A method, medium, and
system encoding and / or decoding an
audio signal by extracting stereo parameters from an input
signal, encoding the stereo parameters, and performing down-mixing on the input
signal to a down-mixed
signal, splitting the down-mixed signal into a low band signal and a high band signal, determining whether to
encode the low band signal in a
time domain or a
frequency domain, if the low band signal is determined to be encoded in the
time domain, encoding the low band signal in the
time domain, if the low band signal is determined to be encoded in the
frequency domain, generating an encoded bitplane by converting the low band signal from the time domain to the
frequency domain by using a first
conversion method and performing quantization and context-dependent encoding on the low band signal converted to the frequency domain by using the first
conversion method, converting each of the low band signal and the high band signal from the time domain to the frequency domain or a time / frequency domain by using a second
conversion method, generating and encoding
bandwidth extension information that represents a characteristic of the high band signal converted by the second conversion method by using the low band signal converted by the second conversion method, and outputting the encoded stereo parameters, the encoded bitplane, and the encoded
bandwidth extension information a result of encoding the input signal. Accordingly,
high frequency components and stereo components may be efficiently encoded and decoded at a potential restricted
bit rate, thereby improving the quality of an
audio signal.