Orban OPTIMOD-FM 5518, Руководство по эксплуатации

Страница: 29 / 217

5518-FM DIGITAL INTRODUCTION
1-9
configure the 5518 to lock its 19 kHz pilot tone and output sample frequency to this
input.
The sample frequency at the 5518’s digital output does not have to be the same as
the reference frequency. If the output frequency is different, the output sample fre-
quency will be the product of a quotient of integers times the reference 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 ref-
erence 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.
Location of the 5518
Optimal Control of Peak Modulation Levels
An OPTIMOD-FM audio processor 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 fil-
tered at 15 kHz to protect the 19 kHz pilot and prevent distortion caused by alias-
ing-related non-linear crosstalk. If this signal is fed directly into a stereo encoder
without overshoot limiters, peak modulation levels on the air will nevertheless be
precisely controlled. However, if the audio processor’s signal is fed to such a 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 aver-
age modulation will not. The 5518’s overshoot limiters must then be enabled, avoid-
ing the need to reduce the modulation level to accommodate the larger peaks. Re-
duced average modulation level will cause reduced loudness and a poorer signal-to-
noise ratio at the receiver.
Because the 5518’s overshoot limiters (like any such limiters) can introduce undesir-
able audible artifacts, minimizing the overshoot in the signal path between your
audio processor and the 5518 will make your transmission as clean-sounding as pos-
sible.
Landline equalizers, transformers, and 15 kHz lowpass 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 lowpass 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.