Re: [Discuss-gnuradio] quadrature_demod losing weak signals

[Tom Rondeau]

On Sat, Nov 30, 2013 at 10:55 AM, Kevin Reid <address@hidden> wrote:
> 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?

Yes, same behavior if I turn it down; not unexpected giving what you
found below.

>> 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/>

Agreed. This threshold should be reduced. Can you experiment with that
threshold to see if there's any lower limit that might be problematic
(my default threshold, for some reason, is 1e-20; would want to make
sure that doesn't cause something else in the math to blow up).

Tom