That's pretty awesome :)
On 06/02/2015 09:44 AM, jean-michel.fri...@femto-st.fr wrote: > If it can be of any help, I use POCSAG to illustrate one of the SDR labs I > teach: the flowcharts are pp.4 and 5 of http://jmfriedt.free.fr/tp_sdr.pdf > (don't > mind the text in French, only look at the pictures) and the associated videos > are at http://jmfriedt.free.fr/ (first two ones from the top). > > JM > >>> 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 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 >>> <mailto: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 >>>> <mailto: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 <http://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 <mailto: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 <mailto:Discuss-gnuradio@gnu.org> >>>>> > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >>>>> >>>>> >>>>> _______________________________________________ >>>>> Discuss-gnuradio mailing list >>>>> Discuss-gnuradio@gnu.org <mailto: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
