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AM-10HD / Aug 2008
A M - 1 0 H D G U I
LF Tilt Correction pre-distorts the shape of the waveform generated by the
processor in order to make it “complimentary” to what the transmitter will do to
the waveform later. In essence, LF Tilt Correction is a specialized, extremely low
frequency equalizer that “tilts” the waveform in the direction opposite to how the
transmitter will tilt it later, and the result is flatter tops of the clipped waveforms
and therefore more accurate modulation of the transmitter.
Such correction is not without caveats, however, and one of them is that it
requires that the transmitter have very good power supply regulation and suf-
ficient RF drive available at the RF power amplifier. If these conditions cannot
be met by the transmitter’s design (or it’s capabilities hindered by “soft” tubes)
carrier shift and modulation control can be worse after LF pre-correction than it
was beforehand!
Our LF Tilt corrector is a bandpass design with variable Q, Frequency, and
Amplitude Boost/Cut. We chose a bandpass filter for this function rather than
the usual lowpass because the lowpass design can create extremely high gains
at very low frequencies, which can even further exacerbate transmitter design
inadequacies such as power supply resonances, which then increase “bounce”
and carrier shift.
The bandpass design has a faster phase shift characteristic on the high side
of its response curve which can more closely compensate for the low frequency
phase characteristics of many transmitters. Finally, the bandpass design is “safer”
in that it provides correction down to frequencies that are appropriate, and then
the response rolls off quickly below that. If a transmitter cannot be adequately
corrected with our bandpass design then it would likely perform even worse with
a lowpass design in the tilt corrector!
Note that the LF equalization generated by the tilt equalizer is NOT intended
to boost low frequency energy in program material to make it more audible to
the ear. Rather, LF Tilt Correction occurs in the subsonic audio region, typically
between 10Hz and 50Hz and simply aids the transmitter’s ability to more ac-
curately follow low frequency clipped waveforms emitted by most competitive
audio processors.
Adjusting the Tilt Corrector
Adjusting the LF Tilt Corrector is a simple process.
The first step is to determine if the transmitter will require Tilt
Correction in order to modulate well at low frequencies, and the
following procedure will determine this:
1. Connect the AM-10HD to the audio input of the transmitter
to be tested.
2. Connect the transmitter to either a dummy load of rated power
or to the antenna system.
3. Turn on the AM-10HD’s Test Oscillator. Adjust it to a frequency
of 75Hz. Adjust the Test Oscillator Level control and the Ana-
log 1 or Analog 2 output (as appropriate) until approximately
80% modulation is indicated on the station’s modulation monitor.
Ensure that the Tilt Correct “L: (level) controls are set at 0dB.
