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. > > > > > > > > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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