1. An apparatus determining a weighting function, comprising: a coefficient conversion unit to convert a linear prediction coding (LPC) coefficient of an input signal into 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 one of importance of the ISF coefficient and importance of the LSF coefficient; and a quantization unit to quantize one of the converted ISF coefficient and the converted LSF coefficient using the determined weighting function, and to convert one of the quantized ISF coefficient and the quantized LSF coefficient into a quantized LPC coefficient.