An interphone system for "hands-free" operation in high ambient noise

A new interphone system has been developed providing a high degree of intelligibility and listener comfort in ambient levels up to 125 dB sound-pressure level. This paper discusses some of the novel electronic features employed, including 1) Automatic Gain Control (AGC) with recovery inhibition to prevent noise "pumping" during speech pauses, 2) Automatic Level Compensation (ALC) for automatic adjustment of listening level with changing ambient, and 3) highly reliable voice-actuated switching (VOX) at widely varying ambient levels.