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

[CEL]

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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