Sec.  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
    Sec. 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  Sec. 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:
   [MATH:  :MATH]

   where:

   L(t)=audio signal left channel,

   R(t)=audio signal right channel,

   m=modulation factor, and

   m[peak](L(t)+R(t))=1 for 100% amplitude modulation,

   m[peak](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 H[2], 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:
   [er20de93.001.gif]

   where:

   A=the unmodulated carrier voltage

   m=the modulation index

   C[sn]=the magnitude of the nth term of the sum signal

   C[dn]=the magnitude of the nth term of the difference signal

   ω[sn]=the nth order angular velocity of the sum signal

   ω[dn]=the nth order angular velocity of the difference signal

   ω[c]=the angular velocity of the carrier
   [MATH:  :MATH]

   A[sn] and B[sn] are the n^th sine and cosine coefficients of C[sn]

   A[dn] and B[dn] are the n^th sine and cosine coefficients of C[dn]

   [ 58 FR 66301 , Dec. 20, 1993]