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DataPRO/FftSharp/Filter.cs

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+using System;
+
+namespace FftSharp
+{
+    public static class Filter
+    {
+        /// <summary>
+        /// Silence frequencies above the given frequency
+        /// </summary>
+        public static double[] LowPass(double[] values, double sampleRate, double maxFrequency) =>
+            BandPass(values, sampleRate, double.NegativeInfinity, maxFrequency);
+
+        /// <summary>
+        /// Silence frequencies below the given frequency
+        /// </summary>
+        public static double[] HighPass(double[] values, double sampleRate, double minFrequency) =>
+            BandPass(values, sampleRate, minFrequency, double.PositiveInfinity);
+
+        /// <summary>
+        /// Silence frequencies outside the given frequency range
+        /// </summary>
+        public static double[] BandPass(double[] values, double sampleRate, double minFrequency, double maxFrequency)
+        {
+            Complex[] fft = Transform.FFT(values);
+            double[] fftFreqs = Transform.FFTfreq(sampleRate, fft.Length, oneSided: false);
+            for (int i = 0; i < fft.Length; i++)
+            {
+                double freq = Math.Abs(fftFreqs[i]);
+                if ((freq > maxFrequency) || (freq < minFrequency))
+                {
+                    fft[i].Real = 0;
+                    fft[i].Imaginary = 0;
+                }
+            }
+            return InverseReal(fft);
+        }
+
+        /// <summary>
+        /// Silence frequencies inside the given frequency range
+        /// </summary>
+        public static double[] BandStop(double[] values, double sampleRate, double minFrequency, double maxFrequency)
+        {
+            Complex[] fft = Transform.FFT(values);
+            double[] fftFreqs = Transform.FFTfreq(sampleRate, fft.Length, oneSided: false);
+            for (int i = 0; i < fft.Length; i++)
+            {
+                double freq = Math.Abs(fftFreqs[i]);
+                if ((freq <= maxFrequency) && (freq >= minFrequency))
+                {
+                    fft[i].Real = 0;
+                    fft[i].Imaginary = 0;
+                }
+            }
+            return InverseReal(fft);
+        }
+
+        private static double[] InverseReal(Complex[] fft)
+        {
+            Transform.IFFT(fft);
+            double[] Filtered = new double[fft.Length];
+            for (int i = 0; i < fft.Length; i++)
+                Filtered[i] = fft[i].Real;
+            return Filtered;
+        }
+    }
+}