§  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 § 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 § 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 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 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 § 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 § 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 antennas, without
   consideration of adjustments specified in paragraphs (b)(1) and (b)(2)
   of § 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 × 100

         ————––—––—

             c

   In a negative direction:

   M = C−MIN × 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 § 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;  66 FR 20755 , Apr. 25, 2001]