16-bit IIR filters

UM0585

16/25

Doc ID 14988 Rev 2

Figure 4.

Block diagram of the canonical form of a second-order IIR filter

The DSP library implements the two forms (direct form I and the canonical form) of the IIR

filter:

●

Direct form I is used to design an IIR filter of order 4 (ARMA IIR)

●

The canonical form is used to design an IIR filter of order 8, by using 4 second-order

IIR filter sections (biquads) arranged in series (biquad IIR filter).

5.2

DSP library functions

The DSP library provides two IIR filters:

1.

iiarma_stm32

: an ARMA IIR filter, designed with 4 auto-regressive and 5 moving-

average filter coefficients, so, in Equation 1, M = 4 and N = 4. The function code is

written in assembly.

2.

iir_biquad_stm32

: a biquad IIR filter, designed by connecting 4 biquads in series.

The function code is written in C.

5.2.1

iiarma_stm32

function

Table 9 describes the iirarma_stm32 function.

+

+

+

+

b

2

ai15450

Table 9.

iirarma_stm32

function

Function name

iirarma_stm32

Prototype

void iirarma_stm32(short *y, short *x, short *h2, short

*h1, int ny)

Behavior description

16-bit, auto-regressive moving-average IIR (ARMA) filter.

Input parameter

– y: output array of length ny+4

– x: input array of length ny+4

– h2: filter coefficient vector, moving-average part.

– h1: filter coefficient vector, auto-regressive part.

– ny: number of output samples

Output parameter

None

Return parameter

None