[Discuss-gnuradio] quadrature_demod losing weak signals

[Kevin Reid]

This seems like a bug, but I want to check first that I'm not having 
unreasonable expectations. I'm using GNU Radio 3.7.2.

If the amplitude of the input to quadrature_demod_cf is less than about 
10^(-2.23), then the output samples are zero instead of the demodulated signal.

Since such an exponent is well within the range for (complex) single-float 
samples, this seems like a problem with the arithmetic used by quadrature_demod.

I noticed it because my analog FM receiver would produce complete silence 
instead of static, despite not having any squelch enabled; unfortunately, it 
also produces silence on weak signals that would otherwise be intelligible. I 
have a notion that this did not occur in older GNU Radio versions (e.g. 3.6) 
but I'm not in a good position to test exactly where the problem might have 
been introduced.

I've attached a GRC flowgraph for experimenting with the behavior, and a 
screenshot of what the demodulated output looks like just at the threshold of 
failure (the signal is 200 samples per cycle).

fm_weak_test_simple.grc
Description: Binary data

-- 
Kevin Reid                                  <http://switchb.org/kpreid/>