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

[Stephan van Beerschoten]

I will play with that live cd if I can find it. It will probably work
fine on my laptop, but not so much on the ARM platform where I want to
use the POCSAG decoder utility.

My FD gets over 1200 calls a year, but I want to do big-data analytics
(somewhat) on all calls going out to all the EMS and FD agencies in my
county. There have been 9 calls in the last 15 minutes. It's a really
heavily trafficed pager system, so there will be plenty of data for me
to play with once I figure out my reception issues,

On Tue, Jun 2, 2015 at 11:48 AM, Marcus Müller <address@hidden> wrote:
> Hey Stephan,
>
> probably, you've even got a fully fledged GR installation around. If
> not, I'd say: get the uber-cool LiveSDR DVD image [1], boot from it.
> As soon as you have a full GR installation, you can run the GNU Radio
> companion, which feels a lot like connecting together "real world
> filters" with "cables".
>
> Actually, how many messages did you get in your firefighting days? Back
> in my catastrophe protection service time, I didn't have a pager, but I
> remember these things rarely going off -- luckily, not too many
> catastrophes around southern Germany. That's why I didn't try getting
> the signal myself -- I didn't assume I had a good chance of seeing it.
>
> Best regards,
> Marcus
>
> [1] http://gnuradio.org/redmine/news/52
>
> On 06/02/2015 04:24 PM, Stephan van Beerschoten wrote:
>> I've been an IT geek for over 15 years, but the frequency chatter and
>> filter settings are daunting to me. About 10 years ago almost got my
>> HAM radio license to up my personal geek factor, but I couldn't get
>> myself to learn the electronic circuitry needed. I bow before you :)
>>
>> To answer your question: I only use gqrx (http://gqrx.dk/) right now,
>> and I just realized it's something built on top of GR and not GR
>> itself.
>> This tool doesn't really allow any custom filters. It's a use as-is
>> kinda tool, but it has a lot of options.
>>
>> I mean, it could just be that my physical location is just not that
>> good to receive these localized (county-wide only) pager broadcasts.
>> When I still had my fire department pager I remember that I'd
>> sometimes get a few alphanumeric characters garbled, indicating bad
>> reception. I would like to try and improve the SNR to make the
>> decoding work.
>>
>>
>>
>> On Tue, Jun 2, 2015 at 2:13 AM, Marcus Müller <address@hidden> wrote:
>>> Hi Stephan,
>>>
>>> so how do you get the samples into GNU Radio?
>>> I guess you use the gr-osmosdr source?
>>> What does your flow graph look like?
>>>
>>> Best regards,
>>> Marcus
>>>
>>>
>>> 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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