Maybe I am wrong, but here is the idea:
the original taps are "taps".
then inside the freq_xlating filter new "combined" taps are generated
as follows
comb_t = taps_t *exp(-j A t)
then the COMBINED filter is reversed.
The effect of that is that essentially we have the filter
reversed_t = taps_{-t} *exp( + j A t)
If we drop the reversing part from the process (commenting out one line of
code) we will end up
with the filter nonreversed with
nonrevered_t = comb_t = taps_t *exp(-j A t)
Comparing the reveresed and non-reversed we see that even when taps are
symmetric, the frequency sign gas changed so we need to change it back!
let me know if i am missing something,
Achilleas
On Wed, Oct 9, 2013 at 11:02 AM, Tom Rondeau <t...@trondeau.com> wrote:
> On Wed, Oct 9, 2013 at 10:45 AM, Achilleas Anastasopoulos
> <anas...@umich.edu> wrote:
> > I will submit the patch.
> >
> > regarding the sign change in frequency, I didn't mean to change the
> > convention:
> > the sign change IS REQUIRED in order to KEEP the convention because now
> > the taps are not reversed...
> >
> > Achilleas
>
> Sorry, Achilleas, I'm not seeing it. In the common case of a symmetric
> FIR filter, the reverse function doesn't change any behavior, but the
> minus sine definitely does.
>
> I don't see how reversing the order of the filter taps and changing
> the sign have anything to do with each other.
>
> Tom
>
>
> > On Wed, Oct 9, 2013 at 9:20 AM, Tom Rondeau <t...@trondeau.com> wrote:
> >>
> >> On Tue, Oct 8, 2013 at 9:39 PM, Achilleas Anastasopoulos
> >> <anas...@umich.edu> wrote:
> >> >
> >> > I was playing around with
> >> >
> >> > fir_filter_XXX
> >> >
> >> > and
> >> >
> >> > freq_xlating_fir_filter_XXX
> >> >
> >> > and noticed that the two do not give the same output
> >> > for the same input (and center_freq=0 in the xlating filter).
> >> >
> >> > Looking at the implementation of the latter
> >> > it is obvious why: the taps are reversed in the line:
> >> >
> >> > std::reverse(ctaps.begin(), ctaps.end());
> >> >
> >> > For consistency the taps should not be reversed (as in all other
> >> > filters)
> >> > We also need to set
> >>
> >> Yes, please submit a patch for this. The taps are reversed inside the
> >> fir filters, so this is redundant and confusing. Most people probably
> >> use symmetric filter taps, which is why it has not been found.
> >>
> >> > float fwT0 = 2 * M_PI * d_center_freq / d_sampling_freq;
> >> >
> >> > (instead of the minus sign in the code).
> >> >
> >> > unless there is an objection, I will go ahead and push a correction,
> >> > Achilleas
> >>
> >> Don't change the sign of the frequency. I know this is controversial,
> >> but from my experience with users, more people find the current way
> >> easier to understand. We're telling the filter what the center
> >> frequency is, which means that we will take a signal at Fc and
> >> downshift it to DC. To me, if we're on carrier Fc and we specify -Fc
> >> as the "Center Frequency", that's more confusing.
> >>
> >> Tom
> >
> >
>
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