FirFilter

    public class FirFilter : IFilter
    {
        private double[] _coefficients;
        private double[] _queue;
        private int _index;

        public FirFilter()
        {
            _coefficients = new[]
                                {
                                    0, 0.0000717, 0.0001444, 0.0002168, 0.0002873, 0.0003538, 0.0004134, 0.0004627,
                                    0.0004977, 0.0005138, 0.0005067, 0.0004718, 0.0004056, 0.0003052, 0.0001698, 0,
                                    -0.0002009, -0.000427, -0.0006702, -0.0009192, -0.0011609, -0.0013801, -0.0015604,
                                    -0.001685, -0.001738, -0.0017049, -0.0015742, -0.0013381, -0.000994, -0.0005449, 0,
                                    0.0006246, 0.0013064, 0.0020166, 0.0027211, 0.0033818, 0.0039577, 0.0044075, 0.004691,
                                    0.0047721, 0.0046204, 0.0042141, 0.0035416, 0.0026033, 0.0014134, 0, -0.0015943,
                                    -0.003313, -0.0050866, -0.0068351, -0.0084696, -0.0098961, -0.0110186, -0.0117432,
                                    -0.0119819, -0.0116567, -0.0107036, -0.0090761, -0.0067478, -0.0037156, 0, 0.0043531,
                                    0.0092736, 0.0146682, 0.0204226, 0.0264051, 0.0324702, 0.0384635, 0.0442269, 0.0496037,
                                    0.0544445, 0.0586122, 0.061987, 0.0644708, 0.065991, 0.0665028, 0.065991, 0.0644708,
                                    0.061987, 0.0586122, 0.0544445, 0.0496037, 0.0442269, 0.0384635, 0.0324702, 0.0264051,
                                    0.0204226, 0.0146682, 0.0092736, 0.0043531, 0, -0.0037156, -0.0067478, -0.0090761,
                                    -0.0107036, -0.0116567, -0.0119819, -0.0117432, -0.0110186, -0.0098961, -0.0084696,
                                    -0.0068351, -0.0050866, -0.003313, -0.0015943, 0, 0.0014134, 0.0026033, 0.0035416,
                                    0.0042141, 0.0046204, 0.0047721, 0.004691, 0.0044075, 0.0039577, 0.0033818, 0.0027211,
                                    0.0020166, 0.0013064, 0.0006246, 0, -0.0005449, -0.000994, -0.0013381, -0.0015742,
                                    -0.0017049, -0.001738, -0.001685, -0.0015604, -0.0013801, -0.0011609, -0.0009192,
                                    -0.0006702, -0.000427, -0.0002009, 0, 0.0001698, 0.0003052, 0.0004056, 0.0004718,
                                    0.0005067, 0.0005138, 0.0004977, 0.0004627, 0.0004134, 0.0003538, 0.0002873, 0.0002168,
                                    0.0001444, 0.0000717, 0
                                };
             _queue = new double[_coefficients.Length];
        }

        public unsafe double Process(double sample)
        {
            int n = _coefficients.Length;
            double result = 0.0;
            fixed (double* coefficients = _coefficients, queue = _queue)
            {
                if (--_index < 0)
                    _index = n - 1;
                queue[_index] = sample;
                for (int i = 0; i < n; i++)
                {
                    result += queue[_index] * coefficients[i];
                    if (++_index >= n)
                        _index = 0;
                }
            }
            return result;
        }
    }