Re: [Discuss-gnuradio] Filter delays and direction of time.

[Eric Blossom]

On Thu, Jan 20, 2005 at 06:54:38PM -0500, Achilleas Anastasopoulos wrote:
> Dear all,
> 
> I was wondering what is the direction of time in the input/output
> streams used in gnuradio.

Later samples are at higher indexes.

> In the C files (eg, gr_fir_fff_generic.cc) the output
> of a filter is evaluated as
> 
> y[k]= sum_i=0 to N    h[i] x[k+i]
> 
> If the stream indexing coincides with actual time
> (ie, higher index means later sample), then
> it seems that the filter coeficients have to be
> fliped around 0 (non-causal filter).

Yes this is correct.  If you're using a non-symmetric filter, you must
reverse the taps before handing them to gr_fir_filter.  For common
cases, it doesn't matter because the taps are symmetric.

This behavior is either a bug or a feature, but at least we're
consistent.

Eric