Handbook for Sound Engineers

Consoles 905

peaks are removed (brutally by a clipper or more subtly

by a timed processor with short time constants), the

average transmitted level can be increased correspond-

ingly. The shorter the time constants, the more apparent

loudness can be squeezed out. Usually, though, this is at

the expense of quality.

Here belies the principal reason broadcasters like

dynamics processing—the louder they seem on the air,

the more listeners are attracted to the station. To this end

excruciating amounts of gain reduction are common

on-air in radio. It’s a well-known effect that something

that sounds louder—even marginally—is perceived as

sounding better, at least in the short term.

It is also a strong reason compression is prevalent in

individual recording channels within a console; each

sound can be made not only more controlled in level,

which helps balancing, but also denser and more solid

sounding. The downside is that it’s so easy to squash

vitality out of a sound, trading liveliness and depth for

something more up front but ultimately less interesting.

25.12.2 Gating

Gating is to a degree the inverse of limiting. It is the
removal of an output signal unless it is of a sufficient
strength; in other words, if the input signal is above a
threshold level it is permitted to pass, but if it falls
below the threshold it is attenuated.
Its purpose is usually to remove or reduce in level a
signal when it is no longer usefully contributing to a
mix, remove noise in between wanted sections of
program and to generally act as an automatic mute. A
true gate totally removes the undesired signal but in
practice—for noise reduction in particular—a lesser
amount of attenuation is invoked; this is set by a control
and indicator called depth or maybe just attenuation.
Gentle amounts of depth make the operation of a gate
far less obvious together with the benefit that there is
less intermodulation distortion if the gain is asked to
change through less of a range.
The gate attack or wake-up time is generally adjust-
able and determines how quickly the gate opens in
response to a signal tripping the threshold. It is usually
set very fast, though, such that none of the leading edge
of the signal is missed. The hold time (if there is one
available, usually) determines how long the gate
remains open after the signal drops below the threshold
and the release or decay time sets how quickly the atten-
uation returns. That these are a direct parallel to their
behavior in the limiter is no accident; nearly all
dynamics processing sections carry these controls. It
only needs to be remembered with a gate that the attack

time has to do with how quickly attenuation is removed

rather than applied as is the case with a limiter.

The ranges of time-constant values are typically

similar to those for a limiter, but the threshold range can

extend from 0 dBu (or even as high as +20 dBu in some

cases) down to about 40 dBu or below. The higher

thresholds are mostly for key triggering while the low

extremes are for noise reduction. Automuting settings

are somewhat critical, needing to be above the general

background level yet below the desired signal’s typical

level; there is always some tuning to be done, but

figures of 10 dBu to 20 dBu are typical. Depth can be

adjustable between 0 dB and 40 dB attenuation (some

manufacturers optimistically state infinity).

Practical uses include automatic microphone muting

(backup singers), spill removal (e.g., a snare drum

microphone is usually gated so that when the snare isn’t

actually being hit, the microphone isn’t picking up the

rest of the kit), and noise reduction (just enough gating

applied to tape track returns to subdue tape hiss or air

conditioning rumble). In all cases the parameters are set

up to be as unobtrusive as possible. These vary from

lightning fast attack and decay on a snare drum to fairly

leisurely ramps in noise reduction.

In addition to the hold, or hang, time which prevents

the gate from chattering on a marginal signal, an addi-

tional tool to prevent falsing is hysteresis between the

signal level necessary to open the gate (open threshold)

and that below which the gate considers the signal to

have gone away (close threshold); this hysteresis (a few

dB) is generally concealed from the operator.

25.12.2.1 Gating Feed-Forward Side Chain

Naturally, a gate cannot possibly operate with its side

chain taken from the amplifier output as is the case with

the feedback limiters described earlier—it would never

open. It has to sense prior to the attenuator in the signal

chain. This arrangement is called feed-forward side-

chain sensing and is the prevalent method of generating

control voltages in today’s dynamics processors. Fig.

25-82 shows a typical gate circuit using this method; the

input signal as well as going to the attenuator hits a vari-

able gain amplifier, which determines the threshold. The

more gain in the amplifier, the sooner the detector

threshold is reached. Following the threshold

detector—which is in this case a comparator type yes/no

level sensor—are the various time constants. Depth is

controlled by placing a limit on the amount of attenua-

tion possible.