Making Music
  Computer Music Software and Hardware

Filter tips

Somewhere within the gamut of filters hovering between paras and graphics, there resides several popular filter types. They are easy to understand as they perform only one filtering function. The first four are most often, but not exclusively, found in synthesis while the shelving filters are more commonly used in recording.

	Low Pass. This passes frequencies below the cutoff frequency and attenuates the higher ones. It is the most natural-sounding filter as higher frequencies are typically the first to be lost in natural environments.
	High Pass. This is the opposite of the low pass filter - it passes frequencies above the cutoff frequency and attenuates lower ones. It is useful for removing unwanted bass frequencies and heavy use can remove the fundamental of a tone resulting in a very thin sound.
	Band Pass/Peak/Bell. As its name suggests, this passes a band of frequencies around the cutoff frequency and attenuates those either side. It's highly selective and particularly useful for homing in on specific frequencies such as tones produced by solo instruments or for tackling problem areas in a mix such as hum and noise. It is, in effect, a parametric EQ.
	Band Reject/Notch. This is the opposite of the band pass filter. It attenuates frequencies either side of the cutoff frequency and passes the others, effectively notching out a frequency band.
	High Shelf. This usually boosts frequencies above the cutoff frequency. It's used as a tone control to shape the upper section of the frequency spectrum.
	Low Shelf. This is the opposite of the high shelf filter and boosts frequencies below the cutoff frequency.

On the shelf

The terms high shelf and high pass are often used synonymously, as are low shelf and low pass. Technically, however, a high shelf filter boosts frequencies above the cutoff point whereas a high pass filter simply passes them and attenuates the frequencies below. The nett result may appear to be the same but the resulting frequencies within the filtered sound will not be the same.

Having said that, modern shelving filters, particularly in software, can cut as well as boost so cutting with a high shelf filter is effectively the same as using a low pass filter (got that!). It's just one of the ways in which modern technology blurs the lines.

Passive and active filtering

One final filter thing - you may occasionally hear some filter devices called active or passive. Passive simply means that they can only attenuate frequencies, not boost them, much like low and high pass filters. You'll find typical passive filters in guitar tone controls and the like.

There are so many EQ variants that it's important to know which ones do what. Armed with this information, you will be better equipped to use them in recording and synthesis.