Proportional-Q Filter Response - Klark Teknik Square One Graphic Operator's Manual

Proportional-Q filter response

The most important design decision for the Square ONE Graphic was determining the equaliser response.
Proportional-Q equalisation, as used on previous Klark Teknik analogue graphic equalisers, offers some key
advantages over the more numerous Constant-Q equalisers on the market. 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 4 and Figure 5 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.
Square ONE Graphic
Proportional-Q filter response
25