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

[Stephan van Beerschoten]

I ran the volk profile generator, but with the resulting volk_config
file (all other settings untouched) I don't get any audio out of gqrx

On Tue, Jun 2, 2015 at 11:32 AM, Philip Balister <address@hidden> wrote:
> 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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