SIGMA EQ Station - Manual (Page 15)

13
GRAPHICAL EQ TYPES IN THE EQ STATION
Introduction
A simple question
Why are graphical EQs so different? Even with the same
gain range and center frequencies they sound different and
respond differently to user settings. Why?
This section describes the fundamental differences of the 3
graphical EQ structures provided within the EQ station.
To fully utilize the Graphic EQ section of the EQ station a
basic understanding of these structures and how they
influence the processed signal, is necessary.
Figure 2 - Block diagram of single-band BP-feed-
forward EQ where Gain is slider setting.
Boost/cut symmetry
Suppose we move the slider of our 1-band EQ to -6dB?
Intuitively this ought to be just a matter of subtracting the
same amount of BP signal rather than adding it?
Not quite so:
Figure 3 - Asymmetry between '1 + BP' and '1 - BP'
This gross asymmetry is due to the non-linearity of the dB
measure (and to some extent human hearing): Doubling
the gain amounts to +6dB, while reducing it to 0 amounts
to minus infinity dB. To obtain -6dB at fc we need to
subtract only half, leaving one half, or -6dB:
What does an equalizer do?
A graphical EQ in neutral position should act just like a
signal buffer within the audio frequency range, albeit
possibly with a short delay for digital implementations.
When one slider, e.g. the 1kHz slider, is moved from 0dB
to - say - +6dB, the EQ's gain at 1 kHz should increase to
6dB corresponding to a gain doubling. The total magnitude
response in this situation may look like this (Figure 1):
Figure 1 - Magnitude response of typical 1 kHz EQ
band set to +6dB with Q = 1
In principle the EQ achieves this effect by adding a 1-kHz
bandpass filtered version of the signal to the signal itself as
shown in block diagram form in Figure 2.