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:
[GRAPHIC] [TIFF OMITTED] TR20DE93.000

    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.


[[Page 33]]


    (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:
[GRAPHIC] [TIFF OMITTED] TR20DE93.001

    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
sn=the nth order angular velocity of the sum signal
dn=the nth order angular velocity of the difference 
    signal
c=the angular velocity of the carrier

[[Page 34]]

    [GRAPHIC] [TIFF OMITTED] TR20DE93.002
    
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]