Orban OPTIMOD-FM 8500S Operating Manual page 175

3-11
OPTIMOD-FM DIGITAL OPERATION
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 8500S'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
8500S's Half-Cosine Interpolation composite limiter, which is located after the look-
ahead limiter in the system block diagram.
The group delay of the phase-linear filters needed to create the SSB/VSB waveform
and the audio delay in the look-ahead limiter together add approximately 12 ms to
the delay of the stereo encoder. When diversity delay is applied to the 8500S's com-
posite output, the 8500S adjusts the delay automatically so that it is constant re-
gardless of mode.
SSB stereo encoder mode can be selected from the M drop-down in
T
ODULATION YPE
the I screen. Choose SSB to turn on SSB/VSB operation or
/O > C
NPUT UTPUT OMPOSITE
to turn on normal DSB operation. It can also be controlled via the 8500S's
S
TEREO
GPI inputs and by PC Remote.
The look-ahead overshoot controller is always active in SSB mode, while the Half-
Cosine Interpolation Composite Limiter is controlled by the C
L D
OMPOSITE IMIT RIVE
control as usual
Composite Limiter/Clipper: Orban has traditionally opposed composite clipping
because of its tendency to interfere with the stereo pilot tone and with subcarriers,
and because it causes inharmonic aliasing distortion, particularly between the stereo
main and subchannels. Protecting the pilot tone and subcarrier regions is particu-
larly difficult with a conventional composite clipper because appropriate filters will
not only add overshoot but also compromise stereo separation—filtering causes the
single-channel composite waveform to "lift off the baseline."
Nevertheless, we are aware that many engineers are fond of composite clipping. We
therefore undertook a research project to find a way to peak-control the composite
waveform without significantly compromising separation, pilot protection, or sub-
carrier protection and without adding the pumping typical of simple gain-control
"look-ahead" solutions.
We succeeded in our effort. The 8500S offers a patented "Half-Cosine Interpolation"
composite limiter that provides excellent spectral protection of the pilot tone and
SCAs (including RDS), while still providing approximately 60 dB of separation when a
single-channel composite waveform is clipped to 3 dB depth. To ensure accurate
peak control, the limiter operates at 512 kHz sample rate.