Sonnox Oxford Restore - Manual (Page 13)

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The second tool to help isolate the fundamental is the large FFT display. The FFT displayed by
default in this graph is no ordinary FFT of the input signal, but can accumulate FFT data. It is
designed to accentuate any frequency components that are consistently present, and average out
everything else. Any buzzes, hums, whines or whistles that are consistently present tend to stand
out in the plot.
A third method of isolating the nominal frequency is a tone generator. In the same way as you
can tune a guitar, as you home into the fundamental, the beat frequency between the tone and the
fundamental will become slower and slower the closer you get.
The FFT graph will automatically mark the three highest peaks found. You can click on one of
these peaks to directly set the nominal frequency to the exact frequency found in the graph. You
can also drag the marker in the graph, or use the rotary control. The small red triangle on the
frequency axis notifies the nominal, and it can be dragged to re-position the frequency knob.
Once the nominal frequency is found and set, the removal section uses this frequency to calculate
the coefficients required for the comb filter to remove this frequency and its harmonics from the
input signal.
4.2
Tracking Modes
By default the plug-in assumes Auto Mode. This means that the plug-in is continuously hunting
either side of the nominal frequency for a peak profile, and then steps the centre of the profile
closer to the peak. Once the lock strength rises to be above a certain level, the plug-in
automatically sets the nominal frequency to the frequency of the peak found. In Auto mode, the
filters follow the detected frequency, not the nominal frequency set by the position of the knob.
So if the hum is wandering in frequency, the detector will follow.
If the nominal is set to within about 5Hz of a 50Hz or 60Hz hum frequency, the plug-in will
automatically move towards the correct frequency and set itself up to remove that frequency.
Once you hear the hum disappear, you can then click on Freeze to set the nominal to this exact
frequency and stop the detector from hunting.
For removing whines and whistles it is best to set Freeze Mode, and then use the FFT to find the
frequency of the whines. These show up as peaks on the plot, and then you can click on a peak to
set up the plug-in to remove that frequency.
Auto Mode is suitable for tracking a weak, slowly wandering fundamental and operates over all
three frequency bands. Fast tracking mode allows the removal filters to automatically follow a