Handbook for Sound Engineers

904 Chapter 25

the discharge is removed; if there is no discharge path,
the reservoir can’t discharge, the control voltage
remains static, as, consequently, does the attenuation.
Recent refinements to dynamics include this feature; if
the side chain is attacking in response to an increasing
signal over the threshold, the constant-current discharge
is turned off automatically. It remains turned off for a
preset amount of time after the attacking has ceased and
when the circuit would ordinarily be releasing. Instead,
the attenuation remains static for that preset time after
which the discharge is reinstituted and normal release
decay occurs, Fig. 25-81D. The advantages are straight-
forward; there is no release time-constant-related distor-
tion in the period of time the attenuation is frozen and
the subsequent release can be independently set to
return gain as quickly or as slowly as desired. Tailoring
the attack, hold-and-release times around a given
program source can render processing virtually trans-
parent in many cases. Hold or hang time is quoted
usually as a direct time in milliseconds or seconds.

25.12.1.8 Limiting and Compression for Effect

The principal creative purpose for limiting or compres-
sion (as opposed to the precautionary and
damage-control functions outlined earlier) is to make
things loud. Suitable parameters can also imbue
low-frequency chunkiness or weight on the sound.
Given a certain maximum head room level in a trans-
mission or recording medium, it is often desirable to
increase the program density or reduce the dynamic
range. A case in point (again) is broadcasting. Their
legitimate purpose for compression is to render audible
portions of program that may otherwise be too quiet and
buried in ambient noise, interference, or static at the
receiver end. Automobiles have a notoriously small
dynamic window between receiver output capability
and cabin noise. A compressor (or usually air chain
processor) is set to give sufficiently high output for a
quiet program and to automatically reduce the gain
when it gets louder.
An interesting subjective side effect of this lies in
psychoacoustics; if the ear hears something it knows to
be quiet ordinarily at a certain volume, then a sound that
it knows to be louder still seems louder even though a
limiter may be compressing the two signals to the same
level. A classic example is with reverberation
tails—these are one of many means by which we
subconsciously gauge relative loudness; if the original
sound spawning the reverb tail is compressed to be
closer to the level of the tail, the whole overall sound
seems louder.

Normal program material consists of quite high tran-

sients and peaks above the mean average level. If these

Figure 25-81. Active release side-chains.

D. Attenuation versus time constant; current discharge with

nonlinear attenuation/control voltage characteristics.

A. Attenuation versus time for simple capacitive

discharge (with VCA).

B. Constant current discharges using current mirror.

C. Attenuation versus time for constant current

discharge (with VCA).

E. Attenuation versus time showing deliberately

introduced hold or hang time prior to release.

Attenuation

Attenuation

Attenuation

Attenuation

Rapid initial

recovery

Time

Linear

recovery

Time

Hold period

Attack

ramp

Recovery

ramp

Quasi hold

characteristic

Rapid disposal of

unwanted attenuation

Constant current

set resistor

+15 V Removing potential

turns discharge off

Attenuation

Attack

Reservoir

capacitor

Identical

transistors

Time

Time