Auto Wah and Envelope follower audio effect

Gabriel Rivas November 29, 2011 Coded in C

This is a variation of the Auto Wah effect that instead of using a fixed frequency LFO like in my past entry ( to control the band pass filter, this code emulates a envelope follower that is a rectifier and a low pass filter to obtain the envelope of the audio signal and use it as the control signal.


#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;
static double a[3];
static double b[3];
double x[3],y[3];

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; 
	//Lowpass filter parameters	
	   C = 1/TAN(PI*Fc/Fs)
	C = 1018.59;

	b[0] = 1/(1+2*0.7071*C+pow(C,2));
	b[1] = 2*b[0];
	b[2] = b[0];

	a[1] = 2*b[0]*(1-pow(C,2));
	a[2] = b[0]*(1-2*0.7071*C+pow(C,2));

double AutoWah_process(double xin) {
	double yout;

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

void AutoWah_sweep(double xin) {
	unsigned int filter_index;
	double yout;
        double detect;

	/*The input is 16 bit unsigned so it
	has to be centered to 0*/
	detect = (xin - 32767.0); 
	x[0] = x[1]; 
	x[1] = x[2]; 
	/*Here the input to the filter
	is half rectified*/
	x[2] = (detect > 0)?detect:0;
	y[0] = y[1]; 
	y[1] = y[2]; 
	y[2] =    b[0]*x[2];
	y[2] +=   b[1]*x[1];
	y[2] +=   b[2]*x[0];
	y[2] -=   a[1]*y[1];
	y[2] -=   a[2]*y[0];

	if (!--counter) {
		/*The output of the LPF (y[2]) is scaled by 0.1
		in order to be used as a LFO to control the band pass filter
	    filter_index = (double)min_freq + y[2]*0.1;
            /*The scaling value is determined as a value
            that would keep the filter index within the
            range of max_freq
            if(filter_index > max_freq) {
                filter_index = max_freq;
            if(filter_index < 0) {
                filter_index = 0;


	    counter = counter_limit; 


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