New York SCA Subcarrier Radio Stations

 The FCC's description of the service is a bit dry

"A subcarrier, known also as Subsidiary Communications Authority or SCA, is a separate audio or data channel that is transmitted along with the main audio signal over a broadcast station.  These subcarrier channels are not receivable with a regular radio; special receivers are required"

These subcarriers are a sideband of a radio frequency carrier wave, which is modulated to send additional information. A subcarrier can be used for all sorts of different purposes, including paging, inventory distribution, bus dispatching, stock market reports, conveying horse race results, traffic control signal switching, point-to-point or multipoint messages, foreign language programming, radio reading services, radio broadcast data systems (RBDS), station control and meter reading, utility load management, and muzak. I've even seen it used for to trigger fireworks displays. 

I have never found a canonically complete list of US subcarrier radio stations. But I have found diligent amateurs maintaining their own local databases. Take for example this website for New York City. It lists the below stations:

New York SCA Subcarrier Radio Stations:

CALLS	FREQ	SUBCARRIER	FORMAT	CITY
WSOU	89.5	67 kHz	Reading for the Blind	Orange, NJ
WKCR	94.3	67 kHz	Reading for the Blind	New York, NY
WXNY	98.7	67 kHz	Radio Omega (Haitian)	New York, NY
WQHT	106.7	67 kHz	Hellas World Radio (Greek)	New York, NY
WQHT	106.7	92 kHz	Radio Jesus es Senor	New York, NY
WSKQ	97.9	67 kHz	Radio Soleil d'Haiti	New York, NY
WBAI	99.5	67 kHz	Radio Maria (Italian)	New York, NY
WBAI	99.5	92 kHz	Radio Maria (Spanish)	New York, NY
WCBS	101.1	67 kHz	Radio Maria Hispana	New York, NY
WRXP	101.9	67 kHz	Radio PaHou (Haitian)	New York, NY
WRXP	101.9	92 kHz	Radio L'Amour Int'l (Haitian)	New York, NY
WAXQ	104.3	67 kHz	Korean Christian prog.	New York, NY
WWPR	105.1	67 kHz	Radio Verite (Haitian)	New York, NY
WBLS	107.5	67 kHz	Radio Shammah (Haitian)	New York, NY

It's worth noting that the original WPAT on 103.5 had two subcarriers. On 67 kHz was Chinese Radio NYC and on 92 kHz was ICN Radio. Today WPAT 93.1 is a HD station so I believe their subcarrier was repurposed for the bandwidth presently dedicated to the WGNK simulcast on HD2. 

So the information above is probably right, but also probably not complete. Many services are short-lived. Back in 1983 NAB printed up a sales sheet encouraging FM broadcasters to use their subcarriers to turn a quick buck. Simultaneously they encouraged the FCC not to over-regulate the service.  [LINK] the resulting landscape let to a rabbit warren of nominally regulated non-advertised subcarrier services. In 1983 the NAB had such clout that it was legal to sell the use of your subcarrier even while your primary signal was offline.

 Effectively there are two reasons that it's so difficult to get a good accounting of subcarrier stations.

1. The FCC does not keep records of which broadcast stations are using subcarriers. So there is no central database attached to licensing. 

2. The FCC requires no special authorization, notice, application, or license is from the licensee planning to transmit a subcarrier signal (see 47 CFR Section 73.127 for AM stations or 47 CFR Section 73.293 for FM stations).  The only exception to this is if the subcarrier is used for non-broadcast purposes. More here.

But then why are there comparatively so few subcarriers?  Similar to the above that boils down to two primary reasons: 

1.  Use of an FM subcarrier requires a reduction  in deviation of the main channel by as much  as 10%. That does not mean that the subcarrier reduces the coverage of the primary signal but an actual 0.92 dB reduction in the signal-to-noise ratio. It sounds scarier than it is.

2. The subcarrier can interfere with the primary signal. Any non -linearity in the transmission system - exciter, cable and antenna, or receiver - can mix the subcarrier  signal with the other signals present in the total FM signal and cause audible interference.  This was a very real problem. 

NAB tried to spin the above "One beneficial  side effect of using an FM subcarrier is to  force a station, if it proves necessary, to  clean up its transmitter." Perhaps it was over optimistic for a service which can only be sold to a few buyers, and only be heard by very few listeners. 

What's in a signal part 2

So that's all the stuff you can hear. Here's everything else. But, let me warn you now, that under Electronic Communications Privacy Act of 1986, it is illegal for you to monitor these transmissions. I have no idea why. It's not really that exciting and they can't tell you're doing it anyway. Unenforcible laws are just silly. So here's what you can't hear.

Stereo Beacon or Reference Signal: This unmodulated subcarrier is transmitted at 19 kHz. This is lets the FM stereo receiver know that the broadcast is in stereo. The receiver doubles the frequency of the pilot tone and uses it as a phase reference to demodulate the stereo information.

Subcarrier
A subcarrier is totally different than a reference tone. The SCA can carry data. This separate signal is carried on the main radio transmission. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth. This is in some ways similar to the multiplexing that's the big excitment (ho-hum) in HD Radio. I've talked about that before as well.

But there's all kinds of data in an SCA. That Horrible easy listening music you hear in the evevator or dentist office, Reading services for the blind, Foreign language news services, and theres a growing variety of data transmissions.

A typical FM broadcast may contain anywhere from 2 to 12 subcarrier voice channels in addition to the main signal, and subcarriers may havesubcarriers of their own. Sounds wildly complicated right? A lot of surveilance devices use these frequencies as well http://tscm.com/bugfrqVLF.html

I also caught an interesting note about Fireworks in Los Angeles using a 92 kHz SCA:
"a man was just reciting numbers, mostly one-at-a-time, but sometimes two." I'm not from KXLU, nor was I involved in that particular fireworks show, but as a fireworks pyrotechnician (one of my sidelines), that sure sounds like firing queues to me. It's becoming far more common to coordinate fireworks shows with music from radio stations and using sub-carriers is a common way to do that" I cut the identifiers out of that because it's not legal to have heard it. oy.

What's in the Signal Pt 1

There is more to radio than what you hear. I dont mean all the goings on behind the mike. I'm referring to the signals in the broadcast that you can't hear. The audio you hear is data on a carrier wave. There is more to it than audible sound.

There are 100 possible FM channels between 88.1 and 107.9 MHz. These are allocated in 200 kHz increments. Because of limited bandwidth a radio station can only transmit audio which is up to 100 kHz. But hey, that's still much wider than the range of human hearing. In practice, FM audio is usually restricted to 15 kHz. This leaves a lot of spectrum for other uses. FM Stations are permitted to place subcarriers in this unused portion of the spectrum.

Here's what's on that carrier wave that you can hear:
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.

Difference signal: This is harder to imagine, it's the difference between the right and left channels. When a songs in stereo both ears are getting very similar but not exactly the same audio. This is just the parts that are different or (L-R). This is transmitted on a 38 kHz subcarrier using FM modulation.

Mind blowing isnt' it? FM radio does not broadcast the Left and Right Channels. It's not a damn think like the channels on a cassete tape. Here is how how stereo audio is decoded. Of course, as much of a genius as Edwin Armstrong was, it was nto his system that gave us stereo audio. The Armstrong system was rejected by the FCC because it did not allow sub-carrier services. The the Zenith system has gone on to become the standard method in most countries.
The Armstrong system was more noise resistant I understand. Instead today's stereo FM signals are far more susceptible to noise and multipath distortion than mono FM signals. This is due to several factors, including the following:

1. the addition of the two sidebands of the difference subcarrier to the baseband signal increases the noise bandwidth of the signal by a factor of three (9.5 dB) as compared with a mono signal.
2. The pre-emphasis is applied to the audio signals results in the pre-emphasis acting in the wrong direction on the lower sideband of the difference subcarrier, i.e. decreasing the level as the frequency rises, which will have a further deleterious effect on the S/N of the difference signal. yadda yadda , I know thats too technical. Btu I cant think of a way to simplify it.

So heres what we got and how it works:
If you fed the baseband audio into a receiver's antenna input it will demodulate the signal. This demodulated signal would be the R+ L audio. If you combine this with the Difference (R-L) signal you get a mess, but that's kind of how it works. What it does is add the R+L and R-L signals, leaving just the right (R) channel. the remainder being the LEft channel left (L) channel. ta-dah!

The upside of this complexity and reduced reception qualityis that mono and stereo stations can all operate with this system as can all FM tuners. Also side bands can exist and aren't those nifty. More tomorrow!