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VP-8IP / Sep 2013
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P R O G U I
HPF - High Pass Filter
The
High Pass Filter
is used to remove undesired subsonic energy from the audio
signal prior to being processed. By removing this energy, processing is cleaner and
modulation energy is not wasted by transmitting sounds that would not be perceptible
to a listener.
The
High Pass Filter
also removes subsonic energy that could upset the operation
of equipment later in the audio chain, such as the AFC loop in the FM exciter.
The
High Pass Filter
has three operating modes:
Off – No
High Pass Filter
boxes are checked. The High Pass Filter is not enabled.
Stereo – If Stereo is checked the High Pass Filter operates on both left and right
audio channels equally by separate high pass filters operating at the frequency set by
the single frequency tuning control. In this mode the Diff Freq slider is not visible.
Sum/Diff – Stereo is not checked, opening a second tuning slider to reveal adjustments
for the stereo difference channel. Separate check boxes, one for the Sum Freq slider and
one for the Diff Freq
slider are visible. Checking a box activates a high pass filter on
that channel, sum or difference (L+R or L-R). Separate frequency sliders are available
for adjusting the high pass filter corner frequency for each signal path. The mono (L+R)
and stereo (L-R) signal paths may have different high pass filter cutoff frequencies.
Why the Sum and Difference Option?
In most program material there is little very low frequency energy in the difference
(L-R) signal. In fact, what energy is there is not typically correlated with the program
material. Therefore the difference channel can operate with a much higher high pass
filter cutoff frequency than the sum channel, and in fact 180Hz is a good tradeoff setting
for the majority of program material.
The sum (mono) channel, on the other hand, carries most of the bass energy in the
majority of program material and therefore it cannot utilize as high a cutoff frequency
as the difference channel without seriously impacting the bass feel of the program.
A good starting setting when using Stereo mode is 30Hz. Good starting settings
when using Sum/Diff mode is 30Hz for Sum and 180Hz for Diff.
Both HPF controls operate over a range of 20Hz to 300Hz.
