rtl_fm for POCSAG : decode several frequencies

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JayMore
Posts: 6
Joined: Wed Aug 19, 2020 3:00 pm

rtl_fm for POCSAG : decode several frequencies

Post by JayMore » Fri Aug 28, 2020 10:23 am

Hello All.

Do you know wich parameters and wich version of rtl_fm I need to have the capability to include a bandwidth settings to the nfm demodulator or to accuracy scan several close POCSAG frequency ?

I want to decode some POCASG signals, but the frequency are close to each other. I have transmission on 466.050, 466.075, 466.0175 and 466.205.

If I tune rtl_fm on 466.050, I can hear the audio signal of the 466.205 frequency, but very noisy and undecodable by multimon-ng.

If I tune on the middle (466.0775), I hear the 4 frequencies but with a lot of noise and undecodable too.

So I tried to use the scanner feature of rtl_fm to hop from the 4 frequencies, but I have the impression the NFM demodulator is not very "narrow". When the scanner stop on a frequency, the signal is so noisy that it is probably the signal of a close frequency.

I use the Development branch of librtlsdr (https://github.com/librtlsdr/librtlsdr/tree/development), and rtl_fm have option to set bandwidth, but I dont know which one to use :

Code: Select all

-w tuner_bandwidth in Hz
-O bw=<bw_in_kHz>        set tuner bandwidth
For the moment, I use a bandwith of 6Khz but the audio sound the same whatever the value is :

Code: Select all

rtl_fm -g 40 -M nfm -f 466.050M -f 466.075M -f 466.175M -f 466.205M -s 22050 -l 1000 -L 10 -w 6000 | "C:\Program Files\VideoLAN\VLC\vlc.exe" --demux=rawaud --rawaud-channels 1 --rawaud-samplerate 22050 --audio-visual visual --effect-list scope -
The previous command stream audio from rtl_fm to VLC. Then I plug PDW on the Digital Stereo Mix input/output of the audio card, but decoding is not very accurate (same result with multimon-ng).

Do you know a better method to scan several POCSAG frequencies at the same time ?

JayMore
Posts: 6
Joined: Wed Aug 19, 2020 3:00 pm

Re: rtl_fm for POCSAG : decode several frequencies

Post by JayMore » Sat Sep 19, 2020 3:33 pm

Answering to my own question : finally, it is better to use SDR# in Wide-FM mode (to cover 4 or 5 POCSAG channel), and PDW as the decoder.
The trick is to check "Filter audio" to remove the DC component of the audio signal when the bandwith is so large. If you dont filter audio, PDW receive an audio signal with a DC component (especially if the Pocsag signal is far from the center frequency), and it cant decode the messages.
A video here : https://www.youtube.com/watch?v=GAgDROwZGS4

Username
Posts: 611
Joined: Sun Oct 09, 2016 7:27 am

Re: rtl_fm for POCSAG : decode several frequencies

Post by Username » Tue Sep 29, 2020 9:36 am

Hi
A new Version of SDR# should support Multiple sessions. And there is a Russian Tool to Output Multiple Audio Channels.
You need just buy VB Cable to have Multiple Virtual Audio Channels.
And there you go... :D Pls report if it work for you.

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