/// <summary>
/// Computes a bi-linear frequency warped version of the LPC cepstrum from the LPC cepstrum. The recursive algorithm
/// used is defined in Oppenheim's paper in Proceedings of IEEE, June 1972 The program has been written using g[x,y]
/// = g_o[x,-y] where g_o is the array used by Oppenheim. To handle the reversed array index the recursion has been
/// done DOWN the array index.
/// </summary>
/// <param name="warp">The warping coefficient. For 16KHz speech 0.6 is good valued..</param>
/// <param name="nbilincepstra">The number of bilinear cepstral values to be computed from the linear frequencycepstrum.</param>
/// <returns>A bi-linear frequency warped version of the LPC cepstrum.</returns>
public double[] GetBilinearCepstra(double warp, int nbilincepstra)
{
//double[][] g = new double[nbilincepstra][cepstrumOrder];
var g = new double[nbilincepstra][];
// Make a local copy as this gets destroyed
var lincep = Java.CopyOf(_cepstra, _cepstrumOrder);
_bilinearCepstra[0] = lincep[0];
lincep[0] = 0;
g[0][_cepstrumOrder - 1] = lincep[_cepstrumOrder - 1];
for (var i = 1; i < nbilincepstra; i++)
{
g[i][_cepstrumOrder - 1] = 0;
}
for (var i = _cepstrumOrder - 2; i >= 0; i--)
{
g[0][i] = warp * g[0][i + 1] + lincep[i];
g[1][i] = (1 - warp * warp) * g[0][i + 1] + warp * g[1][i + 1];
for (var j = 2; j < nbilincepstra; j++)
{
g[j][i] = warp * (g[j][i + 1] - g[j - 1][i]) + g[j - 1][i + 1];
}
}
for (var i = 1; i <= nbilincepstra; i++)
{
_bilinearCepstra[i] = g[i][0];
}
return(_bilinearCepstra);
}
