Listening to SCA with HDSDR, SDR# and an RTL-SDR

In the USA and Canada a subcarrier called SCA (Subsidary Communications Authority) is used to add additional services to a broadcast FM signal. Some examples of the extra services provided are live financial stock telemetry, audio books for the blind, specialized audio radio programs for doctors etc and background music for supermarkets and stores. These SCA signals are modulated into standard broadcast FM radio signals, but require a special radio to receive them. Subcarrier signals can easily be spotted in the audio/baseband waterfall and spectrum plots available in most SDR software.

Over on the new RTL-SDR DX blog, the author (Jay Moore) has uploaded an article showing how to use an RTL-SDR dongle to listen to audio SCA signals. The process involves using HDSDR to receive the broadcast FM signal, then using Virtual Audio Cable to pipe the audio into SDR#, where it is then possible to tune to the audio SCA signal. The same process could also be used to receive different subcarriers used in other countries such as Finland where a subcarrier is used to transmit DARC encoded bus stop sign telemetry.

SCA audio received via a combination of HDSDR and SDR#
SCA audio received via a combination of HDSDR and SDR#

4 comments

    • Jay Moore

      You can use SDR# if you can change minsamplerate in the config to allow you to open the NFM IF to at least 150khz. You need to feed the FM Baseband in to the second copy of SDR#. If you don’t see the 19khz subcarrier; you are likely in WFM mode, which feeds filtered audio.

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