Analog Land Line (Ptt/Post Office Line); Dual Microwave Stls - Orban OPTIMOD-FM 8200 Operating Manual

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OPTIMOD-FM 8200
INTRODUCTION

Analog land line (PTT/post office line):

Analog land line quality is extremely variable, ranging from excellent to poor. Whether land
lines should be used or not depends upon the quality of the lines locally available, and upon
the availability of other alternatives. Even the best land lines tend to slightly veil audio
quality, due to line equalizer characteristics and phase shifts. Slight frequency response
irregularities and non-constant group delay characteristics will alter the peak-to-average
ratio, and will thus reduce the effectiveness of any peak limiting performed prior to their
inputs (see Optimal Control of Peak Modulation Levels on page 1-11).

Dual microwave STLs:

Dual microwave STLs offer greater noise immunity than composite microwave STLs.
However, problems include gain- and phase-matching of the left and right channels,
overloads induced by pre-emphasis, and requirements that the audio applied to the micro-
wave transmitters be processed to prevent over-modulation of the microwave system.
Unless carefully designed, dual microwave STLs can also introduce non-constant group
delay in the audio spectrum, thus distorting peak levels.
Some left/right microwave links can be modified to meet the specification for frequency
response and phase linearity stated in Optimal Control of Peak Modulation Levels on page
1-11. Many such links have been designed to be easily configured at the factory for
composite operation, where the entire FM stereo baseband is passed, including the pilot tone
and stereo subchannel. The requirements for maintaining stereo separation in composite
operation 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 config-
ured for composite operation (even if a composite FM stereo signal is not actually being
applied to the link).
Nevertheless, in a dual-microwave system, the 8200 is usually located at the main FM
transmitter and is driven by the microwave receivers. The microwave transmitters at the
studio are protected from overload by one of Orban's Studio Level Control Systems, such
as the Co-Operator 464A. These units also perform the gain riding function ordinarily
executed by the AGC section of the 8200's processing, and optimizes the signal-to-noise
ratio obtainable from the dual-microwave link.
If the STL microwave uses pre-emphasis, its input pre-emphasis network will probably
introduce overshoots that will increase peak modulation without any increases in average
modulation. If the Studio Level Control System is capable of producing a pre-emphasized
output, we strongly recommend that microwave STL's pre-emphasis be defeated, and
pre-emphasis performed in the Studio Level Control System. This frees the system from
political overshoot. (The Orban 464A Co-Operator can be easily configured to produce a
pre-emphasized output.)