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Montage Acoustics BT4480
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Montage Acoustics HD9001: Subsidiary communications authorization
In the United States, services (other than stereo, quad and RDS) using subcarriers are sometimes referred to as subsidiary communications authorization (SCA) services. Uses for such subcarriers include book/newspaper reading services for blind listeners, private data transmission services (for example sending stock market information to stockbrokers or stolen credit card number blacklists to stores) subscription commercial-free background music services for shops, paging ("beeper") services and providing a program feed for AM transmitters of AM/FM stations. SCA subcarriers are typically 67 kHz and 92 kHz.

The limitation on AM fidelity
The limitation on AM fidelity comes partly from current receiver design, and efforts have been made to improve this, notably the AMAX standards. Moreover, to fit more transmitters on the MW broadcast band in the United States, maximum transmitted audio bandwidth is limited to 10.2 kHz by a National Radio Systems Committee (NRSC) standard adopted by the FCC in June 1989, resulting in a channel occupied bandwidth of 20.4 kHz. The former audio limitation was 15 kHz resulting in a channel occupied bandwidth of 30 kHz.Montage Acoustics

Montage Acoustics BT4480: The first commercial FM broadcasting stations
The first commercial FM broadcasting stations were in the United States, but initially they were primarily used to simulcast their AM sister stations, to broadcast lush orchestral music for stores and offices, to broadcast classical music to an upmarket listenership in urban areas, or for educational programming. By the late 1960s FM had been adopted by fans of "Alternative Rock" music ("A.O.R.—'Album Oriented Rock' Format"), but it wasn't until 1978 that listenership to FM stations exceeded that of AM stations in North America. During the 1980s and 1990s, Top 40 music stations and later even country music stations largely abandoned AM for FM. Today AM is mainly the preserve of talk radio, news, sports, religious programming, ethnic (minority language) broadcasting and some types of minority interest music. This shift has transformed AM into the "alternative band" that FM once was. (Some AM stations have begun to simulcast on, or switch to, FM signals to attract younger listeners and aid reception problems in buildings, during thunderstorms, and near high tension wires. Some of these stations now emphasize their presence on the FM dial.)

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United States, digital radio services
In the United States, digital radio services are being deployed within the FM band rather than using Eureka 147 or the Japanese standard ISDB. This in-band on-channel approach, as do all digital radio techniques, makes use of advanced compressed audio. The proprietary iBiquity system, branded as "HD Radio", currently is authorized for "hybrid" mode operation, wherein both the conventional analog FM carrier and digital sideband subcarriers are transmitted. Eventually, presuming widespread deployment of HD Radio receivers, the analog services could theoretically be discontinued and the FM band become all digital. Montage Acoustics BT4480

Montage Acoustics HD9001:Constructing a "bug"
Constructing a "bug" is a common early project for electronics hobbyists, and project kits to do so are available from a wide variety of sources. The devices constructed, however, are often too large and poorly shielded for use in clandestine activity.

In addition, much pirate radio activity is broadcast in the FM range, because of the band's greater clarity and listenership, the smaller size and lower cost of equipment.