"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.
| 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.