Re: [Discuss-gnuradio] Request for comment: Adding fixed point to VOLK

[Albin Stigö]

What about naming these functions... any ideas? Here are some
suggestions of the top of my head:

volk_32i_convert_32f
volk_q1.32_convert_32f
volk_q1_32_convert_32f


--Albin

On Fri, May 31, 2019 at 10:30 AM Albin Stigö <address@hidden> wrote:
>
> Thanks for the feedback Marcus!
>
> Pretty much all SDRs I've seen use either 8, 16 or 32 bit full range
> integers (or 24 bits packed in the most significant bits of a 32 bit
> int so can use same conversion as 32 bit).
>
> Yes Q1.15 means 1 sign bit and 15 fractional bits, in practice this
> just means that a full range int16_t will map to +-1.0. An N-tap FIR
> filter which has L-bit data values and M-bit coefficients requires L +
> M + log2(N) bits in the accumulator.
>
> A CIC decimator/interpolator might also be interesting, although I'm
> not sure if there's already an implementation.
>
>
> --Albin
>
> On Fri, May 31, 2019 at 10:13 AM Müller, Marcus (CEL) <address@hidden> wrote:
> >
> > Hi Albin,
> >
> > I'd love this, especially if we end up with a kernel that we can use to
> > build decimating FIR filters that inherently convert to higher-bitwidth
> > int or floating point.
> >
> > I think this could be super useful in hardware flow graphs, where one
> > would get integer numbers from hardware anyway; in a low-SNR scenario
> > only processing gain (e.g. through filtering / correlation) would make
> > it sensible to use a larger integer or floating point's larger dynamic
> > range.
> >
> > Not quite sure how that would preferably look like, but I'd imagine
> > Q1.15 going in, getting weighted internally with Q1.15 (or Q1.31 with
> > limited amplitude) coefficients to add Q1.31 numbers to a Q1.31
> > accumulator. (I'm assuming Q1.15 means 15 fractional binary digits and
> > one sign bit.)
> >
> > (Oh, and yes, x86_64 / SIMD extensions have relatively nice conversion
> > instructions.)
> >
> > Best regards,
> > Marcus
> >
> > On Fri, 2019-05-31 at 09:50 +0200, Albin Stigö wrote:
> > > Hi,
> > >
> > > Volk has functions for converting between int and float by casting and
> > > multiplying with a scalar. I'd like to purpose functions for the
> > > special case where scalar is 1/INT32_MAX (and 1/INT16_MAX etc) as this
> > > is a very common use case and might be available to an optimization.
> > > Certain CPUs (armv8 in particular [1], not sure about x86? [2]) has
> > > hardware instructions for converting fixed point to float and back.
> > >
> > > In particular I'm interested in adding the common Q1.31 and Q1.15
> > > formats (full range signed int32_t and full range int16_t to
> > > (-1.0)-1.0 float).
> > >
> > > Would be grateful for input regarding this and in particular ideas for
> > > naming the functions.
> > >
> > >
> > > --Albin
> > >
> > > [1] 
> > > http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0801h/pge1427897637493.html
> > > [2] 
> > > https://software.intel.com/en-us/articles/fast-floating-point-to-integer-conversions
> > >
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