Using The Philtre; What Is Filtering - Alesis ModFX Reference Manual

Using the Philtre

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This section defines the different kinds of filters and modulations,
and explains the functions of the Philtre's controls in greater
detail. The technical information in the next few pages will help
you get the most out of your gear...it's really pretty simple.

What is filtering?

An audio filter is a special kind of tone control that filters out
certain frequencies. Just as a water filter lets water through but
stops dirt and rust particles, audio filters let the frequencies you
want flow through, while stopping the rest. Every kind of audio
equipment has filtering or filter sections in it. If you have two-
way speakers in your studio, each tweeter is protected by a high-pass
filter that lets the treble through while keeping the bass out (so you
don't blow the tweeter), and the woofer may be connected to a low-
pass filter that keeps the high frequencies out. If you have three-
way speakers, a band-pass filter lets only the midrange frequencies
get through to the midrange driver.
While most filters are used for equalization and crossovers,
engineers discovered that powerful filters moved around
dynamically could be a useful and creative special effect. The
Philtre falls into this special class of filters, that includes wah-wah
pedals and voltage-controlled filters on synthesizers.
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