Orban OPTIMOD 6200 Operating Manual page 26

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INTRODUCTION
very transparent provided they do not truncate the digital words produced by the devices
driving their inputs and they do not require downward sample rate conversion.
Downward sample rate conversion can cause overshoot due to spectral trun-
cation and asynchronous re-sampling of the 48kHz peak-controlled samples.
If the link does not have an AES/EBU input, you must drive its analog input from the
6200's monitor output. This is not recommended because the 6200's monitor output will
overshoot in the analog domain because of the physics of the system.
Peak control in the 6200 occurs at a 48kHz sample frequency. This is suffi-
cient to prevent any samples from exceeding the threshold of limiting. How-
ever, after reconstruction, the analog output may overshoot the nominal
100% level because these overshoots "fall between the samples," so the
processing cannot be aware of them. If you use this output to feed the analog
input of a digital STL, the new samples in the STL will not be synchronous
with the samples inside the 6200. Therefore, they may well fall on the over-
shoots, causing loss of peak modulation control. It is therefore very impor-
tant to use a link with an AES/EBU input to ensure correct peak control.
The same sort of thing can happen if you use the output sample rate con-
verter, because the output samples are no longer synchronous with the peak-
controlled samples in the processing. Always use 48kHz output sample rate
to achieve best peak control.
If you must use an analog input, you may bypass any anti-aliasing filters in
digital links driven by the 6200 because the 6200's output spectrum is
tightly controlled. This ensures the most accurate possible transient response,
given the limitations of asynchronous sampling described 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 pre-emphasis.
Older technology converters (including some older NICAM encoders) may exhibit quan-
tization distortion unless they have been correctly dithered. Additionally, they can ex-
hibit rapid changes in group delay around cut-off because their analog filters are ordinar-
ily not group-delay equalized. The installing engineer should be aware of all of these
potential problems when designing a transmission system.
Any problems can be minimized by always driving a digital STL with the 6200's
AES/EBU digital output, which will provide the most accurate interface to the STL. The
digital input and output accommodate sample rates of 32kHz, 44.1kHz, and 48kHz.
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 characteristics of
such links are likely to be notably poorer than 16-bit digital even if propagation condi-
tions are ideal.
ORBAN Model 6200