135148B2Automatic gain control for mobile microphone

Publication Date: 2012.03.13 IP GEM GRP LLC

AI summary

An automatic gain control unit controls the gain applied to an input signal produced by a microphone subject to ambient noise. The automatic gain control circuit continually monitors the signal level of said input signal. A first gain control circuit decreases the gain applied to the input signal in increments of a first size when the input signal exceeds a first predetermined level. A second gain control circuit increases the gain applied to said input signal in increments of a smaller size when the input signal falls below a second predetermined level and in response to the presence of a speech present signal. The second predetermined level is less than said first predetermined level. In one embodiment, the first gain control circuit controls the analog gain of a codec and the second gain control circuit controls the digital gain of the codec.