using System;
using NWaves.Filters.BiQuad;

namespace RedBookPlayer
{
    public class DeEmphasisFilter : BiQuadFilter
    {
        static double B0, B1, B2, A0, A1, A2;

        static DeEmphasisFilter()
        {
            double fc = 5277;
            double slope = 0.4850;
            double gain = -9.465;

            double w0 = 2 * Math.PI * fc / 44100;
            double A = Math.Exp(gain / 40 * Math.Log(10));
            double alpha = Math.Sin(w0) / 2 * Math.Sqrt((A + 1 / A) * (1 / slope - 1) + 2);

            double cs = Math.Cos(w0);
            double v = 2 * Math.Sqrt(A) * alpha;

            B0 = A * ((A + 1) + (A - 1) * cs + v);
            B1 = -2 * A * ((A - 1) + (A + 1) * cs);
            B2 = A * ((A + 1) + (A - 1) * cs - v);
            A0 = (A + 1) - (A - 1) * cs + v;
            A1 = 2 * ((A - 1) - (A + 1) * cs);
            A2 = (A + 1) - (A - 1) * cs - v;

            B2 /= A0;
            B1 /= A0;
            B0 /= A0;
            A2 /= A0;
            A1 /= A0;
            A0 = 1;
        }

        public DeEmphasisFilter() : base(B0, B1, B2, A0, A1, A2)
        {
        }
    }
}