Re: [Discuss-gnuradio] Works with GR 3.6, breaks with 3.7

[Andy Walls]

On Mon, 2014-11-03 at 12:01 -0500, address@hidden
wrote:

>    6. Re: Works with GR 3.6, breaks with 3.7 (Luke Berndt)
> 
> On the off chance that someone has some free time to look into this, I
> have built the Smartnet trunk control channel in GRC. I have also made
> a capture of the signal. Everything is working the way it does on my C
> ++ program. It is able find a lot of frames where the preamble
> matches, but barely any of them bas the CRC test. The expected
> behavior is that there would be 2-3 frames that pass the CRC test
> every second, and that is how it works under 3.6.5.
> 
> The SmartNet stuff is rolled into 3.7 blocks. Git clone, cmake . ,
> make, sudo make install
> 
> If anyone sees some obvious mistakes let me know. I know this not the
> ideal way to decode an FSK signal, but it works perfectly in 3.6.

Hi Luke,

I have not built the SmartNet blocks yet, but:

Your low pass filter looks way too wide and you'll get aliases when you
decimate by 185.  2 Msps / 185 = 10.81 ksps, so the Nyquist frequency is
5.405 kHz.  I winged this is as a low pass filter and things looked
better:

firdes.low_pass_2(1, samp_rate, 4500, 1000, 60, firdes.WIN_HANN, 6.76)

The 60 dB down may be overkill, you can make it smaller for a filter
with less delay.

I found an offset slider value of -15k made things look about centered
manually.

You might want to put a waterfall sink before and after the FLL
Band-Edge filter to observe how it is making the spectrum wobble around
a little. If you change your offset slider, you can see the FLL
band-edge filter centering things back up; so that looks like it is
working.

Regards,
Andy


> Thanks for any pointers people have!
> 
> 
> Capture of the signal:
> http://we.tl/ZMD8R5uQfR <http://we.tl/ZMD8R5uQfR>
> 
> GRC Blocks for Smartnet
> https://github.com/robotastic/gr-smartnet
> <https://github.com/robotastic/gr-smartnet>
> 
> 
> 
> GRC to decode Smartnet Tunking channel
> 
> 
> 
> 
> > On Oct 20, 2014, at 3:19 AM, Martin Braun <address@hidden>
> wrote:
> > 
> > On 10/18/2014 05:57 PM, Luke Berndt wrote:
> >> The Band Edge FLL works great for me too, on 3.6. Does anyone know
> if
> >> there were changes to it or surrounding blocks in 3.7 that would
> make it
> >> stop working?
> > 
> > None of the DSP was changed, if it worked before it should(TM) still
> > work. The reason we say it's a bad choice is that it was derived for
> > PSK/PAM signals. The reason it still might work is that the
> > implementation runs 2 filters at the band-edges and compares the
> energy
> > content of those, and that difference might be sufficient to correct
> > your frequency offset if you have sufficient averaging (i.e. low
> enough
> > loop bandwidth).
> > 
> > M
> > 
> > 
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> 
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