A method of producing a composite signal from first and second sound signals, comprising the following steps:
coding said second sound signal to obtain a coded second sound signal that can be reproduced by sound reproducing means in a manner that is inaudible to the human ear, and
combining said first sound signal and said coded second sound signal into a composite sound signal that can be reproduced by sound reproducing means in such a manner that said first sound signal is audible to the human ear and said second sound signal is inaudible to the human ear.