Audio decoder for audio channel reconstruction

An apparatus for reconstructing N original audio channels from M audio channels, the audio decoder comprising:

• an input interface for receiving a bitstream containing a parameter, φ, and an encoded audio signal representing the M audio channels;
• a decoder for obtain a frequency domain representation of the M audio channels;
• an extractor for obtaining the parameter, φ, from the bitstream;
• and
• a recontructor for generating at least one of the N original audio channels from the M audio channels based on:
• a′=m cos φ+s sin φ,
• where a′ is the at least one reconstructed audio channel, m is one of the M audio channels, s is derived from a weighted or unweighted combination of the N original channels, φ is an angle representing amounts of signal m and s present in a′, and M is equal to two.