Handbook for Sound Engineers

898 Chapter 25

alone sparked by the AM radio loudness wars of the
’60s and ’70s; the deleterious effect will have future
musical historians scratching their heads.
Every recording and transmission medium has defi-
nite head room limitations, a maximum level beyond
which the signal just plain overloads, distorts, or
becomes a serious liability. AM radio overmodulation
not only sounds horrid but causes interfering splatter up
and down the dial, FM transmitter overdeviation causes
adjacent channel interference and runs the risk of distor-
tion in receiver discriminators; disk cutters can produce

grooves that run into each other (long after they become

unplayable by any normal pickup), PA loudspeakers fry,

tape saturates, distorts, and screams, and best of all

anything digital just plain runs out of bits and cracks up.

The answer? A device that senses when enough is

nearly enough and automatically reduces the source

level such that a proscribed output level is not trans-

gressed. This is a limiter.

Fig. 25-76A shows the all-time basic limiter—a pair

of back-to-back diodes. These clamp the input signal

from the source resistor to within their nonconductive

range; beyond 700 mV of either polarity one or other of

the diodes conducts, sawing off any excessive signal.

Brutal, but effective. The downside is gross distor-

tion—serious waveform modification is going on, prof-

ligate audible distortion products are generated. Fig.

25-76B shows the same idea with germanium diodes.

These tend to have a lower turn-on voltage

(200–300 mV) but a gentler knee, with the effect

shown. This sounds considerably less harsh—fewer

high-order distortion products are being created. In situ-

ations where ultimate signal quality is not necessary, but

increased signal density (translated: loud) is required,

these clipping circuits work like a charm; communica-

tion circuits often use this technique to saw the top

10 dB or so peaks off speech and thus gain a nearly

corresponding degree of increased apparent loudness.

The trick is to filter away or contain and control the

resulting distortion products such that they become less

agonizing, while retaining the high signal density clip-

ping affords. Such is meat for a whole other saga.

The ideal circuit is one that knows its input is going

over the top and can reduce its gain such that the signal

is left relatively undistorted but as loud as it can be

within the given constraints. Fig. 25-76C is a block

diagram of such a device. The side chain circuit is a

tripwire in this instance; if the amplifier output exceeds

a stipulated level (just below what the destination can

handle) the side chain develops a control signal that

Figure 25-72. Frequency response of the low-frequency
section of Fig. 25-71 (control at maximum gain).

Figure 25-73. Characteristics of the high-frequency section
of Fig. 25-71 (boost-cut control at maximum boost).

Shelving response when capacitor is shorted

+20

dB

+10

35 Hz

min

200 Hz

max

100 1k 10k

Frequency–Hz

+20

+10

1k 2k 5k 10k 20k

Min

5 kHz10 kHz

Max

20 kHz

Frequency–Hz

Figure 25-74. Typical insert points for dynamic processing.

1 Preequalizer

2 Postequalizer (prefader)

3 Post fader (Rare)

4 Group

Mic

amp

1

Equalizer

2 3

Main

fader

Pan

Mix buses 4L

Pan

Mix

amps

R

Group

fader

4R

L L

Line

amps Outputs

Left

RRRight

High pass

filter