| A CAPACITOR cuts off low frequencies. |  |
| An INDUCTOR cuts off high frequencies. |  |
| A RESISTOR obstructs the flow of electrons. |  |
EQUALISATION
is any intentional variation to a signals frequency response.
FREQUENCY RESPONSE
is an indication or graph of the relative amplitudes of' frequencies across the audio spectrum.
| A CAPACITOR cuts off low frequencies. | |
| An INDUCTOR cuts off high frequencies. | |
| A RESISTOR obstructs the flow of electrons. | |
Passive filter networks are generally referred to as a TONE CONTROL, such as those commonly found on standard guitars and guitar amplifiers.
These type of tone controls only cut the level of high frequencies, thus giving the impression of an increase of bass. Therefore, when recording a bass guitar, in particular, the tone pot on the guitar should be set on full treble, as this gives a clear and defined sound with just as many low frequencies. If some of the brightness is not required in the long run then it is simple to decrease the high frequency level upon mixdown. If the brightness was not recorded to start with then any increase of high frequency during the mix will result in an increase of hiss
Passive filter circuits are also found in some low grade Hi-Fi's and on many portable cassette players, Walkmans and C.D.'s.
As their name implies, these are filters where certain frequencies are passed at full level while others are attenuated. With any equaliser the frequencies which are attenuated by less than 3dB are said to be inside the PASSBAND, while those attenuated by more than 3dB are in the STOPBAND. The frequency at which the signal is attenuated by exactly 3dB is called the TURNOVER or CUTOFF or INDICATED FREQUENCY and is used to name the filter.
Some may say that ideally attenuation should become infinite immediately outside the passband, but in practice this is not attainable and would sound too abrupt, rather than a smooth transition. In the simplest and smoothest case, attenuation increases at a rate of 6dB per octave. This rate is called the SLOPE of the filter. Other slopes include the most common, 12dB/octave, and much steeper 18dB/octave and 24dB/octave.
There are two types of equalisation characteristics: SHELVING or PEAK-DIP (or BELL). The characteristic refers to the shape of the curve as shown on a frequency response graph, which in practice determines the number of' frequency response graph, which in practice determines the number of frequencies (bandwidth) the equalisation control will affect.
SHELVING EQUALISATION characteristic gives equal boost or cut to a band of' frequencies.
PEAK-DIP EQUALISATION gives more effect to the centre frequency than any other frequency in the band.
Shelving is the most common type of equalisation, generally utilised in good quality Hi-Fi's, fixed EQ pots on consoles (e.g. low frequency and high frequency pots), and graphic EQ's. Neither the centre frequency nor the number of' frequencies (bandwidth) can be varied, only the amplitude (boost or cut) can be varied. The BANDWIDTH is measured from indicated frequency to indicated frequency, and as the amplitude is boosted the bandwidth will not actually vary as the indicated frequency is measured at the point 3dB down from maximum level.
Peak-dip equalisation is the most common EQ characteristic used in studios. Peak-dip is utilised in parametric equalisers and is more discerning and / or discrete (i.e. it picks out/affects less frequencies).
The Q FACTOR is a number which refers to the slope of the curve, or the rate of roll-off of the curve. A variation of the Q will result in a variation of the overall bandwidth. On most parametric equaliser this control is commonly known as the bandwidth control.
FIXED EQUALISERS are those whose curve characteristic is not at all variable and it is either ON or OFF. These include:
GRAPHIC EQUALISERS utilise shelving characteristic in successive bands to cover the complete audible frequency spectrum. In professional equipment they are usually octave increments (10 band) or 1/3 octave increments (31 bands).
A graphic equaliser is most commonly used in what can be termed broadband applications such as P.A.'s and Hi-Fi. They are not so commonly used in the studio or on a console, but they may be. In a P.A. application they are used to correct for room anomalies such as room resonance (e.g. booming low frequencies due to room shape).
The divisions of a standard 1/3 octave graphic equaliser are (in Hertz):
20, 30, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1k, 1.25k, 1.6k, 2k, 2.5k, 3.15k, 4k, 5k,
6.3k, 8k, 10k, 12k, 16k, 18k, 20k.
The standard Q factor (slope) of a graphic equaliser's shelving curve is 12dB/octave. This produces a smooth transition at the crossover point of each band and thus a flat frequency response when all bands are set to nil boost or cut.
PARAMETRIC EQUALISERS utilise the peak-dip (bell) characteristic and usually have three of four sections to achieve adjustment over several specific areas of the audio spectrum. The parametric EQ is more discrete or discerning than the graphic EQ, allowing more subtle control if required. This is achieved by the parameters (variable controls) of the unit. These are:
| 1. AMPLITUDE - boost or cut of the frequencies. |  |
| 2. CENTER FREQUENCY SELECTION - a variable sweep control. |  |
| 3. Q SELECTION - a variable bandwidth. |  |
The Q range available ranges from 0.4 to 4. The smaller the number the wider the bandwidth, and the larger the number the narrower the bandwidth.
FULL PARAMETRIC EQUALISER units have control over all three parameters while there exists units which only have control over the amplitude and the centre frequency, but not the Q, and these are known as SEMI (or QUASI) PARAMETRIC
A parametric EQ can be used to achieve notching, which is the attenuation of a very narrow band of frequencies. Units exist which are only for this use and they are known as NOTCH FILTERS.
This type of filter is quite often necessary is a semi-acoustic guitar and amplifier combination as there often occurs a feedback of a narrow band of low frequencies, known as HOWLING.