Orban OPTIMOD-FM 8500S Operating Manual page 174

3-10
OPERATION
Note that the speech detector will detect most speech mixed with music
as "music" unless the music is at a very low level compared to the speech.
Speech must also be centered in the stereo sound field to be detected as
"speech."
By using the S
detector. See To Override the Speech/Music Detector on page 3-63.
DSP-derived Stereo Encoder: The 8500S's stereo encoder is derived from algo-
rithms first developed for the high-performance Orban 8218 stand-alone encoder.
The 8500S's stereo encoder operates at 512 kHz-sample rate to ease the perform-
ance requirements of the D/A converter's reconstruction filter, making it possible to
achieve excellent stereo separation that is stable over time and temperature. DSP-
based group delay and magnitude equalizers for the entire composite analog path
further improve separation.
The 8500S has two independent composite outputs, whose levels are both software-
settable. For convenience, two SCA inputs sum into the 8500S's analog composite
output amplifier. The second SCA input can be configured to provide a 19 kHz-
reference output for subcarrier generators that need it.
The 8500S does not digitize SCAs.
SSB Stereo Encoder Operation: Starting with version 1.1, the 8500S 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 8500S'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 8500S's composite output signal extends to
38,150 Hz when the 8500S'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.
control, you can override the speech/music
D
PEECH ETECT
ORBAN MODEL 8500S