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Manual-2

Dynamics 101 Revisited

Introduction

Compressors and limiters are in the business of automatically

controlling the volume or dynamics of sound, just like your

hand on the fader, or the fat man dancing in front of the mid-

range cabinet. Used wisely, often in conjunction with each other

or with equalization or ltering, dynamics processors improve

the intelligibility of voice and the subjective eect of music.

However, in the wrong hands they can sound terrible.

Heavy compression (low threshold and a high ratio) often

sounds nasty. e timbre of the sound changes, becoming hard

and closed and not nearly as sweet and open as the sounds

you envisioned when you got into this business. On the other

hand, attack times optimized for pleasant compression may

not track initial transients quick enough. In addition, pump-

ing and breathing may accompany heavy compression, i.e., the

background noise rises way out of proportion to the foreground

sound as the compressor releases. Result: it just does not sound

good.

erefore, no matter how you slice it, compressors and limit-

ers are just fancy electronic volume controls. ink of them as an

extra hand on a control, turning the volume down and turning

it back up again. Luckily, this electronic hand is quick and ac-

curate, but it is just adjusting a volume control.

Compression

A compressor, when the input signal reaches the level set by the

threshold control, begins turning down the signal by an amount

set by the ratio control. Modern compressors make the loud sig-

nals quieter, but do not make the quiet parts louder. (However,

by keeping the loud signals under control, you can turn up the

output level, which will make the quiet parts louder along with

the rest of the signal.)

Primary uses are 1) reduce dynamic range of vocalists and

other musical instruments that exceed the recording or repro-

duction capability; 2) prevent clipping and distortion in live

sound systems or recording chains; 3) smooth and balance an

instrument such as a bass guitar with wide dynamic range and

string-to-string level variations; 4) reduce vocal sibilance (de-

esser); 5) produce louder recordings for broadcast; and 6) even-

out paging variations due to dierent speakers in large systems.

Signal Path

A compressor has two internal paths: the signal and the side-

chain. e signal path is the route the main signal takes through

the unit: from the input circuits to the gain control section and

exits through the output circuits. e signal chain goes through

the “volume control” in the “hand on a control” analogy.

Side-chain

e side-chain is the hand that controls the volume. Side-chain

circuitry, also known as the detector, examines the input signal

and issues a control message to adjust the gain of the main signal

path.

Full-featured compressors, like the C4, oer both internal

and external side-chain options. When the internal option is

selected the compressor’s input signal feeds the detector. is

arrangement works well for most applications, and is especially

eective with the C4 due to the parametric EQ built into the

side-chain. e external side-chain allows any signal source, con-

nected to a dedicated input jack, to feed the detector, thereby de-

termining the compressor’s response. is external signal may be

a specially ltered version of the input signal, using an outboard

EQ for example, or it may be another signal altogether.

It is important that the side-chain signal is not heard. For

instance, if you added treble boost to the side-chain audio (either

by using the C4’s built-in PEQ or an outboard EQ), it would not

aect the high frequencies in the main signal path, but it would

cause them to cross the threshold sooner and more often. Large

peaks of treble would cause heavy compression with no compres-

sion at other times. is example is the basic de-esser, a circuit to

remove excess sibilance. (ere is a much more sophisticated and

eective de-esser built-into the C4, but more on that later.) With

a bass boost, you can make a de-thumper, and with a midrange

boost a de-nasaler.

ere are a number of parameters governing side-chain activ-

ity, but the four primary ones are reshold, Ratio, Attack time

and Release time.

INPUT OUTPUT

SIDE-CHAIN

BROADBAND

GAIN CONTROL

RMS

DETECTOR

ATTACK

GAIN COMPUTER

FILTER

THRESHOLD RELEASE

SCALE

RATIO

GAINFREQ BW

SIDE-CHAIN EQ

Figure 2. Frequency sensitive compressor block diagram.

The Ratio controls

the amount of gain

reduction above

the Threshold

The Limiter Threshold

controls the hinge point

of a Ratio set at

∞

The Compressor Threshold

controls the hinge point

Input Level dBu

Output Level dBu

No processing — all Ratios set at 1:1

Compressor Ratio = 2:1

Limiter =

∞

-30

-40

-50

-60

-20

-10

+10

+20

-20

-10

-40

-30

+20

+10

dBu

COMP THRESHOLD

n:1

1.5

RATIO

-20

-10

-40

-30

+22

+10

dBu

LIMIT THRESHOLD

COMPRESSOR LIMITER

Figure 1. Compressor and Limiter functions

1 2 3 4 5 6 7 8 9 10 ... 14 15

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