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FM SCA Information

An Important Note From The Lawyers: Under US Law, it may not be legal for you to monitor certain SCA transmissions. You should check in advance with the provider of the service.

What is SCA?

SCA stands for Subsidiary Communications Authorization. It is commonly referred to as "subcarrier", because a subcarrier is used to transmit the information. A definition of subcarrier is now in order:

A subcarrier is another modulated signal transmitted with the baseband audio. Think of it as another transmission modulating the station's audio. By the way, this is how the stereo information is also transmitted.

There are 101 FM channels allocated, 87.9 to 107.9 MHz in 200 kHz increments. It is possible for a station to transmit audio which is up to 100 kHz. This is obviously much wider than the range of human hearing. In fact, FM audio is generally restricted to 15 kHz. This leaves a tremendous amount of spectrum for other uses.

FM Stereo

The normal baseband audio consists of the right and left audio mixed together (R+L). This is so that when listening on a mono receiver, you hear both channels of sound.

The difference signal, which is the difference between the right and left channels (R-L) is transmitted on a 38 kHz subcarrier using FM modulation.

How is this signal received? Imagine you had another FM radio receiver, tuned to 38 kHz. If you fed the baseband audio into this receiver's RF (antenna) input, the demodulated signal would be the R-L audio. Of course, it would be unintelligible. But, if you mixed the R+L and R-L signals, you would be left with just the right (R) channel. Likewise, if you took the difference, you would have just the left (L) channel. That is how stereo audio is decoded. In addition, another subcarrier (with no modulation) is transmitted at 19 kHz. This is the "stereo beacon" which lets the FM stereo receiver know that the broadcast is in stereo. In addition, the receiver uses the 19 khz (which is half of 38 kHz) as a reference frequency when demodulating the 38 kHz stereo subcarrier.

SCA Broadcasts

But this still leaves a lot of the 100 kHz audio spectrum unused. FM Stations are allowed to place other subcarriers in the unused portion of the spectrum, to transmit other information, either audio or data.

Generally, up to two subcarriers may be transmitted. The most common frequency is 67 kHz. The other frequency is 92 kHz. 67 khz signals are usually received with more clarity and less interference than 92 khz signals.

Data transmissions sometimes use 71 kHz. Other frequencies obviously may be used too.

What types of information is transmitted using SCA?

Many different types. Actually, there's no limit as to what can be transmitted. Here are some examples:
  • Background music. This is probably the most common transmission found.
  • Reading services. Also very common. Newspapers/books/etc are read for the visually impared, along with other information.
  • Foreign language services. Music, news, etc in a variety of foreign languages, aimed at ethnic groups.
  • Data transmissions. Stock market data, sports results, etc can be sent. This data is supposedly encoded in some way. Any details would be greatly useful to the monitoring community.

    What is necessary to receive SCA transmissions?

    First, obviously, an FM receiver. Second, a second receiver that demodulates the subcarrier signal.

    Any sort of FM receiver may be used. It doesn't have to be stereo, the baseband audio is all we're interested in. The radio's normal audio output may not be used though, it is filtered to cut out frequencies above about 15 kHz. So you will need to open up the radio and tap off the audio before the filtering. Generally just after detection is the best place. Careful poking around will usually turn up the correct point in the circuit. Obviously, be careful of electrocution if the radio is AC powered. I have found that old car radios are good to work with. They are small, and powered by 12 VDC, so you can't get shocked. Again, be careful.

    I have also sucessfully used a Radio Shack Pro-2004 Scanner. You must place the scanner in Wide FM mode.

    The subcarrier receiver is a bit harder to come by.

  • Build your own. The LM565 PLL IC is the heart of the circuit. It sells for about $3. Little else is required.
  • Ramsey Electronics (793 Canning Parkway, Victor, NY 14564, 716-924-4560) sells their SCA-1 kit for about $30. It is based on the LM565, and includes an audio amp. It is also sold by JDR Microdevices. I have found performance to be mediocre to fair, and have been unable to tune in a 92 kHz subcarrier.
  • Communications receiver. Many shortwave radios can be used, if they can receive FM, and can tune below 100 khz. I have found that this solution provides the best performance. See below for details.

    LM565 PLL IC Based SCA Decoder

    The following decoder circuit comes from the Signetics Applications Manual:

    Using your shortwave communications receiver to tune SCA broadcasts

    If you own a shortwave radio that tunes below 100 kHz, and can demodulate FM, it can be used to listen to SCA broadcasts. In fact, I have found the audio quality to be far superior to any SCA decoder. Plus, if you own one, you can listen to SCA broadcasts without buying any extra hardware!

    You need to feed the baseband audio into the receiver's RF (antenna) input. A capacitor in series should be used for AC coupling. You may want to add either inductors or resistors to ground to build a high pass filter, to attenuate the audio component, to reduce splattering. Just tune your FM radio to the station, tune your shortwave radio to either 67 kHz or 92 kHz (in FM mode), and you're there.

    Using an Icom R-71A Receiver

    Many shortwave listeners use Icom's R71A receiver. The R71A has an optional FM module. But it doesn't tune below 100 Khz. Well, actually, it does, or rather can be tricked into doing this.

    Go to memory mode. Clear a memory channel. Switch back and forth between that channel and another channel with the MEMORY-CH knob, which you rotate the main tuning knob back and forth. Eventually, 0.0 kHz will come up on the display. Store that frequency into a memory. Now, you can tune up to the desired frequency. You can even store 67 kHz and 92 kHz into memories so you don't have to repeat this procedure each time you want to listen. Don't tune DOWN though, or the radio resets to 30 MHz. Simple, eh?

    In addition, a company (Willco is the name I think) sells a replacement memory board that allows tuning below 100 kHz. It also gives more memories, and eliminates the battery backed RAM containing the tuning range info which, if it fails, makes your R71A brain-dead.

    How to find SCA broadcasters

    Just tune around the FM band to each station in your area. Check the common SCA frequencies (67 and 92 kHz) and see if anything's there. Bare in mind that the subcarriers are weaker than the normal audio. Just as you can't listen to stereo from a distant station (even when it sounds OK in mono), you can't hear the subcarriers well from a weak station. You need a lot of signal to get good quality reception. An outside antenna helps a lot.
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