rtl_airband Suddenly Picking Up Noise - Makes Scanning Unlistenable

[dlchicago]

Hello,

Any help is appreciated. Thank you in advance!

I provide the Chicago Police feed that scans all their freqs at Broadcastify via rtl_airband...it's been relatively fine up until recently, not sure what happened; conf file stayed the same.

I'm getting a TON of noise all the sudden, like the squelch is open in addition to long, digital-sounding feedback. Forgive me, I don't know the official verbiage for these noises.

Take a listen to the MP3 I've attached...just a short sample of what I'm getting.

Running rtl_airband 4.0.2 with this conf file:

devices:
({
  type = "rtlsdr";
  index = 2;
  gain = 25.4;
  mode = "scan";
  #multiple_demod_threads = true;
  send_scan_freq_tags = true;
  shout_metadata_delay = 0;
  channels: (
      {
      freqs = ( 460.02500, 460.05000, 460.07500, 460.10000, 460.12500, 460.15000, 460.20000, 460.22500, 460.37500, 460.40000, 460.42500, 460.45000, 460.47500, 460.50000 );
      labels = ( "Zone 09 - Districts 5 and 22 - 460.025", "Zone 02 - District 19 - 460.050", "Zone 07 - District 3 - 460.075",  "Zone 10 - Districts 10 and 11 - 460.100",  "Citywide 1 - 460.125",  "Zone 04 - Districts 1 and 18 - 460.150",  "Zone 08 - Districts 4 and 6 - 460.200",  "Zone 03 - Districts 12 and 14 - 460.225", "Zone 11 - Districts 20 and 24 - 460.375", "Zone 06 - Districts 7 and 8 - 460.400",  "Zone 12 - Districts 15 and 25 - 460.425", "Zone 13 - District 9 - 460.450",  "Zone 01 - Districts 16 and 17 - 460.475", "Zone 05 - District 2 and HQ - 460.500" );
      modulation = "nfm";
      outputs: (
        {
          type = "icecast";
          server = "192.168.50.101";
          port = 4200;
          mountpoint = "mountpoint";
          name = "Chicago Police";
          genre = "Police";
          username = "source";
          password = "password";
          send_scan_freq_tags = true;
          shout_metadata_delay = 0;
        },
        {
          type = "icecast";
          server = "audio9.broadcastify.com";
          port = 80;
          mountpoint = "mountpoint";
          name = "Chicago Police";
          genre = "Police";
          username = "source";
          password = "password";
          send_scan_freq_tags = true;
        }
      );
    }
  );
});

Again, any help is appreciated and I can provide whatever additional details needed.

Thank you!

 

[wgbecks]

I'm getting a TON of noise all the sudden, like the squelch is open in addition to long, digital-sounding feedback. Forgive me, I don't know the official verbiage for these noises.

Take a listen to the MP3 I've attached...just a short sample of what I'm getting.

I listened to your audio clip, albeit short in duration, it sounds very much like conventional P25. This may be due to propatation
(band opening) but it might also become presistant in the event that a new system has gone into operation in your region.

Unfortunately, there's nothing you can do short of employing a receiver that has PL/DPL squelch whereby your can program matching
codes in use on those channels use by the Chicago Police.

There is also a possibility that your SDR is suffering intermodulation interference whereby it may help to lower LNA gain or perhaps
employ a preselector ahead of the SDR to limit the RF spectrum applied to it. You'd need to verify this as a possibility using a separate
receiver of know selectivity and and immunity to oveload or intermodualtion response.

 

[wgbecks]

I listened to your audio clip, albeit short in duration, it sounds very much like conventional P25. This may be due to propatation
(band opening) but it might also become presistant in the event that a new system has gone into operation in your region.

Disregard my post. I just looked up the CPD and several of the frequencies in your scan list are shown to be using P25 with encryption.
I am located in NW Wisconsin and can't verify this information, but it looks like you'll need to drop those channels from your list if indeed
they are using P25 mode.

 

[maus92]

Chicago has been in the process of migrating to a P25 trunked system, but in the 800 band. I think I recall hearing they were rebuilding the legacy system with P25 conventional. If you look in the RRDB, a frequency that has a NAC code in the Tone column signifies P25, and those channels have either full or part time encryption.

 

[dlchicago]

Disregard my post. I just looked up the CPD and several of the frequencies in your scan list are shown to be using P25 with encryption.
I am located in NW Wisconsin and can't verify this information, but it looks like you'll need to drop those channels from your list if indeed
they are using P25 mode.

Well crap. That would explain the digital noises I've been hearing - like you said sounds almost P25 indeed. Can't believe I didn't think to check something like that, thanks for being more keen than I.

Chicago has been in the process of migrating to a P25 trunked system, but in the 800 band. I think I recall hearing they were rebuilding the legacy system with P25 conventional. If you look in the RRDB, a frequency that has a NAC code in the Tone column signifies P25, and those channels have either full or part time encryption.

I've been listening to CPD since I was a kid; they've had the same frequencies since I can remember, literally. Unfortunate they're going encrypted now, but I can also understand officer safety is of paramount importance.

Thanks again!

 

[wgbecks]

I've been listening to CPD since I was a kid; they've had the same frequencies since I can remember, literally. Unfortunate they're going encrypted now, but I can also understand officer safety is of paramount importance.

Unfortunately, more and more agencies are going encrypted. I would have suggested moving to OP25 for your feed had it not been for the apparent move to encryption.

 

[dlchicago]

Ah well...memories will have to do; started listening to CPD on an old Bearcat 210 back in the early 90s. Moved up to a RadioShack Pro-82 and then a RadioShack Pro-433, the latter I used to stream to Broadcastify before I switched to SDRs.

Now on to mapping out the new P25 channels and see what I can do about encryption

Thank you again!