Orban OPTIMOD 5750 Operating Manual page 22

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Orban 5750 Technical Manual Installation
limited to 15 kHz, where downward sample rate conversion will remove spectral energy and will therefore introduce
overshoot.
If the link does not have an AES3 input, you must drive its analog input from the 5750's analog output. This is less
desirable because the link's analog input circuitry may not meet all requirements for passing processed audio
without overshoot.
If you use a digital link to pass the digital composite output, the link must be uncompressed. We recommend not
using sample rate conversion in such a link, as sample rate converters may introduce filters that compromise stereo
separation.
Composite Baseband Microwave STLs (Analog and Digital): The composite baseband microwave STL carries the
standard pilot-tone stereo baseband 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 5750'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 5750 installation because they allow you to use
the 5750'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 requirements 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 5750 is sometimes located at the studio and any overshoots
induced by the link are tolerated or removed by the transmitter's protection limiter (if any). The 5750 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 or OPTIMOD 6300 protect the link
from overload. These are the preferred choices because their AGCs are identical to the AGC in the 5750.
Some microwave links can be modified so that the deviation from linear phase is less than +10 from 20Hz to 15kHz
and frequency response is less than 3dB down at 0.15Hz and less than 0.1dB down at 20kHz. 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 baseband is passed. The requirements for maintaining
stereo separation in composite operation are similar to the requirements for high waveform fidelity with low