A custom IC for automatic gain control in LPC vocoders

An automatic gain control technique for the audio input path of a vocoder has been developed based on a digitally controlled audio attenuator placed immediately before the a-d converter. The technique is more flexible than traditional analog approaches in that attenuation changes can be synchronized to vocoder frame boundaries thereby maintaining the integrity of the vocoder frame analysis. The digital controller for the attenuator has been implemented in a custom NMOS integrated circuit and was designed with the MACPITTS silicon compiler, a high level IC design tool. The circuit contains 2190 transistors occupying 4.5 by 5.4mm.