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A receiver for an a.c. electrical supply signalling arrangement of the kind in which information is transmitted by impressing on the supply voltage waveform voltage reductions which bridge the zero crossings in the waveform wherein detection by the receiver of an impressed voltage reduction is achie

A receiver for an a.c. electrical supply signalling arrangement of the kind in which information is transmitted by impressing on the supply voltage waveform voltage reductions which bridge the zero crossings in the waveform wherein detection by the receiver of an impressed voltage reduction is achieved by comparing the integral of the supply voltage over a period immediately preceding a voltage zero crossing when a voltage reduction is present with a corresponding integral obtained when no voltage reduction is present.

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A receiver for use in an a.c. electrical supply signalling arrangement of the kind wherein information is conveyed as a pattern of time-spaced voltage reductions impressed on the a.c. supply voltage waveform, each reduction being for a fraction of a cycle of the waveform including a voltage zero cro

A receiver for use in an a.c. electrical supply signalling arrangement of the kind wherein information is conveyed as a pattern of time-spaced voltage reductions impressed on the a.c. supply voltage waveform, each reduction being for a fraction of a cycle of the waveform including a voltage zero crossing, the receiver comprising means for detecting the presence of an impressed voltage reduction by comparing the integral of the supply voltage waveform over a period extending over a small fraction of a cycle ending not later than the voltage zero crossing included in that voltage reduction with the integral over a corresponding period when no voltage reduction is present.