RTL-SDR

Notes on using the RTL-SDR dongle with Amateur Radio.

Sept. 5, 2020: Succeeded in streaming VHF & UHF captured on a Linux workstation over the network. Tested for FM broadcast band, UHF repeater, and UHF simplex.

The computer systems used:

• Server: a Linux desktop host with the RTL-SDR dongle plugged into a USB port. Ethernet connection to the LAN. OS: Linux Mint 19.3
• Client 1: a second terminal on the same host.
• Client 2: a Linux laptop connecting via WiFi. OS: PureOS (from Librem, based on Debian 10).
• Home LAN running over 75-Ohm cable TV coax vial MoCa adapters, plus WiFi.

• On server: rtl_sdr, sox + libsox-fmt-all, socat repository packages.
• On client (laptop): sox + libsox-fmt-all repository packages.

• On server:
  • Plug in the dongle and test with rtl_test.
  • Tune frequency with $ rtl_fm -M wbfm -f 446.0M | sox -traw -r32k -es -b16 -c1 -V1 - -tmp3 - | socat -u - TCP-LISTEN:8080. Change -f value for other frequencies. (See Rtl_fm Guide for more detail on switches and their values.)
    • Note 32k rate, needed to get true audio. (Example code used 24k – too slow')
    • Uses mp3 format. Will try some others in future.
    • TCP-LISTEN:8080 sets socat to listen for a connection on port 8080. Works both on 127.0.0.1 and LAN/WIFI connection.

• On client (laptop):
  • Listen to the stream with nc <mike ip address> 8080
    • nc is netcat.
    • Note space between IP address and port.

On the server host with the RTL-SDR attached:

• Original test was on an FM broadcast station. Had to adjust the rate under Sox to get true audio; 32k sounded right.
• Ran netcat 127.0.0.1 8080 | play -t mp3 - in another terminal on the same host to listen, and it worked fine. (The play utility is part of Sox.)
• Tried again with a UHF repeater and it worked the same, both for repeater ID and my voice.
• I noted a delay of about 8 seconds between reception and audio output. This is caused by buffering for the MP3 stream.

With the second (client) host:

• Again tried the repeater, with the same result.
• Tried UHF simplex, with the same result.

• The output of rtl_fm is audio, which is converted to MP3 format by sox and then streamed by socat.
• The 8-second time delay would not work for 2-way communication, but could be acceptable for monitoring. Perhaps another streaming format could reduce that – more testing needed.
• The sound quality of the streamed audio was decent.
• It would be interesting to test AM and SSB, which rtl_fm also can handle.

• Rtl_fm Guide. See: “Network Streaming”
• socat -- blog entry on using.
• How to Play MP3 Files from Debian 10 Command Line. Install Sox and use its play command.