Oreson
INSTRUMENT design  simple firstorder IIR filter object
Oreson duplicates the functionality of the older CMIX
functions
reson and
rsnset.
It builds a simple IIR filter given a desired center frequency
and bandwidth, very useful in parallel for building complicated
filter response curves (see the
IIR
instrument for an example of Oreson use).
The general recursive filter equation used by this object
is:
y[n] = a*x[n] + b0*y[n1]  b1*y[n2]
where y[n], y[n1] and y[n2] are the current
and two previous outputs of the equation, and
x[n] is the
current input. a, b0 and b1 are the coefficients to
the equation that determine the characteristics of the filter, depending
upon the parameters set in the constructor. Presently the Oreson
filter object does not allow for dynamic changes to the filter
equation.
Constructors
Oreson(float SR, float centerFreq, float bandwidth[, Scale scaling])

SR is the current sampling rate (an
Instrument
class variable).
centerFreq is the center frequency of the bandpass filter
constructed.
bandwidth is the distance in Hz between the halfpower points
of the passband centered around the centerFreq.
scaling is an optional parameter to set how the
filter normalizes the filter equation. The flags for this parameter
are defined in RTcmix/genlib/Oreson.h as:
typedef enum {
kNoScale = 0, // no scaling of signal
kPeakResponse, // peak response factor of 1; use for harmonic signals
kRMSResponse // RMS response factor of 1; use for white noise signals
} Scale;
The default value used is kPeakResponse.
Access Methods
void Oreson::clear()

sets all delayed signals in the filter to 0.0.
float Oreson::next(float sig)

enters the incoming sample value into the filter equation,
computes the equation and then
returns the next filtered sample value as output.
float Oreson::last()

returns the current output of the filter without computing the
equation.
Examples
#include <Ougens.h>
Oreson *thefilt;
int MYINSTRUMENT::init(float p[], int n_args)
{
...
// center freq at 478 Hz, 14Hz wide bandwdith
thefilt = new Oreson(SR, 478.0, 14.0);
...
}
int MYINSTRUMENT::run()
{
float out[2];
float sample;
...
for (int i = 0; i < framesToRun(); i++)
{
sample = someSampleGeneratingProcess();
out[0] = thefilt>next(sample);
}
...
}
See Also
