TDM Audio 32CL-2 User Manual

32CL-2
32CL-4

COMPRESSOR/LIMITER

OWNER’S MANUAL

TDM DESIGN INC.
12800 NW BISHOP RD
HILLSBORO, OR 97124
PHONE: [phone]

FAX: [phone]

TDM-DESIGN.COM

IMPORTANT!

*** Read Before Using ***

CAUTION: The following must be observed to prevent

malfunctioning and/or possible equipment damage.

• Before plugging the unit into the main AC line, make

sure that all of the equipment following the compres­sor/limiter output lines is turned off or all of the inputs
are turned down.

• The unit should be plugged in only when it has been

established that the main AC line is supplying the cor­rect voltage and frequency. US models are set up for
110 VAC at 60 Hz.

• Keep the unit away from excessive moisture.

• Allow only authorized technicians (consult your dealer)

to open the unit. TDM Design assumes no liability for
damage or injuries.

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 2

Table of Contents

INTRODUCTION................................................................................................................................................ 4

COMPRESSOR FUNDAMENTALS ............................................................................................................................ 4

LIMITER FUNDAMENTALS.................................................................................................................................... 6

MOUNTING THE UNIT IN A RACK................................................................................................................ 7

USING THE SECURITY COVER............................................................................................................................... 7

HOOKING UP THE COMPRESSOR/LIMITER.............................................................................................. 8

WHAT YOU’LL NEED........................................................................................................................................... 8

MAKING ADAPTER CABLES.................................................................................................................................. 8

THE BASIC HOOK-UP........................................................................................................................................... 9

COMPRESSING AND/OR LIMITING A MIX ............................................................................................................... 9

USING CHANNEL OR SUBGROUP INSERTS.............................................................................................................10

COMPRESSING ELECTRONIC INSTRUMENTS..........................................................................................................11

OPERATING THE COMPRESSOR/LIMITER...............................................................................................12

FRONT PANEL CONTROLS...................................................................................................................................12

BASIC COMPRESSING..........................................................................................................................................13

BASIC LIMITING.................................................................................................................................................14

COMPRESSING SPEECH .......................................................................................................................................14

COMPRESSING VOCALS.......................................................................................................................................15

COMPRESSING MUSICAL INSTRUMENTS...............................................................................................................16

COMPRESSING PROGRAM MATERIAL...................................................................................................................16

USING SIDE CHAINS .......................................................................................................................................17

HOW SIDE CHAINS WORK...................................................................................................................................17

USES FOR SIDE-CHAIN COMPRESSION..................................................................................................................18

CONNECTIONS FOR SIDE CHAIN OPERATION........................................................................................................19

TDM OPTION CARDS.........................................................................................................................................20

TROUBLESHOOTING AND SUPPORT..........................................................................................................21

NO SIGNAL OUTPUT...........................................................................................................................................21

DISTORTION ......................................................................................................................................................21

EXCESSIVE NOISE...............................................................................................................................................22

60 HERTZ HUM OR BUZZ ....................................................................................................................................23

CONTACTING TDM............................................................................................................................................24

SPECIFICATIONS.............................................................................................................................................25

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 3

Introduction

T

hank you for purchasing the TDM 32CL series compressor/limiter. These units are made from

the finest components and engineered to exacting standards. Precision components are used
in all critical circuitry for the finest sonic quality and performance. To get the most out of your
new TDM compressor/limiter, please take a few minutes to review this manual and familiarize
yourself with the proper operation of the unit.

The remainder of this section provides background information about the theory of compressor
and limiter operation. Individuals that do not need this information may skip directly to the next
section: Mounting the Unit in a Rack.

Compressor Fundamentals

Compressors are used to reduce the dynamic range of a sound signal. A signal’s dynamic range is
the difference between the loudest part of the signal and the quietest part. In theory, compressing
the dynamic range of a signal results in the quietest parts getting a little louder and the loudest
parts getting a little quieter. The way most compressors actually work, however, is by reducing
the volume of the loudest parts. If desired, the overall volume of the output of the compressor
may then be raised to bring the quietest parts up in volume without the loud parts being too loud.

The main use of a compressor is to even out the volume of a signal. For example, in a radio sta­tion, sometimes the announcer speaks softly while other times he or she might shout. This
presents a problem because there is a limited modulation range that is acceptable for broadcasting.
Radio stations use compressors so the signal is strong enough when the announcer is speaking
quietly without causing overmodulation during the loud parts. The quality of the radio an­nouncer’s voice changes, but the actual volume of the signal is relatively constant. Even without
the modulation limits, it is desirable while you are listening to the radio in your car to be able to
hear the announcer over the road noise without being blown away when they get excited.

Compressors are used extensively in broadcasting, music recording, film-making, live sound
reinforcement, and just about anywhere else sound equipment is found. In order to understand
and operate a compressor, you need to be familiar with a few terms.

The first term you need to understand is compression ratio. The compression ratio is the mathe­matical expression of how much compression is taking place. For example, for a 2:1 compression
ration (read “two to one”), the dynamic range is halved. The first number represents the dynamic
range of the sound before compressing, and the second represents the dynamic range after com­pressing. Two to one means that if the dynamic range were two before compressing it would be
one after compressing. A more realistic example would be a symphonic program which might
have a dynamic range of around 70 dB before compressing. This same program would have a
dynamic range of 35 dB after compressing at 2:1. If you flip the numbers over and place a line
between them you get the fraction of the original dynamic range that describes the result. For
example the result of 2:1 compression is ½ the original dynamic range. Although this is the theory,
the reality is a little more complicated, so read on.

The problem with achieving a pure compression ratio comes from the way compressors work.
Without knowing what the loudest and softest parts of the program are going to be, how can a

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 4

compressor adjust its gain to achieve an exact 2:1 compression ratio? Well, the answer is that it
can’t. The way compressors work in the real world is with a threshold. The next term to familiar­ize yourself with is threshold.

The threshold is your compressor’s way of letting you tell it what the softest part of the program
will be. With this knowledge, the compressor can accomplish its goal. Take the 2:1 example again.
A compressor with a 2:1 compression ratio reduces the gain by 1 dB for every 2 dB the input
signal is over the threshold. If the threshold is -60 dBm, and the input signal is 20 dBm, which is
80 dB above the threshold, gain reduction will be 40 dB so the resulting output signal is -20 dBm.

-20 dBm is 40 dB above the threshold so the dynamic range of the input is reduced by half. If you
were to set the output gain to be exactly ½ of your expected maximum gain reduction (in this case
¼ of your expected input dynamic range), you would achieve true 2:1 compression. In such a
setup, the loudest sound (at +20 dBm) would output 0 dBm, and the softest sound (at -60 dBm)
would output -40 dBm. From -40 to 0 is a 40 dB dynamic range compressed equally at either half
of the spectrum. It rarely really works this way, though.

One problem with the above scenario is that it raises the noise floor by 20 dB which is usually
unacceptable.

Another problem is that such a scheme usually doesn’t sound the way you would want it to.
Generally, there is a range of sounds near the bottom end of the dynamic range of the program
that you want to leave uncompressed in order to let the quietest parts of the program decay natu­rally without a lot of “breathing.” Breathing describes the sound of the compressor bringing up
the gain as things get quieter. You can hear the program noise increase, and the program decays
unnaturally.

Another problem is that if the sound source is a microphone being reproduced through a loud­speaker, low thresholds cause feedback problems. The problem is that the microphone gain is
much hotter when there is no sound than when there is. When a person speaking into such a
microphone pauses, the compressor does the equivalent of cranking the volume of the micro­phone way up. This often results in feedback.

Low thresholds also cause unnatural sounding attacks. Suppose the threshold is at -60 dBm and
the compressor is set to a 2:1 compression ratio. If the nominal level is 0 dBm, the signal gain is
30 dB hotter when there is no signal than when the nominal level is present. When the signal first
hits, the initial output is really loud, and then you can hear the gain being reduced by the compres­sor. This is because it takes time to reduce the gain. In an ideal compressor, this gain reduction
would happen instantaneously, but in the real world it never does.

The solution to these problems is to raise the threshold. In most setups, the compressor doesn’t
actually start compressing until the level is much higher than -60 dBm. This leaves a large portion
of the dynamic range uncompressed, eliminates breathing, reduces output noise, provides for
natural sounding attacks, and just generally works better. What it also means, though, is that the
ratio no longer really represents the actual compression of the dynamic range. If the threshold is
set at -20 dBm, and the loudest signal is +20 dBm, then only the portion of the range from -20
dBm to +20 dBm is compressed. For 2:1 compression, the gain reduction at +20 dB is 20 dB. If
the dynamic range of the input signal ranges from -50 dBm to +20 dBm, the range is compressed
from 70 to 50 dB. This is really only a 7:5 ratio, and compression only happens in the high 40 dB
of the 70 dB range. Although this fact might be mathematically offensive, it results in much better
sound quality.

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 5

If all of this math confuses you, you are not alone. You don’t need to thoroughly understand it to
work with compressors, though. It is presented so that you have the background necessary to
understand how compressors work. Normally, you set the compression ratio based on how severe
you want the compression (or gain reduction) to be, and you set the threshold to where you want
the compression to begin. Ratios are usually set to a ballpark value based on the kind of sounds
being compressed, and thresholds are generally fine-tuned to achieve exactly the desired affect.

Limiter Fundamentals

A limiter is really just a compressor with a really high compression ratio. One of the main real­world differences between a limiter and a compressor is that a limiter is a hard-knee device. The

knee here is the knee of the curve that graphs input level vs. output level. Compressors tend to be
soft-knee devices. A compressor eases across the threshold gently and then begins compressing

the signal at the specified ratio. A limiter hits the threshold like a brick wall and then refuses to let
the level go any higher. Limiters are used to set an absolute maximum level and then guarantee
that the level will not pass it. A limiter has a ratio nearing infinity to one.

While compressors are often used to enhance sound quality, limiters are used mostly for protec­tion purposes. A radio station might use a nice 4:1 compression ratio to even out the sound of the
announcer’s voice, but a limiter might be set to make sure that no matter what happens, the level
doesn’t go over the maximum modulation level allowed by law. Here the limiter is a protection
mechanism. Limiters are used in large concert sound systems to prevent damage to speakers (and
ears).

Limiters are much easier to use than compressors. It is not necessary to understand how the
output level, compression ratio, and threshold interact to produce a certain effect. A limiter is
used by simply setting the absolute maximum level as the threshold.

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 6

Mounting the Unit in a Rack

T

DM 32CL series compressor/limiters can be mounted in any standard 19” rack. Each TDM

32CL series compressor/limiter takes up one rack space. To make mounting easier, lay the
rack on its back with the equipment front panels facing up. Remove any rack screws from the part
of the rack where you are planning to mount the compressor/limiter. Position the TDM 32CL
series compressor/limiter in the rack as desired. Make sure the mounting holes in the compres­sor/limiter line up with the screw holes in the rack rails. Use four standard 10-32 rack screws for
each compressor/limiter. We recommend that the rack screws have plastic washers to prevent
damage to the paint on the face of the compressor/limiter. Install each screw loosely through a
mounting hole in the compressor/limiter and into the rack. Do not tighten the screws until they are
all in place. After all four screws are installed loosely, make sure the compressor/limiter is placed
exactly as you desire and then tighten the four screws until they are nice and snug, but not overly
tight.

Using the Security Cover

The 32CL series compressor/limiter can be ordered with an optional security cover. The security
cover will only fit on units shipped for use with it. Units shipped without the security cover have
knobs and buttons that protrude from the face of the unit for easy access and operation. Units
shipped for use with a security cover have knobs and buttons that are recessed into the face of the
unit so that the security cover can be installed.

The security cover is used to prevent unwanted tweaking of the compressor/limiter settings. If
your compressor/limiter will be installed permanently for one particular purpose, and will be set
up once to operate properly, use the security cover. An example of this kind of installation would
be a compressor limiter installed for a public address/paging system. The unit would be set up so
that the volume of speech coming through the system would be sufficient to hear and understand
over ambient noise levels, but not too loud. Perhaps a professional contractor would set up the
system correctly using special measurement equipment to guarantee compliance with local laws.
Once this system was set up, you would not want anyone besides the contractor to change it.
Security covers are recommended for cases like these.

Do not use the security cover if your unit will be used for a lot of different purposes, or will re­quire adjustment often. For example, if you are installing your compressor/limiter in a recording
studio, the security cover is a bad idea.

If you purchased your unit with a security cover, make sure all of your settings are correct, then
install the cover. There are two holes in the face plate of the compressor/limiter—one on either
end. Position the cover so that its two screws line up with the two holes. Then, using an Allen
wrench, tighten the screws so that they are snug, but not overly tight.

If you purchased your unit without a security cover, but you wish to be able to use one, the unit
can be converted. Contact your vendor, or call TDM (see Contacting TDM at the end of this
manual) for information on how you can do this.

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 7

Hooking Up the Compressor/Limiter

O

nce your TDM compressor/limiter is installed in the rack, you are ready to hook it up to your

equipment. Of course, the method used to hook up any compressor or limiter depends a
great deal on how it will be used. We will try to give you the basics in this manual, but you may
need to tailor the methods described here to your particular application.

What You’ll Need

To connect your TDM 32CL series compressor/limiter to your equipment, you will need the
following.

• Power: A power outlet should be located close enough to the unit so that you can plug it in.

The TDM 32CL series compressor/limiters require a grounded (3-prong) outlet. If an outlet is
not close enough, an extension cord or power strip may be used. Make sure you check the
power rating on the extension cord or power strip to make sure that it exceeds the power re­quirements of all units plugged into it combined. TDM units have their power requirements
marked on the rear of the unit.

• Signal Cables: Each channel to be connected requires two signal cables. One cable transmits

the signal from your signal source to the compressor/limiter. The other transmits the signal
from the compressor/limiter to the next item in the signal chain. This might be an amplifier, a
mixing device, or more signal processing equipment.

Making Adapter Cables

In order to hook up your compressor/limiter to the rest of your equipment, it may be necessary to
use special adapter cables. TDM 32CL series compressor/limiters can be shipped from the factory
with either ¼” phone jacks, or with XLR connectors. In either case, inputs and outputs are bal­anced. The ¼” phone jacks are tip-ring-sleeve type for balancing. It is common to need adapters
to go from ¼” to XLR or vice versa. It is very important that if you build your own adapters, you
adhere to certain rules to make sure your TDM unit functions correctly.

If you have a TDM compressor/limiter with XLR connectors and you need to connect it to some
other piece of equipment that has ¼” connectors, you might need to build XLR to ¼” adapters. If
the other piece of equipment is balanced with tip-ring-sleeve jacks, then do the following.

• Wire pin 1 of the XLR connector to the sleeve of the tip-ring-sleeve plug.

• Wire pin 2 of the XLR connector to the tip of the tip-ring-sleeve plug.

• Wire pin 3 of the XLR connector to the ring of the tip-ring-sleeve plug.

If the other piece of equipment is not balanced, but has plain ¼” connectors, then do the follow­ing.

• Wire pins 1 and 3 both to the sleeve of the ¼” phone plug.

• Wire pin 2 to the tip of the phone plug.

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 8