Studio-Transmitter Link; Transmission From Studio To Transmitter; Digital Links - Orban OPTIMOD-FM 5500 Operation Manual

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OPTIMOD-FM DIGITAL INTRODUCTION
The shorter the baseband cable from OPTIMOD-FM to exciter, the less likely that
ground loops or other noise problems will occur in the installation. If you require a
long cable run, you can use Orban's CIT25 Composite Isolation Transformer to break
any ground loops. This transformer will ordinarily cure even the most stubborn hum
or noise caused by the composite connection between OPTIMOD-FM and the exciter.
Its instruction manual contains complete information on its installation and applica-
tion.
If a separate stereo encoder must be used, feed the encoder directly from the 5500's
left and right analog or (preferably) digital outputs. If possible, bypass the pre-
emphasis network and the input low-pass filters in the encoder so that they cannot
introduce spurious peaks. Because of their special design, OPTIMOD-FM's pre-
emphasis network and low-pass filters perform the same functions while retaining
tight peak control.

Studio-Transmitter Link

Transmission from Studio to Transmitter

There are five types of studio-transmitter links (STLs) in common use in broadcast
®
service: uncompressed digital, digital with lossy compression (like MPEG, Dolby , or
®
APT-x ), microwave, analog landline (telephone/post line), and audio subcarrier on a
video microwave STL.
STLs are used in three fundamentally different ways. They can either (1) pass un-
processed audio for application to the 5500's input, (2) they can pass the 5500's
peak-controlled analog or digital left and right audio outputs, or (3) they can pass
the 5500's peak-controlled composite stereo baseband output. The three applica-
tions have different performance requirements. In general, a link that passes un-
processed audio should have very low noise and low non-linear distortion, but its
transient response is not important. A link that passes processed audio does not
need as low a noise floor as a link passing unprocessed audio. However, its transient
response is critical. At the current state of the art, an uncompressed digital link using
digital inputs and outputs to pass audio in left/right format achieves best results. We
will elaborate below.

Digital Links

Digital links may pass audio as straightforward PCM encoding, or they may apply
lossy data reduction processing to the signal to reduce the number of bits per sec-
ond required for transmission through the digital link. Such processing will almost
invariably distort peak levels, and such links must therefore be carefully qualified
before you use them to carry the peak-controlled output of the 5500 to the trans-
mitter. For example, the MPEG Layer 2 algorithm can increase peak levels up to 4 dB
at 160kB/sec by adding large amounts of quantization noise to the signal. While the
desired program material may psychoacoustically mask this noise, it is nevertheless
large enough to affect peak levels severely. For any lossy compression system the
higher the data rate, the less the peak levels will be corrupted by added noise, so
use the highest data rate practical in your system.