Receiving the ‘Hidden’ Broadcast FM SCA Audio Subcarrier with an RTL-SDR and SDR#

Broadcast FM channels can often contain additional subcarriers hidden within the bandwidth. A common subcarrier is Radio Data System (RDS), and this is what provides song and radio station text information to your radio.

Another less commonly seen subcarrier is the Subsidiary communications authority (SCA), which is a separate audio channel hidden within the broadcast FM signal. SCA is typically used for niche radio programs, elevator music, music for doctors offices, and niche services such as reading for the visually impaired. In the past you needed a special hardware SCA radio to receive these channels, however receiving these channels with an SDR is relatively simple. Not all broadcast FM stations will have an SCA service, but the video shown below explains how to find one.

Over on YouTube channel Double A has uploaded a video showing how to decode these SCA subcarriers using an RTL-SDR, two SDR# instances and the MPX Output plugin. The idea to to use a virtual audio cable to pipe the FM Multiplex (MPX) audio output from one instance of SDR# to another. In the second SDR# instance you can then directly tune into the SCA channel. In his video he also explores the FM MPX spectrum, showing the different components, and also how to install and use RDS Spy for decoding RDS.

Tuning an FM Audio Subcarrier (SCA) & Decoding RDS Data with RTL-SDR USB

One comment

  1. Frank Kienast

    This reminds me a demo I did for my radio club years ago. I had regular FM receiver I had tapped the discriminator output for. I fed the signal to my HF rig, which receives down to 30kHz. I was able to “tune in” the SCA subcarrier using FM mode at 67kHz. I also remember being able to “hear” the RDS data (it sounded like an old analog modem) by tuning to 57kHz sideband. I seem to recall also being able to hear the stereo multiplex channel on sideband – it would be at 38kHz.

    Receiving SCA today is trivial using any SDR (including RTL-SDR) and GnuRadio. But the only thing I hear nowdays (in the US) is reading for the blind on my local NPR station. I built a PLL decoder back in the early 1980s, and recall then it was possible to hear “elevator music” on the subcarrier of some FM stations. I also remember tuning in a VHF TV station and hearing what sounded like digital data (telemetry?) on a subcarrier. I remember reading the financial papers in the 1980s and seeing ads for devices that could receive stock quotes from anywhere (well before the days of cell phones). These were apparently transmitted on an FM broadcast station subcarrier. Some current GPS devices get live traffic updates this way as well.

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