47 CFR § 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)).
(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.
(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:
(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: