Handbook for Sound Engineers

902 Chapter 25

changing and rapidly. They are independent of the kind
of device doing the attenuation, whether it is a humble
transistor or an expensive VCA and they are just as
obnoxious. By and large these dynamically induced
distortions subjectively far outweigh the steady-state
distortion characteristics of the devices. These only
become important if the circuit is to sit in a signal path
with little or no dynamics processing taking place. Once
things start moving one is as good or bad as the other
and the subjective quality of a unit is determined by
how well the various timings tailor around the program
material or how well it does so automatically.
Release settings are generally quoted in millisec-
onds or seconds, and sometimes as a decay rate such as
dB/ms or dB/s. In short, a dynamics processor lives or
dies by its side chain.

25.12.1.4 Compound Release Time Constants

The hole-punching problem of a big transient hitting a
limiter with a long release time constant can be attacked
in a couple of ways. If subtlety is none too great a crite-
rion—for instance, in an AM transmitter limiter where
There Shall Be No Overmodulation—it is always
possible to put a variation of the back-to-back diode
limiter on the output of a timed one, Fig. 25-79A. Set to
clip immediately above the normal operating output
level of the feedback limiter, it not only cast-iron stops
excessive output signal swing but also prevents the tran-
sient from entering the side chain and digging too big a
hole in the following audio.
The circuit in Fig. 25-79C is used extensively.
Instead of a single reservoir capacitor in the side chain
with one attack defining resistor and one release
defining resistor, Fig. 25-79B, a compound circuit can

be arranged, Fig. 25-79C. A small value resistor and

capacitor form additional, shorter attack and release

time constants working in conjunction with a slower set.

The extended attack and release times follow the

general loudness envelope of the program material

while the shorter ones, riding on the top take care of any

short-term discrepancies and transients, generally to the

tune of the top 5 dB or so of processing. If a

general-purpose, hands-off, no-tweaks limiter is needed,

this arrangement with carefully chosen values can work

very nicely and is the basis of some commercial

outboard limiters in the auto mode.

25.12.1.5 Multiband Processing

A complex but very effective side step to the inapt time

constant with frequency problem is shown diagrammat-

ically in Fig. 25-80. Here the input signal is split a

number of ways by frequency (in this case three, into

bass, mids, and highs). Each band passes into a limiter

Figure 25-78. Effect of long release times on the output sig-
nal when subject to a large transient.

Slight overshoot

Normal limited

program level Transient

Long level

recovery

Figure 25-79. A feedback limiter with a compound time

constant.

Vin

Vo

Variable

attenuator Amplifier

Rectifier

Transient

clamping

diodes or

zeners

A. Feedback limiter with output clamping.

Attack

Rectifier Attenuation

Release

Reservoir

capacitor

B. Simple attack/release sidechain.

Rectifier Attenuation

100 nF

1 MF

C. Compound time-constant side chain arrangement.

470 7

470 k 7

4.7 k 7