/// <summary>
///
/// </summary>
/// <param name="audio"></param>
/// <param name="samplingRate"></param>
/// <param name="passbandFrequency"></param>
/// <param name="stopbandFrequency"></param>
/// <param name="passbandAttenuation"></param>
/// <param name="stopbandAttenuation"></param>
/// <returns></returns>
public static double[] LowpassFilteringType1(double[] audio, uint samplingRate, uint passbandFrequency, uint stopbandFrequency, float passbandAttenuation, float stopbandAttenuation)
{
const double PI = Math.PI;
double wPass = (2 * PI * passbandFrequency) / samplingRate; //same problem with negative N's as with the highpass filter
double wStop = (2 * PI * stopbandFrequency) / samplingRate;
double omegaPass = Math.Tan(wPass / 2);
double omegaStop = Math.Tan(wStop / 2);
double ePass = Math.Sqrt(Math.Pow(10, (passbandAttenuation / 10)) - 1);
double eStop = Math.Sqrt(Math.Pow(10, (stopbandAttenuation / 10)) - 1);
double w = omegaStop / omegaPass;
double e = eStop / ePass;
double nExact = (Math.Log(e + Math.Sqrt(Math.Pow(e, 2) - 1))) / (Math.Log(w + Math.Sqrt(Math.Pow(w, 2) - 1)));
int nUsed = (int)Math.Ceiling(nExact);
int k = (int)Math.Floor(nUsed / 2d);
double a = 1d / (double)nUsed * Math.Log(1d / ePass + Math.Sqrt(1d / Math.Pow(ePass, 2) + 1));
double omega0 = omegaPass * Math.Sinh(a);
double[] omega = new double[k];
double[] theta = new double[k];
for (int i = 0; i < k; i++)
{
theta[i] = (PI / (2 * nUsed)) * (nUsed - 1 + (2 * (i + 1)));
omega[i] = omegaPass * Math.Sin(theta[i]);
}
double[] filterCoefficientG = new double[(int)k + 1];
double[,] filterCoefficientA = new double[(int)k + 1, 2];
double[,] filterCoefficientB = new double[(int)k + 1, 3];
double[] matrixBHelp =
{
1,
1,
0
};
for (int i = 0; i < k; i++)
{
filterCoefficientG[i + 1] = (Math.Pow(omega0, 2) + Math.Pow(omega[i], 2)) / (1 - 2 * omega0 * Math.Cos(theta[i]) + Math.Pow(omega0, 2) + Math.Pow(omega[i], 2));
filterCoefficientA[i + 1, 0] = (2 * (Math.Pow(omega0, 2) + Math.Pow(omega[i], 2) - 1)) / (1 - 2 * omega0 * Math.Cos(theta[i]) + Math.Pow(omega0, 2) + Math.Pow(omega[i], 2));
filterCoefficientA[i + 1, 1] = (1 + 2 * omega0 * Math.Cos(theta[i]) + Math.Pow(omega0, 2) + Math.Pow(omega[i], 2)) / (1 - 2 * omega0 * Math.Cos(theta[i]) + Math.Pow(omega0, 2) + Math.Pow(omega[i], 2));
filterCoefficientB[i + 1, 0] = filterCoefficientG[i + 1] * 1;
filterCoefficientB[i + 1, 1] = filterCoefficientG[i + 1] * 2;
filterCoefficientB[i + 1, 2] = filterCoefficientG[i + 1] * 1;
}
if (nUsed % 2 != 0)
{
filterCoefficientG[0] = omega0 / (omega0 + 1);
filterCoefficientA[0, 0] = (omega0 - 1) / (omega0 + 1);
filterCoefficientA[0, 1] = 0;
for (int i = 0; i < 2; i++)
{
filterCoefficientB[0, i] = filterCoefficientG[0] * matrixBHelp[i];
}
}
return(FilteringClass.SOSFiltering(audio, filterCoefficientA, filterCoefficientB, nUsed));
}
/// <summary>
/// Butterworth lowpass filtering with parameters calculated from
/// <paramref name="passbandFrequency"/>, <paramref name="stopbandFrequency"/>, <paramref name="passbandAttenuation"/> and <paramref name="stopbandAttenuation"/>.
/// </summary>
/// <param name="audio">Audio to filter.</param>
/// <param name="samplingRate">Sampling Rate of the audio to be processed.</param>
/// <param name="passbandFrequency">The ordinary frequency of the passband.</param>
/// <param name="stopbandFrequency">The ordinary frequency of the stopband.</param>
/// <param name="passbandAttenuation">The attenuation in dB of the passband.</param>
/// <param name="stopbandAttenuation">The attenuation in dB of the stopband.</param>
/// <returns>Returns the filtered version of <paramref name="audio"/></returns>
static public double[] LowpassFiltering(double[] audio, uint samplingRate, uint passbandFrequency, uint stopbandFrequency, float passbandAttenuation, float stopbandAttenuation)
{
const double PI = Math.PI;
uint fPass = passbandFrequency;
uint fStop = stopbandFrequency;
float aPass = passbandAttenuation;
float aStop = stopbandAttenuation;
double wPass;
double wStop;
double omegaPass;
double omegaStop;
double ePass;
double eStop;
double w;
double passE;
double nExact;
int nUsed;
double omega0;
double[] theta;
double[] thetaD;
double[] filterCoefficientG;
double[,] filterCoefficientA;
double[,] filterCoefficientB;
wPass = (2 * PI * fPass) / samplingRate; //same problem with negative N's as with the highpass filter
wStop = (2 * PI * fStop) / samplingRate;
omegaPass = Math.Tan(wPass / 2);
omegaStop = Math.Tan(wStop / 2);
ePass = Math.Sqrt(Math.Pow(10, (aPass / 10)) - 1);
eStop = Math.Sqrt(Math.Pow(10, (aStop / 10)) - 1);
w = omegaStop / omegaPass;
passE = eStop / ePass;
nExact = Math.Abs(Math.Log(passE) / Math.Log(w));
nUsed = (int)Math.Ceiling(nExact);
int k = (int)Math.Floor(nUsed / 2d);
theta = new double[k];
omega0 = omegaPass / (Math.Pow(ePass, 1d / (double)nUsed));
thetaD = new double[2 * nUsed - 1];
for (int i = 0; i < k; i++)
{
theta[i] = (PI / (2 * nUsed)) * (nUsed - 1 + (2 * (i + 1)));
}
for (int t = 0; t < k; t++)
{
thetaD[t] = -2 * Math.Cos(theta[t]);
}
if (nUsed % 2 == 1)
{
thetaD[0] = 1 + omega0;
}
filterCoefficientG = new double[(int)k + 1];
filterCoefficientA = new double[(int)k + 1, 2];
filterCoefficientB = new double[(int)k + 1, 3];
double[] matrixBHelp =
{
1,
1,
0
};
for (int i = 0; i < k; i++)
{
filterCoefficientG[i + 1] = Math.Pow(omega0, 2) / (1 - 2 * omega0 * Math.Cos(theta[i]) + Math.Pow(omega0, 2));
filterCoefficientA[i + 1, 0] = (2 * (Math.Pow(omega0, 2) - 1)) / (1 - 2 * omega0 * Math.Cos(theta[i]) + Math.Pow(omega0, 2));
filterCoefficientA[i + 1, 1] = (1 + 2 * omega0 * Math.Cos(theta[i]) + Math.Pow(omega0, 2)) / (1 - 2 * omega0 * Math.Cos(theta[i]) + Math.Pow(omega0, 2));
filterCoefficientB[i + 1, 0] = filterCoefficientG[i + 1] * 1;
filterCoefficientB[i + 1, 1] = filterCoefficientG[i + 1] * 2;
filterCoefficientB[i + 1, 2] = filterCoefficientG[i + 1] * 1;
}
if (nUsed % 2 != 0)
{
filterCoefficientG[0] = omega0 / (omega0 + 1);
filterCoefficientA[0, 0] = (omega0 - 1) / (omega0 + 1);
filterCoefficientA[0, 1] = 0;
for (int i = 0; i < 2; i++)
{
filterCoefficientB[0, i] = filterCoefficientG[0] * matrixBHelp[i];
}
}
return(FilteringClass.SOSFiltering(audio, filterCoefficientA, filterCoefficientB, nUsed));
}