RTL SDR multiple FM channels

This page describes how to use a single RTL-SDR dongle on a Raspberry Pi to simultaneously monitor multiple FM radio channels at the same time. The demodulated audio from each radio channel can be sent to a speaker or headphones for listening, or can be sent to downstream applications such as TwoToneDetect.

Background

RTL-SDR dongles are capable of monitoring a ~2.2 MHz while chunk of radio spectrum in the VHF and UHF bands. The rtl-airband software can be used to create multiple FM "tuners" at different frequencies within this 2.2 MHz wide swath of spectrum. Unlike a traditional scanner that switches between multiple frequencies but can only listen to one at a time, an RTL-SDR is capable of simultaneously demodulating multiple frequencies at the same time as long as they are all within the RTL-SDR's ~2.2 MHz bandwidth.

Example

An RTL-SDR whose center frequency is tuned to 156.000 MHz could simultaneously demodulate and "listen" to FM two-way radio transmissions on 155.430 MHz and 156.800 MHz.