PM0014 datasheet(41/46 Pages) STMICROELECTRONICS

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PM0014

Signal processing library functions

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6.4

Infinite impulse response filter

6.4.1

iir2_q15_q15_q15

iir2_q15_q15_q15 (struct Filter *IirPtr).

Description:

Real IIR filter of 16-bit fractional inputs, coefficients and outputs.

The filter implemented is a direct form II.

Argument:

IirPtr

Filter pointer.

Algorithm:

w(n) = x(n) +

a(k) * w(n - k)

Notes:

The w(i) are the filter internal states and are saved in the delay line

during computation.

The delay line must be stored in DPRAM. Before the first call to the

function, the delay line must be initialized to 0. The length of the

delay line must be equal to the number of b(i) coefficients. As the

number of b(i) coefficients is supposed to be equal to the number of

a(i) coefficients, the length of the delay line is equal to

IirNumberCoeffs/2.

Library users must care about the a(i) coefficients. If their modulus

are too great, the filter internal states w(i) can saturate. The output

samples are in this condition false.

The coefficient a0 must be initialized to 0.

There is a saturation to 1 (respectively -1) if the computed output

sample is greater than 1 (respectively lower than -1).

Example of the use for iir2_q15_q15_q15:

#define IirNumberCoeffs NumberOfCoefficients

#define NbOutput NumberOfOutput

iram short Delayline[IirNumberCoeffs/2];

iram short Coeff[IirNumberCoeffs];

void main()

{

// Data definition

short Input[NbOutput];

short Out[NbOutput];

// Here should be the coef[] and input[] initialization.

See the Test part...

// Declare a filter structure

struct Filter Filt;

//Filter initialisation

Filt.NumberCoeffs = IirNumberCoeffs;

Filt.NumberOutputSamples = NbOutput;

Filt.CoeffPtr = Coeff;

Filt.InputPtr = Input;

Filt.OutputPtr = Out;

Filt.DelayLinePtr = Delayline;

//Filter processing

iir2_q15_q15_q15(&Filt);

}

IirNumberCoeff

----------------------------------------------

–

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