dsps_fird_s16_ansi.c File Reference

#include "dsps_fir.h"

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Functions
int32_t	dsps_fird_s16_ansi (fir_s16_t *fir, const int16_t *input, int16_t *output, int32_t len)
	16 bit signed fixed point Decimation FIR filter

Function Documentation

dsps_fird_s16_ansi()

int32_t dsps_fird_s16_ansi	(	fir_s16_t *	fir,
		const int16_t *	input,
		int16_t *	output,
		int32_t	len )

16 bit signed fixed point Decimation FIR filter

Function implements FIR filter with decimation The extension (_ansi) uses ANSI C and could be compiled and run on any platform. The extension (_ae32) is optimized for ESP32 chip.

Parameters

fir	pointer to fir filter structure, that must be initialized before
input	input array
output	array with the result of the FIR filter
len	length of the result array

Returns

: function returns the number of samples stored in the output array depends on the previous state value could be [0..len/decimation]

Definition at line 9 of file dsps_fird_s16_ansi.c.

{
    int32_t result = 0;
    int32_t input_pos = 0;
    long long rounding = 0;
    const int32_t final_shift = fir->shift - 15;
 
    rounding = (long long)(fir->rounding_val);
 
    if (fir->shift >= 0) {
        rounding = (rounding >> fir->shift) & 0xFFFFFFFFFF;         // 40-bit mask
    } else {
        rounding = (rounding << (-fir->shift)) & 0xFFFFFFFFFF;      // 40-bit mask
    }
 
    // len is already a length of the *output array, calculated as (length of the input array / decimation)
    for (int i = 0; i < len; i++) {
 
        for (int j = 0; j < fir->decim - fir->d_pos; j++) {
 
            if (fir->pos >= fir->coeffs_len) {
                fir->pos = 0;
            }
            fir->delay[fir->pos++] = input[input_pos++];
        }
        fir->d_pos = 0;
 
        long long acc = rounding;
        int16_t coeff_pos = fir->coeffs_len - 1;
 
        for (int n = fir->pos; n < fir->coeffs_len ; n++) {
            acc += (int32_t)fir->coeffs[coeff_pos--] * (int32_t)fir->delay[n];
        }
        for (int n = 0; n < fir->pos ; n++) {
            acc += (int32_t)fir->coeffs[coeff_pos--] * (int32_t)fir->delay[n];
        }
 
        if (final_shift > 0) {
            output[result++] = (int16_t)(acc << final_shift);
        } else {
            output[result++] = (int16_t)(acc >> (-final_shift));
        }
 
    }
    return result;
}

References fir_s16_s::coeffs, fir_s16_s::coeffs_len, fir_s16_s::d_pos, fir_s16_s::decim, fir_s16_s::delay, n, fir_s16_s::pos, fir_s16_s::rounding_val, and fir_s16_s::shift.