A method and an apparatus for quantizing/dequantizing frequency amplitude data, and a method and an apparatus for encoding/decoding audio signals by using the same are provided to support scalability for plural layers by using frequency amplitude information and phase information of a wideband error audio signal, and quantize a wide frequency band into a small number of bits, thereby providing scalability to audio quality. A method for quantizing frequency amplitude data comprises the followings steps of: calculating and quantizing power of frequency amplitudes of an audio signal(710); normalizing the quantized power by using frequency amplitude data(720); and quantizing even-numbered or odd-numbered data of the normalized frequency amplitude data(730). The method further comprises the following steps of: generating data that are not quantized among the normalized frequency amplitude data by interpolating the quantized data(740); and quantizing an interpolation error acquired by difference between the data, which are not quantized among the normalized frequency amplitude data, and the interpolated data(750).
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