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Re: [Discuss-gnuradio] Xlate FIR filter broken in gr 3.7

From: Iain Young, G7III
Subject: Re: [Discuss-gnuradio] Xlate FIR filter broken in gr 3.7
Date: Thu, 08 Aug 2013 21:37:03 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130623 Thunderbird/17.0.7

On 08/08/13 21:14, Johnathan Corgan wrote:
On 08/08/2013 01:01 PM, Andrew Davis wrote:

It's not just you, I noticed this too. This change broke gr-atsc ( but I
fixed that ).
The problem is on line 80 of
gr-filter/lib/freq_xlating_fir_filter_XXX_impl.cc.t: the 2 was not
negative in 3.6 but is now, i'm not sure why.

Good catch, thanks.

The "old style" version of the block in gnuradio-core in works
correctly.  The "new style" version of the block in gr-filter in
has the sign change.  When all the old-style blocks were removed for
3.7, this of course left only the changed one.

Since it was Tom who did the conversion on that block, I'll wait until
he chimes in on whether his change was an accident or intentional.

I'll defer to those who know the code better than I, however I will note
that the fact that it was "reverse" (ie negative values went "up" in
frequency within the filter" confused me at first...

...But then I spoke to others that used other SDRs, and found that they
had the same behaviour. So I stuck a sign in front, and carried on,
believing it was some strange function of the way things worked deep
in SDR theory, and didn't worry about it any more.

Would suggest that if others do it this way (ie negative values in the
FIR filter go up), it may be worth to keep that behaviour for
convention's sake if there's no better reason to change it

Best Regards


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