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Software Geared to the Shortwave Hobbyist

SCA Info

SCA stands for Subsidiary Carrier Authorization, a hidden channel (or channels) on some FM broadcasts. Technically, these subchannels may reside on 57, 67 or 92 kHz away from the main carrier, however, the 67 kHz channel is used most often. Some of these broadcasts may contain what's commonly known as Muzak (elevator music), Physician's Radio Network, Radio Talking Book (or Radio for the Blind), or some other pay service. Some services consist of data streams that consist of stock and commodities quotes, which may be encrypted.

Sample SCA Schematic Diagram
Sample SCA Schematic Diagram (50% scale)

Parts List
Resistors (0.25w)   Capacitors
3 - 1k   6 - 220pF
2 - 1.8k   2 - 270pF
2 - 4.7k   2 - 0.001µF
10 - 10k   3 - 0.027µF
3 - 15k   7 - 0.1µF
1 - 18k   2 - 10µF (Tantalum)
1 - 22k    
3 - 30k   Potentiometers (10 Turn)
1 - 36k   1 - 10k
1 - 56k   1 - 50k
3 - 62k   1 - 100k
1 - 100k    
1 - 150k   Integrated Circuits
    5 - LM741 Op Amp
    1 - LM565 Phase Locked Loop

Theory of Operation.
Above is a sample schematic for the detection of SCA subscarriers (shown at half-scale for brevity), something I designed in 1980. To download the image to your hard disk, right-click on it and choose "Save Picture As". The circuit allows for low-level inputs, such as from the earphone jack of a portable radio, however, it is usable with larger signals such as that found at the post-demodulated output of a receiver. The signal is high-pass filtered through a series of 4 Op Amp stages with the equivalent of a 10-pole shape factor and has 10dB of gain at 67kHz. This eliminates the majority of the main-band audio (0-15kHz), stereo pilot (19kHz) and difference channel (23-53kHz) portions that leak through a demodulated FM signal.

The resulting signal is then sent to an LM565 Phase Locked Loop (PLL) for further demodulation. The PLL may then be tuned to lock on any of the possible SCA carriers at 57, 67 or 92 kHz by turning the 10k potentiometer. The demodulated SCA signal is then de-emphasized through a series of simple RC low-pass filters, in an attempt to further remove any possible cross-talk or chatter from the FM station's main programming. A final Op Amp stage is used to bring the SCA audio up to standard line levels via the 100k potentiometer across IC U6.

The circuit is powered by a standard 12v DC supply. This may consist of a simple power transformer, 4 x 1N4001 diode bridge, a 470µF 25V capacitor, LM7812 Voltage regulator, and 10µF 15V output filter capacitor. Because of the gain of and sharpness of the front-end filter, the power supply must be clean and fairly ripple-free. The SCA decoder will work fairly well with most portable radios that have fairly good fidelity. If the reciever has tone controls, it helps to lessen the bass as much as possible to emphasize the high end. The user may have to experiment in order to get the right mix of output level from the earphone jack, and attenuation using the 50k potentiometer in the circuit.

Conclusion and Disclaimer.
This information is provided by the author solely for the purposes of entertainment and education. Readers are forewarned that the use of this curcuit will violate the Electronic Communications Protection Act of 1986 in the United States of America. Since construction may imply its use by some law officials, the reader is further cautioned that this action may likewise be considered illegal. For more information on SCA and potential SCA broadcasts in your area, you may consult the FM Atlas, by Bruce Elving, FM Atlas Publishing, Esko, Minnesota 55733-0336.

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Last Updated: 29 April 2004.