Orban OPTIMOD-FM 8700, Operating Manual

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OPTIMOD-FM DIGITAL INTRODUCTION
1-13
ASCII strings through an RS-232 output. See Connecting via Serial Port #2 Using a
Terminal Program on a PC on page 2-48.
100 Mbps Ethernet Port for Remote Connectivity
This port will connect to any Ethernet-based network that supports the TCP/IP pro-
tocol. See step 1 on page 2-63.
Dante Audio-Over-IP Ethernet Ports
These ports support dedicated Dante (100% AES 67-compatible) audio-over-IP net-
work connection, main and backup.
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

s or 75

s standard preemphasis 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 nonlinear 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 consequently be reduced to accommodate the larger peaks.
Reduced 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 preemphasis 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 preemphasis 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 preemphasis 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.