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

[Stephan van Beerschoten]

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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>>
>