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Function API

RM0020

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Description:

Computes the second-order IIR filter on frac16 data. The input samples are stored in

the array x, output samples are written to the array y. The filter coefficients are stored

in the array c divided by 2. There is used the arithmetic with saturation so the output

will never overflow.

Algorithm:

Equation 51

for each n from 0 to N-1

Note: The routine requires these conditions are met:

-1 =< b1/2-a1*b0/2 < 1

-1 =< b2/2-a1*b1/2 < 1

-1 =< -a1*b2/2 < 1

-1 =< a2/2-a1*a1/2 < 1

-1 =< -a1*a2/2 < 1

Performance: See Section 7 and Table 61.

Example 30. iir_frac16_2nd_hc

#include "libdsp2.h"

/* #pragma and ALIGN macros used to align to 4 bytes */

#ifdef __ghs__

#pragma alignvar (4)

#endif

ALIGN_DCC(4)

short x[256] ALIGN_MWERKS(4);

#ifdef __ghs__

#pragma alignvar (4)

#endif

ALIGN_DCC(4)

short y[256] ALIGN_MWERKS(4);

Table 30.

iir_frac16_2nd_hc arguments(1)

The x,y,c,s data are in fractional 16-bit format in range -1 to 1. The arrays x,y,s must be word

aligned, the array c must be half-word aligned (implicit alignment of short data). There must be 4

bytes of readable memory behind the array x. There are returned new values of x(-2), x(-1), y(-2),

y(-1) in the array s to be ready for next function call.

Nin

number of input and output samples (N must be multiple of 4, N must

be >= 8)

array of input samples of length N, x(0) x(1) ... x(N-1)

out

array of output samples of length N, y(0) y(1) ... y(N-1)

cin

array of filter coefficients divided by 2, c[5] = {b(0)/2, b(1)/2, b(2)/2,

a(1)/2, a(2)/2}

in/out

array of previous input and output values (state vector), s[4] = {x(-2),

x(-1), y(-2), y(-1)}



-----------xn

 b1



-----------xn 1

–

 b2



-----------xn 2

–

 a1



-----------yn 1

–



–



-----------yn 2

–



–





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