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
<<inline: Screen Shot 2013-11-29 at 17.42.21.png>>
-- Kevin Reid <http://switchb.org/kpreid/>
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