About Filters and EQ

See topic, Filter Operation, for more information.

Filters and EQ

A filter is a device that attenuates or removes a user-defined range of frequencies from an audio waveform, while passing other frequencies. Typical filters are low pass, high pass, and bandpass.

An equalizer (EQ) is a type of filter originally designed to correct for losses in the transmission of audio signals, making the output equal to the input, or making "flat" an otherwise inconsistent frequency response and giving all frequencies equal energy - hence, the term equalizer.

Parametric EQ

A parametric EQ is a variable equalizer that offers control of all "parameters," including amplitude (boost/cut), center frequency (frequency), and bandwidth (Q). This allows the user to control the amplitude of each band, shift the center frequency, and widen or narrow the affected area.

High Pass

A high pass filter passes a band of frequencies extending from a user-defined cutoff frequency (greater than zero) up toward the high end of the frequency spectrum. All frequencies above a specified cutoff frequency are allowed to pass, attenuating all frequencies below.

Low Pass

A low pass filter passes a band of frequencies extending from a user-defined cutoff frequency (less than infinite) down toward the low end of the frequency spectrum. All frequencies below a specified cutoff frequency are allowed to pass, attenuating all frequencies above.

Band Pass Filter

A band pass filter is a combination of high pass and a low pass filters, allowing the band of frequencies between the low and high cutoff points to pass.

Bass & Treble Tone Filters

Also known as a shelving filter, a tone filter gives the ability to cut or boost gain evenly above or below a given frequency, with the end-band shape giving the visual appearance of a shelf. Tone filters are generally applied to program material, and are expressed as Bass and Treble control.

Filter Defaults and Ranges

The table below shows each filter type with relevant parameters and default settings. The table immediately following shows parameters for each filter type and a typical settings range for each parameter.

Type	Frequency	Parameter 1	Parameter 2
Parametric	1000.0 Hz	Boost/Cut: 0.0 dB	Q (freq ÷ bandwidth): 1.000
High Pass	100.0 Hz	Slope: 6dB/Octave	x
Low Pass	10000.0 Hz	Slope: 6dB/Octave	x
Bass	100.0 Hz	Boost/Cut: 0.0dB	Slope: 6dB/Octave
Treble	8000.0 Hz	Boost/Cut: 0.0dB	Slope: 6dB/Octave

Filter Parameter	Settings Range
Frequency	20 Hz to 20 kHz
Boost/Cut	-24.0dB to +24dB
Q (EQ only)	0.707 to 15.000
Slope (Filters only)	1st Order (6dB) to 2nd Order (12dB)

Definitions of filter parameters are as follows:

• Frequency - the point in the frequency spectrum at which a filter or EQ is applied. For parametric EQ, this point would be referred to as the center frequency. For all other filters, this would be the cutoff frequency (band pass filters contain high and low cutoff frequencies), and cutoff or "corner" frequencies for tone controls. Cutoff frequencies begin at the half-power point, or "3dB down" point.

• Boost/Cut - the amount of gain (boost) or gain reduction (cut) that is applied at a given frequency. Applies only to parametric and shelving filters (tone filters).

• Slope - in a high-pass or low-pass filter, the amount of attenuation in dB per octave. Slope values are in 6dB increments, corresponding to the number of circuits in the filter. For example, a single circuit filter, also known as "1st order," will have a 6dB slope, while a 2nd order filter (two circuits) will have a 12dB slope. Multiple filters may be added together in series to increase the slope.

• Q - the ratio of the center frequency divided by the bandwidth. Q stands for "Quality Factor."