Rane C4 User manual

Manual-1
C4
QUAD COMPRESSOR / LIMITER
Quick Start
Rejected Rane advertising slogan #1743: “C4. It’ll blow your mind!”
We’re not entirely sure why the marketing department didn’t like the idea, but be assured: unlike its explosive material namesake,
the C4 Quad Compressor will not blow up. Heck, it won’t even spark or sizzle under normal circumstances. e only thing smokin’
will be your mix, thanks to the C4’s smooth compression, precise dynamic EQ and ultra-fast limiting.
Each of the C4’s four channels offers a full-featured compressor with selectable Compressor and Dynamic EQ modes. In ad-
dition, an oversampled, brick wall LIMITER is included for catching signal peaks. COMP, THRESHOLD, RATIO, ATTACK,
RELEASE and KNEE give full control over the audio envelope. e compressor is toggled on and off using the ACTIVE/BYPASS
switch; the Limiter is engaged at all times and is not affected by this switch. Limiter operation is completely independent of the com-
pressor with its own LIMIT THRESHOLD control and Headroom meter. Channels 1 & 2, and 3 & 4 can be LINKed to maintain
a proper left/right stereo image, with channels 1 and 3 acting as the MASTERs, respectively.
For the ultimate in dynamics processing flexibility, the C4 includes both internal and external side-chain functions, with a full
PEQ (parametric EQ) section included for good measure. External side-chain mode is automatically selected whenever a connector is
plugged into the rear panel SIDE-CHAIN jack. Selecting LISTEN mode routes the side-chain audio to the channel output, allow-
ing you to hear the effect of PEQ adjustments when doing frequency-sensitive compression or de-essing. If you'd prefer to not use the
side-chain PEQ, simply set the PEQ GAIN to 0 dB.
Setting up the C4 is a breeze thanks to the fast, accurate side-chain and Gain Reduction combination meter. For users unfamiliar
with de-essing, the tpointer on each control indicates a recommended setting for this specialized application. e Auto attack and
release mode speeds up sound check, and the Tips and Tricks on page Manual-8 serve as a handy reference guide for getting started
with compression. ank you for taking the time to read this Quick Start. We now return you to your regularly scheduled sound
check or recording session.
OPERATORS MANUAL
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C4
SIDE-CHAIN
RELEASEATTACK KNEE
GAIN FREQUENCYBANDWIDTH
LIMIT THRESHOLD
RATIO GAIN COMP THRESHOLD
1 2
LINK
Auto
Auto
SoftHard
dBr Reduction dB
ListenComp
LIMIT
BYPASS ACTIVE
PEQ
SIDE-CHAIN
RELEASEATTACK KNEE
GAIN FREQUENCYBANDWIDTH
LIMIT THRESHOLD
RATIO GAIN COMP THRESHOLD
Auto
Auto
SoftHard
dBr Reduction dB
Listen Dynamic
EQ
Comp
LIMIT
BYPASS ACTIVE
SIDE-CHAIN
RELEASEATTACK KNEE
GAIN FREQUENCYBANDWIDTH
LIMIT THRESHOLD
RATIO GAIN COMP THRESHOLD
3 4
LINK
Auto
Auto
SoftHard
dBr Reduction dB
Listen Dynamic
EQ
Comp
LIMIT
BYPASS ACTIVE
SIDE-CHAIN
RELEASEATTACK KNEE
GAINFREQUENCYBANDWIDTH
LIMIT THRESHOLD
RATIO GAIN COMP THRESHOLD
Auto
Auto
SoftHard
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Listen Dynamic
EQ
Comp
LIMIT
BYPASS ACTIVE
PEQ PEQ PEQ
POWER
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QUAD
COMPRESSOR
WEAR PARTS: This product contains no wear parts.
Contents
Quick Start....................................................................1
Dynamics 101 Revisited ............................................2
Compression.....................................................................2
Dynamic EQ ......................................................................3
De-essing ......................................................................3
Dynamic EQ Applications .........................................4
Peak Limiting.....................................................................5
Making Connections..................................................6
Front Panel Controls .................................................6
Side-chain and Gain Reduction Meters.....................6
MASTER / SLAVE Stereo Link.....................................7
ACTIVE / BYPASS ..........................................................7
COMPressor THRESHOLD, RATIO and GAIN......8
ATTACK, RELEASE and KNEE....................................8
SIDE-CHAIN Mode and Listen ....................................9
SIDE-CHAIN Parametric EQ ........................................9
Limit Threshold and Headroom Meter......................10
Tips and Tricks...........................................................10

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 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. 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 different 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, offer 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
effective 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 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. is example is the basic de-esser, a circuit to
remove excess sibilance. (ere 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.
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.
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Figure 1. Compressor and Limiter functions

Manual-3
Dynamic EQ
Dynamic EQ differs from the forms of compression listed above
in that it dynamically controls the boost/cut of a parametric
filter rather than broadband frequency gain. e basic dynamic
EQ uses a bandpass filter in the side-chain with variable center
frequency and bandwidth. e side-chain detector is sensitive
only to the passband frequencies. A parametric filter with match-
ing bandwidth and center frequency is placed in the main signal
path and the boost/cut of the filter is controlled the same way a
broadband compressor boosts or cuts broadband gain.
Relative Threshold Dynamic EQ
Relative reshold Dynamic EQ is a special form of dynamic
EQ where the rms level of the bandpass signal in the side-chain
is compared to the rms level of the broadband signal. e differ-
ence between the bandpass and broadband levels is compared to
the threshold rather than the absolute rms value of the bandpass
signal. e advantage of this type of dynamic EQ is that the
relative amplitude of a band of frequencies, as compared to the
broadband level, is maintained regardless of broadband ampli-
tude. e typical topology is shown in Figure 3.
INPUT OUTPUT
SIDE-CHAIN
ATTACK
GAIN COMPUTER
FILTER
THRESHOLD RELEASE
SCALE
RATIO
FREQ BW
BANDPASS BROADBAND
BANDPASS
RMS
DETECTOR
PARAMETRIC EQ
BROADBAND
RMS
DETECTOR
Figure 3. Relative threshold dynamic EQ block diagram.
De-essers
De-essing limits or controls the sibilant content of speech.
Sibilance produces a hissing sound. English sibilant speech
sounds are (s), (sh), (z), or (zh). De-essing is often confused as
a type of dynamics processor. It’s actually a specific application
that is accomplished using many different types of dynamics
processors. And contrary to popular belief, successful de-essing
is not as simple as placing a bandpass or treble-boost filter in the
side-chain and calling it done. Frequency Sensitive Compression,
Split-Band Compression, Dynamic EQ and Relative reshold
Dynamic EQ are all used for de-essing.
True de-essing involves comparing the relative difference
between the troublesome sibilants and the overall broadband
signal, then setting a threshold based on this difference, there-
fore it is our experience that Relative reshold Dynamic EQ
(as described above) is the best dynamics processor for this task
as it is able to maintain proper sibilant to non-sibilant balance
regardless of level.
A good de-esser looks at the average level of the broadband
signal (20 Hz to 20 kHz) and compares it to the average level of
a bandpass filter in the side-chain. e threshold setting defines
the relative threshold, or difference, between broadband and
bandpass levels, which result 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 4.
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Increasing Level De-essing
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Figure 4. The C4’s De-essing performance remains consistent with
varying input levels.
is means that the de-essing performance is consistent and
predictable, regardless of how loud or quiet the singer/talker is.
Taming sibilance when the talker is quiet is just as important as
when the talker is at a fevered pitch.
Figure 5 shows what happens using a primitive de-esser
with a side-chain EQ. Sibilance during loud passages is attenu-
ated, but there is no gain reduction during quiet passages, even
though there may still be a significant amount of “sss” in the
person’s voice. For a given threshold, this often results in an
overly aggressive effect during the loud choruses, and a com-
pletely ineffective result during the hissy, whispered verses.
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Increasing Level
De-essing
No De-essing
Figure 5. Primitive de-esser with a simple side-chain.
Varying input levels adversely aects de-essing.

Manual-4
Dynamic EQ Applications
Auto-Correct Microphone Proximity Effect
Use Dynamic EQ to automatically correct for timbre changes
due to the low frequency boost caused by the proximity effect
of cardioid microphones (see Figure 6), which occurs when a
singer/speaker does not remain a consistent distance from the
microphone.
Two opposite applications share this problem:
1. e first occurs when the mic is located far enough from the
person that the proximity effect has no bearing (typically
a podium situation), then the speaker leans in closer to the
microphone causing a low frequency boost.
Start with a 100 – 250 Hz center frequency, a bandwidth of 2
octaves and a ratio of 3:1. Set the threshold high enough so when
the person is the normal distance there is no response, and only
when they move closer to the mic does the threshold kick the
filter into action.
2. e second application compensates for the loss of proximity
effect as the person moves the microphone away from their
mouth; typical of most hand held mics.
Solve this problem by perform the opposite routine as the po-
dium mic example above. Have the person hold the microphone
at the farthest distance from their mouth that can occur. Set the
Dynamic EQ so that it is just below threshold. Once they move
the mic closer to their mouth it will reduce the low frequency
boost. Since we have become accustomed to hearing the proxim-
ity effect try low ratios so that the tonality change is slight, but
remains more consistent than without the Dynamic EQ. Use
appropriate EQ on the input channel to add back in any missing
warmth – this warms the signal while the Dynamic EQ keeps
the tonality consistent.
Typical starting points are a frequency of 100 – 250 Hz, a band-
width of 2.0 octaves and a ratio of 2:1 depending on the desired
change in tonality.
Auto-EQ Changing Sound Sources
A great example is evening out the tone, or timbre, of a guitar
amplifier. Using two channels set up for different tones is very
common to switch between a rhythm tone and a lead tone. Of-
ten the musician sets the lead tone brighter than the rhythm tone
so it cuts through better. e problem comes when the sound
system amplifies this all out of proportion, resulting in too much
energy around 2 kHz – 4 kHz (a really nasty frequency range
due to the ear’s maximum sensitivity to this octave).
Setting the Dynamic EQ for a center frequency of 3 kHz and
a bandwidth of about 1 octave cleans this up. Set the threshold
high enough so that during normal playing nothing is happen-
ing. If the device uses relative threshold, once the lead channel
is used it will automatically see the change in timbre and apply
the Dynamic EQ to reduce the excess energy at 3 kHz relative to
the rest of the audio spectrum. You can also use this technique
to make guitar sounds “thick” and “chunky” without being over-
bearing by using the EQ section set in the 200 Hz range as well.
Improve Vocals
It is common for female singers to have a wide tonality swing
when shifting from a quiet breathy passage to a loud crescendo.
e voice sounds warm and pleasant during the quiet passage
but shows a predominance of frequencies in the 1.2 kHz range
for the loud crescendo. is is exaggerated when the singer
moves the microphone away from her mouth thereby removing
the warming character of the microphone’s proximity effect and
adds to the naturally occurring peak in this frequency range.
To fix this, simply set the EQ section to the problem frequency
(typically 1.2 kHz) and set the threshold so that the compres-
sor only kicks in when she sings the loud passages. Use the ratio
control to determine exactly how much of the original tone
change remains – low ratios leave more change while high ratios
clamp down hard and allow very little change.
Create Radical Sounds
Dynamic EQ lets the user create sounds that change tone with
level, or at extreme settings, which allows the creation of radical
sounds based on the threshold, attack & release times.
Experiment – the results will amaze you.
Figure 6.MicrophoneProximityEect (courtesyofShure Inc.)Low
frequencies riseasthe soundsourcegetscloser toa cardioidmicrophone.

Manual-5
Output
Limit Threshold
Input
Output
Limit Threshold
Peak Limiting
A limiter is just a compressor with a really high ratio, right?
Well…not exactly. Although the two devices use similar termi-
nology (reshold, Attack, Release, and so on), they serve two
completely different purposes and therefore operate in different
manners. Fundamentally, a compressor uses an rms detector to
keep the average level of a signal under control, while a limiter
uses a peak detector to act on the instantaneous level of the sig-
nal. Compare Figure 2 to Figure 7.
Primary uses of limiters: 1) preventing clipping and distor-
tion in power amplifiers, 2) protection of loudspeakers from
damage resulting from destructive transients [like the proverbial
dropped microphone] 3) preventing overs [clipping] during
digital recording 4) preventing overmodulation of the transmit-
ted signal in broadcast.
Figure 8 shows the effect of limiting a signal. e top line is
the original, full volume input signal we need to limit – perhaps
the unfortunate blast of noise when the vocalist inadvertently
unplugs the phantom-powered mic. e bottom line shows the
limited output. At no point does the output signal ever exceed
the threshold, demonstrating the limiter’s brick wall capability.
Contrast this with Figure 9, which shows a compressor set to a
high ratio (∞:1), and a fast attack time. Due to the compressor’s
averaging effect, the output easily overshoots the threshold, and
transient peaks are missed altogether. And it’s these peaks which
can potentially ruin your speakers or the perfect recording take,
and possibly your reputation.
Overshoot
Input
Figure 8. Limiter performance showing brick wall characteristic.
Figure 9. Compressor used as a limiter. Signal peaks can still exceed
threshold, even at high ratio settings.
INPUT OUTPUT
SIDE-CHAIN
BROADBAND
GAIN CONTROL
PEAK
DETECTOR
GAIN COMPUTER
FILTER
THRESHOLD RELEASE
Figure 7. Peak Limiter Block Diagram
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Input dBu
-40
+20
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-20
-15
-10
-5
0
+5
+10
+15
O
u
t
p
u
t
d
B
u
Threshold
Figure 10. Peak Limiter Response Graph

Manual-6
Making Connections
e C4 features auto balanced/unbalanced XLR and ¼" TRS
input and output jacks, permitting connection to virtually any
mixing console. Side-chain inputs are also auto balanced/unbal-
anced, on ¼" TRS only. Dynamics processors typically connect
using a mixing console’s insert points (send/return), although
they can be used in-line, such as when compressing or limiting
an overall mix, for example.
MIC INPUT
INSERT SEND
INSERT RETURN
INSERT SEND
INSERT RETURN
INSERT SEND
INSERT RETURN
INSERT SEND
INSERT RETURN
INSTRUMENT IN
SUB GROUP 1
SUB GROUP 2
CH 1 SIDE-CHAIN
CH 1 INPUT
CH 1 OUTPUT
CH 2 OUTPUT
CH 2 INPUT
CH 2 SIDE-CHAIN
CH 3 SIDE-CHAIN
CH 3 INPUT
CH 3 OUTPUT
MASTER
STEREO LINK
SLAVE
CH 4 SIDE-CHAIN
CH 4 INPUT
CH 4 OUTPUT
MIXER C4
BASS
MIC
Figure 11. Connection to a mixing console.
MIC INPUT
INSERT SEND
INSERT RETURN
CH 1 SIDE-CHAIN
GRAPHIC EQ
CH 1 INPUT
CH 1 OUTPUT
MIXER C4
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4
8
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MASTERCONTROL
POWEROL LEVEL RANGEBYPASS
±12
12
–
8
5
2
8
0
2
5
MICROGRAPHIC
EQUALIZER
+
12
Figure 12. Using an external equalizer in the side-chain.
Front Panel Controls
Each channel of the C4 features the following meters and con-
trols:
Side-chain and Gain Reduction Meters
8-3 -1 12 4 2-6-12 118
dBr Reduction dB
TH
is triple-function meter displays the side-chain level, threshold
indication and amount of gain reduction being applied. It allows
you to see at a glance exactly what the compressor is doing at all
times.
Side-chain Level Display
In Compressor mode, the floating meter displays the side-chain
level relative to the reshold (dBr), making it easy to gauge how
close the signal is to the onset of compression. e dBr indicators
light from left to right as the signal approaches the threshold. At
threshold, the yellow TH indicator is lit. Above threshold the
TH indicator remains lit and gain reduction occurs. Side-chain
meter ballistics follow the rms detector time constant of 30 ms.
In Dynamic EQ mode, the side-chain meter displays the
relative difference between the broadband program material and
the bandpass signal defined by the PEQ, as compared to the set
reshold.
Example: With THRESHOLD set to -15 dB, the TH indica-
tor lights when the sibilant portion of the program material is 15
dB below the broadband program material. If the sibilants are
18 dB below the broadband material, then the –3 dBr indicator
is lit.
Gain Reduction Display
e Gain Reduction meter displays the amount of gain correc-
tion presently applied to the signal, lighting from right to left as
the amount of gain reduction increases. In Compressor mode,
this meter display the amount of gain correction being applied
to the broadband program material. In De-ess mode. this meter
shows the amount of gain correction being applied to the dy-
namic EQ (DEQ) in the main signal chain. is meter does not
reflect any gain reduction occurring due to limiting.
Gain Reduction meter ballistics follow the actual gain cor-
rection value as determined by the THRESHOLD, RATIO,
ATTACK, RELEASE and KNEE controls. Gain reduction may
be indicated when the reshold indicator is off, as attack and
decay rates may dictate continued gain reduction even when the
rms level is below threshold.

Manual-7
MASTER / SLAVE Stereo Link When channels are Linked
• Bypassing the Master channel does not automatically bypass
the Slave channel. Channel ACTIVE / BYPASS switches are
the only controls which do not follow the Master channel,
making it possible to bypass each channel individually.
• Selecting Listen on the Master channel automatically puts
both channels into Listen mode. Side-chain function switch
indicators are turned off on the Slave channel.
• Meters for Master and Slave channels remain active, indicat-
ing both channels are being processed.
ACTIVE / BYPASS
e MASTER / SLAVE switches link channels 1 & 2, or
channels 3 & 4 for stereo operation. When linked, the associ-
ated LINK indicator lights and only the controls of the Master
channel (channel 1 or channel 3) are active, with the exception
of channel ACTIVE / BYPASS switches. is feature helps pre-
serves a stable left/right image when compressing a stereo source,
such as a vocal or horn submix.
MASTER SLAVE
1 2
LINK
MASTER SLAVE
3 4
LINK
Figure 13. Active controls in Linked Mode
Stereo Linking in Compressor Mode
Linked Compressors use the rms sum of the two channels’ side-
chains, and display this summed result on the side-chain dBr
meters of both channels. e Master channel settings determine
the actual gain reduction characteristics.
Linked Controls in Dynamic EQ Mode
When channels are linked in Dynamic EQ mode, both channels
use the Master channels’ control settings, but the dynamic EQ
essentially functions independently for each channel. Combin-
ing the rms values of broadband and bandpass signals, as is done
in Compressor mode, is of no advantage in Dynamic EQ mode.
Stereo-Linked Limiters
When channels are linked, the Master channel Limit reshold
setting determines the maximum output level for both chan-
nels. However, the greater of the two peak-detected signals is
used to control the gain of both Master and Slave channels. For
example, if the Slave channel signal exceeds the Mater channel’s
Limit reshold by 3 dB, both channels are limited by 3 dB.
e Limit Headroom meters remain independent for both chan-
nels, as the dynamics of the Master and Slave channels may be
quite different.
0
0
6
3
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24
dB
-20
-10
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-30
+20
+10
-20 -10 0
-40
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+10
msms dBu
dBu
dBHzoct
Headroom
dB
8-3 -1 12 4 2-6-12 1
n:1
MASTER SLAVE
10
2
1.5 5 -6 +6 -6 +6
1
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25
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+12-12
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24
dB
-20
-10
0
-40
-30
+20
+10
-20 -10 0
-40
-30
+22
+10
msms dBu
dBu
dBHzoct
Headroom
dB
8-3 -1 12 4 2-6-12 1
n:1
10
2
1.5 5
1
500
100
50 250
25
10
2
1 5
0.5 20k
640
125 3.15k
20
500
125 1 s
2 s50
25
+12-12
+12-12
18 18
SIDE-CHAIN
RELEASEATTACK KNEE
GAIN FREQUENCYBANDWIDTH
LIMIT THRESHOLD
RATIO GAIN COMP THRESHOLD
1 2
LINK
Auto
Auto
SoftHard
dBr Reduction dB
Listen Dynamic
EQ
Comp
LIMIT
BYPASS ACTIVE
PEQ
SIDE-CHAIN
RELEASEATTACK KNEE
GAIN FREQUENCYBANDWIDTH
LIMIT THRESHOLD
RATIO GAIN COMP THRESHOLD
Auto
Auto
SoftHard
dBr Reduction dB
ListenComp
LIMIT
BYPASS ACTIVE
PEQ
TH TH
Dynamic
EQ
Selecting BYPASS disables all Compressor/Dynamic EQ
processing, including the GAIN control. e side-chain dBr
meter, threshold indicator and gain reduction meter continue
to operate when a channel is bypassed. Similarly, the side-chain
Listen function is also available. Channels can be individually
bypassed, even in Linked mode.
Note: e Limiter is active at all times and is not affected by
the BYPASS switch. To bypass the Limiter, simply rotate the
Limiter reshold control fully clockwise (+22 dBu setting).
BYPASS ACTIVE

Manual-8
COMPressor THRESHOLD, RATIO and GAIN
0
-20
-10
0
-40
-30
+20
+10
dBu dBn:1
10
2
1.5 5 -6 +6
1 +12-12
RATIO GAIN COMP THRESHOLD
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e threshold, like crossing through a doorway, is the beginning
point of gain adjustment. When the input signal is below the
threshold, a compressor acts like a straight wire. Above threh-
sold, the side-chain asserts itself and turns the volume down.
In Compressor mode, COMP THRESHOLD defines the
absolute level, adjustable from –40 dBu to +20 dBu, above which
compression begins. If soft-knee settings are used, the threshold
is defined as the point at the center of the knee. (See the KNEE
description for more details).
In Dynamic EQ mode, this control defines the relative
difference between the broadband program material and the
bandpass signal defined by the PEQ. e tpointer between –10
dBu and –20 dBu shows the typical setting for De-essing.
RATIO defines the ratio of Input change to Output change,
and is adjustable from 1:1 (straight wire, no change) to 10:1
(heavy compression), as shown in Figure 14. For example, a
Ratio of 4:1 means that a 4 dB change at the input causes only a
1 dB change at the output.
e tpointer at 10:1 shows the typical setting for De-
essing.
Figure 14. Input versus Output for various Ratio settings (hard knee)
Once you reduce the dynamic range using compression, you
can increase or decrease the overall loudness by adjusting the
GAIN. is control features a center-detent marking the unity
gain point, with a range of +12 dB or -12 dB on either side.
ATTACK, RELEASE and KNEE
ATTACK determines how fast the gain is turned down once the
signal exceeds reshold, and is adjustable from 25 ms (fast) to
500 ms (slow). e tpointer at 25 ms shows the typical setting
for De-essing.
RELEASE determines the rate of gain increase as less gain
reduction is required, and is adjustable from 25 ms (fast) to
2 seconds (slow). It is important to understand the difference
between release rate – as determined by this control – and release
time. e Release setting determines how long it takes for the
gain to change by 10 dB, not how long it takes to return to unity
gain (no gain reduction).
Example: with the Release control set to 1 s, when a signal
with 5 dB of gain reduction presently applied suddenly drops
below threshold, the release time is 0.5 s, calculated as follows:
Gain Reduction x Release = 5 dB x 1 s = 0.5 s
10 dB 10 dB
Auto Attack and Release
Unsure about how to set the Attack and Release controls? Venue
doors opening in 15 minutes and you need to speed through
sound check? No problem! Simply rotate the Release control
fully clockwise until Auto is lit, indicating the Attack and Re-
lease settings are appropriately fixed for most types of program
material. e C4’s not entirely on auto pilot, though – you still
have full control over the reshold, Ratio, Gain, and Knee set-
tings.
We are the Knights of KNEE
KNEE controls the action of the compressor above and below
the threshold point. Hard knee does nothing until the signal
exceeds the threshold point, then applies full compression. Soft
knee begins applying a small amount of compression just before
the threshold point is reached, continues increasing compres-
sion through the threshold point and beyond, finally applying
full compression to the highest level signals. Depending on the
application and source material, soft knee settings can sound
more natural. On the other hand for maximum loudness before
compression (for equipment protection for instance), use hard
knee settings.
Knee span is adjustable from 0 dB (Hard) to 10 dB (Soft),
with the reshold always being at the center of this span. e
tpointer at the Hard Knee setting shows the typical setting for
De-essing.
msms
500
100
50 250
25
500
125 1 s
2 s50
25
RELEASEATTACK KNEE
Auto
Auto
SoftHard

Manual-9
Figure 15. The Hard Knee compression characteristic has a 0 dB span.
The Soft Knee compression characteristic has a 10 dB span.
SIDE-CHAIN Mode and Listen
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SIDE-CHAIN Parametric EQ
Comp selects Compressor mode for the channel.
Dynamic EQ selects this mode for the channel.
Tip: A small tpointer next to a control indicates a typical set-
ting for De-essing.
Listen routes the side-chain signal to the channel output, allow-
ing you to hear exactly what the detector “hears,” and is especial-
ly useful when making precise PEQ adjustments for frequency
sensitive compression or de-essing. e yellow indicator flashes
when Listen is engaged, as a friendly reminder to toggle back to
Comp or Dynamic EQ mode before show time.
e Listen source depends on the selected mode (Comp or
Dynamic EQ), so the switch operates as follows: when switch-
ing from Comp to Listen, the Comp indicator remains lit and
the Listen indicator flashes, which signifies you are listening
to the Compressor side-chain PEQ response. When the switch
is moved to Dynamic EQ, the Listen and Comp indicators
are turned off and the Dynamic EQ indicator is lit. When the
switch is then moved from Dynamic EQ to Listen, the Dynamic
EQ indicator remains lit and the Listen indicator flashes, which
signifies you are listening to the Dynamic EQ response.
Remember: once the show begins the audience never hears
the filtered side-chain signal – unless you accidentally leave
Listen mode engaged, of course. If something sounds a bit odd
during the gig, look over at the C4 and see if any of the yellow
Listen lights are flashing. If so, casually saunter over to the rack
and toggle the side-chain out of Listen mode…as if you meant
to have it engaged for the first six songs.
e Side-chain PEQ filter operates in one of two ways, depend-
ing on whether the channel is operating in Compressor mode or
Dynamic EQ mode.
PEQ in Compressor Mode
In Compressor mode, the PEQ section operates as a normal,
2nd-order, parametric filter with independent BANDWIDTH,
FREQUENCY and GAIN controls. e GAIN control has a
center detent position indicating 0 dB, or unity gain. Use the
PEQ to adjust the sensitivity of the rms detector to specific
frequencies. Boosting a particular frequency makes the detector
more sensitive to this frequency, while cutting makes it less sensi-
tive. Broadband compression still takes place; the filter does not
directly affect the frequency response of the main signal, as is
done with the Dynamic EQ used in de-essing.
Example: reducing detector sensitivity to low frequencies
helps avoid the pumping and breathing often associated with
compressing an overall mix of instruments. Start by engaging
Listen mode and set the PEQ Gain to –12 dB with a bandwidth
of 1 to 2 octaves. Adjust the frequency and bandwidth controls
until most of the low frequency sound disappears (125 Hz is a
good start). Oh…and don’t forget to dis-engage Listen mode
when you’re done.
PEQ in Dynamic EQ Mode
In Dynamic EQ mode, the PEQ controls define a bandpass filter
in the side-chain and a Dynamic EQ in the main signal path.
e difference in the rms level of the broadband signal and that
of the bandpassed side-chain signal is compared to the threshold,
and the Dynamic EQ’s gain is automatically adjusted to main-
tain the proper ratio of sibilant to non-sibilant content.
See the examples in the Dynamci EQ Applications section on
page Manual-4.
0
dBHzoct
10
2
1 5
0.5 20k
640
125 3.15k
20 +12-12
GAIN FREQUENCYBANDWIDTH
PEQ
SIDE-CHAIN
Listen Dynamic
EQ
Comp

Manual-10
LIMIT THRESHOLD and Headroom Meter
In addition to the Compressor / De-esser, each channel of the
C4 features an independent, brick-wall Limiter with an instanta-
neous attack time, fixed 25 ms hold time and a fixed 6 dB/sec-
ond release rate. e C4’s limiter uses oversampling in order to
ensure high frequency transients are properly detected and acted
upon.
e Headroom meter displays the difference, in dB, between
the LIMIT THRESHOLD and the present signal level. For
example, with LIMIT THRESHOLD set to +10 dBu, a peak
signal level of +4 dBu results in a display of 6 dB of remain-
ing headroom (+10 dB minus +4 dB equals 6 dB). e LIMIT
indicator lights when the present signal exceeds the set Limit
reshold. When setting the Limiter reshold keep in mind
that music often has peaks which are 12 to 20 dB higher than
the average (rms) value, which is displayed on the side-chain
meter.
Note: e Limiter is active at all times and is not affected by
the bypass switch. To bypass the limiter, simply rotate the Lim-
iter reshold control fully clockwise (+22 dBu setting).
6
3
12
24
dB
-20 -10 0
-40
-30
+22
+10
dBu
Headroom
LIMIT THRESHOLD
LIMIT
Initial Settings
Sometimes it is necessary to start from scratch. e drawing
above shows where to set the controls for no processing (the
black knobs have no affect at this point). en you can adjust
each section one at a time.
Suggested Settings
ere is no magic recipe of compressor settings which work for
every audio source in every performance situation. ere are,
however, a few key ingredients you can add to the ol’ dynam-
ics stew to get things going. Start with the suggested settings in
Table 1, then season to taste by adjusting the reshold until
your ears tell you it’s just right.
Vocals
A tough issue with vocals is the extreme dynamic range of some
singers. ose who can lull you to sleep and then scare you with
an unexpected blast. e difference between the soft crooning
and the loud climax represents too much signal change for many
preamps and mixers, causing them to clip and distort badly.
Compression and limiting comes to the rescue.
Use the limiter to prevent the extreme levels from causing
clipping and distorting the sound. With its automatic fast attack
and quick release mode, all you need to set is the Limit resh-
old. Set it as high as the next piece of equipment in the signal
path allows. e C4 gives you a wide setting range of -40 to +22
dBu that covers all requirements.
Compression is one of the most effective tricks for bring-
ing the vocals up in any mix, live or recorded. is is due to
the increase in perceived loudness which results from reducing
peaks and increasing the average level. Good settings for natural
sounding, yet compressed vocals, are a medium fast (25 – 50
ms) attack and a medium-slow (100 ms – 1 sec) release. Releas-
ing too quickly sounds unnatural, while attacking too slowly
misses the problem surges. Soft knee compression generally best
matches vocal dynamics. Adjust the ratio to match the dynamics
of the singer, with 2:1 being a good starting point.
Tips and Tricks
0
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-20
-10
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-40
-30
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+10
msms dBu
dBu
dBHzoct
Headroom
dBn:1
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1.5 5 -6 +6
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SIDE-CHAIN
RELEASEATTACK KNEE
GAIN FREQUENCYBANDWIDTH
LIMIT THRESHOLD
RATIO GAIN COMP THRESHOLD
Auto
Auto
SoftHard
Listen Dynamic
EQ
Comp
LIMIT
BYPASS ACTIVE
PEQ

Manual-11
Bass
It is common for the sound mixer to reduce the bass signal
because it overwhelms the total system. Use a compressor to
smooth a bass sound by lessening the variations between the
strings or strongly resonant notes, and increasing sustain.
Typical settings for a bass guitar are a ratio of 4:1, with a
fast attack of 25 ms and a slow release of around 500 ms. ese
settings should produce a strong, smooth bass line to start with,
but feel free to adjust further as necessary. A hard knee setting
is often preferred since all that is desired is to tame the excessive
peaks and leave everything else alone.
Where does a compressor belong in a bass player’s signal
chain? Well, that depends on how you want it to function. As a
compressor/limiter for the input signal, it goes after the bass (if
the bass has a line-level output) and before the preamp. If it were
to function as a limiter to protect the speakers in the bass rig, it
would go after the preamp and before the power amp. Another
method is to insert the unit in the effect loop of the preamp.
is allows the bass signal to be affected by the pre-amp first,
then the compressor/limiter, and then sent to the power amp.
is can be desirable with tube pre-amps.
Guitar
Here is a suggestion on how to achieve a lower volume without
sounding as if you are playing out of a transistor radio: set a slow
attack time with a medium to slow release and a relatively low
threshold. Experiment from these initial settings.
One of the favorite uses of compression by guitarists is to in-
crease the sustain, or duration of a note after it is played. Carlos
Santana and Gabor Szabo are two musicians who use sustain to
great acclaim, although they did it the old-fashioned way of cre-
ating feedback by aiming the guitar pick-ups at the loudspeaker
and then jamming over it. A compressor creates a similar effect.
Set a high ratio and low threshold for long sustain, along with
fast attack and slow release. Again, experimentation produces
the best results.
ATTACK RELEASE RATIO KNEE
Vocals Medium to Fast Medium to Slow 2:1 to 4:1 Soft
Clicky Bass Fast Fast 4:1 or higher Hard
Mushy Bass Medium to Slow Medium to Slow 4:1 Hard
Raging Electric Guitar Fast Slow 4:1 or higher
(more sustain)
Hard
Acoustic Guitar Medium to Slow Medium to Slow 4:1 Medium
Brassy Horns Fast Fast 5:1 or higher Hard
Drums (kick, snare) Fast Fast 4:1 Hard
Drums (cymbals) Fast Slow 2:1 to 10:1 Hard
Table 1. Suggested Compressor Settings
Drums
Reducing the leading edge of a drum hit and bringing out more
of the “body” of the drum is another popular use of compressors.
Try ratios between 2:1 and 5:1 accompanied by fast attack and
release times. Listen carefully while changing the attack time to
find the final setting.
Cymbals need a fast attack but a slow release to allow the
sustain through. A low ratio of about 2:1 works well to start.
Digital Recording
For digital recording use the C4 to compress a too-wide dynamic
range into a signal that does not cause digital overload. e
Limiter is the primary tool for keeping things under control, but
a little compression with its threshold set just under the limiter
threshold level helps keep the limiting more subtle. To control a
stereo mix, set the mode switches to Slave.
All features & specifications subject to change without notice. DOC 107487
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