On Air
FM subcarriers: the services hidden above the stereo signal
Above the music and the stereo pilot, an FM station can carry channels that no ordinary radio plays: reading services for blind listeners, background music, stock prices, traffic data. Where they sit, how much of the signal the rules let them take, and what is taking their place.
Pieces in this section
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How an emergency alert interrupts a song
The header burst, the two-tone attention signal, the relay chain that predates the internet, and why the system is deliberately built to survive the failure of everything modern.
13 minute read4 notes
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How far a station is allowed to reach
Height above average terrain, the class table, the protected contour, and why a low-power station on a hill can beat a high-power station on a plain.
13 minute read4 notes
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What is actually riding on an FM carrier
The baseband, layer by layer: mono sum, pilot tone, stereo difference channel, station text, and the digital sidebands that sit outside all of it.
14 minute read5 notes
An FM subcarrier is a second, inaudible carrier mixed into a station's composite baseband above the programme audio, so that it travels on the same main carrier as the music. In the United States a station broadcasting in stereo may place subcarriers between 53 and 99 kHz, and a mono station between 20 and 99 kHz. The familiar ones are RDS at 57 kHz and the subsidiary channels at 67 and 92 kHz, which an ordinary receiver ignores and a dedicated decoder recovers.
What is the difference between a carrier and a subcarrier?
A carrier is the radio-frequency wave a station is licensed to transmit, at 88.1 MHz or 101.5 MHz or wherever its channel sits. A subcarrier is a carrier too, in the plain physical sense: an oscillation that something else is impressed upon. The difference is where it lives. A subcarrier is generated at an ultrasonic frequency, modulated with its own information, and then added to the composite signal that frequency-modulates the main carrier. The prefix describes that nesting, not a lesser kind of wave.
So an FM station transmits one carrier, and inside that carrier's modulation ride several subcarriers. The stereo difference channel folded around 38 kHz is the best known of them, and the account of it, layer by layer, is in what is actually riding on an FM carrier. This piece is about the channels above it, the ones a listener never hears.
A receiver can only reach a subcarrier after it has demodulated the main carrier. That is why every service carried this way has needed a separate box or a decoder chip: the ordinary radio performs the first step and discards the rest.
Where do subcarriers sit, and how much room do the rules give them?
The federal rule for FM subcarriers, 47 CFR 73.319, is short and permissive about method: any form of modulation may be used. Its constraints are about position and power. During stereo programming, subcarriers and their significant sidebands must fall between 53 and 99 kHz, which keeps them clear of the stereo difference channel that ends at 53 kHz. During mono programming, or when no programme is on the air, the range opens to 20 to 99 kHz.
Power is counted as injection, the share of the carrier's 75 kHz deviation that the subcarriers take. In stereo, the arithmetic sum of all subcarriers may modulate the carrier by no more than 20 %; in mono, by no more than 30 %. Anything above 75 kHz may contribute no more than 10 % on its own, which is why the 92 kHz slot has always been the quieter neighbour.
Injection is not free. Every percent given to a subcarrier is a percent the programme audio cannot use, so the rules hand some of it back. Under 47 CFR 73.1570 a station may raise total peak modulation by 0.5 % for each 1 % of subcarrier injection, up to a ceiling of 110 %, or 82.5 kHz of deviation. That ceiling is reached at 20 % injection.
| Baseband frequency | Usual occupant | Carries | Worth knowing |
|---|---|---|---|
| 57 kHz | RDS, called RBDS in North America | Data, 1,187.5 bit/s | Third harmonic of the 19 kHz pilot |
| 67 kHz | SCA audio or data | Either | The most common subsidiary slot |
| About 59 to 75 kHz | DirectBand, centred on 67.65 kHz | Data, about 12 kbit/s net | Microsoft service, closed on 1 January 2012 |
| 76 kHz | DARC | Data, 16 kbit/s | Fourth harmonic of the pilot, used for traffic data in Japan |
| 92 kHz | SCA audio or data | Either | Above 75 kHz, so held to 10 % injection |
Who has actually used them?
The oldest users were private audio channels. FM spent its early decades short of revenue, and stations leased subcarriers for subscription background music in shops and offices, a trade known as storecasting. Muzak-type services were among the tenants before that business moved to satellite delivery. The subsidiary channel was established enough before stereo arrived to shape the stereo standard: when the FCC compared competing stereo systems, the Crosby system was rejected because it was incompatible with existing SCA services, and the GE and Zenith pilot-tone system was approved in April 1961.
The use that has lasted longest is the radio reading service. The Minnesota Radio Talking Book Network, started in 1969 by C. Stanley Potter and Robert Watson, was the first in the United States, and Audio-Reader in Lawrence, Kansas, followed in 1971. Volunteers read newspapers and magazines aloud, and the audio went out on a station's subcarrier to listeners who had been given a special receiver, usually free once they certified that they could not read print. The International Association of Audio Information Services is the member organisation for these services, and the country counts more than a hundred of them.
Data came later and in bursts. The Data Broadcasting Corporation's Signal service sent real-time stock quotes over subcarriers. Microsoft's DirectBand leased a 67.65 kHz subcarrier from commercial stations to push traffic, weather and other data to SPOT data watches, and later to car navigation units, under the MSN Direct service name; in October 2009 Microsoft announced that the service would end on 1 January 2012. In Japan the DARC system at 76 kHz has carried the VICS road traffic service in the Tokyo, Nagoya and Osaka areas since 1996.
Why can't an ordinary radio hear a subcarrier?
Because it was never meant to. A consumer FM receiver demodulates the main carrier, keeps the mono sum and, if a pilot is present, the stereo difference channel, and ignores the rest of the baseband. RDS is the exception only because many receivers now include a decoder for it, and even then the data reaches the display, not the speaker.
To hear a 67 or 92 kHz audio service, you need a receiver with an SCA decoder: a circuit that takes the composite signal after demodulation, isolates the band around the subcarrier, and demodulates that band in turn. Dedicated SCA receivers were built for the reading services and the background music trade. The decoder has a known weakness. The stereo difference channel ends at 53 kHz and can interfere with an SCA decoder, which is why stations normally leave a guard band between the top of the stereo signal and the first subsidiary channel.
Position in the baseband also matters at the edge of coverage. Noise after FM demodulation rises with frequency, so a channel high in the baseband, at low injection, degrades before the main audio does. A listener near the fringe can hear the music clearly while the service riding above it is already noisy.
Does a station need permission to run a subcarrier?
Mostly, no longer. Subsidiary communications were deregulated in 1983, and since then a licensed FM station can generally use its subcarriers without separate authority. Some uses still need their own licence, such as common carrier or land mobile radio services. The rule covers the hardware as well: a subcarrier generator may be connected to a type-accepted FM transmitter without specific FCC authorisation, provided the exciter does not have to be modified.
What does not change is responsibility. If a subcarrier causes spurious emissions or harmful interference, the licensee must correct the problem promptly or stop the subcarrier, and keep the measurements that show the fix at the station. The whole stack still has to fit under the emission mask that governs the main signal.
The composite also has to reach the transmitter intact. A station that generates its subcarriers at the studio needs a link wide enough to carry the full composite baseband, the choice set out in the wiring of a room that talks back; a station that sends only left and right audio inserts the subcarriers at the transmitter site instead.
What is replacing the analogue subcarrier?
The services are moving rather than vanishing. Most American reading services now stream live online, and some use an HD Radio subchannel, the digital multicast stream carried in the sidebands of a hybrid FM signal. DirectBand closed in 2012, and Muzak moved its programming to satellite. RDS is the subcarrier that kept its place, because a decoder for it is built into a great many car radios and it asks nothing of the listener.
The vocabulary that runs through all of this, from baseband and pilot to injection and translator, is defined term by term in the glossary of broadcast and audio terms.
Three short answers
Is RDS a subcarrier?
Yes. RDS is a data subcarrier at 57 kHz, three times the 19 kHz stereo pilot, carrying 1,187.5 bits per second. The European specification was first issued in 1984, and the North American version, RBDS, is published by the National Radio Systems Committee.
Can a normal FM radio receive a 67 kHz SCA channel?
No. An ordinary receiver ignores that part of the baseband. A receiver fitted with an SCA decoder is needed, and radio reading services have traditionally supplied one free to listeners who certify that they cannot read print.
Does a subcarrier make the main programme quieter?
It takes part of the modulation budget, so it can. US rules offset that by letting total peak modulation rise by 0.5 % for every 1 % of subcarrier injection, up to 110 %, which is reached at 20 % injection.
Sources
Frequency ranges, injection limits and equipment rules come from 47 CFR 73.319, FM subsidiary communications, and the modulation allowance from 47 CFR 73.1570, modulation levels. The history of the reading services follows the encyclopedia entry on radio reading services.