Re: [Discuss-gnuradio] quadrature_demod losing weak signals

[Kevin Reid]

On Nov 30, 2013, at 2:09, Alexandru Csete <address@hidden> wrote:
> On Sat, Nov 30, 2013 at 2:50 AM, Kevin Reid <address@hidden> wrote:
>> 
>> 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.
> 
> I tried your grc file and the scope sink shows a perfect sinewave
> here.

Just checking, since I didn't mention it before: the default value of the 
slider is the level well above where it fails. Does it fail for you if you turn 
it down?

> So maybe it is a voilk issue or the gr::fast_atan2f on your
> parchitecture. Mine was using avx_64_mmx_orc

Thank you, I should have mentioned my volk configuration. fast_atan2f doesn't 
appear to have any volk dependency...

...but I think I've spotted the problem. In fast_atan2f, we have this 
recently-introduced (Nov 6) code, where the epsilon used to be a comparison 
with zero:

    /* don't divide by zero! */
    if((y_abs < 1.5E-5) && (x_abs < 1.5E-5))
      return 0.0;

Since the preceding multiply_conjugate should square the magnitude, this choice 
of epsilon accounts for the threshold I found empirically.

Given this, I'm going to file an issue requesting that this epsilon be reduced. 
(The only alternative that comes to mind is to place an AGC block before the 
demodulator, which seems unlikely to be better from a numerical perspective).

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