Microwave Stls - Orban OPTIMOD 6300 Operating Manual

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INTRODUCTION

Microwave STLs

The same sort of thing can happen if you use the output sample rate
converter because the output samples are no longer synchronous with
the peak-controlled samples in the processing. Always use 48 kHz output
sample rate with internal sync to achieve best peak control.
If you must use an analog input, you may bypass any anti-aliasing filters
in digital links driven by OPTIMOD 6300 because OPTIMOD 6300's output
spectrum is tightly controlled. This ensures the most accurate possible
transient response, given the limitations of asynchronous sampling de-
scribed above.
NICAM is a sort of hybrid between PCM and lossy data reduction systems. It uses a
block-companded floating-point representation of the signal with J.17 preemphasis.
Older technology converters (including some older NICAM encoders) may exhibit
quantization distortion unless they have been correctly dithered. Additionally, they
can exhibit rapid changes in group delay around cut-off because their analog filters
are ordinarily not group-delay equalized. The installing engineer should be aware of
all of these potential problems when designing a transmission system.
You can minimize any problems by always driving a digital STL with OPTIMOD 6300's
AES3 digital output, which will provide the most accurate interface to the STL. The
digital input and output accommodate sample rates of 32 kHz, 44.1 kHz, 48, 88.2,
and 96 kHz.
In general, an analog microwave STL provides high 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. However, the noise and non-linear distortion charac-
teristics of such links are likely to be notably poorer than 16-bit digital even if
propagation conditions are ideal.
As discussed above, asynchronous resampling will cause overshoots if any analog
path (even a perfectly transparent one) passes OPTIMOD 6300's processed output to
the transmitter. Lack of transparency in the analog path will cause even more over-
shoot. Unless carefully designed, microwave STLs can introduce non-constant group
delay in the audio spectrum, distorting peak levels when used to pass processed au-
dio. Nevertheless, in a system using a microwave STL OPTIMOD 6300 is sometimes
located at the studio and any overshoots induced by the link are tolerated or re-
moved by the transmitter's protection limiter (if any). OPTIMOD 6300 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, 8200ST Compressor / Limiter / HF Limiter / Clipper, an
4000 Transmission Limiter, or another 6300 protects the link from overload. Of these,
the 1101 and 6300 are currently manufactured as of this writing and are the pre-
ferred choices because their AGCs are identical to the AGC in the 6300.
If OPTIMOD 6300 is located at the transmitter and receives unprocessed audio from
a microwave 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 OPTIMOD 6300.
ORBAN MODEL 6300