Dynamics 101 Revisited; Compression; De-Essing - Rane C4 Operator's Manual

Quad compressor / limiter

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Operator's manual (12 pages)

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Dynamics 101 Revisited

Introduction

Compressors and limiters are in the business of automati-

cally 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 ﬁltering, dynamics processors improve

the intelligibility of voice and the subjective eﬀect of music.

However, in the wrong hands they can sound terrible.

Heavy compression (low threshold and a high ratio) often

sounds nasty. The 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.

Therefore, no matter how you slice it, compressors and limit-

ers are just fancy electronic volume controls. Think 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 signals 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 diﬀerent speakers in large systems.

Signal Path

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

chain. The 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. The signal chain goes through

the "volume control" in the "hand on a control" analogy.

Side-chain

The 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, oﬀer both internal

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

selected the compressor's input signal feeds the detector. This

arrangement works well for most applications, and is especially

Manual-2

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Figure 1. Compressor and Limiter functions

eﬀective with the C4 due to the parametric EQ built into the

side-chain. The 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. This external signal may be

a specially ﬁltered 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

aﬀect 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. This example is the basic de-esser, a circuit to

remove excess sibilance. (There is a much more sophisticated and

eﬀective 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.

There are a number of parameters governing side-chain activ-

ity, but the four primary ones are Threshold, Ratio, Attack time

and Release time.

De-essing

Contrary to popular belief, proper de-essing is not as simple

as placing a bandpass or high-shelf ﬁlter in the side-chain and

calling it done. True de-essing involves comparing the relative

diﬀerence between the troublesome sibilants ("sss" sounds) and

the overall broadband signal, then setting a threshold based on

this diﬀerence. Lucky for you, the C4's De-ess mode uses such

an arrangement.

When the C4 is set to De-ess, the average level of the

broadband signal (20 Hz to 20 kHz) is compared to the average

level of a bandpass ﬁlter as deﬁned by the PEQ frequency and

bandwidth controls. The Threshold setting deﬁnes the rela-

tive threshold, or diﬀerence, between broadband and bandpass

levels that results in compression of sibilants. Because de-essing

depends on the ratio of sibilant to broadband signal levels, it is

not aﬀected by the absolute signal level, allowing the De-esser

to maintain the correct ratio of broadband to sibilant material

regardless of signal level, as shown in Figure 2

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Table of Contents

Dynamics 101 Revisited; Compression; De-Essing - Rane C4 Operator's Manual

Dynamics 101 Revisited

Compression

De-essing

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