CHAPTER 23
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SYSTEM MENU
FM Options Menu
The FM Options Menu contains controls related to the composite and L/R output sections. Included here are adjustments
and settings for the MPX Power Limit, Pilot Level, Auto Pilot, MPX and L/R Output Bandwidth, MPX and L/R Output Levels,
Stereo Mode, RF Bandwidth Controller, L/R Codec Safety settings, and the built-in MPX Test Generator.
♦ Enabling the RF Bandwidth Control can tightly control the RF bandwidth to reduce multipath problems. The Strength
slider determines the amount of control applied.
Because multipath issues are so specific to particular locations, the only practical way to determine the best setting for
your station is by trial-and-error while driving to locations that are known to be problematic. With a Wi-Fi hotspot and a
laptop or tablet running Omnia 9’s remote software, it is entirely possible to make these adjustments on the fly and in real
time.
Note:
The signicance of this feature warrants a detailed explanation. Because certain sounds such as ste-
reo-mic’ed piano (the rst few notes of “My Immortal” by Evanescence serves as a good example) can cause
excessive stereo reception problems.
Looking at the MPX spectrum of this song in particular, you will notice two very tall high frequency peaks:
One at 37890 Hz and another at 38110 Hz. The original audio had a piano note at 110 Hz, recorded with
stereo microphones, and has very little phase correlation between the left and right channels. Thus, due to
the FM stereo system, which amplitude modulates the Left-Right dierence signal by 38000 Hz, these two
high frequency peaks result.
You may also notice that they are several dB’s stronger than the pilot tone, but at twice the frequency. When
this signal gets frequency modulated (as your exciter will do), these very tall high frequency peaks will cause
very wide sidebands in your RF carrier, potentially causing multipath reception problems, which would be
especially audible here due to the sparse audio content.
The careful observer may recognize that although SSB (Single Sideband) normally reduces RF bandwidth,
it will not help in this case. In SSB we lose the 38110 Hz peak, but we still have the 37890 Hz peak, and with
SSB it’s 6dB stronger than before.
One solution would be to statically reduce the L-R signal as some processors do. However, this narrows the
entire stereo image no matter what the audio content and is the equivalent of shooting a y with a cannon.
Instead, Omnia.9’s RF Bandwidth Controller (which is actually a limiter with thousands of bands) allows it
to attenuate only the frequencies that cause problems without aecting the rest of the stereo image. This
means tight control over your RF bandwidth without audible loss of stereo separation!
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