Priority coding system

A communication system including a transceiver for transmitting and receiving a signal comprising a plurality of channels one of which consists of analogue speech signals and at least one other of which consists of data signals transmitted at intervals and interleaved with the speech signals, the data signals each comprising at least one complete cycle of a frequency or frequencies, which lie within the audio frequency spectrum wherein the significance of the data i.e. '0' or' 1', is defined in dependence upon its phase or frequency and wherein the data signal transmission rate is determined in dependence upon error rate detected in relation to the data signals received at a remotely located transceiver.