How can I apply a low-shelving filter using Visualdsp++? - c

Question

I'm very new to DSP. And have to solve the following problem: applying the low shelving filter for an array of data. The original data is displayed in fract16 (VisualDSP++).
I'm writing something as below but not sure it's correct or not.
Does the following code have any problem with overflow?
If 1 is true, how should I do to prevent it?
Any advice on this problem?
fract16 org_data[256]; //original data
float16 ArrayA[],ArrayB[];
long tmp_A0, tmp_A1, tmp_A2, tmp_B1, tmp_B2;
float filter_paraA[3], filter_paraB[3]; // correctness: 0.xxxxx
// For equalizing
// Low-Shelving filter
for ( i=0; i<2; i++)
{
tmp_A1 = ArrayA[i*2];
tmp_A2 = ArrayA[i*2+1];
tmp_B1 = ArrayB[i*2];
tmp_B2 = ArrayB[i*2+1];
for(j=0;j<256;j++){
tmp_A0 = org_data[j];
org_data[j] = filter_paraA[0] * tmp_A0
+ filter_paraA[1] * tmp_A1
+ filter_paraA[2] * tmp_A2
- filter_paraB[1] * tmp_B1
- filter_paraB[2] * tmp_B2;
tmp_A2 = tmp_A1;
tmp_B2 = tmp_B1;
tmp_A1 = tmp_A0;
tmp_B1 = org_data[j];
}
ArrayA[i*2] = tmp_A1;
ArrayA[i*2+1] = tmp_A2;
ArrayB[i*2] = tmp_B1;
ArrayB[i*2+1] = tmp_B2;
}

Answer 1

I don't know what the range is for fract16, just -1 to +1 approx?
The section that stands out to me as possibly generating an overflow is assigning org_data[j] but will be dependent on what you know about your input signal and your filter coefficients. If you can ensure that multiplying filter_paraA[2:0] to signal with values tmp_A2..1 = [1,1,1] is < max(fract16) you should be fine regardless of the 'B' side.
I would recommend adding some checks for overflow in your code. It doesn't necessarily have to fix it, but you would be able to identify an otherwise very tricky bug. Unless you need absolute max performance I would even leave the check code in place but with less output or setting a flag that gets checked.
macA = filter_paraA[0] * tmp_A0 + filter_paraA[1] * tmp_A1 \
+ filter_paraA[2] * tmp_A2;
macB = filter_paraB[1] * tmp_B1 - filter_paraB[2] * tmp_B2;
if((macA-macB)>1){
printf("ERROR! Overflow detected!\n");
printf("tmp_A[] = [%f, %f, %f]\n",tmp_A2,tmp_A1,tmp_A0);
printf("tmp_B[] = [%f, %f]\n",tmp_B1,tmp_B0);
printf(" i = %i, j = %i\n",i,j);
}