A signalling system is provided which can be used to update the prices of
goods which are electronically displayed on supermarket shelves (1). The
signalling system comprises a first signalling device having an input
terminal for receiving electrical message data received from a central
computer system (5), a spread spectrum encoder for encoding the received
message data and an electroacoustic converter (11) for converting the
encoded data into corresponding acoustic signals and for transmitting the
acoustic signals into a transmission medium. The signalling system also
comprises a second signalling device (3) having means for receiving
acoustic signals from said transmission medium and for converting the
received acoustic signals into corrresponding electrical signals, a spread
spectrum decoder for decoding the received signals and a message
regenerator for regenerating the message data transmitted by the first
signalling device from the signals output by the decoder.
