Goto Section: 73.127 | 73.132 | Table of Contents
FCC 73.128
Revised as of October 2, 2015
Goto Year:2014 |
2016
§ 73.128 AM stereophonic broadcasting.
(a) An Am broadcast station may, without specific authority from the FCC,
transmit stereophonic programs upon installation of type accepted
stereophonic transmitting equipment and the necessary measuring equipment to
determine that the stereophonic transmissions conform to the modulation
characteristics specified in paragraphs (b) and (c) of this section.
Stations transmitting stereophonic programs prior to March 21, 1994 may
continue to do so until March 21, 1995 as long as they continue to comply
with the rules in effect prior to March 21, 1994.
(b) The following limitations on the transmitted wave must be met to insure
compliance with the occupied bandwidth limitations, compatibility with AM
receivers using envelope detectors, and any applicable international
agreements to which the FCC is a party:
(1) The transmitted wave must meet the occupied bandwidth specifications of
§ 73.44 under all possible conditions of program modulation. Compliance with
requirement shall be demonstrated either by the following specific
modulation tests or other documented test procedures that are to be fully
described in the application for type acceptance and the transmitting
equipment instruction manual. (See § 2.983(d)(8) and (j)).
(i) Main channel (L + R) under all conditions of amplitude modulations for
the stereophonic system but not exceeding amplitude modulation on negative
peaks of 100%.
(ii) Stereophonic (L−R) modulated with audio tones of the same amplitude at
the transmitter input terminals as in paragraph (b)(i) of this section but
with the phase of either the L or R channel reversed.
(iii) Left and Right Channel only, under all conditions of modulation for
the stereophonic system in use but not exceeding amplitude modulation on
negative peaks of 100%.
(c) Effective on December 20, 1994, stereophonic transmissions shall conform
to the following additional modulation characteristics:
(1) The audio response of the main (L + R) channel shall conform to the
requirements of the ANSI/EIA-549-1988, NRSC-1 AM Preemphasis/Deemphasis and
Broadcast Transmission Bandwidth Specifications (NRSC-1).
(2) The left and right channel audio signals shall conform to frequency
response limitations dictated by ANSI/EIA-549-1988.
(3) The stereophonic difference (L−R) information shall be transmitted by
varying the phase of the carrier in accordance with the following
relationship:
eCFR graphic er20de93.000.gif
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where:
L(t) = audio signal left channel,
R(t) = audio signal right channel,
m = modulation factor, and
mpeak(L(t) + R(t)) = 1 for 100% amplitude modulation,
mpeak(L(t)−R(t)) = 1 for 100% phase modulation.
(4) The carrier phase shall advance in a positive direction when a left
channel signal causes the transmitter envelope to be modulated in a positive
direction. The carrier phase shall likewise retard (negative phase change)
when a right channel signal causes the transmitter envelope to be modulated
in a positive direction. The phase modulation shall be symmetrical for the
condition of difference (L−R) channel information sent without the presence
of envelope modulation.
(5) Maximum angular modulation, which occurs on negative peaks of the left
or right channel with no signal present on the opposite channel (L(t)=−0.75,
R(t) = 0, or R(t)=−0.75, L(t) = 0) shall not exceed 1.25 radians.
(6) A peak phase modulation of ±0.785 radians under the condition of
difference (L−R) channel modulation and the absence of envelope (L + R)
modulation and pilot signal shall represent 100% modulation of the
difference channel.
(7) The composite signal shall contain a pilot tone for indication of the
presence of stereophonic information. The pilot tone shall consist of a 25
Hz tone, with 3% or less total harmonic distortion and a frequency tolerance
of ± 0.1 H2, which modulates the carrier phase ± 0.05 radians peak,
corresponding to 5% L−R modulation when no other modulation is present. The
injection level shall be 5%, with a tolerance of + 1, −1%.
(8) The composite signal shall be described by the following expression:
eCFR graphic er20de93.001.gif
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where:
A = the unmodulated carrier voltage
m = the modulation index
Csn = the magnitude of the nth term of the sum signal
Cdn = the magnitude of the nth term of the difference signal
ωsn = the nth order angular velocity of the sum signal
ωdn = the nth order angular velocity of the difference signal
ωc = the angular velocity of the carrier
eCFR graphic er20de93.002.gif
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Asn and Bsn are the nth sine and cosine coefficients of Csn
Adn and Bdn are the nth sine and cosine coefficients of Cdn
[ 58 FR 66301 , Dec. 20, 1993]
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Goto Section: 73.127 | 73.132
Goto Year: 2014 |
2016
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