Equalization and Filtering
The difference between filtering and equalization (EQ) is generally one of amount and purpose. Since EQ is a form of filtering, I’m going to explain filters first.
Filters
A general definition for a filter is anything that changes the gain of frequencies in a sound. This definition is so general it is useless, since by this definition all audio producing, recording, transmission, and listening devices (including rooms, ears, etc.) are filters.
Typically, we use the term filter to apply to devices that intentionally control the amount of gain in a prescribed frequency range of a sound. Some texts describe filters as only cutting amplitudes. Filters are defined by the frequencies they let through (pass band) or frequencies they reject (stop band). The basic filter types for synthesis are low-pass, high-pass, band-pass, or band-reject (notch).
Low-pass
- allows frequencies below a cutoff frequency to pass through unchanged.
- cutoff frequency is defined as being at the -3 dB point on the downward slope of the filter curve.
- the slope is defined by its steepness. A typical filter slope is 6 dB per octave. Higher slopes produce more targeted filtering, but introduce other potential problems, such as phase distortion.
High-pass
- allows frequencies above a cutoff frequency to pass through.
- other parameters (cutoff and slope) are same as low-pass filter.
Band-pass
- allows frequencies around a center frequency to pass through (or between a low cutoff and high cutoff).
- literally a combination of low-pass and high-pass filters.
- generally address center frequency, rather than cutoff frequencies.
- slopes are added, so a 6 dB slope below the center frequency and a 6 dB slope above the center frequency yield a 12 dB slope for the filter.
Band-reject (notch)
- rejects frequencies around a center frequency (or between a low cutoff and high cutoff), not allowing these frequencies to pass through.
- other parameters same as band-pass.
Shelving Filters (Low Shelf and High Shelf)
- reduce and amplify a band of frequencies equally above (high shelf) or below (low shelf) a cutoff frequency.
- rather than a curve that descends to zero amplitude (or ascends from), the there is a sloping area to the response curve, followed by a flat-line response.
Equalization
Equalization is generally comprised of multiple filter shapes that have gentle bell or shelf slopes, usually of 6 dB per octave, with fixed frequencies. The intent of EQ is to make subtle spectrum modifications to enhance the overall sound.
Parametric EQs, such as what is available in Audition and Digital Performer, usually have adjustable frequency, slope, and filter type. These EQs really are better thought of as filter banks.
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