////////////////////////////////////////////////////////////////////////////////
//
// Function: createDeemphasisFilter
//
// Arguments: length: Filter kernel length.
// sampleRate: System sample rate in Hertz. Must
// be greater than 0.
// doAnalysis: If set to true, this function will
// calculate and print the filter response.
// correctPhase: If set to true, this function corrects the
// phase distortion produced by the pre-
// emphasis filter.
//
// Returns: Returns a pointer to a struct containing pointer to the
// filter kernel plus other information
//
// Description: This function creates a filter kernel that implements
// the de-emphasis filter needed to compensate for the pre-
// emphasis filtering that is present on some CD recordings.
// The filter will correct the frequency response of the input
// to produce an output that should have a spectrum identical
// to the original recording before an emphasis filter was
// applied. If the correctPhase switch is set to true, then
// the phase is also corrected; if not, a linear phase filter
// is created.
//
////////////////////////////////////////////////////////////////////////////////
public static Deemphasis createDeemphasisFilter(int length, double sampleRate,
bool correctPhase, bool doAnalysis)
{
// Make sure the length of the filter is at least 8 samples
if (length < 8)
{
throw new ArgumentOutOfRangeException("length", "" + length);
}
// Make sure the length of the filter is power of 2
if (!Utility.isPowerOf2(length))
{
throw new ArgumentOutOfRangeException("length", "" + length);
}
// Make sure the sample rate is greater than 0
if (sampleRate <= 0)
{
throw new ArgumentOutOfRangeException("sampleRate", "" + sampleRate);
}
// Calculate the bin size
double binSize = sampleRate / (double)length;
// Make sure the binsize is > 0 and <= 1/4 of the sample rate. This
// guarantees at least 3 data points when calculating the filter
// response, i.e. at DC, nyquist/2, and nyquist.
if ((binSize <= 0) || (binSize > sampleRate / 4))
{
throw new ArgumentOutOfRangeException("binSize", "" + binSize);
}
// Allocate memory for the deemphasis filter struct
Deemphasis deemphasis = new Deemphasis(length, sampleRate, binSize,
correctPhase, doAnalysis);
// Calculate the ideal frequency response of the de-emphasis filter
deemphasis.idealFilterResponse =
IdealResponse.calculateIdealResponse(length, sampleRate);
// Create the de-emphasis filter kernel, using the ideal filter response
deemphasis.filterKernel =
createFilterKernel(deemphasis.idealFilterResponse,
deemphasis.correctPhase);
// If we are doing analysis, then calculate the response and deviation
// of this filter kernel, and print out the results
if (deemphasis.doAnalysis)
{
// Calculate the actual filter response
//deemphasis.actualFilterResponse =
// IdealResponse.calculateIdealResponse(deemphasis.filterKernel,
// sampleRate, binSize);
// Calculate and print out deviations for
// amplitude (dB), phase Delay (s), group delay (s)
//deemphasis.deviation = Deviation.calculateDeviation(deemphasis);
}
// Return a pointer to the newly created deemphasis struct
return(deemphasis);
}
////////////////////////////////////////////////////////////////////////////////
//
// Function: calculateDeviation
//
// Arguments: deemphasisFilter: A pointer to a calculated deemphasis
// filter.
//
// Returns: A pointer to a deviation struct.
//
// Description: This function calculates the deviation between the ideal
// deemphasis filter response and the actual deemphasis filter
// response embodied in the calculated filter kernel. In
// particular, the deviation of the amplitude response (in dB),
// phase delay (in seconds), and group delay (in seconds) is
// calculated.
//
////////////////////////////////////////////////////////////////////////////////
public static Deviation calculateDeviation(Deemphasis deemphasisFilter)
{
int length;
// Make sure we have valid pointers
if (deemphasisFilter == null)
{
throw new ArgumentNullException("deemphasisFilter");
}
if (deemphasisFilter.idealFilterResponse == null)
{
throw new ArgumentNullException("deemphasisFilter.idealFilterResponse");
}
if (deemphasisFilter.actualFilterResponse == null)
{
throw new ArgumentNullException("deemphasisFilter.actualFilterResponse");
}
if (deemphasisFilter.idealFilterResponse.frequency == null)
{
throw new ArgumentNullException("deemphasisFilter.idealFilterResponse.frequency");
}
if (deemphasisFilter.idealFilterResponse.amplitudeDB == null)
{
throw new ArgumentNullException("deemphasisFilter.idealFilterResponse.amplitudeDB");
}
if (deemphasisFilter.idealFilterResponse.phaseDelay == null)
{
throw new ArgumentNullException("deemphasisFilter.idealFilterResponse.phaseDelay");
}
if (deemphasisFilter.idealFilterResponse.groupDelay == null)
{
throw new ArgumentNullException("deemphasisFilter.idealFilterResponse.groupDelay");
}
if (deemphasisFilter.actualFilterResponse.frequency == null)
{
throw new ArgumentNullException("deemphasisFilter.actualFilterResponse.frequency");
}
if (deemphasisFilter.actualFilterResponse.amplitudeDB == null)
{
throw new ArgumentNullException("deemphasisFilter.actualFilterResponse.amplitudeDB");
}
if (deemphasisFilter.actualFilterResponse.phaseDelay == null)
{
throw new ArgumentNullException("deemphasisFilter.actualFilterResponse.phaseDelay");
}
if (deemphasisFilter.actualFilterResponse.groupDelay == null)
{
throw new ArgumentNullException("deemphasisFilter.actualFilterResponse.groupDelay");
}
// Allocate memory for the deviation struct
Deviation deviation = new Deviation();
// Calculate the length of the deviation vectors
length = deemphasisFilter.actualFilterResponse.frequency.Length;
// Copy the frequency vector from the actual filter response
deviation.frequency = (double[])deemphasisFilter.actualFilterResponse.frequency.Clone();
// Create the deviation vectors
deviation.amplitudeDB = new double[length];
deviation.phaseDelay = new double[length];
deviation.groupDelay = new double[length];
// Calculate the position of the DC component in the ideal filter
// response vectors
int DCIndex =
(deemphasisFilter.idealFilterResponse.amplitudeDB.Length / 2) - 1;
// Calculate the amplitude deviation
for (int i = 0, j = DCIndex; i < length; i++, j++)
{
deviation.amplitudeDB[i] =
deemphasisFilter.actualFilterResponse.amplitudeDB[i] -
deemphasisFilter.idealFilterResponse.amplitudeDB[j];
}
// Record the minimum and maximum amplitude deviations
deviation.amplitudeDeviationMin = deviation.amplitudeDB.Min();
deviation.amplitudeDeviationMax = deviation.amplitudeDB.Max();
// Calculate the phase delay deviation
// First calculate the offset at f = 0
double offset =
deemphasisFilter.actualFilterResponse.phaseDelay[0] -
deemphasisFilter.idealFilterResponse.phaseDelay[DCIndex];
// Then calculate the deviation for each element
for (int i = 0, j = DCIndex; i < length; i++, j++)
{
deviation.phaseDelay[i] =
deemphasisFilter.actualFilterResponse.phaseDelay[i] -
deemphasisFilter.idealFilterResponse.phaseDelay[j] -
offset;
}
// Record the minimum and maximum phase delay deviations
deviation.phaseDelayDeviationMin = deviation.phaseDelay.Min();
deviation.phaseDelayDeviationMax = deviation.phaseDelay.Max();
// Calculate the group delay deviation
// First calculate the offset at f = 0
offset =
deemphasisFilter.actualFilterResponse.groupDelay[0] -
deemphasisFilter.idealFilterResponse.groupDelay[DCIndex];
// Then calculate the deviation for each element
for (int i = 0, j = DCIndex; i < length; i++, j++)
{
deviation.groupDelay[i] =
deemphasisFilter.actualFilterResponse.groupDelay[i] -
deemphasisFilter.idealFilterResponse.groupDelay[j] -
offset;
}
// Record the minimum and maximum group delay deviations
deviation.groupDelayDeviationMin = deviation.groupDelay.Min();
deviation.groupDelayDeviationMax = deviation.groupDelay.Max();
// Return a pointer to the newly calculated deviation structure
return(deviation);
}
