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

[Albin Stigö]

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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