Orban OPTIMOD-AM 9300, Operating Manual

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OPERATION ORBAN MODEL 9300
LF FREQ determines the center frequency of the equalization, in Hertz.
Range is 20-500Hz.
LF GAIN determines the amount of peak boost or cut (in dB) over a

10
dB range.
LF WIDTH determines the bandwidth of the equalization, in octaves. The
range is 0.8-4.0 octaves. If you are unfamiliar with using a parametric
equalizer, 1.5 octaves is a good starting point. These curves are relatively
broad because they are designed to provide overall tonal coloration, ra-
ther than to notch out small areas of the spectrum.
Although a certain amount of low-frequency boost must be used along with the
high frequency boost in order to obtain a balanced sound on analog AM radios for
MW,
do so conservatively! Use the bassiest auto radios (all of which usually have a
peaky mid-bass when you listen through the standard dashboard speaker) as a
“worst case” reference. Do not boost the bass so much that your reference radio be-
comes muddy or boomy. With correct bass boost, your table radio will have only
moderate bass and your pocket radio will sound thin and tinny.
For example, a 6dB boost corresponds to a 400% increase in power! More than 6dB
of bass boost will strain many transmitters, unnecessarily increasing power supply
bounce and IM distortion problems. (The bass boost is further limited dynamically in
the multiband clipper

see immediately below.) Excessive bass boost will also cause
many dashboard speakers to sound unacceptably muddy.
Use of a narrow bandwidth, a low boost frequency (like 65 Hz), and a relatively
large boost can produce a very punchy sound in a car, or on a radio with significant
bass response. It can also cost you loudness (bass frequencies take
lots of modulation
without giving you proportionate perceived loudness) and can cause thin sound on
radios with only moderate bass response. A smaller amount of boost can often pro-
duce a better compromise.
In HF broadcast , perhaps the most difficult of all processing tradeoffs is choosing
bass equalization. This is why the 9300’s bass equalizer can cut as well as boost.
When propagation conditions are good and the signal strength is high, a certain
amount of bass boost (p3dB) provides the most pleasing sound. However,
robust bass can easily induce intermodulation distortion in the clippers, so the
amount of clipping must be reduced to provide acceptable distortion performance.
In turn, this may compromise loudness by up to 3dB — the equivalent of cutting
transmitter power in half!
Bass boost tends to reduce the life of power tubes in most high-powered transmit-
ters. It will often induce intermodulation distortion in envelope detectors under se-
lective fading, when detection becomes markedly nonlinear because of sideband
asymmetry. In short, the arguments for bass cut are usually more persuasive than
those for bass boost. Yet if an HF broadcasting organization seeks the highest possi-
ble subjective quality regardless of transmitter operating cost and feels that it usu-
ally delivers a strong RF signal, free from selective fading, to its listeners, then such
an organization may still wish to boost bass slightly.