Best Location For Optimod-Fm; If The Transmitter Is Not Accessible - Orban OPTIMOD-FM 8500S Operating Manual

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INTRODUCTION

Best Location for OPTIMOD-FM

If the transmitter is not accessible:

1) The system group delay must be essentially constant throughout the frequency
range containing significant energy (30-15,000Hz). If low-pass filters are present,
this may require the use of delay equalization. The deviation from linear-phase
must not exceed ±10° from 30-15,000Hz.
2) The low-frequency −3 dB point of the system must be placed at 0.15Hz or lower
(this is not a misprint!). This is necessary to ensure less than 1% overshoot in a
50Hz square wave and essentially constant group delay to 30Hz.
3) Any pre-emphasis used in the audio transmission system prior to the stereo en-
coder must be canceled by a precisely complementary de-emphasis: Every pole
and zero in the pre-emphasis filter must be complemented by a zero and pole of
identical complex frequency in the de-emphasis network. An all-pole de-
emphasis network (like the classic series resistor feeding a grounded capacitor) is
not appropriate. However, this network could be corrected by adding a second
resistor between ground and the capacitor, which would introduce a zero.
Low-pass filters (including anti-aliasing filters in digital links), high-pass filters, trans-
formers, distribution amplifiers, and long transmission lines can all cause the above
criteria to be violated, and must be tested and qualified. It is clear that the above
criteria for optimal control of peak modulation levels are most easily met when the
audio processor directly feeds the stereo encoder. In the 8500S, no circuit elements
that might distort the shape of the waveform are interposed between the audio
processor and the stereo encoder. We therefore recommend using the 8500S with its
built-in stereo encoder whenever practical.
The best location for OPTIMOD-FM is as close as possible to the transmitter, so that
its stereo encoder output can be connected to the transmitter through a circuit path
that introduces the least possible change in the shape of OPTIMOD-FM's carefully
peak-limited waveform — a short length of coaxial cable. If this is impossible, the
next best arrangement is to feed the 8500S's AES3 digital output through an all-
digital, uncompressed path to the transmitter's exciter.
Use the 8500S's left and right analog audio outputs in situations where the stereo
encoder and exciter are under the jurisdiction of an independent transmission au-
thority, and where the programming agency's jurisdiction ends at the interface be-
tween the audio facility and the link connecting the audio facility to the transmitter.
(The link might be telephone/post lines, analog microwave radio, or various types of
digital paths.) This situation is not ideal because artifacts that cannot be controlled
by the audio processor can be introduced by the link to the transmitter, by transmit-
ter peak limiters, or by the external stereo encoder.
All audio processing must be done at the studio and you must tolerate any damage
that occurs later. If you can obtain a broadband (0-75 kHz) phase-linear link to the
transmitter, and the transmitter authority will accept the delivery of a baseband en-
coded signal, use the 8500S's internal stereo encoder at the studio location to feed
ORBAN MODEL 8500S