Sec. 73.14  AM broadcast definitions.

    AM broadcast band. The band of frequencies extending from 535 to 
1705 kHz.
    AM broadcast channel. The band of frequencies occupied by the 
carrier and the upper and lower sidebands of an AM broadcast signal with 
the carrier frequency at the center. Channels are designated by their 
assigned carrier frequencies. The 117 carrier frequencies assigned to AM 
broadcast stations begin at 540 kHz and progress in 10 kHz steps to 1700 
kHz. (See Sec. 73.21 for the classification of AM broadcast channels).
    AM broadcast station. A broadcast station licensed for the 
dissemination of radio communications intended to be received by the 
public and operated on a channel in the AM broadcast band.
    Amplitude modulated stage. The radio-frequency stage to which the 
modulator is coupled and in which the carrier wave is modulated in 
accordance with the system of amplitude modulation and the 
characteristics of the modulating wave.
    Amplitude modulator stage. The last amplifier stage of the 
modulating wave amplitude modulates a radio-frequency stage.
    Antenna current. The radio-frequency current in the antenna with no 
modulation.
    Antenna input power. The product of the square of the antenna 
current and the antenna resistance at the point where the current is 
measured.
    Antenna resistance. The total resistance of the transmitting antenna 
system at the operating frequency and at the point at which the antenna 
current is measured.
    Auxiliary facility. An auxiliary facility is an AM antenna tower(s) 
separate from the main facility's antenna tower(s), permanently 
installed at the same site or at a different location, from which an AM 
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).
    Blanketing. The interference which is caused by the presence of an 
AM broadcast signal of one volt per meter (V/m) or greater strengths in 
the area adjacent to the antenna of the transmitting station. The 1 V/m 
contour is referred to as the blanket contour and

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the area within this contour is referred to as the blanket area.
    Carrier-amplitude regulation (Carrier shift). The change in 
amplitude of the carrier wave in an amplitude-modulated transmitter when 
modulation is applied under conditions of symmetrical modulation.
    Combined audio harmonics. The arithmetical sum of the amplitudes of 
all the separate harmonic components. Root sum square harmonic readings 
may be accepted under conditions prescribed by the FCC.
    Critical directional antenna. An AM broadcast directional antenna 
that is required, by the terms of a station authorization, to be 
operated with the relative currents and phases within the antenna 
elements at closer tolerances of deviation than those permitted under 
Sec. 73.62 and observed with a high precision monitor capable of 
measuring these parameters.
    Critical hours. The two hour period immediately following local 
sunrise and the two hour period immediately preceding local sunset.
    Daytime. The period of time between local sunrise and local sunset.
    Effective field; Effective field strength. The root-mean-square 
(RMS) value of the inverse distance fields at a distance of 1 kilometer 
from the antenna in all directions in the horizontal plane. The term 
``field strength'' is synonymous with the term ``field intensity'' as 
contained elsewhere in this Part.
    Equipment performance measurements. The measurements performed to 
determine the overall performance characteristics of a broadcast 
transmission system from point of program origination at main studio to 
sampling of signal as radiated. (See Sec. 73.1590)
    Experimental period. the time between 12 midnight local time and 
local sunrise, used by AM stations for tests, maintenance and 
experimentation.
    Frequency departure. The amount of variation of a carrier frequency 
or center frequency from its assigned value.
    Incidental phase modulation. The peak phase deviation (in radians) 
resulting from the process of amplitude modulation.
    Input power. Means the product of the direct voltage applied to the 
last radio stage and the total direct current flowing to the last radio 
stage, measured without modulation.
    Intermittent service area. Means the area receiving service from the 
groundwave of a broadcast station but beyond the primary service area 
and subject to some interference and fading.
    Last radio stage. The radio-frequency power amplifier stage which 
supplies power to the antenna.
    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 originated 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 AM stereophonic broadcasts.
    Main channel. The band of audio frequencies from 50 to 10,000 Hz 
which amplitude modulates the carrier.
    Maximum percentage of modulation. The greatest percentage of 
modulation that may be obtained by a transmitter without producing in 
its output, harmonics of the modulating frequency in excess of those 
permitted by these regulations. (See Sec. 73.1570)
    Maximum rated carrier power. The maximum power at which the 
transmitter can be operated satisfactorily and is determined by the 
design of the transmitter and the type and number of vacuum tubes or 
other amplifier devices used in the last radio stage.
    Model I facility. A station operating in the 1605-1705 kHz band 
featuring fulltime operation with stereo, competitive technical quality, 
10 kW daytime power, 1 kW nighttime power, non-directional antenna (or a 
simple directional antenna system), and separated by 400-800 km from 
other co-channel stations.
    Model II facility. A station operating in the 535-1605 kHz band 
featuring fulltime operation, competitive technical quality, wide area 
daytime coverage with nighttime coverage at least 15% of the daytime 
coverage.
    Nighttime. The period of time between local sunset and local 
sunrise.
    Nominal power. The antenna input power less any power loss through a 
dissipative network and, for directional

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antennas, without consideration of adjustments specified in paragraphs 
(b)(1) and (b)(2) of Sec. 73.51 of the rules. However, for AM broadcast 
applications granted or filed before June 3, 1985, nominal power is 
specified in a system of classifications which include the following 
values: 50 kW, 25 kW, 10 kW, 5 kW, 2.5 kW, 1 kW, 0.5 kW, and 0.25 kW. 
The specified nominal power for any station in this group of stations 
will be retained until action is taken on or after June 3, 1985, which 
involves a change in the technical facilities of the station.
    Percentage modulation (amplitude)
    In a positive direction:

M = MAX-C  x  100
      ----------------
          c

    In a negative direction:

M = C-MIN  x  100
      ---------------
          c

Where:
    M = Modulation level in percent.
    MAX = Instantaneous maximum level of the modulated radio frequency 
envelope.
    MIN = Instantaneous minimum level of the modulated radio frequency 
envelope.
    C = (Carrier) level of radio frequency envelope without modulation.

    Plate modulation. The modulation produced by introduction of the 
modulating wave into the plate circuit of any tube in which the carrier 
frequency wave is present.
    Primary service area. Means the service area of a broadcast station 
in which the groundwave is not subject to objectionable interference or 
objectionable fading.
    Proof of performance measurements or antenna proof of performance 
measurements. The measurements of field strengths made to determine the 
radiation pattern or characteristics of an AM directional antenna 
system.
    Secondary service area. Means the service area of a broadcast 
station served by the skywave and not subject to objectionable 
interference and in which the signal is subject to intermittent 
variations in strength.
    Stereophonic channel. The band of audio frequencies from 50 to 
10,000 Hz containing the stereophonic information which modulates the 
radio frequency carrier.
    Stereophonic crosstalk. An undesired signal occurring in the main 
channel from modulation of the stereophonic channel or that occurring in 
the stereophonic channel from modulation of the main channel.
    Stereophonic pilot tone. An audio tone of fixed or variable 
frequency modulating the carrier during the transmission of stereophonic 
programs.
    Stereophonic separation. The ratio of the electrical signal caused 
in the right (or left) stereophonic channel to the electrical signal 
caused in the left (or right) stereophonic channel by the transmission 
of only a right (or left) signal.
    Sunrise and sunset. For each particular location and during any 
particular month, the time of sunrise and sunset as specified in the 
instrument of authorization (See Sec. 73.1209).
    White area. The area or population which does not receive 
interference-free primary service from an authorized AM station or does 
not receive a signal strength of at least 1 mV/m from an authorized FM 
station.

[ 47 FR 8587 , Mar. 1, 1982, as amended at  47 FR 13164 , Mar. 29, 1982;  47 FR 13812 , Apr. 1, 1982;  50 FR 18821 , May 2, 1985;  50 FR 47054 , Nov. 14, 
1985;  56 FR 64856 , Dec. 12, 1991;  62 FR 51058 , Sept. 30, 1997]

    Effective Date Note:  At  62 FR 51058 , Sept. 30, 1997, Sec. 73.14 was 
amended by adding the definition Auxiliary facility, effective Dec. 1, 
1997.