SCA Subcarrier Demodulator Page





Background...

FM broadcast stations transmit SCA signals by adding one or two subcarriers to their main modulation signal. These subcarriers normally have centre frequencies of 67kHz and/or 92kHz, and each is frequency-modulated by its own SCA audio channel.

To recover the SCA audio, this demodulator board is connected to your FM receiver's discriminator circuit, and to a convenient source of DC power. The board has pre-tuned active filters switchable between 67kHz and 92kHz by an external DPST switch. The audio output of the board can be fed to the receiver's existing audio amplifier, or to an external amplifier in the case of an FM tuner.

The kit instructions provide a lot of information about how to connect this board, including details of fitting it to a couple of 'ghetto blasters' which used to be sold in Australia. This is still useful as a guide to what to do.

Please note! This circuit doesn't have any muting built in, therefore it's not suitable for demodulating transmissions where the subcarrier switches off in the absense of audio. It produces a lot of noise if there's no subcarrier.



Brief Specifications

Subcarrier frequencies: 67kHz and 92kHz, switchable.
SCA FM demodulation method: Pulse counter circuit
Audio de-emphasis curve: 150us
SCA audio frequency response: -3dB at approx 70Hz and 5.5kHz
Audio output level: Adjustable 0 to approx 500mV RMS.
Power requirements: +6V to +30V DC at less than 10mA
Dimensions: 2.6 x 2.1 inches (66 x 54mm)... a lot smaller than the picture above!!


Note!!
The American SCA system is known as the 'Ancillary Communications Service' or 'ACS' down here in Australia, so this kit was given the title (not by me, either!) of an 'ACS decoder'... and that's what it's known as in the Dick Smith catalog. But rest assured it really is an SCA demodulator. :-)



Wanna build one?

Click here to download the schematic.

You can get the PCB and a reprint of the magazine articles as a package from RCS Radio here in Sydney, Australia. Note that it requires 4 x 1000pF 1% polystyrene capacitors which are fairly uncommon components....

- OR -

Click here to see the Ramsey Electronics SCA decoder kit







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Last update: 10th Feb, 2007