SIGMA EQ Station - Manual (Page 17)

15
GRAPHICAL EQ TYPES IN THE EQ STATION
Q mapping: Constant or
progressive Q, and which?
Two single-band EQs with the same center-frequencies
and the same slider settings may differ greatly depending
on the designer's choice of Q as a function of Gain setting,
as the following 3 examples will show:
Figure 7 a/b/c - Three different examples of Q mapping
Naturally there is an infinite number of possible Q
mappings, the examples shown are rather extreme.
Structure dependent multi-band
gain build-up
Even with identical center frequencies, gain settings and
Qs, the responses of two graphical equalizers may differ
significantly when multiple bands are applied. This is due to
the structure dependent multi-band gain build-up. We will
look at 3 different implementation structures with different
multi-band gain build-up properties.
Parallel BP feed-forward/feed-back structure
This structure which has been used in the TC 1128 and its
EQ Station digital twin produces relatively little gain build-
up. Since it is based entirely on summation of BP filtered
signals, the responses of individual BP filters are added in
the absolute domain, not the dB domain.
Figure 8 - Three-band boost (i.e. feed-forward only)
extract of Parallel BP feed-forward/feed-back EQ
structure.
Figure 9 - Three-band boost extract of Cascade-of-
biquads EQ structure.
Note that the above used " Input + gain•BP" structure from
Figure 2 can be contained in a single bi-quadratic filter,
a.k.a. 'a biquad'. This structure is used in EQ Station's
'Classic27' algorithm.