Omnia Audio Omnia.ONE - Manual (Page 94)

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transmitters. Particularly in plate modulated AM transmitters, interstage audio coupling
transformers, interstage coupling capacitors, the modulation transformer and reactor, and
the type of coupling between them, all contribute to low frequency tilt performance.
Since transmitter-induced low frequency tilt appears as overshoot on the station’s
modulation monitor, minimizing it will improve station loudness. In order to perform the
Tilt Compensation alignment procedure, a sine wave audio oscillator and an oscilloscope
are required.
Note: The Omnia.ONE’s output level must be reduced before starting this procedure.
Raise it only after instructed to do so in order to produce no more than 50% to 75%
modulation. Failure to follow this advice may overstress or even damage older
transmitters. Omnia assumes no responsibility for such damage.
Tilt Adjustment Procedure
Since the purpose of Low Frequency Tilt Equalization of an AM transmitter is to
optimize its performance on low frequency, clipped audio waveforms, proper adjustment
of the Tilt Equalizer is mandatory if optimum modulation performance (maximum
loudness) is the goal. Overcompensation, by using too much tilt equalization, is actually
worse than no tilt equalization at all because it further taxes the transmitter’s modulator
and power supply. In fact, on some transmitters, it may be preferable to not completely
cancel low frequency tilt because of power supply and modulator headroom issues. Only
field experience with the subject transmitter (or experimentation) will reveal whether or
not low frequency tilt can, or should be applied to the transmitter. Note also that by
nature of its operation, Low Frequency Tilt Equalization puts additional demands on the
transmitter which are beyond that created by the Omnia’s competitive audio processing
alone. In fact, the combination of these two stresses, especially when applied to older,
plate modulated transmitters, may cause damage.
The reader is therefore forewarned to use care, caution, and common sense when
adjusting the Omnia’s Tilt Equalizer when it is being used on older, less adequate
transmitters!
1. Navigate to the Omnia.ONE’s Output Menu, and reduce the output levels to 0dBU on both Left and Right
channels.
2. Connect a sine wave Audio Oscillator to the Omnia.ONE AM audio inputs. Set the oscillator to provide a
50 Hz, 0dBu output.
3. Adjust the Omnia’s input level controls so that at least 10 dB of gain reduction is seen on the Wideband
AGC bargraph meter.
4. In the Mixer menu, increase the output of the LF Limiter to its +3.0dB maximum value.
5. In the Clipper Menu, increase the Clipper Drive to the +4.0 dB setting. This will produce sufficient clipping
on the 50 Hz tone to allow the following adjustments to be performed.
6. In the Output Menu, increase the Omnia’s output level to produce no more than 50% to 75% modulation as
shown on the station’s modulation monitor.
7. Connect a RF sample to the oscilloscope input and observe the signal envelope. The modulated carrier
should appear similar to Figure 1. The clipped 50 Hz waveform should have a flat-top if there is no low
frequency tilt in the transmitter. If Tilt is visible, as in Figure 2, then adjustment of the Tilt Equalization
control is required.
8. Use a mixture of the Tilt EQ and Tilt Freq parameters to change the appearance of the tilt in the waveform
so that the top becomes as flat as possible. A properly compensated transmitter’s modulation envelope
should appear similar to Figure 1. If too much Tilt Correction is applied, the modulated envelope will
appear similar to Figure 3.