POCSAG decoding under Linux - how?

[Adam Nielsen]

Hi all,

As I can now finally start using my RTL-SDR properly under Linux thanks to
Balint's port of the BorIP server, I'm starting to investigate some of the
digital signals I can pick up in my area.

There is one that sounds like POCSAG at 1200 baud[1] but I'm not sure how to
go about decoding it under Linux.

I found a program called multimon[2] which claims to be able to decode this,
but when I feed in a .wav file I recorded using HDSDR's AF recording option
(i.e. the .wav file records what I hear out my speakers) multimon doesn't seem
to be able to decode it - I just get the normal intro text and no further
output. There doesn't seem to be any documentation about what sort of data
you're supposed to feed in - are you meant to record the HDSDR audio in AM,
FM, or what?

There are a few other Windows programs that can decode POCSAG but they only
record from a sound card and can't load .wav files so they don't really work
under Linux with WINE.

Has anyone used multimon before, or successfully decoded POCSAG under Linux?

Thanks,
Adam.

[1] POCSAG1200 recording at http://www.kb9ukd.com/digital/
[2] http://www.baycom.org/~tom/ham/linux/multimon.html

[Jasper Ragworth]

I've successfully used PDW under Wine, but that was with a hard radio, not a soft one. I'm sure you could plumb the audio in virtually somehow but I didn't manage to work it out. I think Wine and Pulseaudio don't play well together.

I've got gqrx to speak to multimon and used that to decode some ZVEI [5-tone]. I ran multimon with padsp and then used pavucontrol to get multimon listening to the 'Monitor of Built-in Analogue Audio Stereo'. Having just done it again in order to describe it, I've now found it doesn't seem to work, at least the scope in multimon doesn't show anything which suggests it's not hearing the audio. I think I've upgraded to Ubuntu 12.04 since it worked.

I haven't managed to get multimon to decode POCSAG, either with a hard radio or a soft radio.

As an aside ISTR with PDW I spent ages tweaking the settings to get it to the point where it was decoding messages, and then after about 10 minutes I realised it's pretty dull stuff anyway.

[Sinager]

I've used multimon a lot but never for pocsag as we haven't such signals around here.

You can definitely use it in conjunction with GNURadio: you can find a howto on YouTube.

Running multimon on Ubuntu later than 10.04 requires some tricks: either running padsp -which didn't suit my requirements- or recompiling the kernel with support for the legacy OSS.