lowpass and high pass filter in C# [closed]

Answer 1

LP and HP filter - Musicdsp.org documentation

I implemented the filter in semicode above as follows in our sEMG analyzer software and it works great.

public class FilterButterworth
{
    /// <summary>
    /// rez amount, from sqrt(2) to ~ 0.1
    /// </summary>
    private readonly float resonance;

    private readonly float frequency;
    private readonly int sampleRate;
    private readonly PassType passType;

    private readonly float c, a1, a2, a3, b1, b2;

    /// <summary>
    /// Array of input values, latest are in front
    /// </summary>
    private float[] inputHistory = new float[2];

    /// <summary>
    /// Array of output values, latest are in front
    /// </summary>
    private float[] outputHistory = new float[3];

    public FilterButterworth(float frequency, int sampleRate, PassType passType, float resonance)
    {
        this.resonance = resonance;
        this.frequency = frequency;
        this.sampleRate = sampleRate;
        this.passType = passType;

        switch (passType)
        {
            case PassType.Lowpass:
                c = 1.0f / (float)Math.Tan(Math.PI * frequency / sampleRate);
                a1 = 1.0f / (1.0f + resonance * c + c * c);
                a2 = 2f * a1;
                a3 = a1;
                b1 = 2.0f * (1.0f - c * c) * a1;
                b2 = (1.0f - resonance * c + c * c) * a1;
                break;
            case PassType.Highpass:
                c = (float)Math.Tan(Math.PI * frequency / sampleRate);
                a1 = 1.0f / (1.0f + resonance * c + c * c);
                a2 = -2f * a1;
                a3 = a1;
                b1 = 2.0f * (c * c - 1.0f) * a1;
                b2 = (1.0f - resonance * c + c * c) * a1;
                break;
        }
    }

    public enum PassType
    {
        Highpass,
        Lowpass,
    }

    public void Update(float newInput)
    {
        float newOutput = a1 * newInput + a2 * this.inputHistory[0] + a3 * this.inputHistory[1] - b1 * this.outputHistory[0] - b2 * this.outputHistory[1];

        this.inputHistory[1] = this.inputHistory[0];
        this.inputHistory[0] = newInput;

        this.outputHistory[2] = this.outputHistory[1];
        this.outputHistory[1] = this.outputHistory[0];
        this.outputHistory[0] = newOutput;
    }

    public float Value
    {
        get { return this.outputHistory[0]; }
    }
}

Note that this filter was created for audio DSP purposes. To create a clean output you need to set the resonance to sqrt(2).

Answer 2

here is one it has many modes, HP LP BP peak, and so forth, it is a BiQuad perhaps 2 pole static filter something of that kind, it is a particular filter and has a certain kind of digital result: https://github.com/filoe/cscore/blob/master/CSCore/DSP/BiQuad.cs

    /*
     * These implementations are based on http://www.earlevel.com/main/2011/01/02/biquad-formulas/
     */

    using System;

    namespace CSCore.DSP
    {
        /// <summary>
        /// Represents a biquad-filter.
        /// </summary>
        public abstract class BiQuad
        {
            /// <summary>
            /// The a0 value.
            /// </summary>
            protected double A0;
            /// <summary>
            /// The a1 value.
            /// </summary>
            protected double A1;
            /// <summary>
            /// The a2 value.
            /// </summary>
            protected double A2;
            /// <summary>
            /// The b1 value.
            /// </summary>
            protected double B1;
            /// <summary>
            /// The b2 value.
            /// </summary>
            protected double B2;
            /// <summary>
            /// The q value.
            /// </summary>
            private double _q;
            /// <summary>
            /// The gain value in dB.
            /// </summary>
            private double _gainDB;
            /// <summary>
            /// The z1 value.
            /// </summary>
            protected double Z1;
            /// <summary>
            /// The z2 value.
            /// </summary>
            protected double Z2;

            private double _frequency;

            /// <summary>
            /// Gets or sets the frequency.
            /// </summary>
            /// <exception cref="System.ArgumentOutOfRangeException">value;The samplerate has to be bigger than 2 * frequency.</exception>
            public double Frequency
            {
                get { return _frequency; }
                set
                {
                    if (SampleRate < value * 2)
                    {
                        throw new ArgumentOutOfRangeException("value", "The samplerate has to be bigger than 2 * frequency.");
                    }
                    _frequency = value;
                    CalculateBiQuadCoefficients();
                }
            }

            /// <summary>
            /// Gets the sample rate.
            /// </summary>
            public int SampleRate { get; private set; }

            /// <summary>
            /// The q value.
            /// </summary>
            public double Q
            {
                get { return _q; }
                set
                {
                    if (value <= 0)
                    {
                        throw new ArgumentOutOfRangeException("value");
                    }
                    _q = value;
                    CalculateBiQuadCoefficients();
                }
            }

            /// <summary>
            /// Gets or sets the gain value in dB.
            /// </summary>
            public double GainDB
            {
                get { return _gainDB; }
                set
                {
                    _gainDB = value;
                    CalculateBiQuadCoefficients();
                }
            }

            /// <summary>
            /// Initializes a new instance of the <see cref="BiQuad"/> class.
            /// </summary>
            /// <param name="sampleRate">The sample rate.</param>
            /// <param name="frequency">The frequency.</param>
            /// <exception cref="System.ArgumentOutOfRangeException">
            /// sampleRate
            /// or
            /// frequency
            /// or
            /// q
            /// </exception>
            protected BiQuad(int sampleRate, double frequency)
                : this(sampleRate, frequency, 1.0 / Math.Sqrt(2))
            {
            }

            /// <summary>
            /// Initializes a new instance of the <see cref="BiQuad"/> class.
            /// </summary>
            /// <param name="sampleRate">The sample rate.</param>
            /// <param name="frequency">The frequency.</param>
            /// <param name="q">The q.</param>
            /// <exception cref="System.ArgumentOutOfRangeException">
            /// sampleRate
            /// or
            /// frequency
            /// or
            /// q
            /// </exception>
            protected BiQuad(int sampleRate, double frequency, double q)
            {
                if (sampleRate <= 0)
                    throw new ArgumentOutOfRangeException("sampleRate");
                if (frequency <= 0)
                    throw new ArgumentOutOfRangeException("frequency");
                if (q <= 0)
                    throw new ArgumentOutOfRangeException("q");
                SampleRate = sampleRate;
                Frequency = frequency;
                Q = q;
                GainDB = 6;
            }

            /// <summary>
            /// Processes a single <paramref name="input"/> sample and returns the result.
            /// </summary>
            /// <param name="input">The input sample to process.</param>
            /// <returns>The result of the processed <paramref name="input"/> sample.</returns>
            public float Process(float input)
            {
                double o = input * A0 + Z1;
                Z1 = input * A1 + Z2 - B1 * o;
                Z2 = input * A2 - B2 * o;
                return (float)o;
            }

            /// <summary>
            /// Processes multiple <paramref name="input"/> samples.
            /// </summary>
            /// <param name="input">The input samples to process.</param>
            /// <remarks>The result of the calculation gets stored within the <paramref name="input"/> array.</remarks>
            public void Process(float[] input)
            {
                for (int i = 0; i < input.Length; i++)
                {
                    input[i] = Process(input[i]);
                }
            }

            /// <summary>
            /// Calculates all coefficients.
            /// </summary>
            protected abstract void CalculateBiQuadCoefficients();
        }

        /// <summary>
        /// Used to apply a lowpass-filter to a signal.
        /// </summary>
        public class LowpassFilter : BiQuad
        {
            /// <summary>
            /// Initializes a new instance of the <see cref="LowpassFilter"/> class.
            /// </summary>
            /// <param name="sampleRate">The sample rate.</param>
            /// <param name="frequency">The filter's corner frequency.</param>
            public LowpassFilter(int sampleRate, double frequency)
                : base(sampleRate, frequency)
            {
            }

            /// <summary>
            /// Calculates all coefficients.
            /// </summary>
            protected override void CalculateBiQuadCoefficients()
            {
                double k = Math.Tan(Math.PI * Frequency / SampleRate);
                var norm = 1 / (1 + k / Q + k * k);
                A0 = k * k * norm;
                A1 = 2 * A0;
                A2 = A0;
                B1 = 2 * (k * k - 1) * norm;
                B2 = (1 - k / Q + k * k) * norm;
            }
        }

        /// <summary>
        /// Used to apply a highpass-filter to a signal.
        /// </summary>
        public class HighpassFilter : BiQuad
        {
            private int p1;
            private double p2;

            /// <summary>
            /// Initializes a new instance of the <see cref="HighpassFilter"/> class.
            /// </summary>
            /// <param name="sampleRate">The sample rate.</param>
            /// <param name="frequency">The filter's corner frequency.</param>
            public HighpassFilter(int sampleRate, double frequency) 
                : base(sampleRate, frequency)
            {
            }

            /// <summary>
            /// Calculates all coefficients.
            /// </summary>
            protected override void CalculateBiQuadCoefficients()
            {
                double k = Math.Tan(Math.PI * Frequency / SampleRate);
                var norm = 1 / (1 + k / Q + k * k);
                A0 = 1 * norm;
                A1 = -2 * A0;
                A2 = A0;
                B1 = 2 * (k * k - 1) * norm;
                B2 = (1 - k / Q + k * k) * norm;
            }
        }

        /// <summary>
        /// Used to apply a bandpass-filter to a signal.
        /// </summary>
        public class BandpassFilter : BiQuad
        {
            /// <summary>
            /// Initializes a new instance of the <see cref="BandpassFilter"/> class.
            /// </summary>
            /// <param name="sampleRate">The sample rate.</param>
            /// <param name="frequency">The filter's corner frequency.</param>
            public BandpassFilter(int sampleRate, double frequency) 
                : base(sampleRate, frequency)
            {
            }

            /// <summary>
            /// Calculates all coefficients.
            /// </summary>
            protected override void CalculateBiQuadCoefficients()
            {
                double k = Math.Tan(Math.PI * Frequency / SampleRate);
                double norm = 1 / (1 + k / Q + k * k);
                A0 = k / Q * norm;
                A1 = 0;
                A2 = -A0;
                B1 = 2 * (k * k - 1) * norm;
                B2 = (1 - k / Q + k * k) * norm;
            }
        }

        /// <summary>
        /// Used to apply a notch-filter to a signal.
        /// </summary>
        public class NotchFilter : BiQuad
        {
            /// <summary>
            /// Initializes a new instance of the <see cref="NotchFilter"/> class.
            /// </summary>
            /// <param name="sampleRate">The sample rate.</param>
            /// <param name="frequency">The filter's corner frequency.</param>
            public NotchFilter(int sampleRate, double frequency) 
                : base(sampleRate, frequency)
            {
            }

            /// <summary>
            /// Calculates all coefficients.
            /// </summary>
            protected override void CalculateBiQuadCoefficients()
            {
                double k = Math.Tan(Math.PI * Frequency / SampleRate);
                double norm = 1 / (1 + k / Q + k * k);
                A0 = (1 + k * k) * norm;
                A1 = 2 * (k * k - 1) * norm;
                A2 = A0;
                B1 = A1;
                B2 = (1 - k / Q + k * k) * norm;
            }
        }

        /// <summary>
        /// Used to apply a lowshelf-filter to a signal.
        /// </summary>
        public class LowShelfFilter : BiQuad
        {
            /// <summary>
            /// Initializes a new instance of the <see cref="LowShelfFilter"/> class.
            /// </summary>
            /// <param name="sampleRate">The sample rate.</param>
            /// <param name="frequency">The filter's corner frequency.</param>
            /// <param name="gainDB">Gain value in dB.</param>
            public LowShelfFilter(int sampleRate, double frequency, double gainDB) 
                : base(sampleRate, frequency)
            {
                GainDB = gainDB;
            }

            /// <summary>
            /// Calculates all coefficients.
            /// </summary>
            protected override void CalculateBiQuadCoefficients()
            {
                const double sqrt2 = 1.4142135623730951;
                double k = Math.Tan(Math.PI * Frequency / SampleRate);
                double v = Math.Pow(10, Math.Abs(GainDB) / 20.0);
                double norm;
                if (GainDB >= 0)
                {    // boost
                    norm = 1 / (1 + sqrt2 * k + k * k);
                    A0 = (1 + Math.Sqrt(2 * v) * k + v * k * k) * norm;
                    A1 = 2 * (v * k * k - 1) * norm;
                    A2 = (1 - Math.Sqrt(2 * v) * k + v * k * k) * norm;
                    B1 = 2 * (k * k - 1) * norm;
                    B2 = (1 - sqrt2 * k + k * k) * norm;
                }
                else
                {    // cut
                    norm = 1 / (1 + Math.Sqrt(2 * v) * k + v * k * k);
                    A0 = (1 + sqrt2 * k + k * k) * norm;
                    A1 = 2 * (k * k - 1) * norm;
                    A2 = (1 - sqrt2 * k + k * k) * norm;
                    B1 = 2 * (v * k * k - 1) * norm;
                    B2 = (1 - Math.Sqrt(2 * v) * k + v * k * k) * norm;
                }
            }
        }

        /// <summary>
        /// Used to apply a highshelf-filter to a signal.
        /// </summary>
        public class HighShelfFilter : BiQuad
        {
            /// <summary>
            /// Initializes a new instance of the <see cref="HighShelfFilter"/> class.
            /// </summary>
            /// <param name="sampleRate">The sample rate.</param>
            /// <param name="frequency">The filter's corner frequency.</param>
            /// <param name="gainDB">Gain value in dB.</param>
            public HighShelfFilter(int sampleRate, double frequency, double gainDB)
                : base(sampleRate, frequency)
            {
                GainDB = gainDB;
            }

            /// <summary>
            /// Calculates all coefficients.
            /// </summary>
            protected override void CalculateBiQuadCoefficients()
            {
                const double sqrt2 = 1.4142135623730951;
                double k = Math.Tan(Math.PI * Frequency / SampleRate);
                double v = Math.Pow(10, Math.Abs(GainDB) / 20.0);
                double norm;
                if (GainDB >= 0)
                {    // boost
                    norm = 1 / (1 + sqrt2 * k + k * k);
                    A0 = (v + Math.Sqrt(2 * v) * k + k * k) * norm;
                    A1 = 2 * (k * k - v) * norm;
                    A2 = (v - Math.Sqrt(2 * v) * k + k * k) * norm;
                    B1 = 2 * (k * k - 1) * norm;
                    B2 = (1 - sqrt2 * k + k * k) * norm;
                }
                else
                {    // cut
                    norm = 1 / (v + Math.Sqrt(2 * v) * k + k * k);
                    A0 = (1 + sqrt2 * k + k * k) * norm;
                    A1 = 2 * (k * k - 1) * norm;
                    A2 = (1 - sqrt2 * k + k * k) * norm;
                    B1 = 2 * (k * k - v) * norm;
                    B2 = (v - Math.Sqrt(2 * v) * k + k * k) * norm;
                }
            }
        }

        /// <summary>
        /// Used to apply an peak-filter to a signal.
        /// </summary>
        public class PeakFilter : BiQuad
        {
            /// <summary>
            /// Gets or sets the bandwidth.
            /// </summary>
            public double BandWidth
            {
                get { return Q; }
                set
                {
                    if (value <= 0)
                        throw new ArgumentOutOfRangeException("value");
                    Q = value;
                }
            }

            /// <summary>
            /// Initializes a new instance of the <see cref="PeakFilter"/> class.
            /// </summary>
            /// <param name="sampleRate">The sampleRate of the audio data to process.</param>
            /// <param name="frequency">The center frequency to adjust.</param>
            /// <param name="bandWidth">The bandWidth.</param>
            /// <param name="peakGainDB">The gain value in dB.</param>
            public PeakFilter(int sampleRate, double frequency, double bandWidth, double peakGainDB)
                : base(sampleRate, frequency, bandWidth)
            {
                GainDB = peakGainDB;
            }

            /// <summary>
            /// Calculates all coefficients.
            /// </summary>
            protected override void CalculateBiQuadCoefficients()
            {
                double norm;
                double v = Math.Pow(10, Math.Abs(GainDB) / 20.0);
                double k = Math.Tan(Math.PI * Frequency / SampleRate);
                double q = Q;

                if (GainDB >= 0) //boost
                {
                    norm = 1 / (1 + 1 / q * k + k * k);
                    A0 = (1 + v / q * k + k * k) * norm;
                    A1 = 2 * (k * k - 1) * norm;
                    A2 = (1 - v / q * k + k * k) * norm;
                    B1 = A1;
                    B2 = (1 - 1 / q * k + k * k) * norm;
                }
                else //cut
                {
                    norm = 1 / (1 + v / q * k + k * k);
                    A0 = (1 + 1 / q * k + k * k) * norm;
                    A1 = 2 * (k * k - 1) * norm;
                    A2 = (1 - 1 / q * k + k * k) * norm;
                    B1 = A1;
                    B2 = (1 - v / q * k + k * k) * norm;
                }
            }
        }
    }

Answer 3

You can take a look at source code for Butterworth Low Pass filter here on another stackoverflow question. As others point out EmguCV comes with a lot of filters coded efficiently and avalaible out of the box

Answer 4

I found this online tool that looks promising: Interactive Digital Filter Design: Butterworth / Bessel / Chebyshev Filters

You just enter your requirements:

• Filter design: Butterworth / Bessel / Chebyshev
• Filter type: Lowpass / Highpass / Bandpass / Bandstop
• Filter order
• Corner frequency/frequencies

Click submit, and it computes the following information:

• Gains, poles, zeros
• Recurrence relation
• C code implementing the recurrence relation
• Plots of magnitude, phase, impulse and step responses

You can implement a filter in C# directly from the recurrence relation.

If you only need a few constant filters, you're done. However, if you need to be able to adjust the filter parameters at run time, you will need to do more. Luckily, the professor provided the source code for his tool, and it should be possible to convert to C#.

Answer 5

Here are some c# code examples of a butterworth and chebyshev filter using NMath's fft.

Answer 6

Have a look at OpenCV/EmguCV.

They are both open source so you can have the "code" if that is what you are looking for.