Convert prediction filter coefficients to line spectral frequencies
lsf = poly2lsf(a)
lsf = poly2lsf(a) returns
lsf, of line spectral frequencies from
a, of prediction filter coefficients.
 Deller, John R., John G. Proakis, and John H. L. Hansen. Discrete-Time Processing of Speech Signals. New York: Macmillan, 1993.
 Rabiner, Lawrence R., and Ronald W. Schafer. Digital Processing of Speech Signals. Englewood Cliffs, NJ: Prentice-Hall, 1978.