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look very “square.” If it appears to “tilt” in any manner, then there is a coupling problem in the STL system.
(We’d offer further information here on the “how and why” this occurs, but that could consume a whole chapter
unto itself!) Simply stated, the problem could be due to either AC coupling in the STL system or “bounce”
generated by either the modulator or demodulator of the STL. Sometimes it’s a little of both!
If you notice any “grass” exceeding the peak level of the low frequency waveforms, there could be noise getting
into the system somewhere. That can occur if the RF path is noisy due to RF signal loss. This “grass” can also
be created if the composite path through the STL system is not linear throughout the 53 kHz bandwidth that a
composite signal requires. This would create overshoots to the processed signal.
A simple fix for either of these two problems is to employ a composite clipper at the transmitter site. But
beware: This could lead to additional audible distortion due to the added clipping of the overshoots.
While the above items must be given some thought, there are many advantages to installing the processing at
the studio. If the STL link operates in a solid fashion, superior performance can be achieved, along with having
easy access to the processing for monitoring and adjustment.
Omnia-3fm Turbo Connections
The Omnia can generate audio power in the low frequency region so strong that it may affect the performance
of some exciters. It is imperative that the exciter has a dual speed PLL in the AFC, or it may unlock. This is not
the fault of the audio processor, but of the exciter. For those who wish to have good, solid, powerful bass on the
air, this is a key point that cannot be overlooked, or loudness will be lost due to inefficient modulation.
Interfacing With A Digital Exciter
Although the current rage is to create the
all-digital
broadcast transmission path, there are some key issues to be
aware of or you can sacrifice loudness in an attempt to gain quality. I know what you must be thinking right
now: “How can they say this when
everything
is digital!”
One issue concerns the sampling rate of the audio processor and the input to the exciter. It is possible to have
two differently sampled systems interfaced in the digital domain. This compatibility is made possible through a
neat little device known as the “sample rate converter.” A device that outputs digital audio data using a 48 kHz
sampling rate can be converted
down
to a lower rate of, say, 32 kHz Unfortunately, this can create overshoots,
which rob loudness. Detailed information can be found in the technical papers
Digital Broadcast Audio
Processing: Finally The New Frontier
and
Omnia.fm: An Engineering Study
on the Omnia web page
(www.omniaaudio.com).
We strongly believe that any stereo generator for FM must use a 48 kHz sampling rate. In this manner, any
processor that is connected to the stereo generator can’t overshoot. Unfortunately, there are others who are
building DSP stereo generators that use a lower sampling rate than 48 kHz However, reducing the sampling rate
of the Omnia-3fm Turbo runs the risk of creating the nasty
digital sound.
The key is the sampling rate. If the digital exciter is
actually
operating at a 48 kHz sampling internally, then
there is no problem. But, should the exciter provide a
downward
conversion of the 48 kHz sampling rate from
the Omnia-3fm Turbo, then you could have overshoot problems.
