Handbook for Sound Engineers

Consoles 875

The limiter side chain is true symmetrical peak
detecting, selectable to be able to pick off from either
the high-pass filter output (as an input limiter) or from
after the post-equalization breakpoint downstream (as a
channel limiter). A positive-going and a negative-going
level-detecting comparator are adjustable between clip
detection (dB before system head room) or
program level (nominally) +8 dBu.
A bicolor LED blinks red to indicate limiting in
action, and it blinks green when the limiter is disabled
to signify that the selected level (clip or program) is
being reached or exceeded. In this indicate mode, the
limiter integration time constant is deliberately short-
ened to make the green flashing similar in character to
the red flashing in limit.
The difference is due to the nature of servo loops, of
which a feedback limiter such as this is an example. In
limit, the loop is self-regulating, the gain-control
element holding back the audio level so that it’s just
tickling and topping up the side chain. In indicate, the
loop is broken, and there is no such regulation. The
green light stays on whenever the threshold is exceeded
and tends to hang on while the time-constant capacitor
discharges. With even a minor overdrive, this hangover
could extend for quite a few seconds; hence, the short-
ened time constant.
This limiter is not subtle. The comparators deliver a
full-sized, power-supply wallop to the integrator upon
threshold, softened a bit by the attack preset in conjunc-
tion with the output impedance of the comparators. This
rather unusual approach is to help wake up the photore-
sistor that has a relatively leisurely response time. The
combination can be adjusted to be slow enough such that
it doesn’t clip yet fast enough to prevent an audible snap.
Overshoot is generally within 1 dB on normal program,
given a release time long enough to prevent pumping.
As a rough guide, if it’s intended to use such a
limiter for sporadic transient protection, it’s best to aim
for short attack and release times, bearing in mind that
such settings will behave more as a clipper to the lower
frequencies. For continual effect use, longer time
constants will be less gritting and more buoyant. This
side-chain arrangement certainly behaves differently
from more conventional FET or voltage-controlled
amplifier (VCA) linear proportional systems and needs
a slightly different approach in setting up.

25.10.7.7 High-Pass Filters

Constructed around the line output amplifier of the front

end in Fig. 25-55 is a second-order high-pass filter. It is

a completely ordinary Sallen-Key type filter, arranged to

use a dual-gang equal value potentiometer to sweep the

3 dB down turnover frequency from between 20 Hz and

250 Hz. A click-stop switch at the low-frequency end

(counterclockwise) negates the filter, replacing it with a

very large time-constant, single-order dc decoupler.

These are both tied to reference in order to minimize

clicks. Fortunately, the BiFET op-amp in the filter

barely uses any input bias current, so there is little devel-

oped offset voltage from that source to worry about.

Being an equal-value filter, the Q or turnover would

be very lazy indeed if the feedback were not elevated in

level to compensate for the upset resistor ratio. Here a

compromise is struck. A low Q gives a very gentle roll-

off (which is sonically good), and high Q results in a

much more rapid attenuation beyond the cutoff

frequency at the expense of a more disturbed in-band

frequency response—pronounced bumps—and frantic

temporal and phase responses exhibited as ringing and

smeared transients. Luckily, the majority of

control-room monitors exhibit far worse characteristics

at the low-frequency end.

A maximally flat response midway between the two

extremes is chosen by an appropriate amount of

elevated feedback (around 4 dB). This gain is taken

across the filter as a whole, with the second stage of the

microphone amplifier arranged to sustain a 4 dB loss to

compensate. It all works out in the end, with no

compromise of head room. With minimum gain set,

there is still unity electronic gain front to back. An

added convenience of gain is that it provides a better

chance of shoring up feedback phase margin, which is

quite important in a line amp that may have to drive a

lot of heavily capacitative cable. Also, it provides yet

another single-order low-pass pole to help smooth out

the high-frequency resonance of the microphone

transformer.

25.11 Equalizers and Equalization

The term equalization is strictly a misnomer. It was

originally utilized to describe the flattening and general

correction of the response of systems in which by a

matter of course or design had deviated from the orig-

inal shape (e.g., telephone lines and analog tape

machines). (In the latter case, equalization refers to the

adjustment tweaks to the preemphasis and deemphasis

curves—not necessarily the curves themselves.)

In search of a name for the deliberate modification of

amplitude and phase versus frequency responses for taste