Encoding hidden data channels in sigma-delta bitstreams

A new technique is introduced which allows a hidden data channel to be encoded onto a sigma-delta bit-stream in addition to the primary signal input. The technique is developed so that the primary signal can be demodulated by standard techniques, without interference by the side-channel. Applications include imprinting a 'watermark' onto an archived audio recording, or encoding additional hidden audio or video channels in the bitstream. Results are presented which indicate that side-channel data rates in excess of 500 kbit/s are possible.