Re: [Discuss-gnuradio] Incorrect quantizations when converting from float to char

[Paul Boven]

Hi Marcus,

This machine did not yet have a volk_config when I ran these tests.

I have since run volk_profile and rebooted, and the Float->Char 
quantization bug still occurs.

$ volk-config-info --machine
avx2_64_mmx_orc

$ grep volk_32f_s32f_convert_8i .volk/volk_config
volk_32f_s32f_convert_8i u_sse2 u_sse2

Regards, Paul Boven.

On 06/09/2018 05:30 PM, Müller, Marcus (CEL) wrote:
> Hi Paul,
>
> hm, OK, considering the actual conversion is done in VOLK, can you tell
> us
>
> * whether ~/.volk/volk_config exists (and if so, its contents regarding
> volk_32f_s32f_convert_8i )
> * what the output of `volk-config-info --machine` is?
>
> Thanks,
> Marcus
>
> On Sat, 2018-06-09 at 17:13 +0200, Paul Boven wrote:
> > Hi everyone,
> >
> > I'm trying to perform 2 bit sampling of an RF signal. In one
> > approach,
> > I'm using a float->char block, and noticed that around zero, a number
> > of
> > float inputs become quantized in a bin that's one off from the
> > correct
> > value. The ones that are wrong are always off by one, with their
> > quantization error always in the direction of zero.
> >
> > The problem can be demonstrated with a really simple flowchart,
> > using
> > the following blocks:
> >
> > * Noise Source (Noise Type: Gaussian, Amplitude: 1, Seed: 0, Output
> > type: Float)
> > * Throttle
> > The throttle is then connected to two blocks:
> > * A file-sink (Type Float) and a
> > * 'Float to Char' block.
> > * The float to char block is again connected to a File Sink, now of
> > type
> > Char.
> >
> > As an example, I've plotted all the samples that quantized as zero.
> > These should fall in the range [-0.5:0.5], but occasionally we also
> > get
> > hits that lie within [-1:1]. These mishaps are rare (about one in
> > 2000).
> > It also shows that they only occur at multiples of 8192 samples, and
> > zooming in reveals that they always happen shortly before the next
> > multiple of 8192, not after.
> >
> > For other values than 0, the same applies, but the misquantizations
> > are
> > only in one direction, ending up in a lower bin if the input is
> > positive, or in a higher bin if the input is negative. Again, the
> > misquantizations only occur in half the bin: For a value of 1, the
> > float
> > value should be in [0.5:1.5], but occasionally a value in [1.5:2]
> > also
> > ends up being quantized as 1.
> >
> > This seems to me a bug that is somehow related to internal block
> > boundaries, but I'm not familiar enough with the internals of
> > GnuRadio
> > to figure out just what's going wrong.
> >
> > The problem does NOT occur when converting to Short or Int.
> >
> > This is using GnuRadio 3.7.11 (as packaged with Ubuntu 18.04).
> >
> > Regards, Paul Boven.
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
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