Re: [Discuss-gnuradio] Half band decimator/interpolator

[CEL]

Hi Albin,

Don't know what Derek suggested, but be aware that our "straight" FIR
filter decimators are already polyphase implementations, and use SIMD
(through VOLK) heavily (run volk_profile if you want to make maximum
use of that advantage); it might be that on paper, the number of
operations are unnecessarily high, but in CPU practice, some cures are
worse than the disease.

For example, I haven't tried, but it's thoroughly possible that albeit
the classical case of "half your tap memory by only storing one half of
your symmetrical FIR" performs worse than just working with the full
(half redundant) tap vector, simply because doing so avoids nonlinear
memory access patterns.

Best regards,
Marcus

On Tue, 2019-04-23 at 19:06 +0200, Albin Stigö wrote:
> Thanks for the reply Marcus. Interesting approach for sure! Will have to 
> think about it...
> 
> Derek, your solution works but it will be 4 times slower than the example I 
> linked to.
> 
> 
> --Albin
> 
> 
> On Tue, Apr 23, 2019, 18:24 Müller, Marcus (CEL) <address@hidden> wrote:
> > Hi Albin,
> > 
> > I'm very very tired right now, but me and Felix came to the following
> > conclusions on our *Whiteboard Of Wisdom, Smarts, Expertise,
> > Rationality and Science* (WOWSERS):
> > 
> > 1. A Nyquist-M bandpass filter, esp. a halfband, has zeros on every
> > other tap – except for the one center tap, which is at the maximum of
> > the sinc that you window your prototype filter with. 
> > 2. you'd thus implement that filter as polyphase filter with two
> > different polyphase components:
> >   1. one polyphase component that contains all the zeros, and the
> >      single center tap that's not
> >   2. one component that contains all the non-zero taps
> > 3. you'd thus need to "deinterleave" your input stream into two:
> >   1. Delay by half the number of taps (and multiply by the maximum
> >   2. filter with all the non-zero taps
> > 4. sum up the results
> > 
> > So, for all this, we already have blocks – yay!
> > 
> > Best regards,
> > Marcus
> > 
> > On Tue, 2019-04-23 at 17:45 +0200, Albin Stigö wrote:
> > > Just checking before I write one, is there a half band 
> > > decimator/interpolator in gnuradio I haven't been able to find...
> > > 
> > > Optimized like this:
> > > http://liquidsdr.org/doc/resamp2/
> > > 
> > > Or is there an even better option for interpolation by 2?
> > > 
> > > 
> > > --Albin
> > > _______________________________________________
> > > Discuss-gnuradio mailing list
> > > address@hidden
> > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

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