Audio encoder with selectable L/R or M/S coding

An encoder system for encoding a stereo audio signal having an original left channel and an original right channel, the encoder system comprising:

• a downmixer for generating a downmix signal and a residual signal based on the stereo audio signal, wherein the residual signal covers only a part of an audio frequency range of the stereo signal;
• a parameter estimator for determining one or more stereo parameters based on the stereo audio signal, the one or more stereo parameters describing a stereo image of the stereo audio signal;
• a selector that analyzes the stereo audio signal and selects:
• left/right perceptual encoding, or
• mid/side perceptual encoding,
• in a time variant manner;
• a high frequency reconstruction encoder for generating high frequency reconstruction parameters, the high frequency reconstruction parameters including a spectral envelope of a high band portion of the original left channel and the original right channel;
• and
• a bitstream generator for creating an audio bitstream containing a coded representation of stereo audio signal, the one or more stereo parameters, and the high frequency reconstruction parameters.