Digital automatic gain control with lookahead, adaptive noise floor

Abstract

An automatic gain controller for a digitized audio signal, comprising a buffer with a plurality of subframes. Each subframe contains digitized data samples of the signal, the subframes including at least one future subframe and a current subframe. Signal processing means (such as a DSP) is coupled to the memory for controlling gain of the audio signal represented by the current subframe in the buffer. The signal processing means includes means to control gain of the data samples using a stored program for computing a plurality of mean signal level values from the plurality of subframes, each mean signal level value in the plurality corresponding to one of the subframes. The program includes means for causing decay of gain on the signal represented by the current subframe when a first set of the mean values are each below a low threshold signal level. The program also has means for causing gain attack on the audio signal represented by the current subframe when a second set of the mean values are each above a high threshold signal level. The program also comprises means for shifting the data samples in one subframe in the future group into the current subframe for controlling gain of the audio signal after causing gain attack or decay. Thus, a voice messaging system under control of the program causes gain attack and decay on the audio signal represented by the subframes which attack and decay appear to occur without delay to a user of the audio signal. All control parameters are configurable. The AGC system is initialized using novel floor tracking and decay boost initialization methods which improve performance at the start of a call. Spike resistance can be set as high as desired.