Background communication using shadow of audio signal

摘要

Data is transferred by delaying an audio signal less than fifty milliseconds to produce a shadow signal and combining the shadow signal with the audio signal. The shadow represents the data to be transferred A telephone constructed in accordance with the invention has at least two, separate machine states, each of which includes operating in half duplex mode and two, separate machine states, each of which includes operating in full duplex mode. The state is selected based upon one or more shadows detected at the line input of the telephone.