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Sec. 5–8 FM Broadcast Technical Standards and Digital FM Broadcasting 351
5–8 FM BROADCAST TECHNICAL STANDARDS AND DIGITAL
FM BROADCASTING
There are about 9,000 FM stations in the United States. Table 5–4 gives some of the FCC technical
standards that have been adopted for FM systems. In the United States, FM stations are
classified into one of three major categories, depending on their intended coverage area. Class
A stations are local stations. They have a maximum effective radiated power (ERP) of 6 kW
and a maximum antenna height of 300 ft above average terrain. The ERP is the average transmitter
output power multiplied by the power gains of both the transmission line (a number less
than unity) and the antenna. (See Sec. 8–9 for some TV ERP calculations.) Class B stations
have a maximum ERP of 50 kW, with a maximum antenna height of 500 ft above average
terrain. Class B stations are assigned to the northeastern part of the United States, southern
California, Puerto Rico, and the Virgin Islands. Class C stations are assigned to the remainder
of the United States. They have a maximum ERP of 100 kW and a maximum antenna height of
2,000 ft above average terrain. As shown in the table, FM stations are further classified as commercial
or noncommercial. Noncommercial stations operate in the 88.1 to 91.9 MHz segment
of the FM band and provide educational programs with no commercials. In the commercial
segment of the FM band, 92.1 to 107.9 MHz, certain frequencies are reserved for Class
A stations, and the remainder are for Class B or Class C station assignment. A listing of these
frequencies and the specific station assignments for each city is available [www.fcc.gov/mb/
audio/fmg.html].
Digital FM Broadcasting
In the United States, the FCC has adopted the iBiquity FM in band on channel (IBOC) system
for digital broadcasting in the FM band (i.e., 88.1 to 107.9 MHz). This IBOC system provides
for transmission of digital audio data and auxiliary digital data simultaneously along with a
conventional FM signal (modulated by analog audio). Consequently, a conventional FM
receiver can be used to receive the analog audio. An IBOC receiver can be used, with its
built-in codec, to convert the digital part of the FM IBOC signal into almost CD quality stereo
audio. If the received FM IBOC signal is weak, the listener will hear the conventional analog
audio from the FM part of the signal. If the received signal is strong, the decoded audio will
be of CD stereo quality as decoded from the IBOC data. If the signal strength is somewhere
between weak and strong, the receiver will produce a blend of the audio obtained from the
AM and IBOC parts.
Orthogonal Frequency Division Multiplexing (OFDM), as discussed in Sec. 5–12, is
used to produce the IBOC signal. To generate the IBOC signal, data from the transmitter
codec is partitioned into two OFDM groups placed adjacent to the sidebands of the conventional
FM signal. One of the OFDM groups is placed on the upper sideband and the other on
the lower sideband. The sidebands are independent, but the data are partitioned so that only
one sideband is needed to recover reduced-quality audio. Data from both sidebands (all two of
the OFDM groups) are needed to recover full high-fidelity stereo audio.
The bandwidth of the FM IBOC signal is 400 kHz, but the carrier spacing between
stations remains at 200 kHz. Consequently, there is a potential for interference to stations
assigned to the adjacent channels. However, the existing assigned frequency of a FM station