Patent US 5,652,843

An intermittent analog voice signal is coded which is contained in an analog mixed signal having the intermittent analog voice signal mixed with a background analog noise signal. An analog-to-digital converter receives the analog mixed signal and generates a coded output signal by digitizing the analog mixed signal when a control signal is applied thereto and does not generate a coded output signal when the control signal is not applied thereto. A voice signal detector means receives the analog mixed signal and detects periods during which the analog voice signal is present in the analog mixed signal. A control signal generator generates the control signal applied to the analog-to-digital converter during voice periods detected by the voice signal detector during which the analog voice signal is present in the analog mixed signal. The analog mixed signal is thus digitized by the analog-to-digital converter only during the voice periods detected by the voice signal detector.