Let me add that I don't know anything about the signal, other than that it's broadcast on 155.520MHz.
On Tue, Jun 2, 2015 at 3:19 AM, Marcus Müller <marcus.muel...@ettus.com> wrote: > Hi Stephan, > > I am sure GR can do that, but I can't ;-) > > I can't help but propose you change that ;) No, seriously, cross-compiling > GNU Radio for an ARM sounds more complicated than doing non-coherent binary > FSK demod, but then again, that might just be me :D. > > In fact, you're absolutely right: getting a solid signal quality before > attempting decoding might be a good idea. However, most probably pagers > don't need awesome SNR, so "somewhat noisy" might still be ok. > > so how do you get the samples into GNU Radio? > I guess you use the gr-osmosdr source? which sampling rate? Where in your > base band are your carriers? > What does your flow graph look like? > > Generally: If you have a RF recording, [1] might just profit from one more > entry, and we'd have something more tangible to talk about :) > > I'll outline the steps I'd do to try to achieve better signal: > > > 1. Record a signal and test with that -- doing everything live makes > things complicated and hard to reproduce. > 2. Use a xlating FIR filter to move a single 12.5kHz channel to 0Hz, > so that either symbol is +- 4.5kHz > 1. this will require that you design a filter. Don't worry, that's > relatively easy: > 1. run gr_filter_design > 2. select low pass, enter your source's sampling rate, set the > end of the pass band to let's say 5kHz and the start of the stop > band to > 7.5kHz (If I understand wikipedia correctly, channel spacing is > 12.5kHz, > and symbol deviation is +-4.5kHz, so from the center of the lower > channel > to the lower bit of the upper channel it's 12.5kHz - 4.5kHz = 8kHz). > 3. You'll notice that if you start with a high sampling rate, > your filter gets ridiculously long. If that's the case, you might > want to > reduce the sampling rate of your signal source, or add a stage of > half- or > quarter bandwidth FIR decimation (with a decimation factor of 2 or 4, > respectively) > 2. set the decimation of that xlating FIR to something reasonable, > so that rate_in/decimation > 12.5kHz/2, but not >>. > 1. this way, you'll get "just enough" rate at the output. > 3. set the center frequency to the middle of your two symbol > frequencies in the input spectrum > 3. add visualization sinks here and there, and verify :) > 4. add a real high-pass filter > 1. Your single-channel spectrum looks something like [1] with 0 Hz > in the middle. Since we've filtered away stuff above 5kHz, we'd now > concern ourselves with filtering away everything below 4kHz. > 2. Same procedure as for the xlating fir, but use the reduced > sampling rate and a 4 kHz high-pass with a 2kHz stop band or something. > The > closer the stop band is to pass band, the longer your filter gets. > 3. In principle, a 4-5 kHz real-tapped bandpass xlating fir would > have done the same, but doing this step by step reduces error > probability. > 5. repeat "add visualizations" :) > 6. You should now have a clean signal with only two peaks in your > spectrum at +-4.5kHz; does your external decoder deal well with that? > > In principle, you're extremely close to having your own decoder by now. > Non-coherent BFSK decoding would simply do the same as step 2, but with two > filters, each centered on either symbol frequency, baudrate-wide passband, > decimating to the baudrate, followed by a complex-to-magnituded-squared > conversion each, then something like division of the 1-filter magsquared by > the 0-filter magsquared, followed by a threshold decision (threshold=1). > You'd then be getting a raw POCSAG bitstream :D > Best regards, > Marcus > > > [1] from http://edge.rit.edu/edge/P09141/public/FSK.pdf , Watkins-Johnson > Company "Tech-notes Vol. 7 No. 5 September/October 1980: FSK: Signals and > Demodulation", p. 8 [image: FSK spectrum] > <http://edge.rit.edu/edge/P09141/public/FSK.pdf> > > On 06/02/2015 12:04 AM, Stephan van Beerschoten wrote: > > I am sure GR can do that, but I can't ;-) > Also, I don't have a good waterfall at all of the pocsag broadcast, which > is probably part of why I can't make it out with my ears either. Yes, I > think I have too much noise. > I hope it can be overcome with the right settings and filters. > > I'll try to capture a screenshot of what I see. It's nothing like the > screenshots in Wikipedia. > On Jun 1, 2015 5:47 PM, "Marcus Müller" <marcus.muel...@ettus.com> wrote: > >> Hi! >> I personally think the soundbite from wikipedia is broken, since it's >> 11kHz sampling rate violates Nyquist ;) >> Well, I must admit that my preferred way of analyzing this wouldn't be >> the audible reproduction; if you can see it clearly on the waterfall, and >> "optically" have enough dB between the carriers and noise, then you'll be >> fine decoding it. >> >> Now, I trust you're actually seeing excessive noise -- this might point >> to problems with your receiver (unsuitable antenna, too much noise in the >> amplifier, too little gain, intermodulation). The first step in limiting >> noise is always adding appropriate filtering. Can you add a FIR that >> selects your POCSAG channel out of your sampling bandwidth? >> >> Best regards, >> Marcus >> >> On 06/01/2015 11:28 PM, Stephan van Beerschoten wrote: >> >> You're right in that I need more than GR. The audio of a pocsag broadcast >> is very distinct. It's also clearly visible on a waterfall. >> The problem is that I have too much static in there. Way too much noise. >> I can't get the gqrx module (where I tune and see the waterfall) set right >> so the reception is fine. >> I think the Wikipedia article had a soundbite of a pocsag encoding. If >> you listen to it you'll notice it's very distinct.i just have 90% noise and >> I can hear the broadcast in the very background. >> On Jun 1, 2015 5:25 PM, "Marcus Müller" <marcus.muel...@ettus.com> wrote: >> >>> Hi again, >>> >>> Ok, I'm not familiar with the standard POCSAG, but if you got a signal >>> that you still need to decode with something else, how do you know you >>> don't get clear reception? What is your measure for "good reception"? >>> >>> As far as I read the English wikipedia, POCSAC uses a 4.5kHz binary FSK, >>> so can you see the two alternating frequency e.g. in a waterfall plot of >>> your RX signal? >>> Ideally, you'd directly be able to see the 512, 1200 or 2400 baud. >>> >>> To explain a bit more: >>> GNU Radio is not a decoder for any specific standard; think of it as the >>> LEGO of SDR. You can build amazing things with it, in fact, there's a lot >>> of examples that come with GNU Radio, and useful and complex standard >>> implementations (FM receiver, DTV transmission!), but if you need to have >>> something that's not there, you might need to a) use someone else's >>> Out-Of-Tree module or b) implement that functionality yourself. So I must >>> admit that I don't have the slightest idea which settings you're referring >>> to :) Maybe you're interested in a quick&dirty introduction to GNU Radio >>> [1]. >>> >>> In the case of POCSAG, I remember gr-pocsag being a thing (search for >>> pocsag on cgran.org); I can't remember the original author, and I >>> presume it's pretty much dead -- but I'd love to be proven wrong. >>> Also, pyboms has pocsag-mrt package, but that seems to rely on GNU Radio >>> 3.6.2, if the Readme is correct, so that's pretty dead, too. >>> >>> >>> Best regards, >>> Marcus >>> >>> [1] https://github.com/iZsh/pocsag-mrt >>> On 06/01/2015 10:18 PM, Stephan van Beerschoten wrote: >>> >>> It is. I plan on running the output through a utility that can decode >>> it. However, before that can happen I need to find out how I can get a >>> clear reception of the broadcast. >>> On Jun 1, 2015 4:15 PM, "Marcus Müller" <marcus.muel...@ettus.com> >>> wrote: >>> >>>> I'm a bit confused, I though POCSAG was a text pager system? >>>> >>>> On 06/01/2015 10:04 PM, Stephan van Beerschoten wrote: >>>> > Hi Guys, >>>> > >>>> > I compiled gnuradio for my ODROID ARM platform, and I can listen to >>>> > regular wideband radio just fine. I am using a Generic RTL2832U with >>>> > Rafael Micro R820T tuner. >>>> > >>>> > The radio quality is fine, and even when using the rtl_fm tool >>>> > directly (off topic for this list), it works. >>>> > >>>> > However, when I switch channels to 155.520 to capture POCSAG >>>> > broadcasts I cannot get a clear reception. I can't find any decent >>>> > documentation on GR to tell me what each setting is, and I am not a >>>> > HAM radio operator so some of the basics evade me. >>>> > >>>> > I can't get decent POCSAG reception with the rtl_fm tool either, so >>>> > this is probably a setting thing somewhere. >>>> > >>>> > Why can't I get clear reception? Any pointers? >>>> > >>>> > Stephan >>>> > >>>> > _______________________________________________ >>>> > Discuss-gnuradio mailing list >>>> > Discuss-gnuradio@gnu.org >>>> > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >>>> >>>> >>>> _______________________________________________ >>>> Discuss-gnuradio mailing list >>>> Discuss-gnuradio@gnu.org >>>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >>>> >>> >>> >> >
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