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OPTIMOD-PC
INTRODUCTION
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-PC'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.
Some NICAM hardware is known to have inadequate low frequency response, com-
promising peak control if OPTIMOD-PC's processed output is passed through this
hardware. Such hardware should be tested and qualified.
Microwave STLs
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-PC'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-PC is sometimes lo-
cated at the studio and any overshoots induced by the link are tolerated or removed
by the transmitter's protection limiter (if any). OPTIMOD-PC can only be located at
the transmitter if the signal-to-noise ratio of the STL is good enough to pass un-
processed audio. The signal-to-noise ratio of the STL can be used optimally if an
Orban Studio AGC, another OPTIMOD-PC, or an Orban Transmission Limiter protects
the link from overload.
If OPTIMOD-PC 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-PC.
Some microwave links may be modified such that the deviation from linear phase is
less than +10° 20-20 kHz, and frequency response is less than 3dB down at 0.15Hz
and less than 0.1dB down at 20 kHz This specification results in less than 1% over-
shoot with processed audio. Many such links have been designed to be easily con-
figured at the factory for composite operation, where an entire FM stereo baseband
is passed. The requirements for maintaining stereo separation in composite opera-
tion are similar to the requirements for high waveform fidelity with low overshoot.
Therefore, most links have the potential for excellent waveform fidelity if they are
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