1. An apparatus for quantizing an input signal, the apparatus comprising: a coefficient conversion unit, implemented by a processor, configured to obtain a line spectral frequency (LSF) coefficient from a linear predictive coding (LPC) coefficient of the input signal; a normalization unit configured to normalize the LSF coefficient to a range of spectral bins obtained by time-to-frequency mapping of the input signal; a weighting function determination unit, configured to determine a first weighting function based on a magnitude of a spectral bin corresponding to a frequency of the normalized LSF coefficient, determine a second weighting function based on frequency information from the normalized LSF coefficient, and determine a combined weighting function by using the first weighting function and the second weighting function; and a quantization unit configured to quantize the LSF coefficient based on the combined weighting function, wherein the first weighting function is based on a maximum value of the magnitude of the spectral bin corresponding to the frequency of the normalized LSF coefficient and the magnitude of at least one neighboring spectral bin, wherein the frequency information is based on at least one of perceptual characteristics and a formant distribution of the input signal, and wherein the input signal has at least one of audio characteristics and speech characteristics.