Orban OPTIMOD-FM 8600Si, Operating Manual

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OPTIMOD-FM DIGITAL INTRODUCTION
1-13
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.
Do not apply an AES3id or AES3 to this input.
Location of OPTIMOD-FM
Optimal Control of Peak Modulation Levels
The audio processing circuitry in OPTIMOD-FM produces a signal that is pre-
emphasized to either the 50
μ
s or 75
μ
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
0°
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.