Orban OPTIMOD-FM 8700, Руководство по эксплуатации

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OPTIMOD-FM DIGITAL OPERATION
3-13
er is used, the limiting action will produce energy up to 55 kHz (as it does with nor-
mal 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 occu-
pied 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

. The 8700i’s Multipath Mitigator, which removes interchannel
phase shifts and converts input audio to “intensity stereo,” is important to optimum
SSB/VSB operation because its action minimizes the amount of modulation over-
shoot.
Suboptimal system design can cause additional overshoots. To prevent this type of
overshoot, the 8700i’s SSB/VSB generator uses constant-delay filters and its frequen-
cy 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 8700i’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 8700i’s com-
posite output, the 8700i 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 8700i’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 be-
cause 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-