Lawrence F. Heyl

Apparatus for reproducing sound with a reduced dynamic range

An open-loop speaker amplifier and a dynamic range reduction circuit are disclosed. The speaker amplifier includes a bridge driver and a bridge circuit. The bridge circuit includes a first set of switches and a second set of switches. When the first set of switches are activated, a positive current flows through a speaker load. When the second set of switches are activated, a negative current flows through the speaker load. The bridge driver generates a pulse width modulated signal and a delayed pulse width modulated signal to drive the first and second set of switches of the bridge circuit responsive to a modulation signal. The modulation signal maybe for example, a plurality of pulse code modulated samples representing sound. The dynamic range reduction circuit modifies a selected signal to reduce the dynamic range of the sound reproduced based on the modulation signal. The selected signal is either the modulation signal or a reference signal. In one embodiment, the dynamic range reduction circuit modifies the selected signal by processing the selected signal based on a smooth saturating function. In another embodiment, the dynamic range reduction circuit modifies the amplitude of the reference signal based on an envelope size of the modulation signal.

Audio peripheral mixer circuit and method for noise reduction

A circuit and method for mixing signals from audio sources provide noise reduction by employing a noise gate to selectively attenuate one of the source signals. The noise gate has a low gain state and a unity gain state. The noise gate switches to its unity gain state when a control signal reaches a predefined threshold voltage. The control signal is generated by rectifying and integrating the noisy source audio signal, so that the noise gate enters the unity gain state only when a useful sound occurs. Thus, the noise in the combined audio signal is reduced, without attenuating useful components of the source signals.