OPTIMOD-FM INTRODUCTION
1-11
Best Location for OPTIMOD-FM
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 in-
troduces 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 8400’s AES/EBU digital output through an all-digital, uncom-
pressed path to the transmitter's exciter.
Use the 8400’s left and right analog audio outputs in situations where the stereo encoder
and exciter are under the jurisdiction of an independent transmission authority, and where
the programming agency’s jurisdiction ends at the interface between the audio facility
and the link connecting the audio facility to the transmitter. (The link might be tele-
phone/post lines, analog microwave radio, or various types of digital paths.) This situa-
tion is not ideal because artifacts that cannot be controlled by the audio processor can be
introduced by the link to the transmitter, by transmitter peak limiters, or by the external
stereo encoder.
If the transmitter is not accessible:
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 transmit-
ter, and the transmitter authority will accept the delivery of a baseband encoded signal,
use the 8400’s internal stereo encoder at the studio location to feed the STL. Then feed
the output of the STL receiver directly into the transmitter’s exciter with no intervening
processing.
If an uncompressed AES/EBU digital link is available to the transmitter, this is also an
excellent means of transmission, although it will not pass the effects of the 8400’s com-
posite processor (if you are using it). However, if the digital link employs lossy compres-
sion, it will disturb peak levels.
If only an audio link is available, use the 8400’s left and right audio outputs and feed the
audio, without pre-emphasis, directly into the link. If possible, request that any transmit-
ter protection limiters be adjusted for minimum possible action — OPTIMOD-FM does
most of that work. Transmitter protection limiters should respond only to signals caused
by faults or by spurious peaks introduced by imperfections in the link. To ensure maxi-
mum quality, all equipment in the signal path after the studio should be carefully aligned
and qualified to meet the appropriate standards for bandwidth, distortion, group delay and
gain stability, and such equipment should be re-qualified at reasonable intervals. (See
Optimal Control of Peak Modulation Levels on page 1-10).
If the transmitter is accessible:
You can achieve the most accurate control of modulation peaks by locating OPTIMOD-
FM at the transmitter site and using its stereo encoder to drive the transmitter. You can
usually also obtain good results by locating OPTIMOD-FM at the studio and connecting
the baseband output of its stereo encoder to the transmitter through a composite baseband
STL (see page 1-14). However, many composite baseband STLs do not control peaks
perfectly because of bounce (see page 1-15), and locating OPTIMOD-FM at the transmit-
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