Multichannel audio coding is studied from a rate-distortion theoret- ical
viewpoint. Two practical coding techniques, both of which are based on
rate-distortion optimization, are also proposed. The first technique decorrelates a
multichannel signal hierarchically using el- ementary unitary transforms. The
second method rearranges a mul- tichannel signal into sub-signals and compresses
them at optimized bit rates using a conventional codec. Both objective and
subjective tests were conducted to illustrate the efficiency of the methods.