Code

IIR FIlter and add tone

Ron April 1, 2011 Coded in C for the TI C67x

Filters and then corrupts the signal with a overlaid tone. Your own coefficients "IIRLPF.h" can be made using fdatool in MATLAB.

//iirFilter.c  

// Include the filter coefficients and corresponding variables
#include "IIRLPF.h"
#define A 1		//The amplitude of added wave
#define FREQ 1000	//The frequency of the sine wave in Hz

// The intermediate values in the Direct Form II filter
float delay_w[MWSPT_NSEC][3];		
						// delay_w[i][j] <=> w_i(n-j), j=0,1,2
						// i is the section, j the delay
						
float sectionOut;		// yk[n] <=> sectionout

interrupt void isr()	 //Interrupt function t=125us, f = 8kHz
{	
  short i; 			// i loops through the MWSPT_NSEC number of sections
  int period = 8000 / FREQ;
  float rad = FREQ * 2 * pi;
  int j = 0;

  //  Do the filtering if DIP switch 1 up
  if (get_DIP1() == 1) {
			
	// In the first section, we read in the x-value, apply the first stage gain
	sectionOut = NUM[0][0] * get_sample();

	for (i=1; i<MWSPT_NSEC; i++) { // Loop through all the sections
	
		// Get the new delay_w[0];
		delay_w[i][0] = sectionOut - DEN[i][1]*delay_w[i][1] - DEN[i][2]*delay_w[i][2];

		// Get the output of this section		
		sectionOut = NUM[i][0]*delay_w[i][0] + NUM[i][1]*delay_w[i][1] + NUM[i][2]*delay_w[i][2];
	
		// Delay the w's for the next interrupt
		delay_w[i][2] = delay_w[i][1];
		delay_w[i][1] = delay_w[i][0];

	}

	//Add a tone to sectionOut
	sectionOut = sectionOut + A*sin(rad*j); //Add a sine wave of freq rad to sectionOut
	j = (j + 1) % period; //Increment the sine wave counter

	// Apply the gain, convert to short and send out
	send_output((short)(2 * sectionOut));
	// Gain of 2 chosen heuristically for speech from PC

  } else { // If DIP switch 1 down, == 0, then just pass through signal.

	send_output(get_sample());
  }

 	return;							// return from interrupt
}

void main()
{
	short i,j;

 	for (i=0; i<MWSPT_NSEC; i++)
 		for (j=0; j<3; j++) 
			delay_w[i][j] = 0;      // init intermediate array
			
	init_all();                  	// init all
	while(1);    		  	// infinite loop
}

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