IIR Bandstop Filter

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

Simple bandstop filter with DIP1 filter/passthrough. Your own coefficients "IIRBSF.h" can be made using fdatool in MATLAB.

// Include the filter coefficients and corresponding variables
#include "IIRBSF.h"

// 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

  //  Do the filtering if DIP switch 1 up
  if (get_DIP1() == 1) {
	// In the first section, 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];


	// 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.

 	return;	//interrupt done

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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