Infinite Impulse Response Filters - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 197

conditions, determine the frequency generated. They are scaled for a fixed-point
implementation. Using the difference equation

() = ( ) -- ( )

y n Ay n - 1 y n 2

the output at time n = 0 is

() = () - () =- ( ) ( ) + ( ) =

y 0 y 1 y 2 2 cos ww T T sin sin 2 w T 0

// two_tones.c Generates/adds two tones using difference equations

#include "DSK6713_AIC23.h" //codec-DSK support file
Uint32 fs=DSK6713_AIC23_FREQ_8KHZ; //set sampling rate
short sinegen(void); //for generating tone
short output; //for output
short sinegen_buffer[256]; //buffer for output data
const short bufferlength = 256; //buffer size for plot with CCS
short i = 0; //buffer count index
short y1[3] = {0,-15137,-11585}; //y1(0),y1(-1),y1(-2) for 1.5kHz
const short A1 = 12540; //A1 = 2coswT scaled by 2^14
short y2[3] = {0,-16384,0}; //y2(0),y2(-1),y2(-2) for 2kHz
const short A2 = 0; //A2 = 2coswT scaled by 2^14

interrupt void c_int11() //ISR

{
output = sinegen(); //out from tone generation function
sinegen_buffer[i] = output; //output into buffer
output_sample(output); //output result
i++; //increment buffer count
if (i == bufferlength) i = 0;

//if buffer count=size of buffer

return; //return to main

}

short sinegen() //function to generate tone

{
y1[0] =((y1[1]*A1)>>14)-y1[2]; //y1(n)= A1*y1(n-1)-y1(n-2)

y1[2] = y1[1]; //update y1(n-2)

y1[1] = y1[0]; //update y1(n-1)

y2[0] =((y2[1]*A2)>>14)-y2[2]; //y2(n)= A2*y2(n-1)-y2(n-2)

y2[2] = y2[1]; //update y2(n-2)

y2[1] = y2[0]; //update y2(n-1)

return (y1[0] + y2[0]); //add the two tones

}

void main()
{
comm_intr(); //init DSK, codec, McBSP

while(1); //infinite loop

}

FIGURE 5.19. Program to generate and add two tones ( two_tones.c ).