IIRFilter Class

The IIRFilter class is the 1st order IIR low-pass filter. Implements the difference equation: More...

Header: #include <Filter/Filter.h>
Inherits: CDPBaseObject

Public Functions

IIRFilter()
double Calculate(double x)
double Getb0()
void Init(double b0)
void Setb0(double newb0)
void SetF3dB(double f3dB, double frequency)
void SetValue(double newValue)
double Value()

Reimplemented Public Functions

virtual void Create(const char *shortName, CDPBaseObject *pParent)
virtual std::string GetNodeTypeName() const override

Additional Inherited Members

Detailed Description

The IIRFilter class is the 1st order IIR low-pass filter. Implements the difference equation:

y[n] = b0 * x[n] + (1-b0) * y[n-1];

or in C++ form:

y_n = b0 * x + (1.0 - b0) * y_n1;
or
y_n += b0*(x - y_n)

Selecting a suitable cut-off frequency

The b0 parameter for the filter may either be set directly with Setb0(), or the cut-off frequency (-3dB frequency) may be set using the function SetF3dB().

SetF3dB() sets a new b0 using the following formula:

b = 2 * PI * f3dB / fs

This relationship between b and the cut-off frequency is valid only for b << 1.

It is recommended to use SetF3db(), as this will prevent the filter characteristic from changing if the sampling freqency is changed.

Note: The stability requirement for the first order filter is that b>0 and b<=1. This leads to the following requirement on f3db:

f3db < fs / (2 * PI)

or, expressed as digital frequency F = f/Fs:

F3db < 1 / (2 * PI)'

See also IIRFilter, RateLimitedFilter, and Integrator.

Member Function Documentation

IIRFilter::IIRFilter()

Constructor. Initializes members to safe values.

double IIRFilter::Calculate(double x)

Calculates filter output from x. Call this function periodically at a fixed sample interval, exacly once each period. Returns output value from filter.

Note: To read output from filter more than once each period, use Value() method to prevent a new output from being calculated again.

[virtual] void IIRFilter::Create(const char *shortName, CDPBaseObject *pParent)

Reimplemented from CDPBaseObject::Create().

Creates the IIRFilter with shortName and pParent object.

double IIRFilter::Getb0()

Returns filter constant b0 value.

[override virtual] std::string IIRFilter::GetNodeTypeName() const

Reimplemented from ICDPNode::GetNodeTypeName().

Returns "IIRFilter".

void IIRFilter::Init(double b0)

Initializes the filter and sets filter b0. This function can also be used to reset the filter memory and output. For a frequency-invariant filter,

See also SetF3dB().

void IIRFilter::Setb0(double newb0)

Sets new filter constant b0 from newb0

void IIRFilter::SetF3dB(double f3dB, double frequency)

Sets new filter constant b0 by specifying cut-off frequency f3db and frequency (both in Hz).

void IIRFilter::SetValue(double newValue)

Sets the output directly from newValue

double IIRFilter::Value()

Returns current output value.

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