Proportional-Q Filter Response - Midas XL8 Operator's Manual

222

Proportional-Q filter response

A major consideration in the design of the XL8 Control Centre was determining the
equaliser response. Proportional-Q equalisation (as used on previous Klark Teknik
analogue graphic equalisers) offers some key advantages over Constant-Q equalisation.
Namely, at low amounts of cut or boost, the width of the filter is relatively broad and
becomes narrower as the amount of boost or cut is increased, giving a more 'focused'
response. This differs from a Constant-Q response, which boosts or cuts an
increasingly wide band of frequencies. This is an important consideration in
applications, such as cutting particular problem frequencies, as more of the frequency
spectrum is scooped out when using Constant-Q equalisation. Whereas, Proportional-Q
equalisers are much better at cutting just the problem frequency band. On Constant-Q
equalisers the Q is measured 3dB up from the point of maximum attenuation when in
cut, rather than the correct definition of 3dB down from the point of maximum gain
(0dB when cutting), which results in a notch filter response.
In all types of graphic equalisers, one of the key issues is how the summing of adjacent
EQ bands alters the frequency response. Lower-Q filters will blend together more
smoothly, but higher-Q filters provide more selective control of problems - at the
expense of more frequency response ripple. A weakness of the Constant-Q approach is
that for small amounts of boost and cut (a very typical situation), the EQ bands have to
be comparatively broad to blend smoothly so as to avoid excessive ripple in the
frequency response. However, as the Q of the EQ bands remains constant, larger
amounts of boost or cut will affect an increasingly wide range of frequencies, thus
limiting the precision and usefulness of the equaliser.
Figure 5 and Figure 6 show the responses of a leading Constant-Q equaliser in both its
'wide' and 'narrow' modes of operation for 2dB of boost and cut, which is typical of the
small adjustments made in corrective EQ applications. The 'wide' response gives a very
smooth combined response for the three bands shown. However, the 'narrow' response
shows significant ripple in the combined response which will lead to audible artefacts
such as phase distortion.
Figure 5: Symmetrical-Q equaliser 'wide' mode ±2dB boost and cut
Chapter 11: Graphic Equaliser (GEQ)
XL8 Control Centre
Operator Manual