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. Subcarriers are used for many different purposes, including (but not limited to) paging, inventory distribution, bus dispatching, stock market reports, traffic control signal switching, point-to-point or multipoint messages, foreign language programming, radio reading services (see the Policy Statement  and earlier MO&O and NOI  in MM Docket 87-9), radio broadcast data systems (RBDS), station control and meter reading, utility load management, and muzak. A broadcast station may transmit more than one subcarrier signal. Licensing of subcarrier operations ended in 1983 when the service was deregulated.
The Commission does not keep records of which broadcast stations are using subcarriers.
No Commission authorization, notice, application, or license is required by the broadcast station licensee wishing to transmit a subcarrier signal (see 47 CFR Section 73.127  for AM stations or 47 CFR Section 73.293  for FM stations). If the subcarrier signal is used for non-broadcast purposes, the subcarrier user may require Commission authorization, as follows:
- Common Carrier use of a broadcast subcarrier: The proposed user of the broadcast subcarrier must first seek authorization from the Commission (Wireline Competition Bureau ) under 47 CFR Parts 21 & 22 (using FCC Form 600). After a 30 day notice period, the Commission will issue its decision. The responsibility for initially determining whether a particular activity is common carriage rests with the broadcast licensee. See 47 CFR Section 73.127  for AM stations or 47 CFR Section 73.295  for FM stations.
- Private Carrier proposing use of a broadcast subcarrier: The licensee of the broadcast station (not the proposed subcarrier user) must notify the Wireless Telecommunications Bureau in Gettysburg, PA by letter prior to initiating service. The letter must certify that the subcarrier facilities will only be used for permissible purposes (47 CFR Parts 90 & 94). For additional information, please call the FCC's Call Center at 1-888-225-5322.
- Land Mobile Service proposing use of a broadcast subcarrier: The licensee of the broadcast station (not the proposed subcarrier user) must notify the Wireless Telecommunications Bureau in Gettysburg, PA by letter prior to initiating service. The letter must certify that the subcarrier facilities will only be used for permissible purposes, and must also certify that the service will be open only to eligible users (47 CFR Part 90), and that any interconnection with a telephone exchange (e.g, a paging system) will be made in accordance with Section 331 of the Communications Act. For additional information, please call the FCC's Call Center at 1-888-225-5322.
Private systems and common carrier systems operating over a subcarrier channel may be exempt from state and local regulation. For further information, please see the Memorandum Opinion and Order  in Docket 82-536, FCC 84-187, 98 FCC 2d 792, released May 2, 1984.
Although the subcarrier user may seek the necessary authorization for use, the broadcast station remains responsible for the technical operation, including the subcarriers.
The Commission does not regulate the programming content of subcarriers. However, the broadcast licensee, not the subcarrier user, must retain control of the programming, with the right to reject any material that it deems inappropriate, undesirable or illegal. See 47 CFR Section 73.127(e)  for AM stations or 47 CFR Section 73.295(e)  for FM stations.
Subcarrier use is secondary to the transmission of the main audio channel and must not interfere with the main audio channel. See 47 CFR Section 73.127(c)  for AM stations or 47 CFR Section 73.295(c)  for FM stations.
For FM stations only, subcarrier operation may continue when the main channel (regular FM programming) is off the air. However, the FM licensee must continue the hourly identification procedures on the main channel (see 47 CFR Section 73.1201 ). The hourly announcement may be made by a transmitter duty operator or by a time-clock-activated recording.
Transmissions over the AM or FM station's subcarrier are not required to comply with the station identification, delayed recording, and sponsor identification announcements required by 47 CFR Section 73.1201 , 47 CFR Section 73.1208 , and 47 CFR Section 73.1212 . See 47 CFR Section 73.127(d)  for AM stations and 47 CFR Section 73.295(d)  for FM stations. In addition, there is no program log requirement for broadcast subcarrier use.
For monophonic FM stations, or when the main FM channel is off, the multiplex subcarriers (two or more) must be located between 20 kHz and 99 kHz. The arithmetic sum of all the multiplex subcarriers may not exceed 30% modulation (22.5 kHz) (where 100% is 75 kHz deviation from the carrier (or assigned center) frequency (see 47 CFR Section 73.310 , "Percentage Modulation"). The arithmetic sum of all multiplex subcarriers above 75 kHz may not exceed 10% (7.5 kHz). See 47 CFR Section 73.319(c)(1), (c)(3), (d)(1). and (d)(3) .
For stereophonic FM stations, the multiplex subcarriers (two or more) must be located between 53 kHz and 99 kHz. The arithmetic sum of all multiplex subcarriers may not exceed 20% modulation (15 kHz) (where 100% is 75 kHz deviation from the carrier). The arithmetic sum of all subcarriers above 75 kHz may not exceed 10% (7.5 kHz). See 47 CFR Section 73.319(c)(2) and (d)(2) .
For FM stations only, the total peak modulation may be increased 0.5% for each 1.0% increase in subcarrier modulation. Modulation levels up to 110% (82.5 kHz peak deviation) are permitted for FM stations using subcarriers (75 kHz deviation = 100%). Instantaneous sidebands are permitted up to 99 kHz. See 47 CFR Section 73.1570(b)(2) .
Technical specifications for subcarriers are no longer contained in the FCC's rules. However, because we receive occasional requests for this information, the 1983 rules for subcarriers  have been made available in a PDF document.
Educational stations may use subcarriers for profit-making purposes. However, an educational FM station is required to provide another subcarrier for any radio reading services that may request such use. See 47 CFR Section 73.593 . The educational station is limited to charging the radio reading service for the fixed or operating costs, but may not charge on a for-profit basis. For additional information regarding radio reading services, see the Commission's Policy Statement , Docket 87-9, 3 FCC Rcd 6323, released 10/28/1988.
In general, tunable subcarrier receivers are prohibited because they violate Section 605 of the Communications Act of 1934, as amended, which states that no one may receive, or assist in receiving, any radio communication to which they are not entitled and use that information for their own benefit. In addition, 18 U.S.C. Sections 2510 through 2521 prohibit the manufacture, assembly, possession, and sale of any device primarily useful for the surreptitious interception of such radio transmissions. See KMLA Broadcasting Corp. v. Twentieth Century Cigarette Vendors Corp. 264 F. Supp. 35 (D. Cal. 1967); see also Letter to Dr. Haghighi from Richard B. Engelman, Chief, Technical Standards Branch, OET . (One exception  was made in 1984 for a tunable subcarrier receiver for a data service in which (1) the receivers could not receive other SCA transmissions, (2) subscribers who had not met their contractual obligations to the service provider were denied access.)
Broadcast stations should keep copies of any subcarrier leasing agreements at the station. The agreements must be made available for inspection by any authorized representative of the FCC upon request. See 47 CFR Section 73.1226(c) .
For more information pertaining to AM and FM radio broadcasting and the Audio Division, please call: (202) 418-2700.