System and method for performing combined digital/analog automatic gain

Abstract

A system which performs automatic gain control on received audio data. The system comprises an analog adjustable gain amplifier coupled to a digital gain control loop which is preferably a digital signal processor, with an A/D converter and D/A converter interposed between the amplifier and control loop. The invention thus advantageously amplifies the analog input signal before being converted into a digital signal. This results in better performance than prior art system which amplify an already converted to digital signal which may have already been clipped before the signal could be attenuated. The combined digital/analog method provides improved detection of silence to avoid amplifying background noise. The gain control loop comprises a long-term energy averager and gain calculator as well as a short-term energy averager and gain calculator which receive the digital audio output signal. The gain calculators periodically generate gain adjustment outputs based on the average energy of the audio signal so as to attenuate or amplify the analog audio input signal. Thus the invention advantageously achieves the apparently conflicting objectives of having an automatic gain control system which is both stable and responds quickly to clipping. The gain control loop further comprises a voice activity detector which detects the presence of silence versus voice activity based on ratios of the long-term and short-term energy averages. The long-term energy averager advantageously pauses operation during silence. Additionally, the gain control system amplifies the audio input signal only during the voice activity thus suppressing noise amplification during periods of silence.