Picking up neighboring frequencies despite only one channel set up

[ThisIsntTheWay]

Using an RTL-SDR v4, I am listening in to a local airfields ATIS broadcast.
However, whenever communications occur on a neighboring frequency, I can hear those communications fairly clearly in the output.
(This chatter must be originating from either 121.025 or 127.325 MHz)

radio chatter.wav

My config:

devices: (
  {
    type = "rtlsdr";
    index = 0;
    gain = 16.6;
    centerfreq = 125.0;
    channels: (
      {
        label = "LSZB ATIS";
        ampfactor = 0.8;
        
        freq = 125.130;

        modulation = "am";
        sample_rate = 1;
        bandwith = 1000;
      }
    );
  }
);

I've set up a fairly narrow bandwidth and sample rate on purpose because I assumed that this would ensure that I would only ever hear the ATIS.

If set up a SDR++ session with more or less the same parameters, I only ever hear the ATIS:

Am I misunderstanding the config or how rtl_airband operates in multichannel mode?

[charlie-foxtrot]

sample_rate should be set at the device level, not in one of the channels so it would have no effect where it is currently set . . . . but I don't think that will fix your problem.

The default sample_rate is 2.56 MHz, and with a centerfreq of 125.0 that would give you 123.72 - 126.28 . . . . if the interference is truly coming from 121.025 or 127.325 then something else is wrong.

Are you compiling rtl_airband yourself or using the docker image? The RTL-SDR Blog V4 dongle requires a custom driver, and it's possible that is built into the SDR++ but not on your host system. If you can try the rtl_airband docker image (that should have the right driver) otherwise see https://www.rtl-sdr.com/V4/

[ThisIsntTheWay]

Sorry, I could have expanded on my environment a bit more:
rtl_airband is running in a docker container (tag v5.1.6), but the v4 custom drivers are installed on the system.
rtl_test et al detect the dongle properly.

The airport is closed now so all frequencies are silent, but tomorrow I'll try to generate a raw I/Q file over an extended period of time, maybe that could help.

I noticed that the airport also operates a radio on 123.4 which is meant for glider traffic (and would almost fit into the frequency range).
Though I haven't noticed any such traffic today, that could still have been a source of interference of course.

Thanks for the config hint.

[ThisIsntTheWay]

I managed to record cf32 and MP3 files (recorded simultaneously using the same config as above):
recordings.zip

An interference can be heard ~1:04 minutes into the _08 recordings.

I tried to analyze the I/Q file in gnuradio-companion, but it's my first time using such a program and its scope admittedly goes over my head.
I've set up an (advanced) file source of type "short" and vec set to 1, but according to the file repeater it very quickly reaches the end of line.

Radio setup:
Pi <- SDR dongle <- Wideband LNA <- 118 - 136MHz antenna

[charlie-foxtrot]

I'll try to reproduce but it will be a few days before I can.

Can you provide the config you used to generate the I/Q file as well?

Is the config at the start of this the entire config used or just a portion? (Are you setting fft_size or any of the global parameters?)

What is the full command (or compose files) you using to start up the container?

[ThisIsntTheWay]

compose file:

services:  
  rtlsdr:
    image: ghcr.io/rtl-airband/rtlsdr-airband:v5.1.6
    volumes:
      - ./rtlsdr.conf:/app/rtl_airband.conf
      - ./recordings:/opt/recordings
    devices:
      - /dev/bus/usb:/dev/bus/usb
    depends_on:
      - transcriber
    restart: always
  
  transcriber:
    build:
      context: ./app
    expose:
      - 8000 # UDP socket
    # Rest omitted for brevity

rtlsdr.conf used during file generation:

devices: (
  {
    type = "rtlsdr";
    index = 0;
    gain = 16.6;
    centerfreq = 125.0;
    channels: (
      {
        label = "LSZB ATIS";
        ampfactor = 0.8;
        freq = 125.130;
        modulation = "am";
        bandwith = 1000;
        
        outputs: (
          {
            type = "udp_stream";
            dest_address = "transcriber";
            dest_port = 8000;
            continuous = true;
          },
          {
            type = "rawfile";
            directory = "/opt/recordings";
            filename_template = "LSZB_ATIS";
          },
          {
            type = "file";
            directory = "/opt/recordings";
            filename_template = "LSZB_ATIS";
          }
        );
      }
    );
  }
);

The config you see (including the initial config) is all that I've been passing to the application.

Thankfully chatter is not that frequent so it's not too much of an impairment on my use case.
I greatly appreciate your assistance!