Dual Microwave Stls - Orban OPTIMOD-FM 5500 Operation Manual

1-17
OPTIMOD-FM DIGITAL INTRODUCTION
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 5500'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 a 5500 installation because they allow you to use the 5500's composite
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 pre-emphasis, 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,
dual 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 5500 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 5500 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 pro-
tects 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 5500.
If the 5500 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 5500.
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%
overshoot with processed audio. Many such links have been designed to be easily
configured at the factory for composite operation, where an entire FM stereo base-
band is passed. The requirements for maintaining stereo separation in composite
operation are similar to the requirements for high waveform fidelity with low over-
shoot. Therefore, most links have the potential for excellent waveform fidelity if