Orban OPTIMOD-FM 8600Si, Operating Manual

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OPTIMOD-FM DIGITAL OPERATION
3-11
The 8600Si has two independent composite outputs, whose levels are both soft-
ware-settable. For convenience, two SCA inputs sum into the 8600Si’s analog com-
posite output amplifier. The second SCA input can be configured to provide a 19
kHz-reference output for subcarrier generators that need it.
The 8600Si does not digitize SCAs.
SSB Stereo Encoder Operation: Starting with version 1.1, the 8600Si allows its ste-
reo encoder’s stereo subchannel modulator to operate in an experimental compati-
ble single sideband/vestigial sideband mode. SSB/VSB operation suppresses the up-
per sideband of the stereo subcarrier above 38,150 Hz, which reduces the occupied
bandwidth of the FM-modulated RF signal. In SSB mode, the subchannel modulator
acts as a pure SSB generator for L–R material in the frequency range of 150 Hz to 17
kHz and as a vestigial sideband generator below 150 Hz.
In normal operation, the stereo subchannel modulator produces a double sideband
suppressed carrier signal with pairs of mirror image sidebands around 38 kHz. With
respect to an L+R gain of 1, the gain of each sideband is 0.5. In SSB/VSB mode, the
upper sideband is suppressed by at least 80 dB above a modulating frequency of 150
Hz and the gain of the lower sideband is 1.0. Below 150 Hz, the sum of the gains of
the sideband pairs is 1.0. (The conventional DSB case is a limiting case of this, where
the gains of the upper and lower sidebands are both 0.5 and sum to 1.) This “sum-
mation to 1” criterion is necessary to achieve compatibility with normal FM radios
that use synchronous demodulation of the stereo subchannel. Almost every radio
manufactured since 1973 works like this. We have verified that the 8600Si’s SSB gen-
erator produces more than 60 dB of separation from 50 to 15,000 Hz when meas-
ured on a Belar FMSA-1 “Wizard” modulation monitor, which was originally de-
signed for convention double sideband operation.
In SSB/VSB mode, the bandwidth of the 8600Si’s composite output signal extends to
38,150 Hz when the 8600Si’s composite limiter is not used. When the composite lim-
iter is used, the limiting action will produce energy up to 55 kHz (as it does with
normal DSB operation) but this energy will be much lower in level than the energy
that would have been produced by normal DSB operation in the frequency range
occupied by the upper sideband.
SSB operation causes irreducible, “laws of physics” composite peak modulation over-
shoots to occur with certain combinations of left and right channel signals that are
independently peak limited to 100% modulation, which is the correct limiting tech-
nique for conventional double-sideband transmission. The worst-case irreducible SSB
overshoot occurs when the left and right channels contain correlated signals whose
phase difference is 90
°
.
Suboptimal system design can cause additional overshoots. To prevent this type of
overshoot, the 8600Si’s SSB/VSB generator uses constant-delay filters and its fre-
quency response extends to DC (because of the VSB operation below 150 Hz).
To control irreducible overshoots, the SSB generator includes a look-ahead over-
shoot limiter. To eliminate all overshoots, this limiter must be used together with the
8600Si’s Half-Cosine Interpolation composite limiter, which is located after the look-
ahead limiter in the system block diagram.