RM0020 datasheet(54/69 Pages) STMICROELECTRONICS

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Function API

RM0020

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Arguments:

Description:

Computes the cascade of second-order IIR filters on frac16 data. The input samples

are stored in the array x, output samples are written to the array y. The function calls

the iir_float_2nd function for each second-order IIR filter in the cascade. There is

used the arithmetic with saturation so the output will never overflow.

Algorithm:

The filter equation for the i-order IIR filter (i = 2*m):

Equation 47

for each n = 0,1...N-1

The i-order filter is implemented using the cascade of m IIR filters of second-order:

Equation 48

Equation 49

Equation 50

for each n = 0,1...N-1

Performance: See Section 7 and Table 60.

Table 29.

iir_frac16_casc 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 and y. 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.

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

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 for each second-order IIR filter, c[m*5] =

{b10, b11, b12, a11, a12, b20, b21, b22, a21, a22,

...,

bm0, bm1, bm2, am1, am2}

in/out

array of previous input and output values (state vector), s[m*4] =

{x1(-2), x1(-1), y1(-2), y1(-1), x2(-2), x2(-1), y2(-2), y2(-1),

...,

xm(-2), xm(-1), ym(-2), ym(-1)}

number of second-order IIR filters in the cascade



xn

 b1

xn 1

–

 

xn i

–

 a1

yn 1

–



–

 ai

yn i

–



–



b10

x

 b11

x

–

 b12

x

–

 a11

y

–



–

a12

y

–



–



b20

y

 b21

y

–

 b22

y

–

 a21

y

–



–

a22

y

–



–





bm0

y

–

 bm1

y

–

 bm2

y

–

 am1

y

–



–

am2

y

–



–

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