In this paper, the approximation of the optimal compressor function using spline function of the first and second degree is done. On the basis of the obtained approximate spline functions, a quantizer designing is done. The achieved SQNR is very close to that of the model of quantizer based on the nonlinear compressor function. Also, it is shown that for the medium bit rate a higher value of SQNR is achieved by using quantizer model proposed in this paper for the spline of degree 2 than by optimal compandor.