Location Of Optimod-Fm; Optimal Control Of Peak Modulation Levels - Orban OPTIMOD-FM 5500 Operation Manual

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INTRODUCTION

Location of OPTIMOD-FM

Optimal Control of Peak Modulation Levels

kHz, or a 10 MHz sinewave or squarewave signal, 0.5 to 5 V peak. 10 MHz is a com-
mon output frequency produced by GPS and rubidium frequency standards. You can
configure the 5500 to lock its 19 kHz pilot tone and output sample frequency to this
input.
The sample frequency at the 5500's digital output does not have to be the same as
the reference frequency to be locked to it. If the output frequency is different, the
output sample frequency will be the product of a quotient of integers times the ref-
erence frequency. For example, if the reference frequency is 96 kHz and the output
frequency is set to 32 kHz, the actual output frequency will be 1/3 x the reference
frequency. If the reference frequency is 48 kHz and the output frequency is set to
44.1 kHz, the actual output frequency will be 147/160 x the reference frequency.
The audio processing circuitry in OPTIMOD-FM produces a signal that is pre-
emphasized to either the 50s or 75s standard pre-emphasis curve. It is precisely
and absolutely high-frequency-controlled and peak-controlled to prevent over-
modulation, and is filtered at 15 kHz to protect the 19 kHz pilot and prevent distor-
tion caused by aliasing-related non-linear crosstalk. If this signal is fed directly into a
stereo encoder, peak modulation levels on the air will be precisely controlled. How-
ever, if the audio processor's signal is fed to the stereo encoder through any circuitry
with frequency response errors and/or non-constant group delay, the peaks will be
magnified. Peak modulation will increase, but average modulation will not. The
modulation level must therefore be reduced to accommodate the larger peaks. Re-
duced average modulation level will cause reduced loudness and a poorer signal-to-
noise ratio at the receiver.
Landline equalizers, transformers, and 15 kHz low-pass filters and pre-emphasis net-
works in stereo encoders typically introduce frequency response errors and non-
constant group delay. There are three criteria for preservation of peak levels
through the audio system:
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 1 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
ORBAN MODEL 5500