Enrique is working on a project which would record FM audio as MP3 files. To do this he uses rtl_fm with several RTL-SDR dongles. However, a major roadblock was that he found that adding five or more dongles to his server resulted in all dongles with a USB index over 3 producing the error “Failed to submit transfer 4!”.
After trying to work around the problem with Docker and VMs and ultimately failing he decided to look into other solutions. He found that rtl_test had an option to force synced output, and with this option enabled he was able to use more than four dongles. So he ended up implementing that synchronization code into rtl_fm.
With that code implemented he is now able to run up to 15 dongles on a single server. A higher amount might still be possible, but Enrique did not have that many dongles to test.
rtl_fm / rx_fm: Allows you to decode and listen to FM/AM/SSB radio. rtl_sdr / rx_sdr: Allows you to record raw samples for future processing. rtl_power / rx_power: Allows you to do wideband scans over arbitrarily wide swaths of bandwidth by hopping over and recording signal power levels over multiple chunks of spectrum.
rx_tools is based on SoapySDR which is an SDR abstraction layer. If software is developed with SoapySDR, then the software can be more easily used with any SDR, assuming a Soapy plugin for that particular SDR is written. This stops the need for software to be re-written many times for different SDR’s as instead the plugin only needs to be written once.
In this episode of Hak5, a popular YouTube technology channel, Shannon shows how to use the RTL-SDR on Debian Linux. She shows how to install the RTL-SDR drivers from scratch if using a distribution without them pre-installed and also shows how to install and use rtl_fm, a command line FM demodulator.
Th0ma5w has released his rtl_fm_python program which is a modification of rtl_fm. The added feature is an API and web interface which allows interaction with a running instance of rtl_fm. The API and web interface allows you to change the frequency, modulation and gain settings while rtl_fm is running. This may be useful for remote devices running rtl_fm such as a Raspberry Pi.
Over on our forums, user sysrun has posted about his fork of the rtl_fm command line tool. His fork allows retuning of the rtl_fm program via UDP without the need to restart the program. In the future he hopes to support UDP PCM streaming. He writes
rtl_udp is a copy of rtl_fm with a special feature: It opens a udp control port (currently fixed to 6020) which takes commands like changing the frequency or mode. No need to restart
Over on Reddit user olgierd has posted a video on this thread showing his work on developing a portable software defined radio based on an RTL-SDR R820T, the rtl_fm software and a Raspberry Pi with LCD screen. The software is based on rtl_fm which has the capability to demodulate FM/AM/SSB signals. His video shows him tuning to various signals using a tuning knob.
Note that his setup is capable of playing the audio, but it is not shown in the video as he only had earbuds connected. In the future he hopes to make it more portable by adding a li-ion battery.