§  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

   wsn = the nth order angular velocity of the sum signal

   wdn = the nth order angular velocity of the difference signal

   wc = 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]