[Discuss-gnuradio] grc amplitude demodulation questions

[ematlis]

Hello-

I am using GRC 3.3 to create a flow graph using the USRP2 and a LFRX to do 
amplitude demodulation.  I have a few questions.  

1) What are good values for "Audio Pass" and "Audio Stop" within the "AM Demod" 
block?  

2) I observe a DC offset at the output of the demodulation.  Is this expected, 
and what is the best way to remove it?  I am currently using a high-pass 
filter.  My incoming signal is a 27 kHz sine modulated at 1 kHz.

3) I am sending the result to an FFT sink. I instituted a variable slider to 
control the decimation within the AM Demod block, but when I change that 
decimation, the displayed fft trace does not adjust correctly to the new sample 
rate.  In fact, the fft trace remains in a fixed position; the only thing that 
changes in the plot is the x-axis, ie frequency.  I have a second slider for 
the usrp2 decimation which does change the displayed spectrum correctly, so 
that the peaks move to the correct position.  Any thoughts?

4) Related to the above- I thought I'd mention that if I move the usrp2 
decimation slider bar too quickly, the fft plot then fails to adjust the fft 
trace correctly - I have to move the bar slowly.

5) Occasionally I am seeing a random error which causes the flowgraph to freeze:

Executing: "/home/matlis/Downloads/top_block.py"

Error: failed to enable realtime scheduling.
>>> gr_fir_fff: using SSE
Traceback (most recent call last):
  File 
"/usr/local/lib64/python2.6/site-packages/gnuradio/wxgui/plotter/plotter_base.py",
 line 187, in _on_paint
    for fcn in self._draw_fcns: fcn[1]()
  File 
"/usr/local/lib64/python2.6/site-packages/gnuradio/wxgui/plotter/plotter_base.py",
 line 59, in draw
    self._draw()
  File 
"/usr/local/lib64/python2.6/site-packages/gnuradio/wxgui/plotter/grid_plotter_base.py",
 line 395, in _draw_p
oint_label
    label_str = self._populate_point_label(x_val, y_val)
  File 
"/usr/local/lib64/python2.6/site-packages/gnuradio/wxgui/plotter/channel_plotter.py",
 line 149, in _populate
_point_label
    y_value = (samples[x_index_high] - samples[x_index_low])*scale + 
samples[x_index_low]
IndexError: index out of bounds

>>> Done


Thanks for any suggestions!

eric