Sec. 73.310  FM technical definitions.

    (a) Frequency modulation.
    Antenna height above average terrain (HAAT). HAAT is calculated by: 
determining the average of the antenna heights above the terrain from 3 
to 16 kilometers (2 to 10 miles) from the antenna for the eight 
directions evenly spaced for each 45 deg. of azimuth starting with True 
North (a different antenna height will be determined in each direction 
from the antenna): and computing the average of these separate heights. 
In some cases less than eight directions may be used. (See 
Sec. 73.313(d).) Where circular or elliptical polarization is used, the 
antenna height above average terrain must be based upon the height of 
the radiation of the antenna that transmits the horizontal component of 
radiation.
    Antenna power gain. The square of the ratio of the root-mean-square 
(RMS) free space field strength produced at 1 kilometer in the 
horizontal plane in millivolts per meter for 1 kW antenna input power to 
221.4 mV/m. This ratio is expressed in decibels (dB). If specified for a 
particular direction, antenna power gain is based on that field strength 
in the direction only.
    Auxiliary facility. An auxiliary facility is an antenna separate 
from the main facility's antenna, permanently installed on the same 
tower or at a different location, from which a station may broadcast for 
short periods without prior Commission authorization or notice to the 
Commission while the main facility is not in operation (e.g., where 
tower work necessitates turning off the main antenna or where lightning 
has caused damage to the main antenna or transmission system) (See 
Sec. 73.1675).
    Center frequency. The term ``center frequency'' means:
    (1) The average frequency of the emitted wave when modulated by a 
sinusoidal signal.
    (2) The frequency of the emitted wave without modulation.
    Composite antenna pattern. The composite antenna pattern is a 
relative field horizontal plane pattern for 360 degrees of azimuth, for 
which the value at a particular azimuth is the greater of the 
horizontally polarized or vertically polarized component relative field 
values. The composite antenna pattern is normalized to a maximum of 
unity (1.000) relative field.
    Composite baseband signal. A signal which is composed of all program 
and other communications signals that frequency modulates the FM 
carrier.
    Effective radiated power. The term ``effective radiated power'' 
means the product of the antenna power (transmitter output power less 
transmission line loss) times: (1) The antenna power gain, or (2) the 
antenna field gain squared. Where circular or elliptical

[[Page 129]]

polarization is employed, the term effective radiated power is applied 
separately to the horizontal and vertical components of radiation. For 
allocation purposes, the effective radiated power authorized is the 
horizontally polarized component of radiation only.
    Equivalent isotropically radiated power (EIRP). The term 
``equivalent isotropically radiated power (also known as ``effective 
radiated power above isotropic) means the product of the antenna input 
power and the antenna gain in a given direction relative to an isotropic 
antenna.
    FM Blanketing. Blanketing is that form of interference to the 
reception of other broadcast stations which is caused by the presence of 
an FM broadcast signal of 115 dBu (562 mV/m) or greater signal strength 
in the area adjacent to the antenna of the transmitting station. The 115 
dBu contour is referred to as the blanketing contour and the area within 
this contour is referred to as the blanketing area.
    FM broadcast band. The band of frequencies extending from 88 to 108 
MHz, which includes those assigned to noncommercial educational 
broadcasting.
    FM broadcast channel. A band of frequencies 200 kHz wide and 
designated by its center frequency. Channels for FM broadcast stations 
begin at 88.1 MHz and continue in successive steps of 200 kHz to and 
including 107.9 MHz.
    FM broadcast station. A station employing frequency modulation in 
the FM broadcast band and licensed primarily for the transmission of 
radiotelephone emissions intended to be received by the general public.
    Field strength. The electric field strength in the horizontal plane.
    Free space field strength. The field strength that would exist at a 
point in the absence of waves reflected from the earth or other 
reflecting objects.
    Frequency departure. The amount of variation of a carrier frequency 
or center frequency from its assigned value.
    Frequency deviation. The peak difference between modulated wave and 
the carrier frequency.
    Frequency modulation. A system of modulation where the instantaneous 
radio frequency varies in proportion to the instantaneous amplitude of 
the modulating signal (amplitude of modulating signal to be measured 
after pre-emphasis, if used) and the instantaneous radio frequency is 
independent of the frequency of the modulating signal.
    Frequency swing. The peak difference between the maximum and the 
minimum values of the instantaneous frequency of the carrier wave during 
modulation.
    Multiplex transmission. The term ``multiplex transmission'' means 
the simultaneous transmission of two or more signals within a single 
channel. Multiplex transmission as applied to FM broadcast stations 
means the transmission of facsimile or other signals in addition to the 
regular broadcast signals.
    Percentage modulation. The ratio of the actual frequency deviation 
to the frequency deviation defined as 100% modulation, expressed in 
percentage. For FM broadcast stations, a frequency deviation of 
plus-minus75kHz is defined as 100% modulation.
    (b) Stereophonic sound broadcasting.
    Cross-talk. An undesired signal occurring in one channel caused by 
an electrical signal in another channel.
    FM stereophonic broadcast. The transmission of a stereophonic 
program by a single FM broadcast station utilizing the main channel and 
a stereophonic subchannel.
    Left (or right) signal. The electrical output of a microphone or 
combination of microphones placed so as to convey the intensity, time, 
and location of sounds originating predominately to the listener's left 
(or right) of the center of the performing area.
    Left (or right) stereophonic channel. The left (or right) signal as 
electrically reproduced in reception of FM stereophonic broadcasts.
    Main channel. The band of frequencies from 50 to 15,000 Hz which 
frequency-modulate the main carrier.
    Pilot subcarrier. A subcarrier that serves as a control signal for 
use in the reception of FM stereophonic sound broadcasts.
    Stereophonic separation. The ratio of the electrical signal caused 
in sound channel A to the signal caused in sound channel B by the 
transmission of only a channel B signal. Channels A and B

[[Page 130]]

may be any two channels of a stereophonic sound broadcast transmission 
system.
    Stereophonic sound. The audio information carried by plurality of 
channels arranged to afford the listener a sense of the spatial 
distribution of sound sources. Stereophonic sound broadcasting includes, 
but is not limited to, biphonic (two channel), triphonic (three channel) 
and quadrophonic (four channel) program services.
    Stereophonic sound subcarrier. A subcarrier within the FM broadcast 
baseband used for transmitting signals for stereophonic sound reception 
of the main broadcast program service.
    Stereophonic sound subchannel. The band of frequencies from 23 kHz 
to 99 kHz containing sound subcarriers and their associated sidebands.
    (c) Visual transmissions. Communications or message transmitted on a 
subcarrier intended for reception and visual presentation on a viewing 
screen, teleprinter, facsimile printer, or other form of graphic display 
or record.
    (d) Control and telemetry transmissions. Signals transmitted on a 
multiplex subcarrier intended for any form of control and switching 
functions or for equipment status data and aural or visual alarms.

[ 28 FR 13623 , Dec. 14, 1963, as amended at  39 FR 10575 , Mar. 21, 1974; 
 44 FR 36038 , June 20, 1979;  48 FR 28454 , June 22, 1983;  48 FR 29507 , 
June 27, 1983;  48 FR 37216 , Aug. 17, 1983;  49 FR 45145 , Nov. 15, 1984; 
 57 FR 48333 , Oct. 23, 1992;  62 FR 51058 , Sept. 30, 1997]

    Effective Date Note:  At  62 FR 51058 , Sept. 30, 1997, Sec. 73.310 
was amended by adding the definitions Auxiliary facility and Composite 
antenna pattern, effective Dec. 1, 1997.