A method and apparatus to
encode / decode a low bit-rate
audio signal. The method of encoding a low bit-rate
audio signal, includes quantizing and losslessly-encoding a specific frequency component of an
audio signal in a
frequency domain, generating codebooks using the audio
signal in the
frequency domain, detecting an envelope of a frequency component of the audio
signal other than the specific frequency component in a specific band unit and quantizing and losslessly-encoding the detected envelope of the other frequency component, selecting a
codebook that is most similar to the other frequency component of the audio
signal to be encoded from among the generated codebooks and determining a
codebook index (
fine structure), losslessly-encoding the determined
codebook index, and generating a bit
stream using the specific frequency component, the envelope of the other frequency component, and the determined codebook index. The method of decoding a low bit-rate audio signal, includes restoring and dividing a bit
stream into a specific frequency component and a frequency component other than the specific frequency component, losslessly-decoding and inversely quantizing the specific frequency component, restoring codebook index information and envelope information about the other frequency component, generating codebooks using the specific frequency component which is inversely quantized, and restoring the other frequency component using the restored codebook index information and the restored envelope information about the other frequency component.