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

[Paul Boven]

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.

Float_to_Char.png
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