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Re: [Discuss-gnuradio] fxpt_nco vs rotator ?

From: Douglas Geiger
Subject: Re: [Discuss-gnuradio] fxpt_nco vs rotator ?
Date: Wed, 31 Dec 2014 01:37:36 -0500

On Tue, Dec 30, 2014 at 11:55 PM, Sylvain Munaut <address@hidden> wrote:
If you need high, deterministic, quantifiable precision, you're
probably better off reimplementing the critical blocks yourself the
way you want.

Most of the default blocks will have numerous optimization for speed
that can sacrifice precision. And this will actually depend on which
version of the volk kernel gets selected (generic / SSE / AVX / ....)
because they use different algorithm and opcodes and such.

I'm pretty sure the rotator SIMD version has spurs because of the
periodic renormalization process, but in any case they're like more
than -130 dB down which makes them irrelevant for all but the most
demanding applications.



Just as an aside - and a note for other folks who want to investigate differences between SIMD-optimized vs. generic kernels, VOLK does have an interesting (but perhaps not well-documented) feature in that if you set/create an environment variable called VOLK_GENERIC (e.g. 'export VOLK_GENERIC=1'), then the VOLK dispatcher will only call the generic implementation.
This shows up in most GR and OOT QA tests automatically (at least in the Python-based QA tests) via the GrTest.cmake module.


Doug Geiger

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