Orban OPTIMOD-FM 8600, Руководство по эксплуатации

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3-12
OPERATION ORBAN MODEL 8600
Suboptimal system design can cause additional overshoots. To prevent this type of
overshoot, the 8600’s SSB/VSB generator uses constant-delay filters and its frequency
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
8600’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 8600’s com-
posite output, the 8600 adjusts the delay automatically so that it is constant regard-
less of mode.
SSB stereo encoder mode can be selected from the
M
ODULATION
T
YPE
drop-down in
the
I
NPUT
/O
UTPUT
> C
OMPOSITE
screen. Choose
SSB
to turn on SSB/VSB operation or
S
TEREO
to turn on normal DSB operation. It can also be controlled via the 8600’s 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
OMPOSITE
L
IMIT
D
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 particular-
ly difficult with a conventional composite clipper because appropriate filters will not
only add overshoot but also compromise stereo separation—filtering causes the sin-
gle-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 8600 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.
For those who prefer the sound of conventional composite clipping, we also offer a
defeatable composite clipper. This also provides excellent spectral protection for the
pilot tone and subcarriers. The composite clipper drives the “Half-Cosine Interpola-
tion” composite limiter, which serves as an overshoot compensator for the compo-
site clipper when it is active. (Overshoot compensation necessary to remove over-
shoots introduced by the pilot- and SCA-protection filters following the composite
clipper.)