FIR FILTER DESIGN? inext = (inext+1) & (BUFFER_SIZE-1);

[duskatnine]

Hello There. Im Using te dsPIC PRO 3 Development Board With a dsPIC6014A Running at 80mHz. I, Like Many Others, Have Had Considerable Difficulies Successfully Running The Filter Design Code Generated By The Filter Design Tool. I Have Used The Support Forum To Reference and Resolve My Other Faults. My New Question Is This:

This Is The FIR Filter Code Generated By The Filter Design Tool. Im Sampling Audio Through The ADC, into the dsPIC, and Out The Dac

Code: Select all

BUFFER_SIZE = 16;
FILTER_ORDER = 8;
input[BUFFER_SIZE];
inext = 0;


  input[inext] = ADCBUF0;    // Fetch sample from ADC BUFFER
                                         
                                                                                                   
  CurrentValue = FIR_Radix ( 
                         FILTER_ORDER+1,   // Filter 
                         COEFF_B,          // coefficients                           
                         BUFFER_SIZE,      // Input buffer length        
                         input,            // Input buffer  
                         inext);           // Current sample
                                        

            
  inext = (inext+1) & (BUFFER_SIZE-1);

I Do Not Understand the "inext = ...... " line. If I Remove It, My test tone(s) run clean through the ADC>>Code>>DAC, but they are not filtered. If I Leave This Line of Code In, The DAC Just Puts Out Noise. I'd Dont Understand The Logic Behind It. If We Evaluate It:

• inext+1 & BUFFER_SIZE - 1 = input
  
  0001 & 1111 = 0001
  0010 & 1110 = 0010
  0011 & 1101 = 0001
  0100 & 1100 = 0100
  0101 & 1011 = 0001
  0110 & 1010 = 0010
  ... & .... = ....

So The Way I See It, Your Indexing The Buffer In A Really Sporatic Way {Index 1, then 2, then 1, then 4, then 1, then 2}

...Am I Missing Something or is this another ME Fault?

Regards

[MasterBlaster]

Hello There

Hi duskatnine,

you posted in incomplete code, without all forwarded defines and/or var's declarations...

When anyone shall help you with your problem, it's important to know exactly
whats happend with all defines and var declarations.

[idakota]

You are definitely missing something

inext = (inext+1) & (BUFFER_SIZE-1);

BUFFER_SIZE is a constant which according to the simple filter I just did is 32, so BUFFER_SIZE - 1 = 31 = b0001 1111

Assuming inext starts at 0 you have:

0000 0001 & 0001 1111 = 0000 0001;
0000 0010 & 0001 1111 = 0000 0010;
0000 0011 & 0001 1111 = 0000 0011;
.
.
.
0010 1111 & 0001 1111 = 0000 0000;

What is essentially does is make inext count from 0 to 31 without needing to use a if statement (thereby making it faster)