On Sat, Nov 30, 2013 at 5:09 AM, Alexandru Csete <oz9...@gmail.com> wrote: > On Sat, Nov 30, 2013 at 2:50 AM, Kevin Reid <kpr...@switchb.org> wrote: >> >> 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). >> >> > > I tried your grc file and the scope sink shows a perfect sinewave > here. So maybe it is a voilk issue or the gr::fast_atan2f on your > parchitecture. Mine was using avx_64_mmx_orc > > Alex
Works fine for me, too. Alex is probably right that there's something going on with the SIMD architecture of your processor. The quadrature_demod_cf block uses the volk_32fc_x2_multiply_conjugate_32fc kernel. On my machine, I'm using the SSE3 version of the kernel for both aligned and unaligned architectures (in ~/.volk/volk_config). That kernel only has SSE3 and generic kernels. You might try changing those to 'generic' to see if that makes a difference on your machine. For more details on VOLK and how the configure file is structured, see: http://gnuradio.org/redmine/projects/gnuradio/wiki/Volk Tom _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
