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Output Connection Possibilities
Left/Right Analog
: Given the other choices, this is the least-recommended procedure. The stereo generator in
the Omnia-3fm Turbo provides superior performance, so you should use it instead of the one provided in the
exciter.
Analog Composite
: If you don’t object to going partially analog, this will provide excellent peak control and
sound quality.
AES/EBU
: The new Prediction Analysis Clipper that can be activated within the clipper section mitigates much
of the overshoots resulting from the “sample rate roller coaster” of multiple rate conversions and lowpass
filtering. However, it cannot make up for potential sonic degradations. This method is similar to Left/Right
Analog above in that it is bypassing the Omnia-3’s excellent stereo generator (and composite clipper) relying on
the stereo generator built into the digital exciter instead.
Pre-Emphasis: Where To Insert?
Another item of concern is with pre-emphasis. Most digital exciters can generate pre-emphasis within their DSP
stereo generator. This is overkill! Omnia-3fm Turbo provides not only the proper pre-emphasis for any region
in the world, but also exhibits excellent peak control of the pre-emphasized signal so that no overshoots occur.
Using the pre-emphasis in the digital exciter will not guarantee this type of performance. Here’s some
background in the context of operating in a complete digital environment:
1.
Most exciter manufactures will recommend the use of their pre-emphasis network. If that was done,
then the incoming audio data from the processor must be
de-emphasized
before it reaches the exciter. Why
perform the emphasis function twice? Also, and more importantly, if the pre-emphasis phase delay is not
exactly the same
as the previous de-emphasis in the processor, overshoots will occur. Therefore, it is far more
advantageous to employ pre-emphasis only once—in the Omnia-3fm Turbo—and then set the exciter to accept
a
flat
input. The overshoot problem is greatly reduced.
2.
We understand why the exciter manufacturers have provided these pre-emphasis options. It has to do
with digital codec STLs. Research has shown that pre-emphasized audio routed through a codec is not very
friendly
sounding. Hence the de-emphasis in the processor, and second pre-emphasis in the exciter. That’s a
poor example of a Band-Aid. If you are in a situation where you need to use a codec STL, then locate the
processor at the transmitter. This totally eliminates any codec abnormalities to the processed audio, along with
letting the processor have the best possible electrical coupling to the exciter, be it analog or digital.
You can find more detailed information in the Technical Papers on the Omnia web page
(www.omniaaudio.com).
If your desire is to create an all-digital transmission path, please be aware of the topics that have been discussed
here. Should you experience any problems with overshoot or added distortion, you should look to the function
of the digital exciter and its stereo encoder. As a reference, you can easily make signal comparisons with the
analog outputs from the Omnia-3fm Turbo system, especially the composite output. Viewing the signal on an
oscilloscope will reveal that the output is well controlled, with no overshoots, providing a clean baseband
spectrum.
Digital Exciter Connection Considerations
The physical connection to a digital exciter is through an AES/EBU connection. Using a shielded audio cable
with appropriate XLR connectors on each end, connect the processor to the exciter. The exciter should be set for
flat
input, which means that the exciter is not adding any pre-emphasis to the input signal.
