FILTERS AND EQUALIZATION

Filters are signal processors that affect frequency
balance. At a basic level, filters are used to attenuate or
boost the level of specific frequencies or frequency ranges.
Designed originally to compensate for frequencydependent
loss in telephone lines, some form of frequencydependent
filtering (or equalization) is found in all but the
most basic of sound systems. The simplest form of filter is
the tone control, basically a filter that attenuates high frequencies
above a predetermined frequency. Equalizers are
typically characterized by combining several filter sets to
offer more precise frequency response shaping.
Historically, filters were passive devices capable of
attenuation only. The frequency range and amount of
attenuation were achieved with capacitors, inductors, or a
combination of both. Favorably, passive filters do not
require power and do not generate noise. The large size
and expense of discrete components, however, precludes
the ability to develop equalizers with larger numbers of
filters and more precise control of frequency and level.
Active filters allow for fast, easy tuning and the ability to
add gain, using smaller components at lower cost. Tone
controls employing active filters can be found on even the
most inexpensive home stereo systems. In this scenario
there are typically two controls, treble and bass, which
correspond to filters that affect low frequency and high
frequency response. Since they are active, these tone
controls are capable of cut or boost.
Simple filters that affect a broad range of frequencies
are divided into four types: high pass, low pass, band
pass, and band reject. High pass filters, as the name
implies, allow high frequencies to pass, and low pass filters
do the same for low frequencies. It is often more convenient
to think of these filters in terms of the frequencies that they
cut instead. High pass filters are also known as low cut
filters, and low pass filters are known as high cut filters, but
their function is the same and these terms can be used
interchangeably. Low and high cut filters
have an associated slope that defines how rapidly output
declines below (or above) the filter frequency. Slope is
typically defined in dB/octave. The span of an octave
relates to a doubling (or halving) of frequency, for example,
50 to 100 Hz or 5 kHz to 2.5 kHz. A 6 dB/octave low cut
beginning at 100 Hz, therefore, translates into 6 dB less