Abstract: A page party system for use in industrial environments, having a plurality of stations joined by a page line and by a single party line for providing multiple full-duplex conference calls over the single party line. Multiple full-duplex conferencing is achieved by combining single sideband suppressed carrier amplitude modulation and frequency division multiplexing.

Abstract: An intrinsically safe telephone as provided for use in hazardous areas. The telephone includes a single line instrument located in the hazardous area which, in turn, is coupled by a two-wire transmission path to a shunt diode barrier and a telephone line interface located in a safe environment. The telephone line interface reduces power of incoming ringing and audio signals. The telephone line interface further includes an off-hook indicating circuit which senses when the single line instrument is off-hook. The intrinsically safe telephone is useful in all hazardous environments as classified by the National Electrical Code and requires no explosion-proof housing.

Abstract: A loudspeaker having two horns defining two soundpaths passing therethrough an input for connection to a driver and an output for broadcasting an acoustic signal is shown to include a central channel connected at one end to the input, the soundpaths being coincident therethrough and a first transition member which defines separate channels for the soundpaths. Such separate channels serving to change the direction of the soundpaths. There is also shown a pair of side channels, the cross-section of the output ends of the side channels are defined by first and second dimensions, wherein the first dimension is larger than the second dimension. A pair of second transition members serve to again change the direction of the soundpaths. Such transition members have an output end defined by first and second dimensions wherein the second dimension is larger than the first dimension.

Abstract: An intrinsically safe telephone as provided for use in hazardous areas. The telephone includes a single line instrument located in the hazardous area which, in turn, is coupled by a two-wire transmission path to a shunt diode barrier and a telephone line interface located in a safe environment. The telephone line interface reduces power of incoming ringing and audio signals. The telephone line interface further includes an off hook indicating circuit which senses when the single line instrument is off hook. The telephone line interface further includes a bootstrap power supply for powering the single line instrument whenever the single line instrument is off hook. The intrinsically safe telephone is line powered and requires no auxiliary power in the hazardous environment. The intrinsically safe telephone is useful in all hazardous environments as classified by the National Electrical Code and requires no explosion-proof housing.