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

[Marcus Müller]

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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