Digital audio carrying extra information

A new technique is proposed which makes possible the inaudible addition of extra information to an audio signal. Full compatibility with present-day transmission and reproduction systems is guaranteed, as the resulting signal remains in the same format. The extra information can be retrieved by the use of additional signal processing. The technique of adding and retrieving extra information is described, and applications are presented. Special attention is given to the so-called 4-2-4 coding, as the technique presented offers a very promising scheme for such a coding