Re: [Discuss-gnuradio] Bug in freq_xlating_fir_filter_XXX

[Tom Rondeau]

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.

Tom


> 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
>
>