Abstract: Auxiliary digital data are inserted into a main digital data stream, to be subsequently coded to produce a coded data stream, by identifying a component of the main data stream that will make substantially no contribution to the coded data stream. It is into this component that data from the auxiliary data stream is inserted. The main digital data stream may comprise MPEG coded audio data, and the component (which represents unoccupied sub-bands or being at a level at or below a quantization noise floor) identified by estimating sub-bands that are unoccupied, or estimating quantization levels.