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Re: [Discuss-gnuradio] Improve FSK4 decodes in OP25
From: |
Andy Walls |
Subject: |
Re: [Discuss-gnuradio] Improve FSK4 decodes in OP25 |
Date: |
Tue, 17 Jan 2017 13:10:22 -0500 |
Hi Luke:
On Tue, 2017-01-17 at 12:50 -0500, Luke Berndt wrote:
> Andy - Thank you so much, this was extremely helpful. I switched to
> your RF chain and I think it helped a bit.
I'm glad it helped.
FWIW I tried an alternate FM demod approach - 4 PLLs one at each tone -
but the tones turned out to be not spaced far enough apart for that to
work well. I'd didn't bother with the 4 bandpass filter, non-coherent
approach that Marcus mentioned, since 4 coherent PLL's didn't work out.
Stick with either the FM detector or Quad Demod.
> As you can probably guess, I am more of a programmer than an RF
> expert.
I'm not very good a great many things myself. :)
>
> It does look like Max has a branch of OP25 which has a deemphasis
> filter that has been tuned for P25.
> https://git.osmocom.org/op25/tree/op25/gr-op25_repeater/apps/tx/op25_c4fm_mod.py?h=max
> It might be tricky to port to C++ because it uses Numpy, but I am going to
> give it a shot.
Don't bother. That's just a moving average filter of approximately the
symbol length. The same one I have in the flowgraph I gave you. The
sinc(4800f) in the frequency domain, works out to a boxcar in the time
domain.
Now the APCO P25 transmitters' pre-emphasis filter is a little weird,
since it is 1/sinc(4800f) in the frequency domain and goes infinite.
However it is truncated at 2880 Hz, before it hits it's first infinity.
Luckily you don't need the pre-emphasis filter.
>
> I do think you are right though, the underlying problem is that there
> is a lot of noise. This may sound crazy, but I think it is coming from
> the USB hub I am using. I know USB2 operates around 480MHz and I am
> trying to record at 490MHz. I am going to experiment with my setup a
> bit to see if this is it.
Not crazy; EMI is a real thing in UHF.
The source can also be PC DRAM clocks, PC monitors, etc.
Funny side note: my directional TV antenna gets lots of noise at least
once a day when it points at the galactic disc. Apparently the Milky
Way is a large source of UHF noise.
>
> Thanks again for taking the time to put this all together. I will
> report back on how things are going and if mange to get clean
> recordings.
It was fun. Down where I live they've just upgraded to APCO P25, but
its all new and all Phase II, so no C4FM signals for me. :(
Good luck!
-Andy
>
> - Luke
>
> On Sun, Jan 15, 2017 at 6:58 PM, Andy Walls
> <address@hidden> wrote:
> Hi Luke,
>
>
> See the attached GRC for what can be done in GNURadio.
>
>
> 1. You need the baseband noise filter.
>
> 2. You may or may not need the deemphasis filter. It appears
> to make things better,
> but it introduces receiver side ISI, since the filter doesn't
> get dumped at every symbol
> peak. (APCO 25 expects an integrate and dump deemphasis
> filter, this one just
> integrates).
>
> 3. You may wish to use the FM detector PLL instead of the Quad
> Demod block. You
> can limit symbol peak frequency deviation using the Max Freq
> and Min Freq parameters.
> Currently, I have them set at +/-3 * 600 Hz * 190%. Leave the
> loop bandwidth alone;
>
> it is set at symbol_rate/2 * 1.20 Hz = 2880 Hz, which is where
> the transmitter Nyquist
>
> filter should cut off.
>
>
> The signal you provided really does have a very poor SNR. The
> attached low SNR
>
> png shows the best I could do with it. There are some of the
> 4 levels visible in the chaos,
>
> but it's not good. I have attached a png from a high SNR test
> signal for comparison.
>
>
> So what you can do "outside" of GNURadio:
>
>
> 1. Attach an external receiver bandpass filter for the 470 -
> 512 MHz safety
>
> band to your radio. The only small, reasonably priced, off
> the shelf ones I could find
> are ceramic packages that are expected to be soldered to a
> board. :(
>
>
> 2. Set your gains on your SDR smartly to maximize your
> receiver noise figure.
>
> Set the RF gain as high as you can without clipping the IF
> stage, set the
>
> IF gain as high as you can without clipping the Base Band
> stage. Set the Base
>
> Band gain as high as you can without clipping the ADC inputs.
>
> https://en.wikipedia.org/wiki/Friis_formulas_for_noise
> "... the overall noise figure of a radio receiver is primarily
> established by the noise figure of its first amplifying stage.
> Subsequent stages have a diminishing effect on signal-to-noise
> ratio. "
>
>
>
> Regards,
>
> Andy
>
>
>
> On Thu, Jan 12, 2017 at 12:00 PM,
> <address@hidden> wrote:
> Message: 6
> Date: Wed, 11 Jan 2017 21:56:51 -0500
> From: Luke Berndt
> To: Marcus M?ller , GNURadio Discussion List
> <address@hidden>
> Subject: Re: [Discuss-gnuradio] Improve FSK4 decodes
> in OP25
>
> Thanks Marcus! I will try putting in a low pass in
> first after the Quad demod. That BPF / Polyphase
> Channelizer approach sounds pretty cool too!
>
> I have attached a GRC file that I use for debugging
> and a link the raw dump from a File Sink. The sample
> rate at the sink is 96k/s, the system channel rate is
> 48k/s, with 10 samples per symbol. The FSK Level has
> to be set to about 0.66.
>
> I am sure that some of the problem is that I don?t
> have a great signal coming in, but if there is any
> additional processing I could do to improve the
> reception that would be great.
>
>
>
> Wav file
>
> https://www.dropbox.com/s/ncpz6wt8tubm2v3/32784-1484187802_4.90762e%2B08.wav?dl=0
>
> <https://www.dropbox.com/s/ncpz6wt8tubm2v3/32784-1484187802_4.90762e+08.wav?dl=0>
>
> Raw capture from a file sink
>
> https://www.dropbox.com/s/810zj7pudc66yo3/32784-1484187802_4.90762e%2B08.raw?dl=0
>
> <https://www.dropbox.com/s/810zj7pudc66yo3/32784-1484187802_4.90762e+08.raw?dl=0>
>
>
>
> > On Jan 11, 2017, at 12:12 PM, Marcus M?ller
> <address@hidden> wrote:
> >
> > Hi Luke,
> >
> > you don't happen to have a waterfall plot or a raw
> IQ recording?
> > Quadrature demodulation isn't the most resilient to
> noise. So, a low-pass filter that makes sure that
> events that are sufficiently shorter than a symbol
> after the Quadrature demodulator would sound wise.
> Then again, I don't know the OP25 infrastructure
> overly well, so chances are the FSK4 demod already
> contains one.
> >
> > An alternative to your Quad FM demod would be a
> four-bandpasses-based decider. An easy way to build
> one is to either use four individual BPFs, or use one
> set of filter taps in the Polyphase Channelizer (which
> divides your f_sample_in stream into four
> f_sample_in/4 -rate streams with channel centers).
> Just use a filter that would select the "zeroth"
> carrier. The magic of polyphase math will add three
> f_sample_in/4-shifted versions (you might need to use
> Rotator or similar to put the zeroth subcarrier at
> f=0). I have a "playing with channelizers" flow graph
> [1]. Advantage of the polyphase channelizer is that it
> uses very little more ressources than a single
> bandpass filter would ? basically, you get the three
> additional filters for the cost of a short FFT. Yay!
> >
> > Cheers,
> >
> > Marcus
> >
> > [1]
> https://gist.github.com/babffaa88f54a7fba9b52fc09bda0462
> <https://gist.github.com/babffaa88f54a7fba9b52fc09bda0462>
> >
> > On 01/11/2017 04:44 PM, Luke Berndt wrote:
> >> I am using the C++ OP25 libs to capture and record
> the P25 transmissions from a trunked radio system
> ( https://github.com/robotastic/trunk-recorder
> <https://github.com/robotastic/trunk-recorder> ).
> >>
> >> Transmission can either use 4-Level FSK or CPSK. I
> am getting great decodes from a system using CPSK but
> I am having a lot of trouble with a system using 4FSK.
> >>
> >> At a high level, here is my 4FSK flow graph
>
> (https://github.com/robotastic/trunk-recorder/blob/master/trunk-recorder/recorders/p25_recorder.cc
>
> <https://github.com/robotastic/trunk-recorder/blob/master/trunk-recorder/recorders/p25_recorder.cc>):
> >> Xlating + LPF to roughly capture the channel
> >> ARB Resampler to get the system sample rate
> >> Quad FM Demod
> >> Multiply Const - to get the right levels, -3 to 3
> >> FSK4 Demod - from OP25
> >> FSK4 Slicer - from OP25
> >> OP25 Decode
> >> it then converts it to what is needed to record it
> as a Wav file
> >>
> >> Are there some obvious blocks I should add in to
> help condition the input better? I think the incoming
> levels may not be constant. There also seems to be
> occasional interference. Is there a block that will
> discard values above a certain range? It would be
> great to throw-away samples above 3.5 or below -3.5.
> >>
> >> Attached is a picture with what I think is an
> example of the interference.
> >>
> >> Here is an example of a very clean recording:
>
> https://beta.openmhz.com/wmata?call-id=587652495833790100186198
> <https://beta.openmhz.com/wmata?call-id=587652495833790100186198>
> >>
> >> Here is an example of a bad decode:
>
> https://beta.openmhz.com/wmata?call-id=58765250583379010018619c
> <https://beta.openmhz.com/wmata?call-id=58765250583379010018619c>
> >>
> >> Any tips would be greatly appreciated!!
> >>
> >> - Luke
>
>
- Re: [Discuss-gnuradio] Discuss-gnuradio Digest, Vol 170, Issue 10, Andy Walls, 2017/01/15
- Re: [Discuss-gnuradio] Discuss-gnuradio Digest, Vol 170, Issue 10, Luke Berndt, 2017/01/17
- Re: [Discuss-gnuradio] Improve FSK4 decodes in OP25,
Andy Walls <=
- Re: [Discuss-gnuradio] Improve FSK4 decodes in OP25, Luke Berndt, 2017/01/22
- Re: [Discuss-gnuradio] Improve FSK4 decodes in OP25, Andy Walls, 2017/01/22
- Re: [Discuss-gnuradio] Improve FSK4 decodes in OP25, Andy Walls, 2017/01/22
- Re: [Discuss-gnuradio] Improve FSK4 decodes in OP25, Luke Berndt, 2017/01/22
- Re: [Discuss-gnuradio] Improve FSK4 decodes in OP25, Andy Walls, 2017/01/23