Pass Filters.

These pass through some frequencies while cutting out others. A High Pass filter allows high frequencies, above the set frequency, to pass through unchanged, while any lower than the set frequency are removed. A Low Pass filter allows sounds with frequencies lower than the set frequency to pass through. A Band Pass filter has two set frequencies, and sounds between these are allowed to pass though, all others being blocked.

Shelf Filters

A Shelf filter gives a uniform output within one band, and a uniform output at a different level in the other band. A Low Shelf filter will raise or lower the levels in the low frequency band, covering all frequencies below the set frequency. A high Shelf filter does the same for frequencies above the set frequency.

Comb Filter

A Comb Filter is an effect rather than an adjustment - it is similar to a Flanger. A short delay is applied (much shorter than would be used for an echo effect). When the delayed signal is mixed with the main signal, some frequencies come out 180 degrees out of phase, and are cancelled out. The result is a filter with a series of notches.

3 Band Equaliser

An Equaliser gives an unequal frequency response - some frequencies come out louder or quieter than others. The name comes from the idea that this can be used to correct unequal response in the original signal, which is sometimes possible.

Levels can be boosted or cut independently for low, middle and high sounds. The centre frequency for the middle sounds can be varied - this is useful for pulling some particular instrument out of a mixed recording, or for adding "presence", for which a centre frequency of around 3KHz is best.

Parametric Equaliser

A Parameter is simply a setting. This equaliser has only one band but it has an additional setting, Q. This is the width of the band which is affected. For example, a narrow band cut at mains frequency might be used to reduce hum, or at a higher frequency to reduce a feedback whistle.