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PM0014

Signal processing library functions

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6.4.2

iirBiquad5_q15_q15_q15

iirBiquad5_q15_q15_q15 (struct Filter *IirPtr).

Description:

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

The filter implemented is an IIR cascaded biquad cells form.

Argument:

IirPtr

Filter pointer.

Algorithm:

with Q = filter order/2, the number of cells

Notes:

The filter order must be an even number. Each biquad cells is

implemented in the direct form II.

The delay line must be stored in DPRAM. Its size is 2*3*Q. Before

the first call to the function, the delay line must be initialized to 0.

The coefficient a0 is fixed to 1.

Library users must care about the a(i) coefficients. If they are too

big in modulus, the filter internal states w(i) can saturate. The

output samples are in this condition false. This problem is less

important in iirBiquad5_q15_q15_q15 than in iir2_q15_q15_q15

because there are less a(i) coefficients used to compute biquad

cells. There can be some error bits because of the internal

rounding.

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 iirBiquad5_q15_q15_q15:

#define IirNumberCells NumberOfCells

#define NbOutput NumberOfOutput

iram short Delayline[IirNumberCells*3];

iram short Coeff[IirNumberCells*5];

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

iirBiquad5_q15_q15_q15(&Filt);

}

()

–

∏

b0k + b1k x z

-1 + b

2k x z

-2

1 - a1k x z

-1 - a

2k x z

-2

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