DSP Applications and Student Projects - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 403

// IIR_ctrl.c Selectable IIR filter with scrambling option using DIP SW
. . .
short DIP_Mask = 20; //any DIP SW value except 0-15
short BUFFER_SIZE = 512; //size of buffer
short buffer[512]; //buffer for voice scrambler
short index=0,input_index=0,output_index=0;//index for sample #,buffer
interrupt void c_int11()
{
short i, input;
int un, yn;
input = (short)input_sample(); //external input
if (DIP_Mask == 0) { //output = input (no filtering)
{. . . yn=input; } //like a loop program
}else if (DIP_Mask == 1) { //2kHz filter if DIP=1
for(i=0;i<stages;i++) {un=input-... yn=...update delays- See IIR.c}
...
}else if (DIP_Mask == 2) {...

//...for other filters

}
else if (DIP_Mask == 5){ //for voice scrambler
if((index % 2) == 0) { //every other sample
buffer[input_index++] = input; //input sample->buffer
if(input_index==BUFFER_SIZE) {input_index=0;} //reset when full
}
if (index >= BUFFER_SIZE) { //if buffer is at least half full
yn = buffer[output_index++]; //output next value
if(output_index==BUFFER_SIZE) {output_index=0;} //reset if at end
}
index++; //incr overall sample index
if(index>=(BUFFER_SIZE*2)) {index=0; } //reinit sample index if end
}else { yn = 0; } //no output if other DIP #

output_sample((short)(yn)); // output

return;
}
void main()
{
comm_intr();
while(1) {
short newMask = 0;
newMask += DSK6713_DIP_get(3) * 1;
newMask += DSK6713_DIP_get(2) * 2;
newMask += DSK6713_DIP_get(1) * 4;
newMask += DSK6713_DIP_get(0) * 8;

//hex value of DIP switch

if (DIP_Mask != newMask) { //wait for change

DIP_Mask = newMask; //load DIP switch value

if (DIP_Mask == 5) {
DSK6713_LED_on(3);
DSK6713_LED_off(2);
DSK6713_LED_on(1);
DSK6713_LED_off(0);
} else if (DIP_Mask == 4) { ... //for other SWs

} //and all LEDs off

//end of 1 st if }

} //end of while(1)

} //end of main

FIGURE 10.40. Core C program to select an implement IIR filters using the onboard
switches with an optional scrambling scheme.