Re: [Discuss-gnuradio] Regular FM radio fine, POCSAG horrible

[Philip Balister]

On 06/02/2015 11:22 AM, West, Nathan wrote:
> I've heard a complaint about something similar on ARM before that was VOLK
> related. Can you set your volk_config to use the neon for
> volk_32f_x2_dot_prod_32f and report back?
> 
> If the previous request is confusing just copy this file [0] to
> ~/.volk/volk_config.
> 
> [0]
> https://raw.githubusercontent.com/balister/meta-sdr/f1ce8601482655695cb27b06aefbf9a620a27bd0/recipes-support/volk/files/ettus-e300/volk_config

That is the output of volk_profile run on an E310. You may get better
results running volk_profile on your hardware. Then making the change
Nathan suggests.

Philip

> 
> I'm interested in results and can provide more detailed steps in a few
> hours if needed.
> 
> -Nathan
> 
> On Tue, Jun 2, 2015 at 10:29 AM, Stephan van Beerschoten <
> address@hidden> wrote:
> 
>> 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 <address@hidden>
>> 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" <address@hidden> 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" <address@hidden>
>>>> 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" <address@hidden>
>>>>> 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
>>>>>>>
>>>>>>> _______________________________________________
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>>>>>>
>>>>>>
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>>>>
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