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rtl_fm SDR dongle Scanner

Posted: Sun Mar 18, 2018 2:08 pm
by pmarq2008
I am trying to setup an SDR to monitor the frequency of the fire department I am on. I am running into some issues with rtl_fm. I use this command and it seems to receive the first transmission for a second or two and then it just seems like it freezes. I can do CTRL+C and it says signal caught, exiting! then user cancel, exiting but it doesn't exit. I need to close the terminal window in order to get it to do anything.

I am just looking to use this to play out the speakers.

Any Ideas?

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rtl_fm -l 70 -M fm -f 154.965M | play -t raw -r 24k -e signed-integer -b 16 -c 1 -
Thanks for the help in advance.

Re: rtl_fm SDR dongle Scanner

Posted: Sun Mar 18, 2018 5:03 pm
by Ronaldlees
I've noticed that other people using utility voice narrow band FM are also using these in their command line:

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-s 12k
-l 70
12k sample rate for narrow band
70 squelch level

Have you played with the squelch level?

Re: rtl_fm SDR dongle Scanner

Posted: Mon Mar 19, 2018 7:34 pm
by pmarq2008
Yeah i have tried it with those. I just tried the below. It works fine for the 3.41 seconds that you can see at the bottom and the signal level moves to which i can hear the voice but after the 3.41 seconds it stops.

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[email protected]:~ $ rtl_fm -l 70 -M fm -f 154.965M -s 12k | play -t raw -r 12k -e signed-integer -b 16 -c 1 -
Found 1 device(s):
  0:  Realtek, RTL2838UHIDIR, SN: 00000001

Using device 0: Generic RTL2832U OEM

-: (raw)

  Encoding: Signed PCM    
  Channels: 1 @ 16-bit   
Samplerate: 12000Hz      
Replaygain: off         
  Duration: unknown      

In:0.00% 00:00:00.00 [00:00:00.00] Out:0     [      |      ]        Clip:0    Found Rafael Micro R820T tuner
Tuner gain set to automatic.
Tuned to 155217000 Hz.
oversampling input by: 84x.
Oversampling output by: 1x.
Buffer size: 8.13ms
Exact sample rate is: 1008000.009613 Hz
Create UDP thread
Created UDP thread
Main socket started! :-) Tuning enabled on UDP/6020 
Sampling at 1008000 S/s.
Output at 12000 Hz.
In:0.00% 00:00:03.41 [00:00:00.00] Out:32.8k [      |      ]        Clip:0  

Re: rtl_fm SDR dongle Scanner

Posted: Mon Apr 02, 2018 1:57 pm
by Ronaldlees
The reason I asked about the squelch level is because in the case of FM - the squelch often breaks open for a short bit when a weaker carrier starts, but then it closes again. So, you'd move the squelch around to see if that helps, i.e. not set to 70, but try 20,30,40,50,60,70,80,90. The break-open effect may be a side product of analog designed squelch, but doesn't hurt to try.