1. An encoding apparatus for enhancing a quantization efficiency in linear predictive encoding, the apparatus comprising: a first converter to convert a linear predictive coding (LPC) coefficient of a mid-subframe of an input signal to one of a line spectral frequency (LSF) coefficient and an immitance spectral frequency (ISF) coefficient; a weighting function determination unit to determine a weighting function associated with an importance of the LPC coefficient of the mid-subframe using the converted ISF coefficient or LSF coefficient; a quantization unit to quantize the converted ISF coefficient or LSF coefficient using the determined weighting function; and a second coefficient converter to convert the quantized ISF coefficient or LSF coefficient to a quantized LPC coefficient using at least one processor, wherein the quantized LPC coefficient is output to an encoder of the encoding apparatus.