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Passeq
Frequency Filters
As a rule almost everyone of us has rst made an aquaintance with frequency ltering
through our listening to home stereos. Such elementary kinds of lters are simple ampli-
tude-based lters: When one turns a bass control clockwise, one hears a general or overall
increase in bass frequency energy.
But with the explanation above on the composition of a complex, natural tone, it is clear
that such a low frequency control does not only inuence the energy of the fundamental
frequency, but also always the sound of a tone—the relationship between energy of the
fundamental and harmonics frequencies is changed.
Typically amplitude-based frequency ltering boosts or cuts the energy of a specic audio
frequency band. In such processes it is possible to employ lters with design and function
that are very different from each other: Depending upon the technical construction, such
lters may, for example, process only high or low frequencies in certain way.
Filter Types
There are two types of lters used in the Passeq: wide-band lters which are comparable to
shelf-lter characteristics and bell-formed peak-lters with narrower bandwidths.
Shelf Filters
A shelf lter increases or decreases the energy of all frequencies above or below a chosen
frequency. Depending upon the direction of processing one refers to high frequency (HF)
or low frequency (LF) shelf lters. Beginning with the threshold frequency, the frequency
band is boosted or cut much like a shelf. The maximum boost or cut achieved at the point
furthest from the threshold frequency. The threshold frequency is usually about 3 dB less
(with the overall increase set to maximum). This gives the typical rising form of the shelf
lter’s response curve.
Peak Filters
A peak lter boosts or cuts a chosen frequency‘s energy with a maximum amplitude and a
denable frequency range around this frequency with a fall off of up to 3 dB to both sides.
The chosen frequency with the maximum amplitude is called center frequency—it takes
place in the middle at the peak of the response curve. The response curve forms a bell, thus
peak lters are also often referred to as bell lters.
Bandwidth
The width of a frequency range or band is musically dened in octaves. The technical coun-
terpart to this is the “Quality” of a lter, and the abbreviated “Q” is the most common value
for the bandwidth of a lter.
A high Q value means a narrow bandwidth while a smaller Q factor corresponds to a wider
one:
Bandwidth 2 Octaven: 0.7 Q
Bandwidth 1 1/3 Octaven: 1 Q
Bandwidth 1 Octave: 1.4 Q
Bandwidth 1/2 Octave: 2.8 Q
The Basics of Frequency Filtering