dbx 165 User manual

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dbx Model 165
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INSTRUCTION MANUAL
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Dear dbx customer:
Your new model 165A is identical to the old model 165 except for a
special feature called PeakStop. Until a new manual is produced, . this
memo has all of the necessary PeakStop information and is intended to
supplement the accompanying 165 manual.
The PeakStop feature allows you to control t he maximum peak levels at
the output of the 165A irrespective of any other control . PeakStop comes
after the compression and other circuitry, including the output gain, so
it lets an absolute limit be put on the peak-to-peak excursions of the
output. PeakStop works instantaneously; you are able to apply moderate
amounts of dbx's Over Easy compression and will still be protected from
large transients, nther short-term over loads, and overmodulation.
Technically, PeakStop consists of a sophisticated voltage-controlled
clipper that produces a minimum of audible distortion. It rounds the
corners of a peak rather than cutting it off sharply, as the word
"clipping" usually implies. By making a signal's leading and trailin g
.edges curved instead of sharp corners, it reduces the amount of higher
odd-order, offensive-sounding harmonics that conventional clipping
causes. The level at which PeakStop is activated is adjustable from -2 to
+24 dBm. Note that small signal excursions above the set value of
PeakStop are possible, to allow the rounding to take place; therefore, for
any applications where you must not exceed a given ceiling, set the
PeakStop control 1-2 dB below.it to be sure .
The red PeakStop LED flashes whenever peaks attempt to exceed PeakStop
level and get reduced in amplitude. To disable the PeakStop function
altogether, simply set the control to +24 dBm (which is the maximum output
level of the 165A anyway).
In use, the PeakStop function can prevent an amplifier from being
driven into hard clipping, where it can lose control over the speaker
system. PeakStop is a smooth, well-controlled clipper whose behavior is
sonically similar to the gentleness of Over Easy compression; its clipping
is much preferable to a power amp's. As mentioned, control of speaker
overexcursion, broadcast overmodulation, and harsh electronics clipping
are all applications of PeakStop. With it and Over Easy, you have the
best of both worlds: virtually inaudible rms compression and peak
protection downstream at the end.
Sincerely,
dbx Inc.
Professional Products
dbx. Inco rpor ated/ 71Chapel Street / Box100C I Newton, Massachusetts 02195 / Tel. [phone] /Telex 92-2522

INDEX
BRIEF OPERATING INSTRU CTIONS
Front Panel
Rear Panel
INTRODUCTION
More About Separate Level Detector Access
More About Over EasyTM Compression
SIGNAL CONNECTIONS
Input Connection
Level Detector Input Connection
Output Connection
Grounding
Stereo Coupler Cable
Input Impedance & Terminations
OPERATION & APPLICATIONS
Compression Contro l
Threshold Adjustment & LED Indicators
Auto Switch & Auto Attack/ Release LED Indicator
Attack Rate & ReleaseRate Controls
Where To Set Attack & ReleaseControls
Meter Calibration & Use
Useof an Equalizer in the Level Detector Circuit for Frequency Weighted
Compression, De-essing, or Increasing Sustain
Use of a Filter in the Level Detector Circuit
Useof a Time Delay Line in the Signal Path but not in the Level Detector
Circuit for Zero or Negative Attack & ReleaseTimes
The 165 As A Line Amplifier
SPECIFICATIONS
dbx PRODUCT WARRANTY & FACTORY SERVICE
BLOCK DIAGRAM
SCHEMATIC
dbx GLOSSARY
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WARNING:TOPREVENTFIREORSHOCKHAZARD,
DONOTEXPOSETHISAPPLIANCETORAINORMOISTURE.

BRIEF OPERATIN G INSTRU CT IONS
Fig. 1 - Front Panel
Threshold Indicators There are three LED's
which indicate the relationship of the input
signal level to the threshold of compression.
The green LED is ON when the signal is
below threshold, the red LED is ON when the
signal is above threshold, and the amber LED
is ON when the signal is in the Over Easy
threshold range.* (Refer to Figure 4.) Since
the LED's indicate the contents of the gain
control signal, their response time will be
affected by the selection of automatic or
manual operation, and by the attack/release
rates selected in manual operation.
Meter Function Switches These three interlocking pushbuttons determine whether the meter
displays Input level, Output level or the amount of Gain Change. Input level metering is useful
in deciding how much compression may be needed. Output level metering is useful as a check to
ensure that the levels are not exceeding the desired maximum and the over-all gain of the 165 is
set properly. Input and output levels are displayed on an RMS basis, with fixed time constants
which are unaffected by the front panel attack and release controls. Gain change metering pro-
vides a more precise indicatio n of how much of the program is being compressed than can be
determined from the Threshold Indicator LED's. The meter, when displaying gain change,
indicates the contents of the gain control signal, so its response time will be affected by the ·
setting of the front panel manual/automatic switch and, in manual mode, by the attack and
release rate controls. (Fixed gain changes commanded by the output gain control are not
displayed by the meter's gain change function.)
NOTE : Even though no input signal is
applied, it is normal for the Above Threshold
and Over Easy™ LED indicators to flicker ON
when the power is turned ON or OFF.
Compression Control Clockwise rotation of
this control increases the maximum amount of
compression from 1:1 (unity gain) up to
infinity. The unit can be considered to be a
limiter at compression settings of 10: 1 or
higher, especially with a fast attack rate
Meter The meter is factory set so that OVU
is equivalent to+ 4 dB input or output level,
as selected with the Meter Function switch.
The nominal OVU level may be changed to
anywhere from - 10 dB to +10 dB by
adjusting the rear panel Meter Calibration
trimmer. In Gain Change mode, the meter
pointer moves down scale from OVU to
indicate how many dB of compression the
165 is providing.
(above 100 dB/mS).
Auto Attack/ Release Indicator
Power ON/OFF Switch Engage this
switch to apply AC power to the unit.
The LED (Light Emitting Diode)
indicator above will be illuminated
when the power is ON. No signal flows
when the power is OFF, unless the
System Bypass button is engaged.
This LED is illuminated when the
165 is in Auto Attack/Re leasemode.
NOTE: The screw beneath the meter face is a
zero-set adjustment to assure the needle rests
at OVU when the unit is OFF; it is not
intended for calibration. See page 8 for
meter rei;alibration instructions.
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Stereo Coupler For single-
channel (monaural) operation,
this switch determines
whether the unit will be the
master (controlling unit) or
the slave. The unit becomes a
slave when the button is
pressed IN, and the LED
above the button turns ON to
indicate the slave status. In
stereo operation, all gain,
attack/release, compression
and threshold adjustments are
made.wit h the Master unit's
controls. The Slave follows
the Master's commands with
the exceptions of Meter
switching, System Bypass
switching and Power switch-
ing which remain independent
and must be done on both
units.
Threshold Control Adjust th is
knob to set the threshold of
compression from - 40 dB
(7.8 mV) to +10 dB (2.45 V). ••
The threshold of compression
I
/
Attack Rate Control Clockwise
rotation of this contro l adjusts the
maximum attack rate from
400 dB per millisecond to 1 dB
per millisecond. The control
setting only affects the 165's
operation when the unit is in
manual mode; it has no effect
when the 165 is in Auto Attack/
Release mode.
Auto Switch Engage this switch to
place the 165 in the Auto Attack /
Release mode. The compressor's
attack rate and release rate then
automatically vary to suit the
volume envelope of the input
signal. The attack and release
characteristics may be set manually
with the adjacent front panel con-
trols when the switch is not engaged.
is defined as the approximate
middle of the Over EasyTM region.
Release Rate Control Clockwise
rotation of this control adjusts the
maximum release rate from 4,000 dB
per second to 10 dB per second. The
control setting only affects the 165's
operation when the unit is in manual
mode; it has no effect when the 165 is
in Auto Attack/Release mode.
-••o· ·••
-:0 •10
OUTPUT
000 @N~
INPUT OUTPUT GA.,N 1's vsHM \
CHM~¥ BYPASS
I •
\ • 0
0
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/ utput Gain Con) ol This control adjusts the amount
/ ~f gain in the 165's output amplifier stage. The signal
can be attenuated or boosted by a ful I 20 dB relative
to a "O" center setting (unity gain). This control is
independent of the threshold of compression.
Because 20 dB of gain can be added at the 165
output, it is possible to cause clipping even when the
input level is with in the specified range. Especially
when the COMPRESSION RATIO is set at a low
number (a low "factor"), extreme clockwise rotation
of the OUTPUT GAi N may cause the 165 output
stage to clip program peaks. Therefore, for normal
operation we suggest setting OUTPUT GAi Nat
"O dB" (12 o'clock position). Where the circuit fed
by the 165 has a high input sel]sitivity, lowering the
165's OUTPUT GA iN setting can avoid the need for
an attenuation pad.
System Bypass Switch Engaging this button creates a hard-
wired bypass of the 165's circuitry by connecting the input
directly to the output. The Bypass is useful for A-B com
parisons of the compressed and the "straight" program, or
for rapid restoration of signal flow in the event of a 165
c1rcu1tproblem.
NOTE: Since the system output is normally single-ended,
in normal operation the (+) output carries the signal, while
the(-} output is connected to ground. In the System Bypass
mode, the(+) signal input is directly connected to the(+)
signal output, and the (-) signal input is directly connected
to the (-) system output. Thus, in Bypass mode, the output
is single-ended when the input is single-ended, and balanced
when the input connection is balanced.
*"Threshold" is defined as the point of 6 dB gain reduction when the unit is set for a
maximum (infinite) compression ratio.
**O dB is referenced to 0.775 V RMS.
"dbx" and "Over Easy" are trademarks of dbx Inc. 3

4
Fig. 2 - Rear Panel
Signal Output These term inals feed
the program processed by the 165 to
the next device in the audio path.•
Signal Inp ut These term inals accept
the inco mi ng audio program for
processing by the Mode l 165. •
- SIGNAL -,
, INPUT
+ ! -
+
OfTEC TOA
INPUT OUEC10A
INPUT
SIGNAL
l oorPu1I
+ ! -
Detector Input The 165 comes from the factory
with these term inals strapped to the adjacent
Signal Input(+) and( - ) terminals. If you wish to
gain access to the 165's Detector Inp ut for
insertion of an aux iiiary device, yo u may do so
by remov ing the strappi ng, wiring the auxiliary
device's output to the 165's Detector Input
termina ls, and feeding the auxi liary device's input
with the same signal fed to the 165's Signal Input.
In certa in situations, the signal processor (auxiliary
device) may need to be inserted in the signal pat h,
not in the detector path . (This wou ld be the case
when a delay line is used to achieve a "preview" of
the signal.) In such cases, signal is fed to the input
of the auxi liary device, and also to the detector input,
and the auxiliary device's out put is fed to the 165
signal input.
Unlessthe installation is permanent, the con-
nections described above can be awkward. There-
fore we recommend wiring a few Tip/Ring Sleeve
phone jacks 10 the 165 instead, " normal lin g" the
jacks so that the Signal Inp ut-to-Detector Input link
is normally established via the jacks, but is
automatica lly disconnected whenever a plug is
inserted in the Detector Input jack . A simi lar jack
may be used for the 165 output. (See Figure 6 for
informatio n on wiring of these jacks.)
Stereo Coupler When a suitable cable joins this con-
nector to the Stereo Coup ler connector on anot her
Model 165, and when one of the units is switched to
"slave" mode, the two units may then be utilized for
processing a stereo program. A multi-pin male con-
nector is supplied with each 165 so the coupler cable
can be assembled using the wiri ng diagram in Figure 7.
NOT E: On ly two Mode l 165's can be coup led
together . Four units cannot be coup led togethe r for
quad (4-channel) operation . For this purpose we
recommend the use of two dbx Model 162 stereo
compressor /Iim iters.
Fuse For Mode ls sold in the U.S.A ., th is 1/4 amp,
250 volt AGC type fuse protects the primary side
of the unit's 117V AC power supply. Be sure to
replace a blown fuse with one of the same size and
type. If the fuse blows consistentl y, contact the dbx
factory or yo ur dbx dealer.
Sfl:AE:O COUPL[A
Meter Calibr ation Contro l This recessed,
screwdriver-adjustable trimmer may be
used to precisely calibrate the ,neter
so OVU Input or Output level is
equiva lent to anywhere fro m - 10 dB
(245 mV) to +10 dB (2.45 V).
MODEL 1&5 0
AC Power Cable Connect this
cable to a 117 VAC, 50 or
60 Hz AC power source
only. Models for use wi th
other power sources outside
the United States are
available. Contact the dbx
facto ry for in formation.
The Mode l 165 requires a
max im um of 15 watts AC
power.
*A udio High (+), chassis ground('#-), and Audio Lo w (-) terminals
are provided for connection of the 765 Input and Output to
balanced or floating lines (i.e., two conductor shielded audio cables).
For use with an unbalanced line (i.e., a single conductor shielded
cable}, it is necessary to connect a jumper between the (-} and
(~) input terminals. No jumpers are necessary for the output
terminals when input jumpers are used. The Detector Input has
only audio high (+) and low(-) terminals, and it shares the
chassis terminal with the Signal Input.

INTRODUCTION
The dbx Model 165 is a professiona l single channel
compressor /limiter that features the new dbx Over Easy
compressio n curve. The dbx Over Easy compression
curve permits extremely smooth _, almost inaudible com-
pression due to the gradual change of compression ratio
around the threshold, instead of the customary sharp
threshold. This curve, plus dbx's true RMS level detector
coup led to a wide-range voltage control led amplifier in
a feed-forward circuit, makes it possible to achieve larger
amounts of compression without adverse audib le side
effects.
The RMS IeveI detector is separateIy accessible,
allow ing various signal cond itioners - equalizers, filters,
delay lines, etc. - to be inserted in either the level
detector or signal path independently. This feature aids
in the compression of certa in difficu lt types of materia l,
aswell asin the creation of special effects. In addit ion
to dbx's automatical ly variable attack and releasetime
circu itry (as on the Models 160, 161, 162, 163 and
164), a front panel switc h and contro ls provide for
manually adjustable attack and releaserates over a
very wide range.
The Model 165 also features stereo coupling for.two
165's (at the touch of a front panel button), LED
indicators showing whether the signal is at, above or
bel~w the ~hreshold of compression, a 30 dB dynamic
range meter (indicating input, output and gain change
levels), and a rear panel zero VU calibration adjustment.
A h~rd-wired bypass switch, also located on the front
J:!anel, isconvenient for checking the effect of the
compression, and assuresfailsafe flow of audio through
the unit. The maximum input level is +24 dB (12.3 V
RMS) and maximum output level is +23 dBm. The
output amplifiers have a source impedance of 47 ohms
and will drive input loads of 600 ohms or greater
impedance. Output gain is also adjustable over a very
wide range( + 20 dB), so the Model 165 is compat ible
with virtually all professiona l sound and creative audio
equipment.
Perhaps the most flexible and useful compressor /
+ 20
+ 15
1o+ 10
"O
_J + 5
UJ
>
UJ
_J 0
,-
::>
Cl. - 5
,-
::>
O - 10
- 15
4 :1
20: 1
oo:1
Ro tat ion Point Th reshold
- 15 - 10 - 5 0 + 5 + 10 + 15 + 20
INPUT LEVEL (dB)
3A -Convention al Comp ressor/ Limiter
limiter ever offered, the dbx Model 165 is well suited to
a wide range of applicat ions incl uding: tape record ing,
disc mastering, radio and TV production and broadcast,
Iive concert sound reinfo rcement, and theatrica l
production.
More About Separate Level Detector Access
Becauseauxiliary sound equipment can be used to
process the level detector signal but not the main audio
inpu t signal (or vice-versa), the 165 offers the user an
opportuni ty to create many unusual effects. By con-
necting it to additional signal processors, such asa
parametric equalizer, the Model 165 can be converted
to a de-esser,avocal stresser or a level-sensitive filter.
Certain musical or vocal elements in a program can be
suppressed without affecting othe rs. In addition to these
signal conditioni ng functions , many creat ive special
effects are possible.
More About Over Easy Compression
Conventional compressor /lim iters have a sharp knee
at the thresho ld point (see Figure 3A). That is, when the
input signal is below the threshold, the gain remains
fixed (1: 1 ratio or no compression), but when the input
signal goesabove the thresho ld, the gain abruptly
decreasesaccording to the compression ratio for wh ich
the unit is adjusted (several fixed compression ratios are
shown in Figure 3A). This abrupt change in gain in a
conventional compressor /limiter is of ten audible and
therefore undesirab le to the user.
The dbx Over Easy approach utilizes a soft knee
at the thresho ld of compression (see Figure 3B). The
Over Easy compressor/limiter gradually increasesits
compression ratio from 1:1 towards the set comp ression
ratio as the input signal risesthrough the thresho ld
region. Thus an age old dream can be realized ...
dynamic range restrict ion without audib le, abrupt gain
changes.When properly operated, the Model 165 is a
highly effective compressor /limiter that you don't
hear working.
+ 20
+ 15
co+10
"O
_J + 5
UJ
>
UJ
_J 0
,-
::>
Cl. - 5
,-
::>
O - 10
- 15
oo:1
\Thr esho ld
- 15 - 10 -5 0 + 5 + 10 +1 5 + 20
INPUT LEVEL (dB)
3B - Over Easy Comp ressor/ Limi ter
Fig. 3 - Over EasyTM Ver sus Conve ntional Compression
5

Since dbx Over Easy compressor/limiters have no
distinct point at which the gain changes, the threshold
on such units is defined differently from conventional
units. We define the threshold to occur approximately
mid way between the fixed gain portion of the curve and
the point where the curve "levels off" at the selected
compression ratio. At an infinite compression setting,
threshold is defined as the level at which 6 dB gain
reduction is realized. At this setting, the maximum
permissible output level is 5 dB above the threshold.
At lower compression settings, the threshold represents
somewhat lessgain reduction. (In contrast, a conventional
compressor/limiter yields OdB of gain reduction at the
threshold; gain reduction begins just above the threshold.)
To seehow the 165's Threshold indicator LEDs correlate
with the compression curves, refer to Figure 4.
+ 20
+ 15
iii+ 10
~
~ + 5
UJ
>
UJ
~ 0
I-
:,
a. - 5
I-
:,
O - 10
- 15
1:1
oo:1
"-~ ', RED
~'c>-c.. , Abov e threshold
' ~
', Over easy range
<v~
«,;
0«:- Below threshold
- 15 - 10 - 5 0 + 5 + 10 + 15 + 20
INPUT LEVEL (dB )
Fig. 4 - How The 165 Threshold Indicator LED's
Correlate With The Compression Curve
SIGNAL CONNECTIONS
Make input and output connections to the barrier
strip on the rear panel (Figures 5 and 6).
Input Connection
For balanced or floating lines, connect the signal leads
to the(+) and(-) terminals, and the shield to the chassis
ground (*)terminal. For unbalanced Iines, connect the
signal high lead to the (+) terminal, and jumper the (- )
and chassis ground (~ ) together for connection of the
shield. When using an unbalanced connection, reversing
the(+) and(-) input terminals will cause the output
signal to be 180° out of phase (reverse polarity) relative
to the input signal.
Level Detector Input Connection
For normal compressor operation, leave the factory-
installed straps connected between the Detector( +) and
the Input Signal (+) terminals and the Detector( - ) and
Input Signal( - ) terminals. (Refer to Figure 5.) If you
wish to gain accessto the 165's Detector Input for
insertion of an auxiliary device, you may do so by
removing the strapping, wiring the auxiliary device's
output to the 165's Detector Input term inals, and
feeding the auxiliary device's input with the same signal
fed to the 165's Signal Input. In certain situations, the
auxiliary device may need to be inserted in the signal
path, not in the detector path. In such cases,signal is
fed to the input of that device, and also to the detector
input, and the auxiliary device's output is fed to the
165 signal input. Unless the installation is permanent,
this connection can be awkward. Therefore, we recom-
mend wiring a few Tip/Ring/Sleeve phone jacks to the
165 instead, and using the jacks for Signal and Detector
Inputs aswell asfor the Signal Output. (Refer to
Figure 6.)
(0 PTI ONALI
RING SLEEVE
PHONE
PLUG
SIGNAL
1•NPUTI
SIGNA L
IOUTPU T I
6
T HIGH
R LOW
,s
f >HIElD
2 ----- 3
DETECTOR
INPUl
HIGH
LOW
..L 0
DETECTOR
INPUT
..L
XL ',,<-:C:.SH::,:IEc::L0::........,,.1,____ J RECOMMEND NOT
CONNECTING TH(
~ - SHIELOHERE
!UNLESS HUM
OEVElOPSI
CONNECTOR • ~--~
NOTE: Straps Connecting Signal and
Detector Inputs are shown in place.
HIGH
INPUT
5A - Balanced Configuration
SIGNAL
IIN PUT I
..L
StGNAL
IOU TPUTI
..L
HIGH
LOW
SHIELD
ICNO)
RECOMMEND_/
CONNECT ING
SHIELD HERE
!OPTIONAL I
STD
PHONE
PLUG
1 XL
CONNECTOR
---j-~- ------'~o
HIGH
LOW PHONO
lOW
SHIELD
INP UT
CONNEC T
JUMPER
HERE '--+-,I----+-+--.--- PlU G
SHIElO
DO NOT
CONNECT
SHIELD Al
TH IS ENO
TO JO IN
l.OW ( .) AN O
SHIELD I-ti
5B -Unbalanced Configuration
Fig. 5 - Typical Signal Connections to 165 Input & Output
ICNOI \ _____
US E OUAl CO NDU CTOR
SHIELDED CA81.E & " TIE"
TH E LOW & SHIEL D
TOGETHER AT THI$ POINT

s
0
OPTIONAL (FOR FEED TO AUXI LIARY
INPUT MU LT S IGNA L PROCESSOR>
NO TE: Straps
have been removed .
SIGNAL
INPUT
V
•
DETECTOR
INPUT
V •
. 1
,
ICNAL
UTPUT V
.
I I • ' -
' \ -
~
I~ (?;
•
DETECTOR
INPUT
.
I
~ ~
0
.
DETECTOR
INPUT
SIGNAL
I OUTPUT I
. ..1. .
-
e;r"b (2
~__./
Fig. 6 -Signal Input & Output Jacks With A Normalling
Jack For The Detector Input Dotted lines indicate location of
opt ion al mult jack (parallel wired) for feeding input of auxiliary
device with same program as 165 Signal Input.
Output Connection
The output of the dbx 165 is designed to feed balanced
or unbalanced 600 ohm or greater loads. The output
stage is single ended, so that in normal operation, the
(- ) signal output terminal is interna lly connected to the
(7) terminal. When the system bypass switch is engaged,
the signal inpu ts are directly connected to the signal
outputs (so a balanced input would produce a balanced
output).
Grounding
For maximum hum rejection, avoid common ground-
ing at the inpu t and output (i.e., double grounding). One
method that usually works is to ground the shield at the
165's output Ground (~) terminal and also ground it at
the input of the following device. Do not connect the
shield at the 165's input Ground terminal; leave the
input shield connected only to the output of the device
feeding the 165.
Stereo Coupler Cable
When you wish to link two Model 165's for processing
astereo program, a cable must be constructed to join
the Stereo Coupler connectors on the two units. Usethe
mating connectors supplied with each Model 165, and
wire the cable according to Figure 7. Useconnectors
supplied with units, or equivalent (Cinch-Jones
P-312-CCT). The cable should be 6-pair, twisted 24 ga.
wire with shield (Belden 9506).
Once the cable is connected to the two 165's, it may
be left in place at all times. When both units' Stereo
Coupler switches are placed in "master" mode (button
OUT), the 165's operate complete ly independe ntly;
stereo operation is achieved simply by switching one of
the two units to "slave" mode (button IN, Slave LED
ON).
Input Impedance & Terminations
There is sometimes a misunderstanding regarding the
nature of matching and bridging inputs, the use of
terminating resistors, and the relationship between
actual inp ut impedance and nominal source impedance.
Most electronic outputs work well when "terminated"
165STEREOCOUPLERCABLE
1 BLK -'
( ' RED 1
2 RED BLK 2
3 BLK WH 3
4 WH BLK 4
BLK
5 GN 5
GN
6 BLK 6
7 BLK BLU 7
8 BLU BLK 8
9 BLK BRN 9
10 RRN BLK 10
11 BLK BLK 11
12 YEL YEL 12
SHIELD
Fig. 7 - Stereo Coupler Cable
NOTE: The cable for the dbx Model 162 is not compatible with
the Mode l 165.
by an input (connected to an input) having the same or
a higher actual impedance. Outputs are usually over-
loaded when terminated by an impedance that is lower
than the source impedance. When the input impedance
is nearly the same impedance asthe source, it is known
asa "matching" input. When an input is 10 times the
source impedance, or more, the input is considered to
be a "bridging" input.
The dbx Model 165 signal inp ut has an actual
impedance of 22,000 ohms in balanced configuration
or 11,000 ohms in unbalanced (it has a high-Z* input).
This makes the 165 signal input suitable for use with
virtually any nominal source impedance, low or high.
The dbx signal input will bridge 150-ohm or 600 -ohm
(low-Z) lines.
The dbx 165 detector input has an actual impedance
of 600,000 ohms in balanced configuration or 300,000
ohms unbalanced. This very high impedance means
the level detector will bridge virtually any nominal
source impedance. It also means that when the jumper
connectors link the detector and signal inputs, the
detector has a negligible effect on th e 165's input
impedance as"seen" by the source device. The very high
impedance of the detector does suggest, however, that
cables from an external processor to the detector input
be kept asshort as practical to reduce susceptibil ity to
hum and RFI.
Terminating resistors are not needed for the dbx 165
signal or detector input s to operate correctly, but may
be required at these inputs when they are fed from
devices (such aspassiveequalizers) designed for a
specified load impedance.
The dbx 165 output iscapable of driving loads of
600 ohms or greater. While it may be term inated by a
low impedance, such termination is not requir ed.
*"Z" is an abbreviation fo r "impedance."
7

OPERATION & APPLICAT IONS
Additional Inform ation on the Setting and
Function of Certain Controls.
Compression Control
This contro l is contin uously adjustable from a com-
pression ratio of 1:1 to infinite compression (that is, no
change in output level regardless of changes in input
level above the set threshold). High compression (settings
greater than 6) significantly level out program materials.
Infinite compression virtually stops music levels from
exceeding the thresho ld setti ng. Lower compression
settings (below 4) still permit dynamic range to exist.
They are used to tighten up the sound of a bassguitar,
lead guitar, snare drum, kick drum and vocals. Moderate
overall compression is typically used during stereo
mixdowns. Here a variety of settings could be used.
While any compression ratio can be set with this
control, remember that the Over Easy curve causes
that ratio to be approached gradually asthe input
signal level risesthrough the threshold . Only when the
input signal isconsiderab ly above the threshold does the
165 closely approach the set ratio of compression.
(See Figure 3.)
Threshold Adju stm ent & LED Indicator s
The Threshold contro l sets the level at whic h the 165
begins to compress the signal (i.e., it sets the point
where the Over Easy compression curve intersects the
input signal). The contro l has a wide rangeso that the
desired results can be obta ined with any line level input
signal. When the Threshold contro l is set too low, the
165 wi ll compress most if not all of the input signal
(red LED ON most of the time). At low compression
ratios, the very low threshold sett ing can be used to
gently reduce the overall dynamic range of the program.
Low threshold settings (- 20 to -4 0) are used when
program materials are to be compressed. Low settings,
with high input levels,cause the 165 to "work" all or
more of the time than with lower input levels. The
major portion of program mater ial is processed with a
low threshold setting. Normal compression and "leveling"
of vocals and instruments typical ly use lower settings.
Compression of the whole program, however, may not
sound natural, especially at compression ratios of 1O:1
or greater. High threshold settings (-10 to +10) are
used for limiting program levels or where only peaks are
to be compressed. The major portion of the program
material is not processed with a high threshold setting.
Speaker protection and peak overload prevention are
just two applications.
NOTE: Whenever threshold level and compression ratio
settings are made, it is important to watch the LED's
and meter for reference and confirmation. Remember
that a compressor/limiter is a tool that can provide
desirable effects when used properly. When used to
excess, the results can sound unusual, and may be of
value only for special effects.
With the Compression contro l set for the desired
rnaximum compression ratio (or an approx imation
thereof), rotate the Thresho ld control counte rclockwise
until the desired sound, special effect, or amount of gain
reduction is achieved.
Auto Switch & Auto Attack/ Release LED Indicator
When the Auto switch is IN (Auto Mode), the LED
indicator will be illuminated and the Model 165 will
automatically adjust its attack rate and releasetime to
suit the program envelope. (This Auto Model sets the
Model 165 for the same attack and releasecharacteristics
asdbx Models 160, 161, 162, 163 and 164 compressor/
8
limiters .) When the Auto switch is OUT (manual mode),
the LED indicator above it turns OFF, and the front
panel Attack and Releaserate controls determine the
maximum rate of gain change and the behavior of the
level detector circuitry (see below) .
Attack Rate & Release Rate Controls
The Model 165 is the fir st dbx compressor/ limiter to
offer a choice of automatic or user adjustable attack and
releasecharacter istics. In Auto Mode, the 165 utilizes
the patented dbx RMS level detector with its program
dependent attack/release characteristics to obta in
natural-soundi ng compression or limit ing. For special
effects and certain signal situations, however, it is often
desirable to set fix ed attack and releasecharacteristics.
Manual mode affords this capability. The Auto Mode is
recommended for vocals aswell asinstrume nts. When
determining separate attac k and releaserate control
settings, it prov ides a good starting poin t . Becausethe
Auto Mode has a variable attack rate, the 165 may
compress or limit some program mater ialssmoother
than in the manu.al mode which has a fixed rate of
attack. Th is is especially true on vocals.
Where To Set Attack & ReleaseControls
There is no "right" way to set the Attack and
Releasecontro ls. Generally, one wou ld want a slow
enough Releaseto avoid "pumping" or "breath ing"
sounds caused when background sounds are audibly
modulated by the dominant signal energy, yet the
releasemust be fast enough to avoid suppression of the
desired signal after a sudden transient or a loud note has
decayed. Depending on the desired effect, one might
want avery slow Attac k so that percussive or transient
sounds are not restricted, but averagevolume levels are
held within the desired range.
A very fast attack setting (control maximum counter-
clockwise) will cause the 165 to act like a peak li mite r
even though RMS detection circuitry is used. Slower
attack settings cause the 165 to act like an RMS or
averaging detecting compressor/limiter. Don 't forget,
attac k and releasecontro ls operate toget her and wit h
the compression ratio contro l. Changing any one contro l
may necessitate changing the other settings.
Meter Calibration & Use
The meter in the 165 is factory calibra ted to
indicate " O" when the signal level is +4 dB (1.23 V)
at either the input or outpu t of the 165, depending
on the meter funct ion switch posit ion. (When the
meter is in Gain Change mode, "O" indicates no gain
change; the meter calibration contro l does not affect
this mode.)
To recalibrate the meter, engagethe Input meter
function switch and feed a 1 kHz signal at the selected
nom inal operating level (the level desired for a "O VU "
meter indicatio n) to the 165's signal input. Then adjust
the rear panel METER CAL IBRATIO N cont rol unt il the
meter indicates "O dB".
Useof an Equalizer in the Level Detector Circuit for
Frequency Weighted Compression, De-essing, or
IncreasingSustain
It is possible to "separate" certai n instruments from
a mix by frequency weighted compressio n. This function
is created by insert ing an equalizer ahead of the Model
165's level detector input, but not in the main signal
input path. Peaking the equalizer in a certain frequency
range wil l tend to suppressany freq uencies (notes) in
that register. A relatively high th reshold setting can
allow normal sounds to be unaffected whi le solo or very

loud sounds are held back. Of course, when compression
does occur, the level of the entire program is affected.
For this reason, it may be more useful to reserve this
combined EO/compression technique for isolated sound
sources, such asa single channel of vocal or a single
instrument in a multi-track program. Uniike overalI
program equalization, EQ of the level detector will take
effect only when signals are above threshold (or when
those frequencies affected fall above the set threshold).
Depending on the threshold setting, lower-level funda-
mentals or harmonics will not cause compression, and
the program is not subject to the phaseshift normally
caused by program equalization.
For example, consider a single channel carrying the
preamplified signal from a microphone placed near a
cymbal and a tom-tom. Set up the 165 with an equalizer
in the level detector path, as depicted in Figure 8. The
equalizer can be adjusted for boost with a peak at about
5 kHz, causing the cymbal to be compressed on a very
loud crash, preventing tape saturation at high frequencies
where there is lessheadroom. However, gentle tapping
of a drumstick or brushing of the cymbal will not be
held back. Assuming the tom-tom is a lower frequency
instrument, and can be better tolerated by the tape,
there is less need for compression on it. The equalization
in the detector circuit means that the compressor will
not be triggered asreadily by a loud tom beat as by an
equally loud cymbal crash.
Another application for this type of EQ boost in the
level detector is for increasing the sustain of a guitar, bass,
etc; this requires EO boost in the dominant frequency
range of the instrument, along with a fairly low threshold
and a moderate compression ratio.
Still another related application involves de-essing of
vocals (reduction of sibiIance). Use a parametric equalizer
in the level detector circuit and set it for high frequency
boost in the specific frequency range where the vocal
"hiss" or lisp occurs. This pre-emphasizes the already
"hissy" vocal input to the detector. Used in conjunction
with a moderate to high threshold and compression
ratio, this arrangement greatly attenuates the "essing"
without affecting the basic sound quality or balance of
the voice. While it is true that all frequencies are
lowered in level when the compressor is triggered,
generally the "sss" sound occurs alone, before or after
the dominant tone in the voice. (If this seemsunlikely,
just try to hum and hiss at the same time.)
The converse of the above EQ techniques may be
used; dipping the equalizer will cause any sound in the
affected register to pull the level up because it will seem
to require lesscompression than the other frequencies.
-
-·
i
INPUT OUTPUT
FILTER OR
EQUA LIZER
PROGRAM
SOURCE
,-~.-=::.....
+ t - ..,.,.,_,.
•
-·- 0
Fig. 8 - Equalizer or Filter Used with the Level Detector Input
Use of a Filter in the Level Detector Circuit
The results of inserting a filter in the level detector
circuit are basically the same as obtained with an equali-
zer, as previously described. Those frequencies passedby
the filter are subject to compression (or at least they
are subject to c.onsiderably more compression than those
frequencies outside the passband). Becausea passive
filter can have in$ertion loss, it may be necessary to
lower the 165's Threshold setting to maintain a given
amount of gain reduction within the filter passband; this
can be determined, as usual, by monitoring the 165's
thresho ld indicator LEDs.
Use of a Tim e Delay Line in the Signal Path but not in
the Level Detector Circuit for Zero or Negative Attack
& ReleaseTimes
While the Model 165 can be set for incredibly fast
attack times, there will always be some small transient
that "gets past" the level detector. In some cases- such
asmaximum modulation broadcasting - it may be
desirable to preserve the sonic quality obtained with a
slower attack time, yet it may not be permissible for
even the slightest overshoot to get past the compressor/
limiter. A delay line (digita l or analog) can be used in
this instance. By feeding the program directly to the
165's detector input, but delaying the feed to the 165's
signal input, the unit can "anticipate" the need for a
gain change. (Refer to Figure 9.) With some experimen -
tation, the effect can be that of "zero" attack time.
Additional signal delays beyond the "zero" time
established above would then cause the compressor to
finish changing gain before the leading edge of the loud
passageenters the signal input, suppressing program
which is not above threshold. Also, the 165 would begin
to recover from compression (release) before the input
signal has dropped back to the set threshold, causing the
output to surge higher in level as the note or passageis
decaying. This special effect obtained with the time
delay might sound akin to reverse playback of a tape
recording.
Access to the 165's level detector makes possible
a whole range of effects not normally available. The
more you think about it and experiment, the more use-
ful this capability can become.
PROGRAM
SOURCE
.-I- ---- ~ DELAY
INPUT OUTPUT LINE
,-=.....
-
.,.....
-·
•·••·---·· •
......
......._
-·-
Fig. 9 - Delay Line Used With The Signal Input
The 165 As A Line Ampl ifier
0
To use the 165 asa Iine ampIifier, adjust the
COMPRESSION RATIO control to fully counterclock-
wise (1: 1 position), THRESHOLD to full clockwise
position (+10) and OUTPUT GAIN to whatever setting
is required for the application. Remember that, as with
any amplifier, excessive gain may lead to output clipping
of high level signals. To add compression, adjust the
COMPRESSION RAT IO and the THRESHOLD controls
to the desired settings.
9

SPECI FlCATIONS
Input Characteristics
IMPEDANCE
Signal: 22 kohms, balanced; 11 kohms, unba lanced
Detector: 600 kohms, balanced; 300 kohms, unbalanced
LEVEL
Signal: >+24 dB (12.3 V) maxi mum
Detector : >+28 dB (18.6 V) maximum
Output Characteristics
IMPEDANCE
Less than 47 ohms (active low Z outputl
LEVEL
>+23 dBm (1 1 V) into a 600 ohm or higher Z termination
Performance Characteristics
DISTORTION
2nd harmonic, 0.05%
3rd harmonic, 0.2% (Auto or Manual with Attack &
Release Controls Centered}
NOTE: Distortion figures are typical at infinite compression,
1 kHz, OdBm (0.775 V) input and output. 2nd harmonic is
relatively unaffected by compression ratio, time constants and
frequency while 3rd harmonic decreases with slower time
constants, higher frequency and lower compression ratio .
EQUIVALENT INPUT NOISE
Less than - 90dBm, 20 Hz to 20 kHz
ATTA CK RATE* (63% dB reducti on in Signal Level}
Auto Mode: 15 mS for 10 dB level change, 5 mS for 20 dB
level change. 3 mS for 30 dB level change
Manual Mode: Continuously variable from 1 to 400 dB per
mi IIisecond.
RELEAS E RATE*
Auto Mode: 120 dB/second
Manual Mode: Conti nuously variable fro m 10 to
4000 dB per second.
FREQUENCY RESPONSE
+O, - 1 dB from 20 Hz to 20 kHz
Controls
THRESHOLD
Conti nuously variab le from - 40 dB (7.8 mV} to+ 10 dB
(2.45 V}
COMPRESSION RATIO
Cont inuously variab le from 1:1 to infinity :1
ATTA CK RATE
Continuously variab le from 400 dB per mil lisecond 10
1 dB per mi llisecond
RELEASE RATE
Continuously variable from 4,000 dB per second to
10 dB per second
OUTPUT GAIN
Continuous ly variable from -20 dB 10 +20 dB
Metering
RANGE
30 dB (fro m - 20 to +10 VU)
FUNCTION
Switc hable for input level, output level or gain change
CALIBRATION
Factor y preset at O VU ; +4 dB (1.23 V}; rear panel
potentiometer sets O VU for any level from -1 0 dB
(7.8 mVI to +10 dB (2.45 V}.
Connectors
SIGNAL
Jones type barrier strip for signal input, signal output,
and detector input.
STEREO COUPLER
12 pin Cinch-Jones J-312 -CCT connector for strapp ing
two 165's toget her to process a stereo program.
General
POWER REQUIREMENTS
117 V AC + 10%, 50 or 60 Hz; 234 V AC, 50 or 60 Hz
available on special order for use outside the U.S.
DIMENSIONS
19" wide x 3-1/ 2" high x 10-1/8" deep
(48.3 cm x 8.9 cm x 25.7 cm}; suitab le for rack mounting
NET WEIGHT
8 Pounds (3.6 kg}
dbx PRODUCT WARRANTY &
FACTORY SERVICE
All dbx products are covered by a lim ited warrant y.
Consult your warranty card or your local dealer for full
details.
The dbx Customer Service Depart ment is prepared to
give addit ional assistance in the use of this prod uct. All
questions regardi ng interfacing dbx equi pment with your
system, service information or infor mation on special
applications will be answered. You may call during
normal businesshours - Telephone: [phone],
Telex: 92-25 22, or wri te to :
dbx, Inc_
71 Chapel Street
Newton, MA 02195
Attn: Customer Service Department
Should it become necessary to have your equipment
factory serviced:
1. Pleaserepack the unit, including a note describing
the problem along with the day, month and year of
purchase.
2. Send the unit freigh t prepaid to:
dbx, Inc.
224 Calvary Street
Waltham, MA 02154
Attn: Repair Department
3. We recommend that you insure the packageand
send it via United Parcel Service wherever possible.
4. Pleasedirect all inquiries to dbx Customer Service
Department .
Outside the U.S.A . - contact you r nearest dbx dealer
for the name and address of the nearest autho rized
repair center.
•Measured in the infinite compression region of the over easy curve.
10

_.
Signal
Input
(Differential)
I
I
I
I
I
I
I
I
I
I
(Normally, Signal and Detector
Inputs are tied togetherl
In
Meter
Detector Input
(Differential) ') I In
To From
Stereo
Interconnect
NOTE : Stereo Interconnect is not shown in fu ll detail.
Meter Inputs are 1101 shown in full detai l.
Voltage
Controlled
Amplifier
Out
Slave Master
1
l
From To
J
r
Stereo
Interconnect
Stereo
1
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(RMSI
2
Smal l
Cap I
-
•
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J.
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-
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2 I t
Manua l
Automati c
.j,
Large Cap
L.
Threshold
Compression
Ratio
Output
Gain
i
Bypass
Normal
J
t
Signal Output
(Single -Endedl
(Differentia l in Bypass)
Threshold
Lights
@©~
Automatic
Manual
!
In
Time
Constant
Generator
Attack Release
Out
OJ
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GLOSSARY
Asperity Noise
This is a swishing type of background noise that occurs with tape
recordings in the presence of strong low frequency signals, especially
when there are no high frequency signals to mask the hiss. Asperity
noise is caused by minute imperfections in the surface of the tape,
including variations in the magnetic particle size in the tape's oxide
coating. The imperfections inc rease or decrease the strength of the
magnetic field passing the play head in a random manner, resulting
in audible noise. Asperity noise may be present even when no
program is recorded. When a program is recorded, asperity noise
becomes superimposed on the signal, creating modulated asperity
noise, or "modulation noise." Using high-quality tape with a
calendered surface helps reduce asperity and modulation noise
(calendered tape is pressed smooth by high-pressure rollers).
Attack Time
Attack time may mean different things, depending on the
context. In music, the time it takes for a note to reach its full
volume is the attack time of the note. Percussive instruments have
short attack times (reach maximum volume quickly) and wind
instruments have lon'g attack times (reach maximum volume more
gradually).
When a compressor (or expander) changes the level of an incom-
ing signal, the circuitry actually requi res a finite amount of time to
complete that change. This time is known asthe attack time. More
precisely, the attack time is the interval (usually measured in milli-
seconds or microseconds) during which the compressing or expand-
ing amplifier changes its gain from the initial value to 90% of the
fina l value.
Aux Input (Aux Level)
Aux inputs, an abbreviation for auxiliary inputs, are low
sensitivity jacks provided on most hi-fi and semi-professional
equipment. Aux inputs (also know n as "aux level" or "line level"
inputs) have "flat" frequency response and are intended to be used
with preamplified signals. Aux-level (line-level) signals are medium-
level, higher than microphone levels, but not enough power to
drive a speaker. The advantage to these levels is that they are less
susceptible to hum and noise than are microphone levels. Typ ical
items which might be connected to aux inputs are tape machine
"play" outputs, tuner outputs, and dbx "play" outputs. Mic-level
or phono-level signals are considerably lower in level than aux inputs
(approx. -60 to -40dBV), so they will not produce adequate volume
when connected to an aux input. Moreover, phono cartridge outputs
require R IAA equalization which is not provided by aux inputs .
Bandwidth
Bandwidth refers to the "space" between two specific
frequencies which are upper and lower limits; alternately, band-
width refers to the absolute value of the range of frequencies
between those limits. Thus, a filter which passesfrequencies from
1,OOOHzto 1O,OOOHzmay be said to have a bandwidth of 1kHz -
1OkHz, or it may be said to have a 9kHz bandwidth (10kHz minus
1kHz equals 9kHz).
Bandwidth is not necessarily the same as frequency response.
Bandwidth may be measured at low levels, and frequency response
at higher levels. Moreover, bandwidth may refer only to certain
portions of the circuitry within a piece of equipment, whereas
frequency response may refer to the overall performance of the
equip ment . Thus, while the overall input-to-output frequency
response of dbx type II equipment is 20Hz to 20kHz, the band-
width of the RMS detection circuitry within that equipment is
30Hz to 1OkHz.
Bass
The low audio frequency range below approximately 500Hz.
For the purpose of discuss ion or analysis, the bass range may be
further divided into upper bass (250 to 500Hz), mid bass
(100-200Hz), low bass (50-100Hz), and ultra-low bass (20-50Hz).
BassBoost
An accentuation of the lower audio frequencies (bass frequen-
cies), whereby they are made louder than other frequenc ies.
Biampl if ied
Descriptive of a sound system which utilizes a low level cross-
over network to divide the ful l-spectrum audio signal into low and
high frequency ranges. These ranges are then fed to separate
power amplifiers, which in turn feed low frequency speakers
(woofers) and high frequency speakers (tweeters).
Bias
Bias, as the term is used in tape recording, is a very high fre-
quency signal (usually over 1OOkHz) that is mixed with the
program being recorded in order to achieve linear magnetization of
the tape. If only the audio program were applied to the recording
head, a very distorted recording would result because lower-ene rgy
portions of the program would not be able to overcome the initia l
magnetization threshold of the tape (known as hysteresis).
The frequency of the bias signal is not critica l, so long as the
record and erase bias are synchronized. However, the bias
energy level has a direct effect on the recorded level, background
noise, and the distortion. It is sometimes necessary to reset the bias
level for optimum performance with different types of record ing
tape, and professional tape mach ines are equipped wit h continu-
ously variable bias controls; many consumer tape machines are now
equipped w ith bias selector switches.
Clipping
Clipping is a very distorted sound. It occurs when the output
capabilities of an amplifier are exceeded, and the amp can no longer
produce any more voltage, regardless of how much adciit ional gain
or how much more input signal is present. Clipp ing is relatively easy
to see on an oscil Iiscope, and it is sometimes audible as an increase in
harmon ic distortion. In severe casesof clipping (hard clipping),sine- ·
waves begin to resemble square waves. and the sound quality is very
poor. Often, the maximum output level of an arnplifier is defined
3S that level where clipping begins to occur. There is a phenomenon
known as inp ut clipping, and this may occur where the input signal
is so high in level that it exceeds the level-handling ability of the
transformer and/or of the input amplifier. Clipping also occurs
when tape is saturated by excessive record levels.
So-called "soft clipping" is usually the result of transformer
saturation, and it may be somewhat lessobjectionable than the
"hard clipping" that occurs when output voltage limits are reached.
Aside from degrading the sound quality, clipping can damage loud ·
speakers. Output clipping may be avoided by reducing the level of
the input signal, reducing the gain of the amplifier, or using a
larger amplifier. Input clipping may be avoided by reducing the
level of the incoming signal, and then increasing the gain of th e
amp Iifier.
Clipping Level
This is the signal level at wh ich clipping just begins to occur.
Clippi ng level is not always easy to define. It may be a matter of
visually judging the waveform on an oscilliscope as the level is
increased; alternatP.ly, clipping level may be defined as the level at
which harmonic distortion reaches a given value. Tape clipping, or
saturation, is defined as the 3% harmonic distortion level.
Compression
Compress ion is a process whereby the dynamic range of program
material is reduced. In other words, the differenc e between the
lowest and highest audio levels is "squeezed" into a smaller dynamic
range. A compressed signal has higher average level, and therefore
may have more apparent loudness than an uncompressed signal,
even though the peaks are no high er in level . Compressio n is
achieved with a compressor, a special type of amplifier that
decreases its gain as the level of the input signal increases. The
amount of compression is expressed as a ratio of the input dynam ic
range to the output dy nam ic range; thus, a compressor that takes
a program input with 100dB of dynam ic range and yields an output
program of 50dB dynamic range may be said 10 have a 2:1 com -
pression ratio.
Compressor
A compressor is an amplifier that decreases its ga,n as the level
of the input signal increases to reduce the dynamic range of the
program (see "compression"). A compressor may opera te over the
entire range of input levels, or it may operate only on signals above
and/or below a given level (the threshold level) .
Crossover Frequency
In loudspeaker systems and multi-amp lifi er aui:iio systems, the
transition frequency (actually a frequency range) between bass and
midcange or midrange and treb le speakers or amp lifiers.
Crossover Network
A circ uit wh ic h divides the audio spectrum into two or more
frequency bands for distribution to different speakers (high level
crossover) or different amp lifiers which then feed different
speakers (low level crossover).
High level crossovers are usually built into the speaker cabinet,
and are passive (they require no po1111ersupp ly ). Low level cross-
overs are used in biamplified or triamplified sound systems. They
are usually self-contained, and come before the power amp lifiers
Low level crossovers may be passive or act ive; active lovv level
crossovers are kno\{\/n as "electronic crossovers."
Damping Factor
The ratio of loudspeaker impedanc e to the amplifier 's output
source impedance. Dampi ng describes the amplifier's ability 10
prevent unwanted, residual speaker movement. The higher the
numerical value, the better the damping.
DB (Decibel) also. dBv dBV dB SPL dBm dB
One dB is the smallest change in loudness the average human ear
can detect. OdB SPL is the threshold of human hearing whereas the
threshold of pain is between 120 and 130dB SPL. The term dB is an
abbr~viation for decibe l, or 1/10 of a Bel. The decibel is a ratio , not
an absolute number, and is used 10 express the difference betwee n
two power, voltage or sound pressure levels. (dB is 10 times the

logarithm of a power ratio or 20 times the logarithm of a voltage
or sound pressure ratio.) If the number of "dB's" are referenced to
a given level, then the value of the dB number becomes specific.
dBv expresses a voltage ratio. OdBv is usually referenced
to 0. 775V rms. Thus, OdBv=O.775V, +6dBv=1.55V
(twice OdBv), +20dBv=7.75V (ten times OdBv), etc.
dBV expressesa voltage ratio and is similar to dBv, but
OdBV is usually referenced to 1V rms. Thus, OdBV is 2.2dB
higher than OdBv.
dB SPL expresses a Sound Pressure Level ratio. dB SPL is a
measure of acoustic pressure (loudness), not acoustic povver,
which would be measured in acoustic watts. OdB SPLis
equal to 0.0002 dynes/square centimeter (the threshold of
human hearing at 1kHz). As with dBV, an increase of 6dB SPL
is twice the sound pressure, and an increase of 20dB SPL
is an increase of 10 times the sound pressure.
dBm expressesa power ratio. OdBm is 1 milliwatt (.001
watts), or 0.775V rms delivered to a 600-ohm load. +3
dBm=2 milliwatts, or 1.55V into 600 ohms (twice OdBm),
+10dBm =10 milliwatts, or 7.75V into 600 ohms (ten
times OdBm), etc. dBV and dBm are numerically equal
when dealing with 600-ohm circuits. However. when the
impedance is other than 600 ohms, the value of dBV
remains the same if the voltage is the same, whereas the
value of dBm decreases with increasing impedance.
dB alone, without any suffix, doesn't mean anything
unless it is associated with a reference. It may express the
difference between two levels. Thus, the difference
between 1OdBV and 15d BV. the difference between
OdBm and 5dBm, and the difference between 90dB SPL
and 95dB SPL are all differences of 5dB.
Decay Time
Decay time may mean different things, depending on the con-
text. A compressor's decay time is also known as its release time
or recovery time. After a compressor (or expander) changes its
gain to accommodate an incoming signal, and the signal is then
removed, the decay time is the amount of time required for the
circuitry to return to "normal." More precisely, the decay time
is the interval (usually measured in microseconds) during which
the compressing or expanding amplifier returns to 90% of the
normal gain. Very fast decay times can cause "pumping" or
"breathing" effects, whereas very slow decay times may cause
moderate-level program which follows high-level program or pro-
gram peaks to be too low in level.
Decoder
When a circuit restores an original program from a specially
treated version of that program, the circuit may be said to decode
the program. The equipment or circuit which performs this
function is known asa decoder. Decoders must be used only with
programs which have been encoded by complementary encoding
circuitry. Typical decoders include: FM tuners that use multiplex
decoders to extract left and right stereo signals from left -plus-right
and left-minus-right signals, matrix quadraphonic decoders that
extract four channels of program from the stereo program on
encoded recordings, and dbx decoders that retrieve wide-dynamic
range programs from the compressed programs on dbx-encoded
recordings.
D&-emphasis& Pre-emphasis
De-emphasis and pre-emphasis are related processes that are
usually done to avoid audio noise in some storage or transmission
medium. Pre-emphasis is a boost at specific higher frequencies, the
encoding part of an encoding/decoding system. De-emphasis is an
attenuation at the same frequencies, a reciprocal decoding that
counteracts the pre-emphasis. In dbx noise reduction, de-emphasis
is performed by the decoder (the play circuitry). The de-emphasis
attenuates high frequencies, thereby reducing tape modulation
noise and restoring the original frequency response of the program
before it was dbx encoded. There are other types of pre-emphasis
and de-emphasis. For example, in FM tuners, de-emphasis is used
to compensate for special equalization (known as 75-microsecond
pre-emphasis) applied at the station's transmitter.
Dynamic Range
The dynamic range of a program is the range of signal levels
from the lowest to the highest level. In equipment, the dynamic
range is the "space," in dB, between thr residual noise level and
the maximum undistorted signal level. A program with wide
dynamic range has a large variation from the softest to the loudest
passages,and will tend to be more lifelike than programs with
narrow dynamic range.
Encoder
When a circuit processes an original program to create a
specially treated version of that program, the circuit may be said
II
to encode the program. The equipment or circuit which performs
this function is known as an encoder. Encoded programs must
decoded only with complementary decoding circuitry. Typical
encoded programs include: FM multiplex broadcasts, matrix
quadraphonic recordings, and dbx encoded recordings.
Envelope
In music, the envelope of a note
describes the change in average signal
level from initial attack, to peak level,
to decay time, to sustain, to release
time. In other words, the envelope
describes the level of the note as a
function of time. Envelope does not ,.,,........ '""' ,m,lope,
refer tQ frequency. theJl.t,,.i i1wilhin ~en11tlope ,
In fact, any audio signal may be said to have an envelope. While
all audio frequencies rise and fall in instantaneous level from 40 to
40,000 times per second, an envelope may take many milliseconds,
seconds or even minutes to rise and fall. In dbx processing, the
envelope is what "cues" therms level detection circuitry to com-
press and expand the signal; the peak or average level of individual
cycles of a note would be uselessfor level detection because the
gain would change much too rapidly for audibly pleasing sound
reproduction.
EQ (Equalization)
EQ or equalization, is an intentional change in the frequency
response of a circuit. EQ may be used for boosting (increasing) or
cutting (decreasing) the relative level of a portion of the audible
spectrum. Some EQ is used for achieving sound to suit personal
listening tastes, while other types of EQ are specifically designed
to correct for non-linearities in the system; these corrective EQ
"curves" include tape (NAB or CCIR) equalization, and phono-
graph (RIAA) equalization. In a sense, the pre-emphasis and de-
emphasis used in dbx processing are special forms of equalization.
There are two common types of EQualization curves
(characteristics): PEAKING and SHELVING. Shelving EQ is
used in most Hi-Fi bass and treble tone controls. Peaking EQ is
used in Hi-Fi midrange tone controls, in graphic equalizers, and
many types of professional sound mixing equipment.
EQ is performed by an equalizer, which may be a specially built
piece of equipment, or it may be no more than the tone control
section of an amplifier. Graphic equalizers have many controls,
each affecting one octave, one-half octave, or one-third octave of
the audio spectrum. (An octave is the interval between a given tone
and its repetition eight tones above or below on the musical scale;
a note which is an octave higher than another note is twice the
frequency of the first note.)
Expander
An expander is an amplifier that increases its gain as the level of
the input signal increases, a characteristic that "stretches" the
dynamic range of the program (see "expansion"). An expander may
operate over the entire range of input levels, or it may operate only
on signals above and/or below a given level (the threshold level).
Expansion
Expansion is a process whereby the dynamic range of program
material is increased. In other words, the difference between the
lowest and highest audio levels is "stretched" into a wider dynamic
range. Expansion is sometimes used to restore dynamic range that
has been lost through compression or limiting done in the original
recording or broadcast; expansion is an integral part of com-
pander-type noise reduction systems, including dbx. Expansion is
achieved with an expander, a special type of amplifier that increases
its gain as the level of the input signal increases. The amount of
expansion is expressed as a ratio of the input dynamic range to
the output dynamic range; thus, an expander that takes a program
input with 50dB of dynamic range and yields an output program
of 1OOdBdynamic range may be said to have a 1:2 compression
ratio.
Fundamental
A musical note is usually comprised of a basic frequency,
plus one or more whole-number multiples of that frequency.
The basic frequency is known as the fundamental, and the
multiples are known as harmonics or overtones. A pure tone
would consist of only the fundamental.
Ground Compensated Output
This is a sophisticated output circuit that sensesthe potential
difference between the ground of the dbx unit and the shield
ground of unbalanced inputs to which the dbx unit is connected.
Ideally, the dbx unit and the input of the following device should
be at the same level (potential). However, where grounding is not
"right" (where so-called "ground loops" exist), this circuit calculates
the ground error and adds a correction signal to the high side of the
output, thereby cancelling much of the hum, buzz and noise that
might otherwise have been introduced by ground loops.

Harmonic Distortion
Harmonic distortion consists of signal components appearing
at the output of an amplifier or other circuit that were not present
in the input signal, and that are whole-number mult iples (harmonics)
of the input signal. For examp le, an amplifier given a pure sine-
wave input at 1OOHz may produce 200Hz, 300Hz, 400Hz, 500Hz,
600Hz and even 700Hz energy, plus 100Hz, at its output (these
being the 2nd, 3rd, 4th, 5th, 6th and 7th order harmonics).
Usually, only the first few harmon ics are signi fica nt, and even-order
harmonics (i.e. 2nd and 4th) are le$Sobject ion ab le than odd-order
harmonics (i.e. 3rd and 5th) ; higher harmonics may be
negligible in comparison to the fundamental (100Hz) output.
Therefore, rather than specifying the level of each harmon ic com -
ponent, this distortion is usually expressed as T.H.D . or Tota l
Harmon ic Distortion. Whi le T.H.D. is the total power of all
harmonics generated by the circuitry, expressed as a percentage
of the total output power, the "mixture" of different harmonics
may vary in different equipment w it h the same T .H.D . rating.
Harmonics
Overtones which are integral multiples of the fundament al.
Headroom
Headroom refers to the "space," usually expressed in dB,
between the nomina l operating signal level and the maximum signal
level. The in put headroom of a circuit that is meant to accept
nominal -1OdB levels, but can accept up to +18d8 without
overdrive or excessive distort ion, is 28d B (from -10 to +18 equa ls
28dBl. Similarly, the output headroom of a circuit that is meant
to supply nominal +4dBm dr ive levels, but that can produce
+24d8m before clipping is 20dB . A circuit that lacks adequate
headroom is more li kely to distort by clipping tra nsient peaks,
since these peaks can be 10 to 20d8 above nominal operating
signal levies.
I.M . (Intermo dula t ion Distortion)
Intermodulation distortion consists of signal components
appearing at the output of an amplifier or other circuit that were
not present in the input signal, that are not harmon ically related to
the in put, and that are the result of in teract io n between two or
more in put frequencies. I.M. distortion, like harmonic d istort ion, is
usually rated as a percentage of the total output pow er of the
device. Wh ile some types of harmonic distort ion are musical , and
not particularly objectionable, most I.M. distortion is unp leasant
to the ear.
Impulse Response
Related to the rise 1,me of a circui t, the impu lse response ,s a
measurement of the abil ity of a circuit 10 respond to sharp sounds,
such as percussion instrumen ts or plucked strings . A circuit with
good impluse response would tend to have good tra nsient response.
Level Match
The dbx noise reduct ion system is unl:ke compet it ive systems
in that there is no one threshold at which compress ion or expansio n
begins. Instead, compression occurs linearly, with respect to
decibels, over the full dynamic range of the program. By necessity,
there is an arbitrary signal level which passes through the encoder
and decoder without being changed in level. This level 1sknown as
the level match point (transition point). Some dbx equipme nt
prov ides for user adjustment of the level matc h po int, fo r monitor-
ing purposes only. Although th is is not necessary for proper encode/
decode performance, by setting the level matc h point to be approx i-
mately equal to the nominal (average) signal leve!, there wil l be no
increase or decrease ,n level as you switch from mon,toring "live"
program 10 mon1ton!'lg dbx-processed program.
L imiter
A lim iter is a type of compressor, one wnh a 10 : 1 or greater com-
pression ratio. A lim iter w ith a high compression ra110(120 :1) can
be set so th at no amount of increase in the input signal w ill be able
to ra,~e the outp ut level beyond a preset val ue. Th e difference
between limit ing and compression is that compression gently
"shrinks" dynamic range, whereas limit ing is a way to p lace a fixed
"ceil ing" on maximum leve l, without chang ing the dynamic range
o f program belo w that "ceiling," or threshold.
Line Level (Line Input)
Line level refers 10 a preamp lif,ed audio s,gnal, in contrast to
mic level , wh,ch desc ri bes a lower -level aud io signal. The actual
signal levels vary. General ly, mic level is nom,nally -50d8m (w ,t h
typical dyn am,c range of -64dBm to +1OdBm). Line level signals
vary, depending on the audio system. H1-F1line levels are nom ,nally
-15dBV, whereas professiona l l,ne levels are nominal ly +4dBm or
+8d8m (wi th typical dynam,cs ranging from -50d8m 10 +24d8m ).
Line ,nputs are simply inputs that have sensitivi ties ,ntended for
line level (preamplified) signals. Often, the nominal impedance of a
line leve l inpu t will be different than the nom inal impedance of a
mic level ,nput.
II I
M odulation Noise
Modulation noise is a swishing type of background hiss that
occurs w ith tape recordings in the presence of strong low frt!quency
signals. The noise depends on the level of the recorded signal; the
higher the recorded signal level, the higher the modulation noise.
Modu lation noise has typica lly been "masked," hidden by the
dominant signal and/or by the background hiss of the tape. How-
ever, when the background hiss is removed, as with dbx processing,
modu lation noise could become audible. This would happen
primar il y with strong, low -frequency signals, but in fact it is
minim ized by db x's pre-emphasis and de-emphas is.
Octave
In music or audio, an interval between two f requencies having
a ratio of 2:1.
Overshoot
When a compressor or expander changes its gain in response 10
a fast increase or decrease in level, the rnaximum gain change should
be directly proportional 10 the actual signal level. However, in some
compressors the level detect ion and gain changing circuitry develop
a kind of "inert ia," over-reacting to changes in level, increas ing or
decreasi:1g the gain more than th e fixed ratio asked for. This over-
reaction is known as overshoot, and 11can cause aud ibly non-linear
compression (disto rt ion ). dbx circu its have minimal overshoot, so
they provide high Iy Iinear compression and exp ansion.
Peak Level
An audio signal continuously varies in level (strength . or
maximum voltage) over any period of 1,me, but at any 1n~1an1, the
level may be highe r or lower than the average . Th e maximum
instantaneous value reached by a signal is its peak level (see
RMS level).
Pnase Shift
"Time shift" is another way to describe phase shift. Some
circuitry, such as record elec tro nics and heads, wil l delay some
frequencies of an audio program with respect 10 other portions of
the same program. In other words, phase shift increases or decreases
the delay time as the frequency increases. On an abso lute basis,
phase shift cannot be heard, but when two signals are compared to
one another, one having a phase shift relative 10 the other, the
effec ts can be very noticeable, and not ver'/ desirable. Excessive
phase shift can give a tunnel-li ke quality 10 the sound. Phase shift
also can degrade the performance of compander ty pe no ise
reduc ti on systems wh,ch depend on peak or average level
detection circu itry.
Power A mplifier
A unit that takes a medium-level signal (e.g., from a pre-
amp lifier) and amplifies it so it can drive a loudspeaker . Power
amp lifiers can operate into very low impedance loads (4-16 oh ms) ,
whereas preamplifiers operate only into low impedance (600
ohms) or high impedance (5,000 ohms or higher) loads. Also
known as a main amp lifier, the powe r ampl ifier may be built into
an integrated amplifier or a receiver.
Preamplifier
A device which takes a small signal (e.g., from a m icro phone,
record player), or a medium-level signal (e.g., fro m a tune r or tape
record er), and amplifies it or routes it so it c.;n drive a power
ampl ifie r. Most preamplifiers incorporate tone and volume con-
trols. A preamp may be a separate component, or part of an
inte gra ted ampl ifier or of a receiver .
Pre-Emp hasis (See "de-emp hasis")
Receiver
A single unit that combines tuner, preamp and power amplifier
sections.
Release Time or Release Rate (See "decay time" and "attack time")
Rise Time {Attack Time)
Th is is the ability of a circuit to follow (or "track") a sudden
increase in signal level. The shorter the rise ume, the better the
frequ ency response. Rise 1ime ,s usually specif ied as the interval
(in microseconds) required to respond to the leading edge of a
square-vvave input.
RMS Level
RMS IP.vel (Root Mean Square) is a measurement obtained by
math ematical ly squaring all the instantaneous voltages along the
waveform, adding the squared values together, and taking the
square root of rhat number. For simple sine waves, the RMS value
is approximately 0.707 times the peak value, but fer complex audio
signals, RMS value is more difficult to calculate. RMS leve l is
sim ilar 10 average level, although not identical (Average level is a
slovver measurement).

Sub Harmonic
A sub-multiple of the fundamental frequency. For example,
a wave the frequency of which is half the fundamental frequency
of another wave is called the second sub harmonic of that wave.
Sub Woofer
A loudspeaker made specifically to reproduce the lowest of
audio frequencies, usually between 20Hz and 1OOHz.
Synthesizer
An ELEC T RONIC MUSIC SYNTHESIZER is an audio
processor that has a bui lt-in sound generator (oscillator!, and
that alters the envelope of the sound with voltag e contro lled
circuitry. Synthesizers can produce familiar sounds and serve as
musical instruments, or they can create many un ique sounds
and effects of their own.
A SUB HARMONIC SYN TH ESIZER is a device which is not
used to create music, but to enhance an existing audio program.
In the case of the dbx Model 100, the unit creates a new signal
that corresponds to the volume of the input signal, but is at
1/2 the frequency of the input signal.
Tape Saturatio n
There is a·maximum amount of energy that can be recorded on
any given type of magnetic tape. When a recorder "tries" to record
more energy, the signals become distorted, but are not recorded at
any higher levels. This phenomenon is called tape saturation
because the magnetic ox ide particles of the tape are literally
saturated with energy and cannot accept any more magnetization.
T.H.D. (Total Harmon ic Distortion) (See "Harmonic Distortion")
Threshold
Threshold is the level at which a compressor c,r limiter ceasesto
have linear gain, and begins to perform its gain-changing function
(i.e., where the output level no longer rises and falls in direct
proportion to the input level). In most systems, the threshold is a
point above which the level changes, although there are compressors
that raise signal levels below a threshold point. Some compander-
type noise reduction systems, such asDo lby~• have upper andl
lower threshold between which the gain changes; these systems
require carefu l level calibration for proper encode/decode perfor-
mance. dbx noise reduction systems have no threshold at which
compression or expansion factors change, so level calibration is
not critical.
INPUT I OUTPUT I
-2: I 120:1
ComPfenion Comp,en1on
Uimitin;;il
--- -- -Thres:hold--- -- --- -- ---
~ '- J \.
TrackingAccuracy
Tracking refers to the ability of one circuit to "follow" the
changes of another circuit. When two volume contro lsare adjusted
*•oolby' is a trademark of Dolby® Laboratories, Inc.
Manufactured under one or more of the
following U.S. patents: 3,681 ,618 ;
3,714,462; 3,789,143; 4,101,849; 4,097,767.
Other patents pending. IV
in exactly the same way, the corresponding "sameness" of the
output levelscan be expressed as the tracking accuracy of the
controls.
The level detection circuitry in a dbx encoder sensesthe signal
level, changes the gain, and creates an encoded signal. The corre-
sponding "sameness" of the original signal and the encoded/
decoded signal can be expressed as the tracking accurac'y of the
noise reduction system. (dbx systems are non-critical for the
operator, and are built to close tolerances, so that tracking
accuracy is excellent, even if the encoder and decoder are in
different pieces of db x equipment.I
Transition Level (See Level Match)
When a circuit has uniform compression or expansion through-
out its full dynamic range, t here must be some level which passes
through the unit without being raised or lowered (w here gain is
unity). This unity gain level is the transition level or transition point.
The transition point is a "window" 1dB wide, in a dbx encoder
(compressor). all signals above the transition point are decreased in
level, and all signals below the point are increased in level. Con-
versely, in a dbx decoder (expander), all signals above the
transition point are increased in level, and all signals below the
point are decreased in level. The transition level is simi lar to a
"threshold," except it does not refer to a point at which
compression or expansion factors change.
Triamplifi ed
Similar to biamplified. A sound system where a passive cross-
over network creates three frequ ency ranges, and feeds three power
ampli f iers: one for bass,one for mid, and one for high frequencies.
The amplifiers are connected directly to the woofers, midrange
drivers and tweeters without a passive, high-level crossover network.
Tuner
A unit which receives radio broadcasts and converts them
into audio freq uency signals. May be part of a receiver.
VCA (Vo ltage Controlled Am plifi er)
Traditionally, ampl ifiers have been designed to increase signal
levels (to provide gain). If an amplifier were required to decrease
the level (to attenuate), it could become unstab le, and might even
oscillate. The gain (amount of amplification) in these traditional
amplifiers would be adjusted by one of three methods (1) attenuat-
ing the audio signal fed to the input of the amplifier, (2) attenuating
the audio output of the amplifier, or (3) changing the negative feed-
back (feeding more or lesssignal from the output back to the input,
but in reversed polarity).
The VCA is a special type of amp lifier that can be used to
increase or decrease levels over a wide dynamic range. Instead of
using signal attenuation or negative feedback, the gain (or loss) is
adjusted by means pf an external de control voltage. dbx hasa
unique, patented VCA design that has extremely low noise and
very wide dynamic range;·'the dbx VCA is the heart of dbx noise
reduction equipment.
Woofer
A loudspeaker which reproduces only low frequencies.
9805C-600127