Re: [Discuss-gnuradio] Bug in gr_quadrature_demod_cf, or scheduler?

[Matt Ettus]

Steven Clark wrote:
> Hi all-
> please look at this sequence of eye diagrams:
> http://picasaweb.google.com/steven.p.clark/GMSKFmdemodGlitches
>
> These are from a gmsk mod/demod pair, showing the output of the TX's
> gaussian filter (blue) overlaid with the output of the RX's fmdemod
> (red). BT = 0.35.
>
> At 8 samples per symbol, everything looks ok. Red is pretty much right
> on top of blue, as we'd expect.
> As I increase samples per symbol, however, something strange happens,
> shown in plot sequence. The RX's fmdemod output gets successively more
> "glitchy".
>

This might be because of the use of gr_fast_atan2f.  This function is an
approximation used in order to speed things up since the real atan2f is
slow.  Could you try replacing line 58 of gr_quadrature_demod_cf: 

          out[i] = d_gain * gr_fast_atan2f(imag(product), real(product));


with

        out[i] = d_gain * atan2f(imag(product), real(product));

and see if the problem persists?  When doing very high oversampling like
you are, the difference in phase angles between successive samples is
very small.  If the error when doing the lookup table interpolation
changes a bit when crossing over the endpoints of the table, you could
see this sort of effect.  You'll notice that the errors are always at
the same point in the diagram, around the mid-level, which is where the
frequency is lowest.

Matt