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Re: [Discuss-gnuradio] Bug in freq_xlating_fir_filter_XXX

From: Achilleas Anastasopoulos
Subject: Re: [Discuss-gnuradio] Bug in freq_xlating_fir_filter_XXX
Date: Wed, 9 Oct 2013 23:01:03 -0400

I attach the patch for this correction
(for some reason I cannot git push...)


On Wed, Oct 9, 2013 at 12:59 PM, Achilleas Anastasopoulos <address@hidden> wrote:
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,

On Wed, Oct 9, 2013 at 11:02 AM, Tom Rondeau <address@hidden> wrote:
On Wed, Oct 9, 2013 at 10:45 AM, Achilleas Anastasopoulos
<address@hidden> 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.


> On Wed, Oct 9, 2013 at 9:20 AM, Tom Rondeau <address@hidden> wrote:
>> On Tue, Oct 8, 2013 at 9:39 PM, Achilleas Anastasopoulos
>> <address@hidden> 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

Attachment: 0001-fixed-freq_xlating_filter-block-so-that-it-does-not-.patch
Description: Binary data

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