Location Of Optimod-Fm; Optimal Control Of Peak Modulation Levels - Orban OPTIMOD-FM 8200 Operating Manual

OPTIMOD-FM 8200

Location of OPTIMOD-FM

Optimal Control of Peak Modulation Levels

The audio processing circuitry in OPTIMOD-FM produces a signal that is pre-emphasized
µ
to either the 50 s or 75
trolled and peak-controlled to prevent over-modulation, and is filtered at 15kHz to protect
the 19kHz pilot and prevent distortion 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. But 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 modu-
lation level must therefore be reduced to accommodate the larger peaks. Reduced average
modulation level will result in reduced loudness, and a poorer signal-to-noise ratio at the
receiver.
Frequency response errors and non-constant group delay are typically introduced by land
line equalizers, transformers, and 15kHz low-pass filters and pre-emphasis networks in
stereo encoders.
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
± 10° from 30-15,000Hz.
2) The low-frequency − 3dB 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 encoder 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 is not appropri-
ate).
Low-pass filters (including anti-aliasing filters in digital links), high-pass filters, transform-
ers, 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 8200, 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 8200 with its built-in
stereo encoder whenever practical.
µ
s standard curve, is precisely and absolutely high frequency-con-
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INTRODUCTION