A stereo sound encoding method and system for encoding left and right channels of a stereo sound signal, down mix the left and right channels of the stereo sound signal to produce primary and secondary channels, encode the primary channel, and encode the secondary channel. Encoding the secondary channel comprises analyzing coherence between coding parameters calculated during the secondary channel encoding and coding parameters calculated during the primary channel encoding to decide if the coding parameters calculated during the primary channel encoding are sufficiently close to the coding parameters calculated during the secondary channel encoding to be re-used during the secondary channel encoding.