Orban OPTIMOD-AM 9300, Operating Manual

Page: 153 / 242

OPTIMOD-AM DIGITAL OPERATION
3-21
The high-frequency receiver equalizer is designed to compensate for the high fre-
quency rolloff in average AM radios. The typical AM radio is down 3dB at 2kHz and
rolls off at least 18dB/octave after that. The HF equalizer provides an 18dB/octave
shelving preemphasis that can substantially improve the brightness and intelligibility
of sound through narrowband radios that do not have an abrupt rolloff. The HF
equalizer has two controls: a gain control that determines the height of the shelving
curve (dB), and a curve control, calibrated with an arbitrary number that determines
how abruptly the shelving equalizer increases its gain as frequency increases.
0
pro-
vides the most abrupt curve;
10
provides the gentlest. The
HF C
URVE
control is used
to trade off harshness on wider band radios against brightness in narrowband ra-
dios.
An
HF C
URVE
of 0 provides the same equalization that was originally supplied as
standard on early OPTIMOD-AM 9100 units and was later provided by the 9100’s
green module. Compared to higher settings of the HF Curve control, it provides
much more boost in the 5 kHz region, and tends to sound strident on wideband ra-
dios. However, it can be very effective where narrowband radios remain the norm.
With an
HF C
URVE
setting of 0, an
HF G
AIN
control setting of 22 dB will result in a
perceived bandwidth of 6 kHz on “Group 2” AM radios (see page 3-7); a 15 dB set-
ting yields a 5 kHz perceived bandwidth, 10 dB yields 4 kHz, and 5 dB yields 3 kHz.
Advancing the
HF G
AIN
control will result in a brighter, higher fidelity sound, but it
Figure 3-1: HF Receiver Equalizer Curves