OPTIMOD-AM DIGITAL INSTALLATION
2-37
K) Interactively tweak these controls to maximize separation (by minimizing the
maximum amount of crosstalk into the undriven channel). It may turn out
that no improvement is possible.
L) Navigate to S
ETUP > TEST.
M) Set the M
ODE to OPERATE.
15. Test the polarity and LF transmitter equalization settings under pro-
gram conditions.
A) Apply program material to OPTIMOD-AM's input at normal operating levels.
B) Recall the GEN
MED preset:
a) Press the Recall button.
b) Turn the control knob until you see next: GEN
MED.
c) Press the RECALL NEXT soft key button to select the GEN
MED preset.
C) Turn on the carrier.
D) Navigate to S
ETUP > MODIFY SYS PST > TX1/DAY.
E) Set the P
OS PEAK 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 CALIB .
b) Press the button corresponding to the active output
c) Change the setting of the P
OLARITY control.
d) Navigate to S
ETUP > MODIFY TX PRESET > TX1/DAY.
G) Set the POS
PEAK control to 100%.
The 100% setting yields the cleanest sound. (See page 1-16 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-
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