The AK2511 realizes a very natural hands-free function with an integrated HPF for noise reduction and a noise suppressor. The HPF has an automatic back ground noise detect function for noisy circumstances. It is not necessary to change parameters at different noise levels. A voice switch and adaptive filter provide a stable hands free talk without effects from sound gain and echo amount. The AK2511/2511A is suitable for various kinds of microphones and speakers. Direct internal register access through CPU interface enables to set the voice switch parameters to their optimum values for a given system without a need of external circuit adjustments. It also performs as an analog interface device by connecting an audio CODEC with AtoA function.