AGC Overview

The lib_agc library provides an API to implement Automatic Gain Control within an application. The goal of the AGC algorithm is to provide consistent output levels for voice audio.

The gain control can adapt to maintain the amplitude of the peak of the frame within an upper and lower bound configured for the AGC instance. When used in an application with a Voice to Noise Ratio estimator (VNR), the AGC will adapt only when voice activity is detected, so that speech in the input signal is amplified above other sounds.

The AGC also has a Loss Control feature which can be used when the application has an Acoustic Echo Canceller (AEC). This feature uses data from the AEC to adjust the gain applied to reduce residual echoes by attenuating the audio when near-end speech is not present.

The AGC takes as input a frame of data from an audio channel. This could be the microphone input or the output of another module in the application.

Gain control is performed on a frame-by-frame basis. Each frame consists of 15ms of data, which is 240 samples at 16kHz input sampling frequency. Input data is expected to be in a fixed-point 32-bit 1.31 format.

Before processing any frames, the application must configure and initialise the AGC instance by calling agc_init(). Then for each frame, agc_process_frame() will update the AGC instance’s internal state and produce the output frame by applying the AGC algorithm to the input frame.

The gain values in this module for AGC gain and Loss Control gain are multiplicative factors that are applied to scale the input frame. Therefore, a fixed gain value of 1.0 (without loss control) will create no change to the input.

If multiple channels need to be processed by the application, or multiple outputs are required, an independent instance of the AGC must be run for each channel.