Orban OPTIMOD-AM 9300, Руководство по эксплуатации

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2-32
INSTALLATION ORBAN MODEL 9300
a) Navigate to
S
ETUP
> T
EST .
b) Set the
M
ODE to
O
PERATE .
14. Test the polarity and LF transmitter equalization settings under program
conditions.
A) Apply program material to OPTIMOD-AM's input at normal operating levels.
B) Recall the
GEN PURPOSE MEDIUM
preset:
a) Press the
R
ECALL button.
b) Turn the control knob until you see next:
GEN PURPOSE MEDIUM
.
c) Press the
RECALL NEXT
soft key button to select the
GEN PURPOSE
MEDIUM
preset.
C) Turn on the carrier.
D) Navigate to
S
ETUP
> M
ODIFY
TX P
RESET
> TX1/D
AY
.
E) Set the
P
OS
P
EAK control to 125%:
F) Check modulation asymmetry with the oscilloscope or with your modulation
monitor.
If negative peaks are modulating higher than positive peaks:
a)
Navigate to
S
ETUP
> I/O C
ALIB
> O
UTPUT
.
b) Press the button corresponding to the active output
c) Change the setting of the
P
OLARITY control.
d) Navigate to
S
ETUP
> M
ODIFY
TX P
RESET
> TX1/D
AY
.
G) Set the
POS PEAK
control to 100%.
The 100% setting yields the cleanest sound. (See page 1-15 for an expla-
nation.)
However, if absolute maximum loudness is desired at the expense of
cleanest possible sound, the
POS PEAK
control may be set as high as
your government regulations and transmitter performance will allow. In
the U.S., FCC Rules limit this to 125%.
Note too that the distortion of older transmitters and most receivers
tends to increase radically when negative modulation of more than 85%
is attempted. In the case of receivers, the major cause of this distortion is
cheaply designed envelope detectors with incorrectly biased diodes. Con-
sider reducing clipping in the processing so that the last 15% or so of
modulation consists of low duty-cycle spikes that can be soft-clipped by
the receiver’s detector. This trades off about 1.5dB loudness loss for sub-
stantially cleaner sound.
If you choose to modulate asymmetrically with a transmitter that com-
presses peaks in the positive direction,
do not attempt to modulate be-
yond the performance limitations of your transmitter. Doing so would
only cause distortion beyond the distortion intrinsic to asymmetrical op-
eration.