a logic interface signal input with a high degree of electrical noise immunity , particularly suited for use in an automative environment for detecting an on / off control signal multiplexed with an information signal such as a voice audio signal . the invention provides a fast logic output signal in response to the level of the control signal , which is suitable for driving higher speed circuitry and which permits detection to be highly immune from electrical transients or variations in power supply voltage . the invention permits the information signal to be combined with the control signal through polarized means , such as an electrolytic capacitor .