Application Note 2: What Do We Mean By All These Q Types - Klark Teknik HELIX SYSTEM DN9344 Operator's Manual

Application note 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
centre _ frequency
as ,
bandwidth
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, and 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.
lark Teknik
+12
+10
+8
+6
+4
+2
d
+0
B
u
2
4
6
8
10
12
20
.
In the example above, the filter is centred on 1 kHz, the lower 3dB point is at approximately 800 Hz, and the
upper one is at approximately 1.25 kHz.
This filter therefore 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.
where the bandwidth (in Hz) is the range of frequencies affected by the filter.
f=1.00kHz Q=2.2 Level= +12dB
50 100 200
1000
=
2
-
( 1250 800 )
3 dB
Bandwidth
500 1k 2k
Hz
2 .
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