Finite Impulse Response Filters - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 160

// Scrambler.c Voice scrambler/de-scrambler program
#include "dsk6713_aic23.h" //codec-dsk support file
Uint32 fs=DSK6713_AIC23_FREQ_8KHZ; //set sampling rate
#include "sine160.h" //sine data values
#include "LP114.cof" //filter coefficient file
short filtmodfilt(short data);
short filter(short inp,short *dly);
short sinemod(short input);
static short filter1[N],filter2[N];
short input, output;
void main()
{
short i;
comm_poll(); //init DSK using polling
for (i=0; i< N; i++)
{
filter1[i] = 0; //init 1st filter buffer
filter2[i] = 0; //init 2nd filter buffer
}
while(1)
{
input=input_sample(); //input new sample data
filtmodfilt(input); //process sample twice(upsample)
output=filtmodfilt(input); //and throw away 1st result
output_sample(output); //then output
}
}
short filtmodfilt(short data) //filtering & modulating
{
data = filter(data,filter1); //newest in ->1st filter
data = sinemod(data); //modulate with 1st filter out
data = filter(data,filter2); //2nd LP filter
return data;
}
short filter(short inp,short *dly) //implements FIR
{
short i;
int yn;
dly[N-1] = inp; //newest sample @bottom buffer
yn = dly[0] * h[N-1]; //y(0)=x(n-(N-1))*h(N-1)
for (i = 1; i < N; i++) //loop for the rest
{
yn += dly[i] * h[N-(i+1)]; //y(n)=x[n-(N-1-i)]*h[N-1-i]
dly[i-1] = dly[i]; //data up to update delays
}
yn = (yn >>15); //filter's output
return yn; //return y(n) at time n
}
short sinemod(short input) //sine generation/modulation
{
static short i=0;
input=(input*sine160[i++])>>11; //(input)*(sine data)
if(i>= NSINE) i = 0; //if end of sine table
return input; //return modulated signal
}
FIGURE 4.28. Voice scrambler program ( Scrambler.c ).