SIGMA EQ Station - Manual (Page 16)

14
GRAPHICAL EQ TYPES IN THE EQ STATION
Figure 4 - Asymmetry between '1 + BP´ and '1 - 0.5 BP'
and symmetry error.
But it's still not symmetrical! If we send the signal through
one of these one-band EQs set to +6dB and then through
another EQ set to -6dB, the result will be the dashed
(middle) curve in Figure 4 which deviates from neutral by
nearly 2dB!
In some EQ designs the ability to use the same BP filter
both for adding in boost cases and subtracting in cut cases
is considered a benefit, and the asymmetry is not
considered a problem. But we prefer symmetry, so that an
EQ with one setting is cancelled out perfectly by another
identical EQ with the opposite setting. To make the cut
curve (blue dotted in Figure 4) the exact mirror image of
the boost curve (black solid in Figure 4) we need to do one
of two things:
Restructure the circuit in Figure 2 from feed-forward to
feed-back topology (Figure 5).
Figure 5 - Single-band EQ in Cut-mode implemented
by BP negative feed-back.
- or
Reduce the Q of the BP filter by a factor.
Either of these methods solve the boost/cut symmetry
problem to mathematical perfection:
Figure 6 - Boost/cut symmetry problem solved
Now that we've chosen boost/cut-symmetric EQ filters, we
will discuss the remaining properties with only boost filters
as example.