An encoder is arranged to add a binary code bit to block of a signal by selecting, within the block, (i) a reference frequency within the predetermined signal bandwidth, (ii) a first code frequency having a first predetermined offset from the reference frequency, and (iii) a second code frequency having...http://www.google.es/patents/US6272176?utm_source=gb-gplus-sharePatente US6272176 - Broadcast encoding system and method
