Klark Teknik DN9340E, DN9344E What do we mean by all these Q types?, Application Notes

Application Notes

16.2. What do we mean by all these Q types?

The “Q” of an audio equaliser describes the steepness of the filter - the degree to which it will affect signals either side of its nominal or “centre” frequency. In general, the Q of a peaking filter is defined mathematically:

as,	where the bandwidth (in Hz) is the range of frequencies affected by the
	filter.

Because the frequency response of such a filter is a smooth curve (not a sharp “brick wall” filter like the ones in an analogue-to-digital converter) we have to decide how we choose to define the bandwidth. The established convention is that we use the bandwidth to the “-3dB” points on either side of the centre frequency, where the gain is 3dB less than the maximum gain.

In the example above, the filter is centred on 1kHz, the lower 3dB point is at approximately 800Hz, and the upper one is at approximately 1.25kHz.

Therefore, this filter has a Q of:

In a typical parametric equaliser (and in the case of the Helix system the graphic and dynamic sections too) we have a manual control for the Q of the filter, and this allows us to set any Q that we require. In general, high-Q, narrow filters are used for notching out problem frequencies without affecting the programme material too much. While gentler low-Q filters are useful for adjusting the tonal balance. In the case of graphic equalisers there is another issue - that of interaction between adjacent bands. In general, 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.

So far so simple - but why the different types? This is due to the way in which the Q of the filter varies (or not) when the gain control is adjusted. There are three modes available in the Helix system, which we term Proportional, Constant, and Symmetrical Q.

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