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AM-10HD / Aug 2008
A M - 1 0 H D G U I
HPF - High Pass Filter
The High Pass Filter is used to remove non‑audio and 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 is also useful in
removing subsonic energy that could upset the operation of equipment later in
the audio chain.
The High Pass Filter has three operating modes:
Off – The High Pass Filter is not in operation.
Stereo – The High Pass Filter operating mode is stereo, and in this mode both
left and right audio channels are treated equally by setting of the high pass filter.
Sum/Diff – The High Pass Filter is operating in Sum and Difference mode,
which means that the sum (mono) information and difference (stereo) information
may be processed by different values of high pass filter.
The High Pass Filter mode is selected as follows:
‑ If the Stereo checkbox is checked then the mode is Off if the High Pass Filter
checkbox is not checked, or Stereo if High Pass Filter is checked
‑ If the Stereo checkbox is not checked then the mode is Off if both High Pass
Filter checkboxes are unchecked, or Sum/Diff if either (or both) High Pass
Filter checkbox is checked
Why the Sum and Difference High Pass Filter Option?
In most program material there is little very low frequency energy in the
difference (L‑R) signal. In fact, what energy there is below about 100Hz is not
generally 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 set‑
tings when using Sum/Diff mode is 30Hz for Sum and 180Hz for Diff.
Both HPF controls operate over a range of 20Hz to 300Hz.
