/// <summary>
/// Returns a new waveform that has been filtered using the Bessel algorithm.
/// </summary>
/// <param name="waveform">The original waveform.</param>
/// <returns>The Bessel filtered waveform.</returns>
internal static Waveform FilterWaveform(Waveform waveform)
{
// If there are not more than 3 data points, then no bessel filtering can occur
if (waveform == null || waveform.DataPoints.Count() <= 3)
{
return(new Waveform(waveform.DataPoints));
}
double samplingFrequency = waveform.SamplingFrequency();
int paddingCount = 3;
List <DataPoint> inputDataPoints = new List <DataPoint>(waveform.DataPoints);
// The filter should be started in the prepulse noise with y[0]=y[1]=y[2]=average of (x[0],x[1],x[2]).
double paddingY = 0.0;
for (int i = 0; i < paddingCount; i++)
{
paddingY += inputDataPoints[i].Y;
}
paddingY = paddingY / (double)paddingCount;
for (int i = 0; i < paddingCount; i++)
{
inputDataPoints.Insert(0, new DataPoint(inputDataPoints[0].X, paddingY));
}
List <DataPoint> outputDataPoints = new List <DataPoint>(inputDataPoints.Count);
Tuple <double, double, double, double, int> coefficientSet = BesselDigitalFilter.GetCorrectSamplingCoefficientSet(samplingFrequency);
double a1 = coefficientSet.Item1;
double a3 = coefficientSet.Item1 * 3.0;
double b = coefficientSet.Item2;
double c = coefficientSet.Item3;
double d = coefficientSet.Item4;
int delay = coefficientSet.Item5;
// Calculate how much each datapoint's X value must be shifted by
double horizontalShift = (1 / samplingFrequency) * delay * -1;
for (int n = 0; n < inputDataPoints.Count; n++)
{
DataPoint newDataPoint;
if (n < 3)
{
newDataPoint = new DataPoint(inputDataPoints[n].X + horizontalShift, inputDataPoints[n].Y);
}
else
{
double y =
(a1 * inputDataPoints[n - 3].Y) +
(a3 * inputDataPoints[n - 2].Y) +
(a3 * inputDataPoints[n - 1].Y) +
(a1 * inputDataPoints[n - 0].Y) +
(b * outputDataPoints[n - 3].Y) +
(c * outputDataPoints[n - 2].Y) +
(d * outputDataPoints[n - 1].Y);
newDataPoint = new DataPoint(inputDataPoints[n].X + horizontalShift, y);
}
outputDataPoints.Add(newDataPoint);
}
// Remove the 3 data points that were padding to get the digital filter started
outputDataPoints.RemoveRange(0, paddingCount);
return(new Waveform(outputDataPoints));
}