Orban OPTIMOD-FM 8700, Operating Manual

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INTRODUCTION ORBAN MODEL 8700i
Composite Baseband Microwave STLs (Analog and Digital)
The composite baseband microwave STL carries the standard pilot-tone stereo base-
band and therefore receives the output of a stereo encoder located at the studio
site. The receiver output of the composite STL is the stereo baseband signal, which is
applied directly to the wideband input of the FM broadcast transmitter’s exciter.
Thus, no stereo encoder is needed at the transmitter.
In general, a composite microwave STL provides good audio quality, as long as there
is a line-of-sight transmission path from studio to transmitter of less than 10 miles
(16 km). If not, RF signal-to-noise ratio, multipath distortion, and diffraction effects
can cause serious quality problems. Where a composite STL is used, use the 8700i’s
stereo encoder to drive the composite STL transmitter.
Uncompressed digital composite baseband microwave STLs, if properly designed,
have excellent performance and we recommend them highly. They are particularly
desirable in an 8700i installation because they allow you to use the 8700i’s compo-
site limiter to increase on-air loudness. However, the fact that they are digital does
not eliminate the requirement that they have low frequency response that is less
than 3 dB down at 0.15 Hz. Any such STL should be qualified to ensure that it meets
this specification.
Dual Microwave STLs
Dual microwave STLs use two separate transmitters and receivers to pass the left and
right channels in discrete form. Dual microwave STLs offer greater noise immunity
than composite microwave STLs. However, problems include gain- and phase-
matching of the left and right channels, overloads induced by preemphasis, and re-
quirements that the audio applied to the microwave transmitters be processed to
prevent over-modulation of the microwave system.
Lack of transparency in the path will cause overshoot. Unless carefully designed, du-
al microwave STLs can introduce non-constant group delay in the audio spectrum,
distorting peak levels when used to pass processed audio. Nevertheless, in a system
using a microwave STL, the 8700i is sometimes located at the studio and any over-
shoots induced by the link are tolerated or removed by the transmitter’s protection
limiter (if any). The 8700i can only be located at the transmitter if the signal-to-noise
ratio of the STL is good enough to pass unprocessed audio. The signal-to-noise ratio
of the STL can be used optimally if an Orban Optimod-PC 1101, Optimod 6300,
8200ST Compressor / Limiter / HF Limiter / Clipper or an 4000 Transmission Limiter
protects the link from overload. Of these, the 1101 and 6300 are currently manufac-
tured as of this writing and are the preferred choices because their AGCs are identi-
cal to the AGC in the 8700i.
If the 8700i is located at the transmitter and fed unprocessed audio from a micro-
wave STL, it may be useful to use a companding-type noise reduction system (like
dbx Type 2 or Dolby SR) around the link. This will minimize any audible noise
buildup caused by compression within the 8700i.
Some microwave links can be modified so that the deviation from linear phase is less
than +10

from 20 Hz to 15 kHz and frequency response is less than 3 dB down at
0.15Hz and less than 0.1 dB down at 20 kHz. This specification results in less than 1%