Abstract: An oral warning system for monitoring mining operations, or the like, comprising a plurality of first sensors each providing a signal representative of an abnormal condition being sensed, at least one additional sensor providing a signal representing an emergency condition being sensed, an oscillator and modulator for generating an emergency warning tone energized upon detection of the emergency condition, a plurality of speakers at selected locations for broadcasting the emergency warning tone, a multi-track tape player with playback head selectively movable to any one of the track positions and having an audio output connected to local and remote speakers, and circuit means effective to energize the tape player and move its head to an appropriate track containing a prerecorded oral message upon detection of an abnormal condition by one of said first sensors.

Abstract: Intercommunication between a plurality of stations is established through a pair of signal lines to which portable units are connected, each unit having its own power supply contributing to a low DC operating voltage maintained across the signal lines through which the audio signals are also conducted. Communicate and pager switches associated with each unit respectively control signal connections, d.c. line-voltage level, and signal input impedance for establishing intercommunication and paging functions.