Digital Links; Video Microwave Stls With Pcm Adapters - Orban OPTIMOD-FM 8200 Operating Manual

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OPTIMOD-FM 8200
INTRODUCTION
Although a microwave STL exhibits satisfactory stereo separation, it is nevertheless not
unusual for it to bounce because of a large infrasonic peak in its frequency response caused
by an under-damped automatic frequency control (AFC) phase-locked loop. This bounce
can increase the peak carrier deviation by as much as 1dB, reducing average modulation.
Many commercial STLs (and FM exciters as well) have this problem.
Some consultants presently offer modifications to minimize or eliminate this problem. If
your exciter or STL has this problem, you may contact Orban Customer Service for the latest
information on such services.

Digital links:

There are several types of digital links presently available. One type encodes the entire
composite stereo baseband on the link, functionally replacing a composite microwave STL.
Such a link should be driven by the 8200's stereo encoder.
Other digital links pass the audio in left and right form, and may apply data-rate-reduction
processing to the signal to reduce the number of bits per second required for transmission
through the digital link. Such processing may distort peak levels, and such links must
therefore be carefully qualified before you use them to carry the peak-controlled output of
the 8200 to the stereo encoder.
Older-technology links may use straightforward PCM (pulse-code modulation) without data
rate reduction. These can be very transparent and can exhibit accurate pulse response
provided that their input anti-aliasing filters and output reconstruction filters are rigorously
designed to achieve constant group delay over the frequency range that contains significant
program energy. This is not particularly difficult to do with modern over-sampled converter
technology.
Older-technology converters usually exhibit rapid changes in group delay around cut-off
because their analog filters are ordinarily not group-delay equalized. Additionally, they may
exhibit quantization distortion unless they have been correctly dithered. The installing
engineer should be aware of all of these potential problems when designing a transmission
system.

Video microwave STLs with PCM adapters:

The video STLs in use typically operate above 20GHz, with consumer PCM adapters (from
Sony or dbx, for example) to encode left and right audio into a video-like signal. The quality
of signal received at the transmitter through this type of STL is high. However, the high
carrier frequencies make these links subject to rain fading. Other potential problems include
very sharp high-frequency cut-off, rapid changes in group delay around cut-off, and
quantization distortion.
The Sony and dbx encoders are no longer manufactured, but may be found on the used
market.