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OMNIA | FM-STEREO TRANSMISSION | TECHNOLOGY ARTICLE
52
SSBSC
FM-STEREO TRANSMISSION USING SINGLE
SIDEBAND SUPPRESSED CARRIER (SSBSC) MODULATION
ABSTRACT
FM-Stereo transmission, employed in worldwide broadcasting,
has been in place since 1961. The system uses double sideband
suppressed carrier (DSBSC), within the multiplex baseband sig-
nal, as means to transport the stereo sound ield to the receiver.
This method, while robust and reliable, is prone to the effects
of multipath. This paper will discuss an optional method utiliz-
ing single sideband suppressed carrier (SSBSC) modulation as
an alternative for broadcasters. SSBSC is backward compatible
with existing radio receivers. Beneits which are perceptible to
a listener include: a reduction in multipath induced distortion,
additional protection to spectrum used for RBDS, SCA, and HD
Radio ® signals. There is an additional, separate beneit in the re-
ceiver which improves the signal-to-noise ratio when SSBSC is
transmitted and the receiver is designed to capture the SSBSC
signal. SSBSC for FM-Stereo has been deployed recently, under
experimental authorization from the FCC, with ongoing testing in
the lab, and in the ield.
COMPETING FOR EVERY POSSIBLE LISTENER
FM radio has a good ight on its hands. As a media transom to the
public, it battles a multitude of additional delivery services like
never before. Until recently, the competition was from television,
phonograph records, compact discs, or tape. Actually FM-Stereo
has outlived numerous media forms such as the long playing re-
cord (LP), cassette and 8-track tape, mini-disc, soon the compact
disc (CD), as well as a few others. Now, with the advent of good
quality portable audio playback devices, and wireless streaming,
there are many additional franchises available to steal the listen-
er away from radio. What can FM radio do, technically, to improve
sonic performance so a listener has less reason to abandon it as
an outlet?
HD Radio ® was introduced to the marketplace within the last
ten years, and it’s still trying to make an impact on the casual
listener. What’s needed, in the meantime, is an improvement to
the existing infrastructure, which does not require any change or
added expense to the listener. Within present day radio listening,
FM is still the preferred choice. Recent technical research and de-
velopment unveiled a unique way to improve the performance of
FM-Stereo. What follows is the result of those efforts, along with
a recommendation for FM broadcasting.
SILVER ANNIVERSARY OF FM-STEREO
In April of 2011, it marked 50 years since the Federal Communica-
tions Commission (FCC) approved FM stereophonic transmission
in the United States. The Commission, after evaluating fourteen
proposals, decided upon a system that was of similar design from
both Zenith and General Electric.
The rules governing stereophonic performance have not been al-
tered since the mid 1980’s (in the USA) when they were modiied
to allow an additional 0.5% total modulation (maximum of 110%
total), for every 1% of SCA modulation, when an SCA was being
utilized. The rules governing the requirements of the FM-Stereo
baseband signal are quite explicit, and leave little room for im-
provement of the stereo transmission system.
A quick refresher course on FM-Stereo transmission, courtesy of
subpart 73, of the FCC Rules and Regulations:
§ 73.322 FM stereophonic sound transmission standards.
a. An FM broadcast station shall not use 19kHz +/-20Hz, ex-
cept as the stereophonic pilot frequency in a transmission
system meeting the following parameters:
1. The modulating signal for the main channel consists of
the sum of the right and left signals.
2. The pilot subcarrier at 19kHz +/-2Hz, must frequency
modulate the main carrier between the limits of 8 and
10 percent.
3. One stereophonic subcarrier must be the second har-
monic of the pilot subcarrier (i.e., 38kHz) and must cross
the time axis with a positive slope simultaneously with
each crossing of the time axis by the pilot subcarrier. Ad-
ditional stereophonic subcarriers are not precluded.
4. Double sideband, suppressed-carrier, amplitude modula-
tion of the stereophonic subcarrier at 38kHz must be used.
5. The stereophonic subcarrier at 38kHz must be suppressed
to a level less than 1% modulation of the main carrier.
6. The modulating signal for the required stereophonic
subcarrier must be equal to the difference of the left and
right signals.
7. The following modulation levels apply:
i. When a signal exists in only one channel of a two
channel (biphonic) sound transmission, modulation
of the carrier by audio components within the base-
band range of 50Hz to 15kHz shall not exceed 45%
and modulation of the carrier by the sum of the am-
plitude modulated subcarrier in the baseband range
of 23kHz to 53kHz shall not exceed 45%.