Handbook for Sound Engineers

Consoles 969

output up as much as is necessary (the attenuator
becomes a gain stage) and avoid the shifts entirely.
EQ boost is achieved by adding in filter; cut is done
by subtracting it away—a negative coefficient is thrown
at the post filter attenuator instead of a positive one.
There is a non obvious criterion for cut coefficients—as
one cuts, the effective Q of the EQ responder tends to
sharpen; the frequency response of this arrangement at,
say, 12 dB of cut is not complementary to that at 12 dB
of boost; one needs to relate and modify the filter Q
with cut level in order to take this into account and
retain customary lift/cut symmetry.

Multiple sections of parametric EQ can be and
usually are simply cascaded, although emulation of
many classic analog designs has been better served by
running the multiple filters in parallel and then adding
their gained results all together with the straight signal.
The band interactions are entirely different, offensive to
a tidy mind, but far closer to the truth!
Given that most parametric EQs use bandpass filters
only (at a push even shelving filters can be faked
reasonably well using such) and that, as we’ve seen,
bandpass filters have their b1 coefficient always at zero,
it can make sense not to perform that multiply at all,

thus saving data fetches and a multiply. Additionally,

since the b0 and b2 coefficients are simply inverse of

each other, only one need be sent from the host

processor to the DSP, the inversion being simply

achieved internally. This is welcome streamlining of the

processing.

25.21.4 Shelving EQ

Real shelving can be achieved by using a full biquad in

the EQ (as opposed to the simplified bandpass-only

variety shown) with low-order high-pass or low-pass

filter coefficient sets, or an even simpler structure as in

Fig. 25-140. Much greater than a single-order response

in the filter tends toward a frequency response with a

“phase-bounce” in it near the turnover frequency, gener-

ally considered undesirable (except perhaps when one is

being very picky emulating a Baxandall). The arrange-

ment shown is a shelving EQ using very short filters.

Advantage is taken of the fact that with single-order

filters one can very easily create a high-pass filter

merely by subtracting away a low-pass from a straight

signal.

Figure 25-138. Cascaded DF 1 Biquads, sharing delay lines.

Figure 25-139. A bandpass parametric EQ stage.

X +

X

A2

X

X X

X +

X

A

X

X X

Input

ID 1

ID 2

B¾

B1

B2

B¾

B1

B2

A1 A1

A2

OD 1

(ID 1)

OD 2

(ID 2)

OD 1

OD 2

Output

12 dB

Direct signal

* *

x0.25

X

X

Filtered

signal

Direct/filter

summer

B0

Shared

Input

delay 1

+

+

X

Output

delay 1

Levelcoefficient

( + or )

ASL

+6 dB

ASL *

+6 dB

Output

delay 2

Input

delay 2^ BO

X

A2

X

A1

X