Abstract: A circuit that combines the functions of audio compression and voice detection comprises an amplifier in a negative-feedback configuration in which the feedback signal is the product of the output of the amplifier and a syllabic voltage derived from that output. The syllabic voltage is compared with a threshold level to provide an output that is a measure of the presence of a voice signal. In an alternate embodiment, the syllabic voltage is compared with the sum of a threshold level and a detected syllabic voltage to generate a signal that is a measure of the presence of voice. The syllabic detector may be either an averaging detector or a valley detector.

Abstract: Multi-state control circuitry is described that can selectively couple an input signal to the positive and negative inputs of an operational amplifier depending on the voltage of a control signal. The input signal is coupled to the operational amplifier by a switch, which is enabled or disabled by the output of a voltage detector. The voltage detector enables the switch when the voltage of the control signal is less than a threshold voltage, and disables the switch when the voltage of the control signal is greater than the threshold voltage. The amplification of the operational amplifier is switched between +1 and -1 by selectively coupling a DC voltage to the positive input of the operation amplifier, depending on the current of the control signal. The DC voltage is coupled to the positive input of the operational amplifier by a second switch, which is enabled or disabled by a current detector.