Handbook for Sound Engineers

Consoles 903

with side chain time constants optimized for that band
of frequencies; this allows the highs to be optimally
treated with short time constants without compromising
the other sections, and so on. The bands are recombined
and passed through an envelope limiter, which only
needs to catch a limited dynamic range and so has little
effect on the path and the overall sound. Two bands is
enough to remove the pumping effect of the usually
energy-intensive bass-modulating higher frequencies;
three bands allow for better time-constants for the
all-important mid range to be established without
compromising the high and low frequencies; more
bands, say five or six, allow considerable program
density (translation: loudness) to be built up while
retaining musicality.
This is a very common technique in radio broad-
casting airchain processing, and allows for better or
worse far deeper processing than possible with broad-
band units and totally avoids side chain pumping effects,
where typically heavy bass modulates the mids and
highs content. It is usual for there to be a number of
multiband stages, preceded by broadband AGC and
succeeded by broadband limiting/clipping. The first
multiband section (five bands is common) being
compression and perhaps multiband AGC, feeds a
second section of multiband limiting (31 bands of
limiting is not unknown!). Needless to say, such devices
can be quite an entertainment to set up; indeed, a whole
subindustry of processor witchcraft has evolved in radio.
Adjusted well, these units can sound startlingly good
(and loud). In corollary, they are far easier to make
sound truly dire. Unfortunately, the multiband tech-
nique, either using discrete units such as air-chain
processors or virtually as software plug-ins to audio
workstations and digital consoles, has found its way
into music production. The results have rarely been
beneficial.

25.12.1.6 Active Release Time Constants

Passively discharging the side-chain reservoir capacitor

with a resistor is not necessarily the best way of going

about things. Looking at Fig. 25-81A shows that the

initial discharge rate is considerably faster than that

farther along in time. With a gain control element (i.e., a

Voltage Controlled Amplifier/Attenuator—VCA)

having a linear control voltage to dB attenuation charac-

teristic, the gain reduction on release would die away

very quickly initially and steadily bottom out. This is

bad news since a longer than necessary release time

constant would need to be applied to preserve adequate

low-frequency distortion. If the reservoir capacitor is

discharged linearly as in Fig. 25-81B by a

constant-current source instead of by a straight resistor

(example in Fig. 25-81C), a tidy linear dB attenuation

release versus time characteristic ensues; less release

time need be wound in for similar LF distortion.

This can be taken a step further. Some gain-control

elements with logarithmic (transistors) or square-law

(FET) control voltage characteristics, for example, can

be made to work with a passive release system to give a

pretty good approximation of linear dB/time release.

Adding constant-current discharge to one of these

circuits gives a slow discharge initially (i.e., good low

frequency distortion) with a more rapid tailoff, Fig.

25-81C, which removes unnecessary gain reduction

quicker than any other arrangement yet. On program

material this works very well, also serving to reduce

pumping and suck outs from transients.

25.12.1.7 Hold or Hang Time

Given active discharge with a constant current source, it

is always possible to turn the discharge path off. This

has the effect of freezing the attenuation at the instant

Figure 25-80. Multiband signal processing.

Threshold or

Filters depth controls

Limiter

Limiter

Limiter

Mix

Overall

Limiter

Hi

Mid

Lo

Input