Optimod-AM 9400 V1.2 Operating Manual - Orban

2-18
INSTALLATION ORBAN MODEL 9400
5. Set output bandwidth.
This step sets the lowpass filter bandwidth in the default transmission preset
(TX1/DAY).
A) Press the NEXT button.
B) Select the lowpass filter cutoff frequency you need by turning the knob.
The setting of the lowpass filter controls your RF occupied bandwidth. It
is very important to set it to meet the government standards in your
country.
OPTIMOD-AM can be readily programmed from its front panel or by remote
control for any lowpass filter cutoff frequency from 4.5 kHz to 9.5
kHz (NRSC) in 0.5 kHz steps. Default is NRSC.
Quick Setup programs the filter so that it is down 0.1 dB at the assigned
cutoff frequency. However, you can later edit any transmission preset to
shape the transition region of the input section of the filter to trade off
ringing against bright sound. This may be particularly useful when using
a low cutoff frequency like 5.0 kHz. (See step 7.C) on page 2-27.)
The LPF SHAPE control in the transmission preset does this by allowing
you to set the cutoff frequency so that it is –0.1 dB, –3 dB, or –6 dB. By
making the transition between the passband and stopband progressively
more rounded and gentle, each step trades off duller sound against less
ringing.
6. Set highpass filter cutoff frequency.
This step sets the highpass filter bandwidth in the default transmission preset
(TX1/DAY).
A) Press the NEXT button.
B) Select the highpass filter cutoff frequency you need by turning the knob.
OPTIMOD-AM can be programmed for any highpass filter cutoff frequency
from 50 to 100Hz in 10Hz steps. Default is 50 Hz.
The appropriate setting of the highpass filter is not determined by government
regulations, but instead depends on both your programming
format and transmitter. Modern transmitters can accommodate frequencies
below 50Hz without loss of performance due to bounce or tilt. However,
virtually no AM radio can satisfactorily reproduce frequencies below
50Hz. Many produce distortion when attempting to reproduce such
frequencies because their RF AGC circuits are poorly designed and they
mistake low-frequency modulation for changes in carrier level. Simultaneously,
50Hz is a low enough cutoff frequency to retain significant bass
punch and slam with music. We therefore recommend that you set the
highpass filter to 50Hz unless you have a good reason for setting it differently.
One reason to set it higher would be if you have an oldertechnology
transmitter that cannot reproduce low frequencies without
bounce or tilt. (You should experiment by setting the highpass filter to
various frequencies higher than 50Hz to determine if you can get more
average modulation from your transmitter by doing so.) Another reason