Sec. 73.681  Definitions.

    Amplitude modulation (AM). A system of modulation in which the 
envelope of the transmitted wave contains a component similar to the 
wave form of the signal to be transmitted.
    Antenna electrical beam tilt. The shaping of the radiation pattern 
in the vertical plane of a transmitting antenna by electrical means so 
that maximum radiation occurs at an angle below the horizontal plane.
    Antenna height above average terrain. The average of the antenna 
heights above the terrain from approximately 3.2 (2 miles) to 16.1 
kilometers (10 miles) from the antenna for the eight directions spaced 
evenly for each 45 degrees of azimuth starting with True North. (In 
general, a different antenna height will be determined in each direction 
from the antenna. The average of these various heights is considered the 
antenna height above the average terrain. In some cases less than 8 
directions may be used. See Sec. 73.684(d)). Where circular or 
elliptical polarization is employed, the antenna height above average 
terrain shall be based upon the height of the radiation center of the 
antenna which transmits the horizontal component of radiation.
    Antenna mechanical beam tilt. The intentional installation of a 
transmitting antenna so that its axis is not vertical, in order to 
change the normal angle of maximum radiation in the vertical plane.
    Antenna power gain. The square of the ratio of the root-mean-square 
free space field strength produced at 1 kilometer in the horizontal 
plane, in millivolts per meter for one kW antenna input power to 221.4 
mV/m. This ratio should be expressed in decibels (dB). (If specified for 
a particular direction, antenna power gain is based on the field 
strength in that direction only.)
    Aspect ratio. The ratio of picture width to picture height as 
transmitted.
    Aural center frequency. (1) The average frequency of the emitted 
wave when modulated by a sinusoidal signal; (2) the frequency of the 
emitted wave without modulation.
    Aural transmitter. The radio equipment for the transmission of the 
aural signal only.
    Auxiliary facility. An auxiliary facility is an antenna separate a 
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).
    BTSC. Broadcast Television systems committee recommendation for 
multichannel television sound transmission and audio processing as 
defined in FCC Bulletin OET 60.
    Baseband. Aural transmitter input signals between 0 and 120 kHz.
    Blanking level. The level of the signal during the blanking 
interval, except the interval during the scanning synchronizing pulse 
and the chrominance subcarrier synchronizing burst.
    Chrominance. The colorimetric difference between any color and a 
reference color of equal luminance, the reference color having a 
specific chromaticity.

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    Chrominance subcarrier. The carrier which is modulated by the 
chrominance information.
    Color transmission. The transmission of color television signals 
which can be reproduced with different values of hue, saturation, and 
luminance.
    Effective radiated power. The product of the antenna input power and 
the antenna power gain. This product should be expressed in kW and in dB 
above 1 kW (dBk). (If specified for a particular direction, effective 
radiated power is based on the antenna power gain in that direction 
only. The licensed effective radiated power is based on the maximum 
antenna power gain. When a station is authorized to use a directional 
antenna or an antenna beam tilt, the direction of the maximum effective 
radiated power will be specified.) Where circular or elliptical 
polarization is employed, the term effective radiated power is applied 
separately to the horizontally and vertically polarized components of 
radiation. For assignment purposes, only the effective radiated power 
authorized for the horizontally polarized component will be considered.
    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.
    Field. Scanning through the picture area once in the chosen scanning 
pattern. In the line interlaced scanning pattern of two to one, the 
scanning of the alternate lines of the picture area once.
    Frame. Scanning all of the picture area once. In the line interlaced 
scanning pattern of two to one, a frame consists of two fields.
    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 the instantaneous 
frequency of the modulated wave and the carrier frequency.
    Frequency modulation (FM). 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.
    Interlaced scanning. A scanning process in which successively 
scanned lines are spaced an integral number of line widths, and in which 
the adjacent lines are scanned during successive cycles of the field 
frequency.
    IRE standard scale. A linear scale for measuring, in IRE units, the 
relative amplitudes of the components of a television signal from a zero 
reference at blanking level, with picture information falling in the 
positive, and synchronizing information in the negative domain.

    Note: When a carrier is amplitude modulated by a television signal 
in accordance with Sec. 73.682, the relationship of the IRE standard 
scale to the conventional measure of modulation is as follows:


                                                                        
------------------------------------------------------------------------
                                                      IRE               
                                                   standard   Modulation
                      Level                          scale    percentage
                                                    (units)             
------------------------------------------------------------------------
Zero carrier....................................         120           0
Reference white.................................         100        12.5
Blanking........................................           0          75
Synchronizing peaks (maximum carrier level).....         -40         100
------------------------------------------------------------------------

    Luminance. Luminous flux emitted, reflected, or transmitted per unit 
solid angle per unit projected area of the source.
    Main channel. The band of frequencies from 50 to 15,000 Hertz which 
frequency modulate the main aural carrier.
    Monochrome transmission. The transmission of television signals 
which can be reproduced in gradations of a single color only.
    Multichannel Television Sound (MTS). Any system of aural 
transmission that

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utilizes aural baseband operation between 15 kHz and 120 kHz to convey 
information or that encodes digital information in the video portion of 
the television signal that is intended to be decoded as audio 
information.
    Multiplex Transmission (Aural). A subchannel added to the regular 
aural carrier of a television broadcast station by means of frequency 
modulated subcarriers.
    Negative transmission. Where a decrease in initial light intensity 
causes an increase in the transmitted power.
    Peak power. The power over a radio frequency cycle corresponding in 
amplitude to synchronizing peaks.
    Percentage modulation. As applied to frequency modulation, the ratio 
of the actual frequency deviation to the frequency deviation defined as 
100% modulation expressed in percentage. For the aural transmitter of TV 
broadcast stations, a frequency deviation of plus-minus25 kHz 
is defined as 100% modulation.
    Pilot subcarrier. A subcarrier used in the reception of TV 
stereophonic aural or other subchannel broadcasts.
    Polarization. The direction of the electric field as radiated from 
the transmitting antenna.
    Program related data signal. A signal, consisting of a series of 
pulses representing data, which is transmitted simultaneously with and 
directly related to the accompanying television program.
    Reference black level. The level corresponding to the specified 
maximum excursion of the luminance signal in the black direction.
    Reference white level of the luminance signal. The level 
corresponding to the specified maximum excursion of the luminance signal 
in the white direction.
    Scanning. The process of analyzing successively, according to a 
predetermined method, the light values of picture elements constituting 
the total picture area.
    Scanning line. A single continuous narrow strip of the picture area 
containing highlights, shadows, and half-tones, determined by the 
process of scanning.
    Standard television signal. A signal which conforms to the 
television transmission standards.
    Synchronization. The maintenance of one operation in step with 
another.
    Television broadcast band. The frequencies in the band extending 
from 54 to 806 megahertz which are assignable to television broadcast 
stations. These frequencies are 54 to 72 megahertz (channels 2 through 
4), 76 to 88 megahertz (channels 5 and 6), 174 to 216 megahertz 
(channels 7 through 13), and 470 to 806 megahertz (channels 14 through 
69).
    Television broadcast station. A station in the television broadcast 
band transmitting simultaneous visual and aural signals intended to be 
received by the general public.
    Television channel. A band of frequencies 6 MHz wide in the 
television broadcast band and designated either by number or by the 
extreme lower and upper frequencies.
    Television transmission standards. The standards which determine the 
characteristics of a television signal as radiated by a television 
broadcast station.
    Television transmitter. The radio transmitter or transmitters for 
the transmission of both visual and aural signals.
    Vestigial sideband transmission. A system of transmission wherein 
one of the generated sidebands is partially attenuated at the 
transmitter and radiated only in part.
    Visual carrier frequency. The frequency of the carrier which is 
modulated by the picture information.
    Visual transmitter. The radio equipment for the transmission of the 
visual signal only.
    Visual transmitter power. The peak power output when transmitting a 
standard television signal.

[ 28 FR 13660 , Dec. 14, 1963, as amended at  35 FR 5692 , Apr. 8, 1970;  36 FR 5505 , Mar. 24, 1971;  36 FR 17429 , Aug. 31, 1971;  41 FR 56325 , Dec. 
28, 1976;  42 FR 20823 , Apr. 22, 1977;  44 FR 36039 , June 20, 1979;  47 FR 35990 , Aug. 18, 1982;  49 FR 18106 , Apr. 27, 1984;  49 FR 38131 , Sept. 27, 
1984;  49 FR 50048 , Dec. 26, 1984;  50 FR 23699 , June 5, 1985;  51 FR 12616 , Apr. 14, 1986;  56 FR 49707 , Oct. 1, 1991;  58 FR 44951 , Aug. 25, 
1993;  62 FR 51059 , Sept. 30, 1997]

    Effective Date Note:  At  62 FR 51059 , Sept. 30, 1997, Sec. 73.681 
was amended by adding the definition Auxiliary facility, effective Dec. 
1, 1997.

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