Orban OPTIMOD-AM 9300, Руководство по эксплуатации

Страница: 29 / 242

OPTIMOD-AM DIGITAL INTRODUCTION
1-9
output of OPTIMOD-AM and then to restore the preemphasis at the
transmitter. The best one can do is to use NRSC preemphasis, apply NRSC
deemphasis before the lossy link’s input, and then re-apply NRSC preem-
phasis at the link’s output.
If only an analog link is available, use a 9300’s audio output and feed the audio di-
rectly into the link. If possible, request that any transmitter protection limiters be
adjusted for minimum possible action — OPTIMOD-AM does most of that work.
Transmitter protection limiters should respond only to signals caused by faults or by
spurious peaks introduced by imperfections in the link. To ensure maximum quality,
all equipment in the signal path after the studio should be carefully aligned and
qualified to meet the appropriate standards for bandwidth, distortion, group delay
and gain stability, and such equipment should be re-qualified at reasonable inter-
vals. (See
Optimal Control of Peak Modulation Levels on page 1-7).
If the transmitter is accessible:
You can achieve the most accurate control of modulation peaks by locating
OPTIMOD-AM at the transmitter site or by connecting it to the transmitter through
an uncompressed digital STL.
Because OPTIMOD-AM controls peaks, it is irrelevant whether the audio link feeding
OPTIMOD-AM’s input terminals is phase-linear. However, the link should have low
noise, the flattest possible frequency response from 30-9,500, and low nonlinear dis-
tortion.
Studio-Transmitter Link
Transmission from Studio to Transmitter
There are several 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 in AM service are used in two fundamentally different ways. They can either:

pass unprocessed audio for application to the 9300’s input, or

pass the 9300’s peak-controlled analog or digital audio outputs for application
to the transmitter.
These applications have different performance requirements. In general, a link that
passes unprocessed audio should have very low noise and low nonlinear distortion,
but its transient response is not important. A link passing 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.