Multichannel audio data encoding/decoding method and apparatus

A multichannel audio data encoding and/or decoding method and apparatus are provided. The encoding method includes: encoding mono and/or stereo audio data; and encoding extended multichannel audio data other than the mono and/or stereo audio data. The decoding method includes: decoding mono and/or stereo audio data; examining whether or not there is extended multichannel audio data to be decoded other than the mono and/or stereo audio data; and if there is extended data to be decoded, decoding the extended multichannel audio data. According to the method and apparatus, with only one bitstream, mono, stereo, and multichannel can be provided according to a user environment. Also in multichannel audio, a fine grain scalability (FGS) function is provided according to the states of a user terminal and a network. Furthermore, enhancement of the performance of multichannel bit sliced arithmetic coding (BSAC), for example, a high sound quality, low complexity, and scalability, is enabled. In particular, a variety of requirements for MPEG standardization (compatibility with conventional BSAC, maintaining the FGS function, and minimum modification) can be satisfied. Also, the method and apparatus can be employed in more lifelike digital multimedia broadcasting and mobile- and home-theater-based services.