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SQUELCH.md

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Adjusting Squelch for Conventional Systems

flowchart TD
 subgraph subgraph_5wys65oxs["Conventional Recorder"]
        node_9tex2m50c["Squelch"]
  end
    A("SDR Source") --> B("Signal Detector") & C("Recorder Selector")
    B -. Turn On Recorder .- C
    C --> subgraph_5wys65oxs
    B -.- nk["Automatic Signal Detection Threshold"]
    style nk stroke-width:2px,stroke-dasharray: 2
Loading

Conventional Systems can use up a lot of CPU. Prior to v5.0, conventional recorders are always "on" waiting for a signal to break squelch. With v5.0 you now have a Signal Detector per source. Its job is to look over the bandwidth for that source and perform an FFT. For each FFT bin that is over a power threshold, it will look for a Conventional Recorder that has similar frequencies. If there is a match, it will Enable the recorder in the Souce's Recorder Selector. The Recorder Selector is just a way to determine if a Recorder should receive samples from the Source. In your logs, look for something like [ 0 ] AnalogC Enabled - Freq: 855.737500 MHz Detected Signal: -63dBM (Threshold: -69dBM) if you see that, it means that the Signal Detector has turned on a conventional recorder.

The Signal Detector automatically tries to find a power threshold to separate signal from noise. It is not perfect. If you are seeing a recorder not turn on when there is a signal, that could be because the Signal Detector threshold is not being set correctly. Or, if the Recorder is always being enabled by the Signal Detector, even when there is not a transmission, that is also a sign that is not automatically being set correctly. You can manually set the Signal Detector Threshold in the config.json file. Just to make things more confusing, the power levels used in the Signal Detector are slightly different than the values used for the Squelch.

Each Conventional Recorder also has a Squelch setting. Once a Conventional Recorder has been enabled and is receiving samples from the source, it will use a Squelch block and only process samples above a certain power level. This prevent noise from being recorded. The Squelch value needs to be hand tuned, and can be set for the entire Conventional System, or per channel.

Configuring a Conventional System

  • Use a visual signal analyzer program, like GQRX to explore the signal you are trying to capture
    • Adjust the gain settings so that you have strong signals without adding a lot of noise. Copy these gain settings into the config.json file.
    • Note the Db value for the noise floor. You should set the squelch value to be in between the noise floor value and the the signal peak values.
    • In the config.json file you can specify the squelch value for the whole system, or do it channel by channel, or a mix:
    • System Wide Squelch
      • config.json 
          {
    "systems": [
    {
        "type": "conventional",
        "squelch": -50 
    }
  }
    • Channel Specific Squelch

      • config.json

          {
    "systems": [
    {
        "type": "conventional",
        "channelFile": "channels.csv",
    }
  }

      • channels.csv

        TG Number Frequency Squelch
        300 462275000 -50 
        TG Number, Frequency, Squelch
300, 462275000, -50
    • Hybrid Squelch

      • config.json

          {
    "systems": [
    {
        "type": "conventional",
        "channelFile": "channels.csv",
        "squelch": -50
    }
  }

      • channels.csv

        TG Number Frequency Squelch
        300 462275000 -70
        301 462300000 
        TG Number, Frequency, Squelch
300, 462275000, -70
301, 462300000,

Debugging a Conventional System

  • The Channels for a System are never being recorded

    • Try setting the squelch value to something very low, like -100. This should cause the Recorder to always be on and recording. When you quit trunk-recorder you should have a long file, possibly of static. If you, it is possible that the Signal Detector is not working correctly.

  • Signal Detector is not working correctly

    • IF you are not seeing a line like: [ 0 ] AnalogC Enabled - Freq: 855.737500 MHz Detected Signal: -63dBM (Threshold: -69dBM) and you know there is a transmission on that frequency, it is a sign the the threshold for the Signal Detector is not being set correctly. To test this out, manually set the signal detector threshold in the sources: [{}] section of the config.json file.

          "sources": [
      {
        "center": 851734375,
        "rate": 2000000,
        "driver": "osmosdr",
        "signalDetectorThreshold": -100
    }]
    This will effectively leave all of the recorders enabled all the time... which is not the most efficient. If this does work, then try to find a value for the signalDetectorThreshold that works reliably. RF power is measured differently between the Signal Detector and the Squelch block. You may need to use a value roughly 10 dB higher for the Signal Detector. Ideally, you only want the Signal Detector to enable a recorder when a there is a transmission that would trigger the squelch block in the Recorder.