Re: [Discuss-gnuradio] Question on fir filter implementation(gr_fir_ccf_generic)

[Eric Blossom]

On Tue, Jun 02, 2009 at 02:49:58PM -0700, isulsz wrote:
> 
> I have two questions regarding to the FIR filter implementation. 
> I understand that the output of a FIR filter is the sum of the
> multiplications of filter taps and previous (and current) inputs, or simply
> speaking, a convolution, y[n] = \sum_{k from 0 to L-1}  h[k]  *  x[n-k]. 
> 
> Take gr_fir_ccf_generic for example, the code introduce a parameter
> N_UNROLL(= 2 or 4) and seems to break the summation into N_UNROLL parts:
> acc0,acc1,acc2,acc3 (if U_UNROLL == 4). and then sum them together. What is
> the reason for doing this?

Manual loop unrolling like this allows some compilers to generate
better code.  The dependency chain is shorter.

> Another confusion is that, the summation is like this:
> acc0 += d_taps[i+0] * input[i+0];
> acc1 += d_taps[i+1] * input[i+1];
> acc2 += d_taps[i+2] * input[i+2];
> acc3 += d_taps[i+3] * input[i+3];
> 
> Why here is input[i PLUS SOMETHING]? I think these are FUTURE(but not
> PREVIOUS) values of the current input (the current input is input[0] if I
> understand the code correctly). 

The delay line is implicitly contained in the input.

See the .h file:

  /*!
   * \brief compute a single output value.
   *
   * \p input must have ntaps() valid entries.
   * input[0] .. input[ntaps() - 1] are referenced to compute the output value.
   *
   * \returns the filtered input value.
   */
  virtual gr_complex filter (const gr_complex input[]);


FWIW, the code you're looking at is the guts of the generic C++
implementation of the filter.  The GNU Radio block that corresponds is
gr_fir_filter_ccf.{h,cc}


Eric