Abstract: A method of quantizing the line spectral frequencies (LSF) when calculating the parameters of an analysis filter is included in a linear predictive coding (LPC) speech coder. The line spectral frequencies form an alternative to the filter parameters with unambiguous correspondence. Sum polynomials (P) and difference polynomials (Q) are constructed from the direct form coefficients of filters. Thereafter, the roots of the polynomials which correspond to the line spectral frequencies are determined, without calculation, by examining the polynomials in light of pre-selected test frequencies (f.sub.tp1, f.sub.tqf1, f.sub.tp2, f.sub.tq2, . . . ) that are speech typical. The polarity of the two polynomials (P, Q) is investigated for each of these test frequencies. When a polarity change occurs between two test frequencies, one of these frequencies, such as the higher frequency, is chosen.