Goto Section: 73.673 | 73.682 | Table of Contents

FCC 73.681
Revised as of September 1, 2021
Goto Year:2020 | 2022
  §  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 § 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

   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
   § 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.

   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

   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

   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 § 73.682, the relationship of the IRE standard scale to
   the conventional measure of modulation is as follows:
   Level IRE standard scale (units) Modulation percentage
   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 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 ±25 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

   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

   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]

   Editorial Note: For Federal Register citations affecting § 73.681, see
   the List of CFR Sections Affected, which appears in the Finding Aids
   section of the printed volume and at


Goto Section: 73.673 | 73.682

Goto Year: 2020 | 2022
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