Auto Wah audio effect (LFO control)

Gabriel Rivas September 24, 2011 Coded in C

This auto wah effect uses a variable band pass stage as a building block and a LFO

to control it. Find the variable band pass block at:



Xin                                                       Yout

/*************** AutoWah.c ********************************/

#include "bp_iir.h"
#include "AutoWah.h"

static short center_freq;
static short samp_freq;
static short counter;
static short counter_limit;
static short control;
static short max_freq;
static short min_freq;
static short f_step;
static struct bp_filter H;

This is the auto wah effect initialization function. 
This initializes the band pass filter and the effect control variables
void AutoWah_init(short effect_rate,short sampling,short maxf,short minf,short Q,double gainfactor,short freq_step) {
	double C;

	/*Process variables*/
	center_freq = 0;
	samp_freq = sampling;
	counter = effect_rate;
	control = 0;

	/*User Parametters*/
	counter_limit = effect_rate;
	/*Convert frequencies to index ranges*/
	min_freq = 0;
	max_freq = (maxf - minf)/freq_step;

	f_step = freq_step; 

This function generates the current output value
Note that if the input and output signal are integer
unsigned types, we need to add a half scale offset
double AutoWah_process(int xin) {
	double yout;

	yout = bp_iir_filter(xin,&H);
		yout += 32767;
	return yout;

This function will emulate a LFO that will vary according
to the effect_rate parameter set in the AutoWah_init function.
void AutoWah_sweep(void) {
	double yout;
	if (!--counter) {
		if (!control) {
			if (center_freq > max_freq) {
				control = 1;
		else if (control) {
			if (center_freq == min_freq) {
				control = 0;

		counter = counter_limit; 

/*************** AutoWah.h ****************************/

#ifndef __AUTOWAH_H__
#define __AUTOWAH_H__

extern void AutoWah_init(short effect_rate,short sampling,short maxf,short minf,short Q,double gainfactor,short freq_step);
extern double AutoWah_process(int xin);
extern void AutoWah_sweep(void);


/************** Usage Example **************************/

#include "AutoWah.h"

void main(void) {
    short xin;
    short yout;
    AutoWah_init(2000,  /*Effect rate 2000*/
                 16000, /*Sampling Frequency*/
                 1000,  /*Maximum frequency*/
                 500,   /*Minimum frequency*/
                 4,     /*Q*/
                 0.707, /*Gain factor*/
                 10     /*Frequency increment*/

    while(1) {
        if (new_sample_flag()) {
            /*When there's new sample at your ADC or CODEC input*/
            /*Read the sample*/
            xin = read_sample();
            /*Apply AutoWah_process function to the sample*/
            yout =AutoWah_process(xin);

            /*Send the output value to your DAC or codec output*/
            /*Makes the LFO vary*/