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OMNIA | FM-STEREO TRANSMISSION | TECHNOLOGY ARTICLE
56
SINGLE SIDEBAND SUPPRESSED CARRIER (SSBSC)
Figure-13, Peak Level, 15kHz, L-Ch
Notice the reduction in utilized RF spectrum. The signal shown
in Figure–12 will pass through narrow cavities, combiners, and
mal-adjusted antennas with better stereo performance than the
broader signal shown in Figure–10.
Additionally, there are less sideband pairs of the carrier signal.
Less sideband pairs equates to less signals, which can be inter-
fered with during instances of multipath.
In the United States, FM channel spacing is maintained at 200kHz.
This is a bit of a luxury compared to the rest of the world where
channel spacing is usually 100kHz. Consider the probability of
less channel-to-channel interference when SSBSC transmission
could be used. This would appear to be an improved alternative
to the ITU BS-412 MPX power regulation, which is now in force in
some European countries.
Another mentioned beneit SSB brings to the transmission
method is added spectral protection to RBDS, SCA, and HD Radio
services as observed in Figure–14. Single channel only pink noise
is used to generate the baseband signal with SSBSC modulation.
Notice the extremely wide guard band that exists between 38kHz
and where the irst SCA carrier would appear at 57kHz. The reduc-
tion in cross-talk to ancillary services is exceptional!
Figure-14, Reduction in Cross-Talk to Ancillary Services.
SSBSC AND MODULATION PEAK CONTROL
Implementing SSB can be accomplished using numerous tech-
niques. The most common method is through use of the Hilbert
function, where a 90 degree broadband phase shift is used to
cancel the undesired sideband. It can also be achieved using
a Weaver modulator, or a low pass ilter set to critically limit
the desired passband, and the undesired sideband is removed
through iltering. All of these methods provide satisfactory
SSB operation, but there is a critical element that must be con-
sidered…peak control of the overall MPX signal. In each of the
afore-mentioned SSB methods, there will be alteration to the
phase relationship of the sideband signal. This alone will gener-
ate overshoot to MPX encoded signal [3]. It is paramount that
SSB modulation must not add any overshoot to the signal, and
it must not add any unwanted non-linear components, in the
form of audible overshoot peak limited harmonic content, i.e.
clipping by-products. The sonic performance of the SSBSC mod-
ulator must perform sonically, exactly the same as the DSBSC
counterpart. Switching from DSBSC mode to SSBSC should not
change the resulting sound in stereo separation, audio quality,
and peak control.
Theory indicates that a 90 degree phase network, in the form of
a Hilbert ilter will cause overshoot to a square wave. What’s in-
teresting is by adding a second Hilbert function, the overshoot
is removed, and the square wave is recovered. Use of the double
Hilbert function has been referred to as the “Dilbert” function [4].
An example of this is provided in Figure-15.
Figure-15, Hilbert Affect on Square Waves
Normalized Amplitude
2
1
0
-1
-2
-3
0
0.001
0.002 0.003 0.004
Time (Seconds)
0.005 0.006 0.007
400 Hz Band Limited Squarewave Hilbert Transform of Squarewave Dilbert Transform of Squarewave
Just as an audio processor is known to employ non-audible
methods to eliminate peak overshoots in the required 15kHz
low pass ilters, there are non-audible algorithms employed
in the SSBSC generator which insures that any overshoots are
eliminated, and done so without any sonic change or affect to
audio quality. Figure-16 is a screen capture from a digital oscil-
loscope that was measuring real world MPX signal at the output
of an audio processor. Figure-17 is the spectral reproduction of
the same signal. Note exceptional peak control along with the
well maintained spectrum around the 19kHz pilot, and the sharp
drop off after 38kHz.