Handbook for Sound Engineers

Filters and Equalizers 801

four sliders for 800 Hz, 1000 Hz, 1250 Hz, and 1600 Hz

were set to +5 dB. The overall peak is greater than

desired and a ripple of 2 dB is induced across the band.

23.6.2.1 Transversal Equalizers

Fig. 23-28 shows an example of how graphic equalizers

based on tuned filters exhibit ripple in the response

when groups of adjacent controls are used. The actual

response that we were trying to create would have been

better achieved using a single filter as in Fig. 23-29. The

Transversal equalizer configures as a graphic equalizer

produces ripple-free response for any equal or flat

setting of the controls. It produces minimum phase

response curves and avoids phase mismatch anomalies

at the band edges that can be a problem in other equal-

izers. The response curve is mathematically a best

match for the desired response.

The FIR filter discussed previously is a digital
implementation of a transversal filter. Whereas a
conventional tuned filter operates in the frequency
domain, a transversal filter operates in the phase or time
domain. If a unity gain all-pass circuit stage, Fig. 23-
19, is substituted for each Z^1 delay element in Fig.
23-22, an analog transversal filter is created. The coeffi-
cients are implemented by summing the outputs of the

successive delays via different weighting resistors to a

summing amplifier.

23.6.3 Parametric Equalizers

Parametric equalizers allow adjustment of the filters in

term of the three main parameters that define a filter.

• The boost or cut in dB.


• The center frequency.


• The bandwidth or Q.

It is difficult to make a parametric filter that provides

completely independent control over all three parame-

ters over a wide frequency range. Several filter compo-

nents have to be varied with one control. For this

reason, parametric equalizers sometimes have one of the

controls as a multiposition switch instead of continu-

ously variable. This allows a band of calibrated compo-

nents to be switched into place rather than having to

worry about how variable component values track.

Parametric equalizers are always active and typically

there are several second-order sections in a unit. Each

band’s center frequency is adjustable over a limited

frequency range so that the parameters’ independence

can be maintained. This means that each section in a

unit typically covers a slightly different frequency

range, each section having a ratio of between 10:1 and

25:1 between the highest and lowest center frequency.

The lowest band will adjust down to 20 Hz and the

highest band up to 20 kHz. Each section will typically

provide more scope for cutting levels than for boosting.

Typical boost level is up to 15 dB while the available

cut may be down to 40 dB. The bandwidth or Q is not

consistent in its labeling between manufacturers. Some

specify bandwidth in Hz, some specify Q, and others

specify octave fraction. In terms of Q, the range of this

control will typically be between 0.3 and 3, with the

critically damped value of 0.707 being in the center

position of the control.

An overall gain is usually provided to help maintain

the average level and to maximize headroom by

avoiding clipping.

23.6.3.1 Semi-Parametric Equalizers

A reduced version of the parametric equalizer is

commonly found on mixing consoles. This is the

semi-parametric or swept frequency equalizer. This type

has only the center frequency and cut or boost controls.

The Q is usually set to be a midrange critically damped

Figure 23-28. Magnitude and phase response of a graphic

equalizer.

Figure 23-29. Magnitude and phase response of a single

bi-quad filter.

Frequency

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Frequency

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