1. An apparatus for quantizing a signal including at least one of a speech signal and an audio signal, the apparatus comprising: a coefficient conversion unit, implemented by a processor, configured to obtain a line spectral frequency (LSF) coefficient or an immittance spectral frequency (ISF) coefficient of the signal from a linear predictive coding (LPC) coefficient; a weighting function determination unit, configured to determine a weighting function based on a magnitude of a spectral bin corresponding to a frequency of the ISF coefficient or the LSF coefficient; and a quantization unit configured to quantize the ISF coefficient or the LSF coefficient using the determined weighting function, wherein the weighting function is based on a maximum value of the magnitude of the spectral bin corresponding to the frequency of the ISF coefficient or the LSF coefficient and the magnitude of at least one neighboring spectral bin.