2 Signal processing
2.7.3 Frequency
These effects alter the frequency characteristic of the sound they process. Below is a detailed explanation of many different kinds of basic filters and variations. In the graphs, both the gain and the frequency scale are logarithmic.
Filter types
Low pass
The most basic of all filters is the low pass. It rolls off all frequencies above its cut-off frequency with a certain slope. In today's electronic music, this filter, with a steep slope, is often used on the total mix to create a kind of varying fidility.
High pass
The inverse of the low pass is the high pass. Mostly used to keep unwanted low frequencies out of a signal. Very low frequencies can't be heard, but can damage loudspeakers and amplifiers, as they will, in vain, try to reproduce them.
Band pass
A band pass filter only puts out a narrow range of frequencies. The loudest frequency is called the "center" frequency, the sharpness of the top is called "Q", and the rate at which the surrounding frequencies fall off is called "slope".
Band stop or notch
The band stop filter cuts a certain frequency range. The notch filter is a relative of the resonance filter. It's a special type of band stop, and filters one frequency or a very narrow band out of the signal. For instance to radically cancel 50 or 60 cycle hum, a notch filter can be applied (50 or 60 Hz depending on which side of the pond you're standing).
Resonance
The purpose of the resonance filter is to isolate one frequency or a very narrow band. An example of this filter is the tuning section of a radio receiver. You (usually) only want to hear one station, and isolating its frequency is therefor necessary. When applied on an audio signal, a resonance filter sounds similar to a phaser, but more nasal.
High shelf
Often parametric equalizers 
have additional shelving bands. Much like a high pass filter, a high shelf deals with everything above a certain frequency, but doesn't cut everything. There's a minimum gain, which is the actual gain setting. The solid line shows boost, the dashed line shows cut.
Low shelf
The counterpart of the high shelf. The solid line shows boost, the dashed line shows cut.
Applications
Equalizer
Although not really an effect, the creative mind could use it as such. The graph shows different settings:
- Solid lines: each band separately boosted
- Dot/dash line: all bands boosted (bands influence each other)
- Dashed line: each band separately attenuated
- Dotted line: all bands attenuated (bands influence each other)
Phaser
A phaser consists of a socalled comb filter. The graph clearly shows how it got its name. You could think of it as a short delay (.1-2 milliseconds) which causes ripples in the frequency characteristic, when mixed back with the original sound. You can create phasing effects while moving your flat hand closely towards your mouth while making an "FFFFFF"-like sound. Because of the delay-like setup in a phaser, it falls in between the time based and frequency based effects. Digital effects processors will often create a phaser with a slightly modulated delay (like a flanger), analog phasers almost always use a comb filter (which is actually a delay. In fact, every filter is a delay, but an explanation is beyond the scope of this document).
Wah-wah
A foot controlled steep low pass filter, sometimes with adjustable resonance. This resonance is put just before the cut-off point of the low pass filter, and moves along with that cut-off frequency. The sound of this effect does its name honour.
Touch filters
An adjustable filter is controlled by an envelope follower . The most common type is the T-wah or touch-wah. It sounds of course much like the wah-wah, but the filter is controlled by playing volume instead of a rocker pedal.
Triggered filters
An adjustable filter is controlled by a trigger, which, in turn, is controlled by playing notes. When triggered (a note is played) the filter opens or closes at a preset rate. In a way a trigger is an envelope follower with just an "on" and an "off" state.
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© Joris van den Heuvel 2001-2009