Page 1
db:>X
Model 2BX
two band
dynamic
range enhancer
INSTRUCTION
MANUAL
Page 2
WARNING:
TO
PREVENT
FIRE
OR
SHOCK
S^
A
A?
D
vPi\
N0T
^OSE
THIS
APPLIANCE
TO
RAIN
OR
MOISTURE
dbx®
is
a
Registered
Trademark
of
dbx.
Inc.
Page 3
INDEX
BRIEF
OPERATING
INSTRUCTIONS
INTRODUCTION
CONNECTIONS
OPERATION
EXPANSION
SIMPLE
PRECAUTIONS
WHEN
USING
ANY
PROGRAM
EXPANDER
HOWdbx
EXPANDERS
WORK
'
SPECIFICATIONS
"
dbx PRODUCT
WARRANTY
FACTORY
SERVICE
"
SCHEMATIC
"
'
GLOSSARY
"
Page 4
BRIEF
OPERATING
INSTRUCTIONS
PILOT
This
L.E.D.
(light
emitting
diode),
above
the
POWER
switch,
glows
when
the
2BX is
ON.
GAIN
CHANGE
L.E.D.'s
These
two
rows
of
L.E.D.'s
indicate
the
amount
of
gain
change
created
in
each
of
the
2BX's two
frequency
bands.
The red
L.E.D.'s
indicate
upward
expansion
(volume
increase),
the
yellow
L.E.D.'s
indicate
downward
expansion
(volume
decrease).
TRANSITION
LEVEL
When an
incoming
signal
is
above
the
level
set
by
this
control,
the
2BX
expands
up-
wards;
when an
incoming
signal
is
below
the
level set
by
this
control,
the
2BX
expands
downward.
Set
the
control
so
that
red
GAIN
CHANGE
L.E.D.'s
glow
during
loud
portions
of
the
program
and
yellow
GAIN
CHANGE
L.E.D.'s
glow
during
the
quiet
portions
of
the
program.
SOURCE
Push
this
switch
and
the
PRE
switch to
expand
programs
coming
from
your
phono-
graph,
FM
tuner
or
other
auxiliary
equipment.
TAPE
Push
this
switch
and
the
POST
switch
to
expand
programs
coming
from
your
tape
recorder.
If
you
do
not
have
a
tape
recorder,
push
the
SOURCE
switch.
PRE Push
this
switch
to
expand a
program
before
recording
it.*
POST
Push
this
switch
and
the
TAPE
switch to
expand
a
program
played
back
from your
tape
recorder.
POWER
Push
this
switch
once (IN) to
turn
the
2BX
ON; push
again
(OUT)
for
OFF.
EXPANSION
Adjust
this
slide
control
for
the
desired
amount
of
expansion.
A
"1.4"
setting
means
that
an
input
signal
with a
40dB
dynamic
range
will
be
expanded
to
56dB,
or
that
an
input
signal
with
a
50dB
dynamic
range
will be
expanded
to
70dB.
A 1.2
setting
would
result
in a
20%
increase
in dynamics;
a
1.4
setting
would
result
in a
40%
increase
in
dynamics,
etc.
(PRE/POST
Summary)
The
PRE switch
expands
the
signal
before
the
tape
recorder
input
(PRE
places
the
expander
before
the
tape
recorder
input);
the
POST
switch
expands
the
signal
from
the
tape
recorder's
output
(POST
places
the
expander
after
the tape
recorder
output).
Pushing
both
PRE and
POST
switches
simultaneously
bypasses
the
expander
functions.
"Expansion
will,
in
most
cases,
increase
the
dynamic
range
of
a
program
beyond
the
capabilities
of
your
tape
recorder.
To
capture
this
increased
dynamic
range
on
tape
requires
a
dbx
noise
reduction
system,
which
a/lows
recording
at
levels
below
the
tape
or
tape
head's
saturation
point
To
expand
a
program
and
then
record
it,
we
recommend
using
any
dbx
tape
noise
reduction
system
and
carefully
setting
record
and
expansion
levels.
If you
do
not
have
a dbx
tape
noise
reduction
system,
we
recommend
expanding
a
program
upon
playback
(place
the
1bx in
TAPE
and
POST
mode).
Page 5
RECORDER
OUTPUTS*
Connect
the
cables from
these
outputs
to
the
Line
inputs
of
your
tape
recorder.
If
you do not have
a tape
recorder,
do not connect
any
cables
to
these
outputs.
RECORDER
INPUTS*
Connect
the
cables
from
your
tape
recorder
Line
or
Monitor
outputs
to
these
inputs.
If
you do
not
have
a
tape
recorder,
do
not
connect
any
cables
to
these
inputs.
INPUTS
Connect
the
cables
from
the
Tape
Outputs
of
your
receiver,
preamplifier,
or
integrated
amplifier
to
these
inputs.
OUTPUTS
Connect
the
cables
from
these
outputs
to the
Tape
Inputs,
or
Monitor
Inputs
of
your
receiver,
pre-
amplifier,
or
integrated
amplifier.
AC
POWER
CABLE
In
the
U.S.A.
connect
this
cable
to
a
1 1
7V
AC,
50
or
60Hz
AC
power source.
The
2BX
requires
a maximum
of
20 watts
of
AC
power.
Models
for
use
with
power
sources
outside
the
United
States
are
available.
Contact the
dbx factory
for
information.
*/f
you have
a
dbx tape
noise
reduction
system
(such
as any
of
our
120 or
150
series)
see
Page
9
for
connection
diagrams.
Page 6
INTRODUCTION
If
you're
a
music
lover
or
an
audiophile
(or
both)
you
have
probably
noticed
that
much
of
the
excitement
of
a
live
performance
is
missing
in
a
recorded
or
broadcast
per-
formance.
The
primary
reason
for
this
loss
of
excitement
is
that
the
dynamic
range
of
the
recorded
or
broadcast
per-
formance
has
been
purposely
restricted
to
fit
the
dynamic
range
limitations
of
the
recording
or
broadcast
medium.
The
2BX
is
a
sophisticated
expander
that
can
restore
the
dynamic
range
and
excitement
to
a
recording
or
radio
broadcast,
adding
considerably
to
your
listening
enjoyment
By
expanding
dynamic
range,
the
2BX
lowers
the
characteristic
noise
levels
of
a
tape,
phonograph
record
or
FM
broadcast.
It
restores
the
"punch"
of
loud
passages
and
the
whisper
of
quiet
ones.
It
can
add
new
life
to
an
old
record
collection,
and
make
FM
broadcasts
worth
listening
to.
The
use
of
a
2BX
with
a
dbx
tape
noise
reduction
system
(such
as
our
120
or
150
series),
lets
you
make
tape
copies
of
records,
FM
broadcasts
or
other
tapes
that
actually
sound
better
than
the
original.
With
these
capabilities,
the
2BX
will
become
one
of
the
most
valued
components'
in
your
home
music
system.
CONNECTIONS
AC
POWER
Connect
the
2BX
to
a
117V
AC,
50
or
60Hz
power
source
only.
The
2BX
requires
20
watts
of
AC
power
(maximum).
As
a
precaution,
do
not
connect
the
AC
power
cable
until
all
signal
connections
have
been
made.
(Models
for
use
with
foreign
power
sources
are
available
Contact
the
dbx
factory
for
information.)
SIGNAL
CONNECTIONS
NOTES:
1.
If
you
do
not
have
a tape
recorder,
do
not
connect
anything
to
the
TO
TAPE
RECORDER,
or
the
FROM
TAPE
RECORDER
jacks.
2.
With
the
setup
in
Figure
2,
changes
in
preamp
volume
will
require
adjustment
of
the
2BX
transition
level.
3.
If
you
do
have
a
tape
recorder
and
wish
to
expand
before
you
record,
use
the
connections
shown
in
Figures
1
2
or
3.
*
4.
If
you
have
a
dbx
tape
noise
reduction
system
see
Page
9.
Connect
the
2BX
to
your
system
according
to
one
of
the
following
diagrams:
Page 7
LOCATION
FOR
OPTIONAL
REVERB
LOCATION
FOR
-
OPTIONAL
GRAPHIC
EQ
*NOTE:
2BX will
reduce
reverb
noise,
but will
also
shorten
decay
time.
—
—
OPTIONAL
RECORDER
—
i
Fig.
1
—
How to
Connect the
2BX
to
Your
Receiver,
Preamplifier
or
Integrated
Amplifier.
This is
the
preferred
hookup
assuming
a
tape
monitor
loop
is
available.
LEFT
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o
2
o
h
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MAIN
AMP
I
I
(.LOCATION
OF
^•"^OPTIONAL
REVERB
|
OR
GRAPHIC
EQ*
I
*NOTE:
2BX will
reduce
reverb
noise,
but
will
also
shorten
decay time.
OPTIONAL
RECORDER
.
Fig.
2
-
How
to Connect
the
2BX if a
tape
monitor
loop
is
unavailable
and
you are
using
a separate
preamp
and
power
amp
(or
between
preamp
out
and
main amp
in
jacks).
Page 8
Fig.
3
-
How
to
Connect
the
2BX
in
the
Tape
Monitor
Loop
of
a
Graphic
Equalizer
or
Speaker
Equalizer.
Page 9
OPERATION
NOTES:
1.
For
a
description
of
control
functions,
see
the
BRIEF
OPERATING
INSTRUCTIONS
at
the
front
of
this
manual.
2.
If
you
do
not
have
a
tape
recorder
connected
as
shown
in
Figures
1
through
3,
press
IN
the
SOURCE
and
PRE
buttons.
3.
To
avoid
repetition,
we
will
use
the
word
"amplifier"
to refer
to
your
receiver,
preamplifier
or
integrated
amplifier.
EXPANSION
To
Expand
an
FM
Broadcast
or a
Conventional
Phonograph
Disc
1.
With
your
amplifier's
master
volume
control
all
the
way
down,
turn
on
the
AC
Power
for
your
entire
system.
2.
Select
the
desired
source
(FM
or
disc)
on
your
amplifier's
selector
switch.
3.
Place
2BX
in
SOURCE
and PRE
mode
(SOURCE
and PRE
buttons
IN).
4.
Set
the
EXPANSION
RATIO
and
the
TRANSITION
LEVEL
(threshold)
controls
to
approximately
mid
position.
5.
With
the
music
playing,
readjust
the
TRANSITION
LEVEL
control
until
red L.E.D.
GAIN
CHANGE
indicators
glow
on loud
passages,
and
the
yellow
L.E.D.
GAIN
CHANGE
indicators
glow
on
quiet
passages*.
*The
L.E.D.
GAIN
CHANGE
indicators
show
the
relative
amount
of
expansion
produced
by
the
2BX
in
each
of
its
two
frequency
bands.
When
one
or
more
red
L.E.D.
's
light
in
a
given
band,
the
2BX
is
raising
the
program
level
in
that
band.
When
one
or
'
more
of
the
yellow
L.E.D.
's
light,
the 2BX
is
lowering
the
program
level
in
that
band.
The
number
of
L.E.D.
's that
light
correspond
to the
relative
amount
of
expansion
up
to the
maximum
dis-
play
able
range.
More
upward
or
downward
expansion
can
be
achieved
than
is
shown
on
the
display.
6.
Slowly
bring
up
your
amplifier's
master
volume
control
to
the
desired
listening
level.
7.
Readjust
the
EXPANSION
control
for
the
desired
amount
of
expansion.
This
will
depend
on
the
program
being
expanded.
For
a
good
classical
phonograph
disc,
an
expansion
ratio
of
1:1.1
or 1:1.2
(settings
of
1.1
or
1.2)
may
be
optimum.
For
a highly
compressed
FM
broadcast,
an
expansion
ratio
of 1
:
1.4
or 1
:
1.5
(settings
of
1.4
or
1.5)
may
produce
better
results.
If
you're
not
sure
where
to
set the
EXPANSION
control,
start
at
a
low
setting,
and
move
it
higher
until
it
sounds
extreme,
then
move
the
control
back
so the
sound
is
natural
again.
The
degree
of
expansion
desired
also
depends
on
the
mood
of
the
listeners.
Generally,
you
will
desire
larger
amounts
of
expansion
when
you
are
totally
involved
with
the
music.
To
Expand
a
Tape
During
Playback
Follow
the
instructions
above
for
expanding
an
FM
broadcast
or
conventional
phonograph
disc,
except
place
the
2BX
in
TAPE
and
POST
mode
(TAPE
and
POST
buttons
IN).
It
is
not
necessary
to
expand
a
dbx-encoded
tape
after
decoding
(during
playback)
if
the
tape
has
already
been
2BX-expanded
prior
to
recording.
If
you
have
two
tape
recorders
only
one
TAPE
input,
and
you
use
one
recorder
primarily
for
playback,
plug
it
into
your
amplifier's
AUX
INPUTs;
then
follow
the
directions
for Ex-
panding
an FM
Broadcast
or
a
Conventional
Phonograph
Disc
(as
already
described).
How
to
Expand
and
Tape
Record
a
Program
NOTE:
Expanding
a
program
and
then
recording
it may
cause
the
dynamic
range
of
the
program
to
exceed
the
dynamic
range
of
your
tape
recorder.
This
could
add
distortion
and/or
excessive
tape
noise
to
the
recording.
Page 10
(An
exception
would
be
for
very
highly
compressed
programs,
where
expansion
prior
to
recording
adds
only
a
modest
margin
to
the
dynamic
range.)
To
avoid
these
problems,
dbx
recommends
the
use
of a
dbx
tape
noise
reduction
system
when
expanding
before
recording
(see
next
page
for
how
to
connect
your
system).
1.
With
your
amplifier's
master
volume
control
and
your
recorder's
input
level
controls
all
the
way
down,
set
your
recorder
to
the
"record
ready"
(RECORD
and
PAUSE).
2.
Select
the
desired
source
on
your
amplifier's
selector
switch.
3.
Place
the
2BX
in
SOURCE
and
PRE
mode
(SOURCE
and PRE
buttons
IN).
4.
Play
the
source
(start
the
phonograph
disc
or
listen
to
the FM
station
you
will
be
recording).
Set
the
TRANSI-
TION
LEVEL
control
so
that
the
red
L.E.D.
GAIN
CHANGE
indicators
glow
on
loud
passages,
and
the
yellow
L.E.D.
GAIN
CHANGE
indicators
glow
on
quiet
passages.
Set
the
EXPANSION
control
for
the
desired
amount
of
expansion.
If
you're
not
sure
where
to
set
the
EXPAN-
SION
CONTROL,
start
at a
low
setting,
and
move
it
higher
until
it
sounds
extreme,
then
move
the
control
back
until
the
sound
is
natural
again.
5.
Bring
up
the
amplifier's
master
volume
control
to
the
desired
listening
level.
6.
Now
adjust
the
input
level
controls
on
your
tape
recorder
for
normal
VU
meter
readings.
You
may
find
that
slightly
lower
record
levels
are
necessary
when
recording
an
expanded
program
in
order
to avoid
tape
saturation.
7.
Restart
the
program
and
record
it
normally.
If
your
tape
recorder
has
tape
monitoring
provisions
(a
three-head
machine),
and
you
wish
to
monitor
the
recording
as it
is
being
made,
place
the
2BX
in
TAPE
mode
(TAPE
button
IN).
This
Monitors
the
signal
coming
from
the
tape
recorder's
outputs
without
changing
the
expander's
input
to the
tape
recorder.
SIMPLE
PRECAUTIONS
WHEN
USING
ANY
PROGRAM
EXPANDER
The
2BX
(or
any
expander)
places
greater
demands,
on
your
power
amplifier
and
speakers.
Whether
or
not
a
given
amplifier
is
of
adequate
power
rating
is
not
always
easy
to
determine;
it
depends
partially
on
the
sensitivity
of
the
speakers,
and
partially
on the
distortion
characteristics
of
the
amplifier.
Set
for
1:1.5 expansion,
the
2BX
will
expand
a
good
60dB
classical
recording
to
about
90dB
of
dynamic
range.
Full
realization
of the
benefit
of this
dynamic
range
requires
both
a hefty
power
amplifier
and
speakers
that
can
take
the
high
power.
If
you
have
such
equipment,
the
results
will
be
breathtaking.
Fortunately,
such
components
are
not
mandatory
for
full
enjoyment
of
the
2BX.
The
most
important
point
is
this:
if
the
speakers
and
amplifier
cannot
handle
wide
dynamic
range,
and
if
the
expander
"tries"
to
drive
them
to
a
wide
dynamic
range,
excessive
clipping
distortion
(overdrive)
may
occur.
To
avoid
this
unpleasant
effect,
use
good
speakers
and
a reason-
ably
large
amplifier.
If
distortion
still
occurs,
it
will
probably
be
noticed
only
with
programs
that
have
a
good
dynamic
range
to
begin
with,
and
which
do
not
need
expansion
to
much
greater
dynamics.
In
such
cases,
a
reduction
in
the
transition
level
and
the
expansion
ratio
setting
will
avoid
distortion.
A good
expander
is
a
powerful
tool,
and,
as
with
any
powerful
tool,
it
can
be
used
to
excess.
Used
properly,
the
expander
can
turn
an
old
record
collection
into
a
treasure
of
new
listening
enjoyment,
and
it
can
turn
a
boring
selection
of
compressed
and
limited
FM
broad-
casts
into
an
exciting
new
source
of
listening
pleasure.
Page 11
Fig.
4
-
Combining
the
2BX
with
a dbx
Tape
Noise
Reduction
System
Page 12
HOW
dbx
EXPANDERS
WORK
Dynamic
Range
Dynamic
range
is
the
difference
in
level
between
the
oudest
and
the
quietest
portions
of
a
program,
expressed
in
dB
.
Since
the
quietest
parts
of
a
recorded
program
are
usually
restricted
by
noise,
the
dynamic
range
of
a
recording
s
usually
defined
as
the
difference
in
level
(in
dB)
between
the
loudest
parts
of
the
program
and
the
noise
level.
Restricting
Dynamic
Range
i™L
h
D
e
c°^
de
Tu
S0Unds
a
live
Performance
may
reach
121WB
SPL.
The
quietest
sounds,
however,
will
not
be
heard
if
they
are
much
quieter
than
the
ambient
room
noise
(people
coughing,
air
conditioning
or
other
noises)
I
he
ambient
room
noise
in
a
very
quiet
auditorium
is
some-
what
over
30dB
SPL
The
useable
dynamic
range
of
a livT
performance
is
therefore
derived
by
subtracting
the
room
noise
(30dB
SPL)
from
our
tolerance
of
extremely
loud
sounds
(120dB
SPL),
giving
a
maximum
of
about
90dB
nr!
rd
'S
di0S
u
3"
6
l6SS
r00m
noise
and
a
d
vnamic
range
of
over
100dB
can
be
realized.
The
dynamic
range
of
a
recorded
program
is
purposely
restricted
to
far
less
than
100dB
in
order
to
fit
within
the
dynamic
range
limitations
of
the
recording
or
broadcast
medium.
For
example,
the
dynamic
range
of
a
studio
quality
tape
recorder
is
about
65dB.
Tape
noise
restricts
the
quietest
sounds
that
can
be
recorded,
and
tape
Z,
h
fn^Z
0r
"
de
f^
et
"
'f
3
Unit
of
We^on
for
sound
level
or
1Z1
ofs
h
ound
:
°ne
decibel
is
usually
described
as
the
smallest
detectable
change
m
sound
level.
The
threshold
of
human
hearina
OOOHT,:an
Und
¥
°U
can
.?
erceive
^emidrange
frequency
oT
7000Hz)
is
approx,mately
"OdB
SPL
"
(Sound
Pressure
Level) and
TouThl
h
d°'
dofpa,n
(the
r
intat
Which
ft*Hȣ2*
tZate
f§
0
Z7pT^
1
3bOU
i
12
°
dB
SPL
-
Some
»*°Ple ™"
&A?<;s
m
ofpain
"
is
*»
—
*
NOTF Thl
«
a"
8e
Limitations
of
the
Tape
Recording
Process.
hL;
T
Fk l
WaVef
°
rm
IS
not
a sin
ewave
signal;
it
is
the
•envelope-
describing
the
program's
volume
changes.
envelope
saturation
(distortion)
restricts
the
loudest
sounds
that
can
be
recorded.
Home
tape
recorders,
especially
cassette
and
cartridge
recorders,
have
an
even
more
restricted
dynamic
range
.
. .
often
.only
50dB.
(dbx
tape
noise
reduction
systems
can
nearly
double
the
dynamic
range
capabilities
of
any
tape
recorder.)
The
maximum
dynamic
range
of
only
the
very
best
phonograph
discs
is
about
65dB,
and
this
is
seldom
achieved
(see
footnote
on
next
page).
The
quietest
sounds
on
a
disc
are
restricted
by
the
"grain"
of
the
vinyl,
and
other
surface
irregularities
that
create
noise;
the
loudest
sounds
are
restricted
by
the
maximum
excursion
of
the
groove.
Loud
levels
are
a
so
restricted
by
the
ability
of
the
phonograph
needle
to
track"
the
record.
To
allow
more
playing
time
per
side,
the
dynamic
range
of
many
records
is
often
restricted
to
less
than
50dB.
Page 13
The
dynamic
range
capability
of
a radio
program
is
about
60dB for
FM
broadcasts,
or
50dB
for AM
broadcasts.
The
quiestest
sounds are
restricted
by
broadcast
interfer-
ence
and
noise, like
FM
hiss;
the
loudest
sounds
are
limited
by the maximum
allowable
modulation
of the
transmitter
(100%).
Above
100%
modulation,
the
trans-
mitted
signal
would
be
distorted,
and
the
station
would
interfere
with
adjacent radio
stations,
near
the
same
radio
frequency.
Compression is used
to
prevent
overmodulation
and
to raise
the
average
level,
thus
raising
apparent
loudness,
so that
most
broadcasts have much
less
than
50
or
60dB
dynamic
range
. .
.
popular
AM
stations
often
compress
the
program
to average
less
than
10dB
dynamic
range.
NOTE:
By
using
dbx
II
noise
reduction
during
the manu-
facture
of phonograph discs,
the
dynamic
range
can
be
extended
to
100dB. Surface
noise is
reduced
to
inaudibility
and
the full
dynamics
of a
performance
can
be
captured,
dbx-encoded
discs are
commercially
available,
and
can be
decoded
with
any
of the dbx
120 and
140
series
noise
reduction
systems.
Compression
and
Limiting
Compression
and
limiting
are
the
electronic
techniques
used
to reduce
the
dynamic
range
of a live
program
to fit
within
the
restrictions
of the
recording
or
broadcast
medium.
A
compressor
may
be
a
LINEAR
COMPRESSOR:
such
a
device
increases
the level
of quiet
passages,
and
decreases
the
level
of loud
passages.
The
COMPRESSION
RATIO
is
the
ratio in
dB
of the
compressor's
input
dynamic
range
to
its
output
dynamic
range.
For
example,
if the
compression
ratio
is
2:1,
the
output
level
will
only
change
1dB
for
every
2dB
change
at the
input
(thus
restricting,
or
"compressing''
the
dynamic
range).
The
THRESHOLD
is the
level
at
which
the
compressor
decides
to
increase
or
decrease
levels.
The
compressor
decreases
the
level
of
input
signals
that
are
above
the
threshold,
and
increases
the
level
of input
signals
that
are
below
the
threshold.
Those
which
act
only
on
levels
that
are
above
the
threshold
level,
and
let
any
signal
that is
below
the
threshold
pass
unchanged
are
called
ABOVE
THRESHOLD
COMPRESSORS.
A
LI
MITER
is
an
above
threshold
compressor
that
has
a
compression
ratio
of
10:1
or
higher.
The
threshold
of
a
limiter
is usually
adjusted
so
that
it acts only
on
musical
peaks,
preventing
them
from
exceeding
the
threshold
by
more
than
a very
small
margin.
The
restriction
of
dynamic
range
created
by
compression
and/or
limiting
is
undesirable
because
it
removes
much
of
the
excitement
from
a
recorded
performance.
However,
without
this
restriction,
the
quietest
parts of
the
program
could
be
lost
in
noise,
and
the
loudest
parts
of the
program
could
be
severely
distorted.
Fortunately,
there
are
ways
to
overcome
this
dynamic
range
restriction,
by
restoring
"lost"
program
dynamics.
program
Fig.
6
—
Linear
Compression
Linear
compression
reduces
the
entire
dynamic
range of
the
music
irrespective
of
input
signal
level.
Page 14
Fig.
7
-
Above
Threshold
Compression
Above
threshold
com-
pression
has
no
effect
on
low
level
signals.
When
signal
level
reaches
the
adjustable
threshold,
the
dynamic
content
of
the
music
is
decreased
but
only
above
that
threshold.
Higher
compression
ratios
may
be
used
in
above
threshold
compression
than
in
linear
com-
pression;
however,
all
ratios
are
available.
This
is
known
as
limiting
if
the
compression
ratio
is
10:1
or
higher.
dbx
Expanders
An
EXPANDER
is
a
device
that
decreases
the
level
of
quiet
musical
passages,
and
increases
the
level
of
loud
musical
passages.
It
is
the
opposite
of a
compressor.
The
EXPANSION
RATIO
is the
ratio
of the
expander's
input
dynamic
range
to
its
output
dynamic
range.
An
expander
with
a
1
: 1
.4 expansion
ratio
will
have
an
output
level
change
of
1.4dB
for
an
input
level
change
of
1.0dB.
Given
an
expander
with
an
expansion
ratio
of 1
: 1
.4,
and
an
input
program
that
has
a
dynamic
range
of
60dB,'
the
output
dynamic
range
will
be
(60
x 1.4
=
84) or
84dB.
The
TRANSITION
LEVEL
(threshold)
is
the
level
at
which
the
expander
decides
whether
to
increase
or
decrease
program
levels.
When
an
input
signal
is
above
the
threshold,
the
expander
increases
its
level;
when
an
input
signal
is
below
the
threshold,
the
expander
decreases
its
level.
All
expanders
have
a
level
detection
circuit.
This
detection
circuit
is used
to
sense
the
input
signal
level
and
to
determine
whether
it
is
above
or
below
the
threshold.
However,
the
method
used
to
detect
input
signal
level
is
different
on
various
expanders.
The
detection
technique
is
vital,
as
detailed
below.
Peak
Detection
Some
expanders
sense
musical
peaks
in
the
input
signal
to
determine
whether
the
input
signal
level
is
above
or
below
the
threshold.
The
effect
of
this
peak
detection
is
that
the
expander
acts
somewhat
erratically,
and
may
expand
the
program
when
it
detects
a
noise
spike
or
brief
musical
transient
that
isn't
really
representative
of
the
program
level.
Average
Detection
Some
expanders
sense
the
average
level
of
the
incoming
program
to
determine
whether
the
signal
is'
above
or
below
the
threshold.
Average
detection
circuits
will
not
overreact
on
musical
peaks,
but
may
respond
too
slowly
to
accurately
expand
a
program.
The
expander
may
respond
too
late
to a
rapid
increase
in
program
level
after
the
actual
input
signal
has
begun
to
decrease
again,
causing
an
unnatural
or
swishing
sound.
RMS
Detection
The
2BX
uses
RMS
detection,
which
acts
on
the
RMS
(Root-Mean-Square)
value
of
the
input
signal.
RMS
detection
is
different
from
either
peak
or
average
detection.
An
RMS
detection
circuit
will
not
overreact
on
musical
transients
or
noise
spikes,
yet it
responds
quickly
to
significant
musical
transients.
In
fact,
the
human
ear
judges
Page 15
sound
levels
by
their
RMS
values,
which
means
that
the
RMS
detection
circuit
in
the
2BX
electronically
parallels
the
way
the
human
ear
hears
music.
Until
recently,
however,
RMS
detection
was
very
complex
and
costly,
dbx
has
pioneered
the
development
of
moderately
priced
RMS
detection
circuitry,
and
has
led
the
industry
in
applying
RMS
detection
to
expanders,
compressor/limiters
and
tape
noise
reduction
systems.
Linear
dB
Expansion
Once the
signal
has
been
"detected,"
the
expander
knows
when to
increase
or
decrease
its
level.
The
circuit
that
actually
performs
this
level
change
is
known
as a
"voltage
controlled
amplifier"
or
"VCA."
The
"AVC"
(automatic
volume
control),
and
"ALC"
(automatic
level
control)
on
many
cassette
recorders
are
examples
of
voltage
controlled
amplifiers,
as
are
the
level
changing
circuits
in
any
modern
expander,
compressor
or
limiter.
The
voltage
from
the
detection
circuit
increases
or
Fig.
8
-
Linear
Expansion
Linear
Expansion
operates
over
the
entire
dynamic
spectrum
of
music
irrespective
of
input
signal
level,
making
loud
passages
louder
and
quiet
passages
quieter,
reducing^
audible
noise.
Expansion
ratios
are
adjustable.
For
example: 1.0
-
0%
increase
in
dynamic
range;
1.2
=
20%;
1.5
=
50%;
2.0
=
100%.
decreases the gain
of
the
VCA
which
increases or
decreases
the
level
of the
program.
While some
expanders
may increase or decrease
the
program level
by
a
fixed
amount, the
2BX increases
or
decreases the
level
of the
program on
a
"linear decibel"
basis. This
means that the
output dynamic
range
and the
input
dynamic range
are
linearly
related
by the
"expansion
ratio" over the
entire
dynamic
range (as described
earlier)
for
a
smooth,
natural
sound.
Attack
and
Release
Times
The
expander
must
decide
how
fast to
react
to
changes
in
program
level.
The
length
of
time
between
an
increase
in
input
signal
level
and
its
corresponding
expansion
is
known
as
the
ATTACK
TIME.
After
the
expansion
of
an
input
signal,
the
expander
allows
the
input
signal
to
return
to
its
normal
level.
The
amount
of
time
to
return
to
normal
is
known
as
RELEASE
TIME.
These
terms
also
apply
for
compressors
and
limiters.
Different
attack
and
release
times
are
desirable
for
different
types
of
music.
For
example,
a
smooth
classical
string
quartet
may
sound
best
when
expanded
with a
slow
attack
and
release
time.
Other
programs
may
sound
best
with
faster
attack
and
release
times.
The
point
is
that
the
_
attack
and
release
times
should
be
allowed
to
vary
according
to
the
program
content
for
the
most
natural
sound.
The
2BX
does
just
that.
The
2BX's
attack
and
release
times
automatically
and
continuously
follow
the
rate
of
change
of
the
"envelope"
of
the
program.*
In
fact,
because
they
are
not
fixed,
the
2BX's
release
times
are
specified
as
rates
which
change
in
response
to
different
program
envelopes.
Furthermore,
the
attack
and
release
rates
are
scaled
differently
in
each
of
the
*The
envelope
is a
graph
of
the
program
level
versus
time.
Page 16
2BX's
two
frequency
bands,
to provide
an
expansion
characteristic
that
best
suits
the
music.
The
result is
a
smooth
action
that
does
not
alter
the
character
of the
music
as
dynamics
are
expanded
and
noise is
lowered.
dbx
Tape
Noise
Reduction
dbx
tape
noise
reduction
systems
allow
a program
of
up
to
100dB
dynamic
range
to be
recorded
on
tape
(or
on an
encoded
phonograph
disc)
without
losing
the
quiet
passages
in the
noise
or
distorting
the
loud
passages.
Professional
recording
studios
throughout
the
world
are using
dbx
professional
tape noise
reduction
systems,
and dbx
has
become
the
new
established
leader in
the
field,
dbx II
noise
reduction
systems,
such
as our
120
series,
are
available
for
use
by
the
audiophile
and home
recordist.
Like
the
original
dbx
tape
noise
reduction
system,
dbx
II
noise
reduction
systems
make
it possible
to tape
record
up
to 100dB
of.
dynamic
range,
and in
addition,
dbx
II
noise
reduction
systems
also
facilitate
playback
of
special
dbx
encoded
discs
(see
footnote,
page
11).
Fig.
9
—
dbx
Tape Noi
below the
level
of
tape
>ise
Reduction
Loud
passages
are
decreased in
level
allowing
them
to be
recorded
saturation;
very
quiet
signals
are
placed
on
tape significantly
above
the
tape
noise level.
Page 17
SPECIFICATIONS
bXrAIMblUlM
KAI IU 1
.0
to
1
.5
(0
to
50%
increase),
I
i near in
decibels
DYNAMIC
RANGE
110dB (peak
signal
to
weighted
background noise ratio)
TRANSITION
LEVEL
RANGE
30mV
to
3V
(threshold)
ATTACK AND
RELEASE
RATES
Variable,
determined
by
program loudness
and
rate
of
change
FREQUENCY
RESPONSE
±0.5dB,
20Hz
to
20kHz
at an
expansion
ratio of
1
:1
.0
TOTAL
HARMONIC
0.1% typical
at
1.0 expansion, 20Hz
to
20kHz
Ulo
1
Un
1
1 UI\J
IM
DISTORTION 0.1 5% typical
irvim it" in/ifir n a Mf»r
INPUT IMPEDANCE
High
(50
kohms)
OUTPUT IMPEDANCE Low
(the 2BX is
designed
to
feed
a
tape
monitor input or tape deck with long
cables)
MAXIMUM OUTPUT
LEVEL
6
volts
RMS
at 1 kHz
CONTROLS Interlocking
Tape &
Source switches, Interlocking
Pre
&
Post switches,
Power ON/OFF,
Transition Level,
Expansion
INDICATORS
Power
ON
L.E.D.,
(10)
Gain
Change L.E.D.'s for each of
2
bands
(20
total)
CONNECTORS
FROM
PREAMP
TAPE
OUTPUT (x
2)
(Phono jacks)
TO TAPE
RECORDER
AUXILIARY
OR
LINE INPUT
(x
2)
FROM
TAPE
RECORDER
OUTPUT
(x
2)
TO
PREAMP
TAPE
OR
MONITOR INPUT
(x
2)
POWER REQUIREMENTS
117V
AC, 50 or 60Hz.
POWER
CONSUMPTION
.
20
watts, maximum
DIMENSIONS
17-3/4"
W
x
3-3/4"
H x
10-1/2"
D
(45.1cm
W x
9.5cm
H x
26.7cm
D)
WEIGHT
8
lbs., 5oz.
(3.8
kg)
Specifications
subject to
change
without notice
or
obligation.
Page 18
dbx
PRODUCT
WARRANTY
All
dbx
products
are
covered
by
a
limited
warranty.
Consult
your
warranty
card
or
your
local
dealer
for
full
details.
FACTORY
SERVICE
The
dbx
Customer
Service
Department
is
prepared
to
give
additional
assistance
in
the
use
of
the
product.
AM
questions
regarding
interfacing
dbx
equipment
with
your
system,
service
information
or
information
on
special
applications
will
be
answered.
You
may
call
during
normal
business
hours
-
Telephone:
617-964-3210,
Telex:
92-2522,
or
write
to:
dbx,
Inc.
71
Chapel
Street
Newton,
MA
02195
Attn:
Customer
Service
Department
Should
it
become
necessary
to
have
your
equipment
factory
serviced:
1.
Please
repack
the
unit,
including
a
note
describing
the
problem
along
with
the
day,
month
and
year
of
purchase.
2.
Send
the
unit,
freight
prepaid,
to:
dbx,
Inc.
224 Calvary
Street
Waltham,
MA
02154
Attn:
Repair
Department
3.
We
recommend
that
you
insure
the
package
and
send
it via
United
Parcel
Service
wherever
possible.
4.
Please
direct
all
inquiries
to dbx
Customer
Service
Department.
Outside
the
U.S.A.
-
contact
your
nearest
dbx
dealer
for
the
name
and
address
of the
nearest
authorized
repair
center.
Page 19
SCHEMATIC
17
Other patents
pending.
Page 20
NOTES
Page 21
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
increase
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 long
attack times (reach maximum volume more
gradually).
When a compressor
(or expander) changes the
level of
an
incom-
ing signal,
the
circuitry actually
requires
a
finite
amount
of time
to
complete that
change. This time is known
as
the
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
63%
of the
final
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 known
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. Typical
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 RIAA
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 passes
frequencies from
1
,000Hz
to
10,000Hz
may
be
said to
have a
bandwidth
of
1
kHz-
10kHz,
or
it
may be said to
have
a
9kHz bandwidth
(10kHz
minus
1
kHz
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
equipment.
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
10kHz.
Bass
The
low audio
frequency
range below
approximately
500Hz.
For the purpose
of
discussion 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).
Bass Boost
An
accentuation
of the
lower audio
frequencies
(bass
frequen-
cies),
whereby
they are made
louder than
other frequencies.
Biamplif
ied
Descriptive of
a
sound
system
which utilizes
a
low level cross-
over network to
divide the full-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
1
00kHz) 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-energy
portions
of the
program
would
not be
able
to
overcome the initial
magnetization threshold
of
the tape
(known
as
hysteresis).
The frequency
of
the bias signal is not
critical,
so
long
as
the
record
and
erase bias are
synchronized.
However, the
bias
Page 22
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
recording
tape,
and
professional
tape
machines
are
equipped
with
continu-
ously
variable
bias
controls;
many
consumer
tape
machines
are
now
equipped
with
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
additional
gain
or
how
much
more
input
signal
is
present.
Clipping
is
relatively
easy
to
see
on
an
oscilliscope,
and
it
is
sometimes
audible
as
an
increase
in
harmonic
distortion.
In
severe
cases
of
clipping
(hard
clipping)
sine-
waves
begin
to
resemble
square
waves,
and
the
sound
quality
is
very
poor.
Often,
the
maximum
output
level
of
an
amplifier
is
defined
as
that
level
where
clipping
begins
to
occur.
There
is
a
phenomenon
known
as
input
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
less
objectionable
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
the
amplifier.
Clipping
Level
This,
is the
signal
level
at
which
clipping
just
begins
to
occur.
Clipping
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;
alternately,
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
Compression
is
a
process
whereby
the
dynamic
range
of
program
material
is
reduced.
In
other
words,
the
difference
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
higher
in
level.
Compression
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
dynamic
range
to
the
output
dynamic
range;
thus,
a
compressor
that
takes
a
program
input
with
100dB of
dynamic
range
and
yields
an
output
program
of
50dB
dynamic
range
may
be
said
to have
a
2:1
com-
pression
ratio.
Compressor
A
compressor
is
an
amplifier
that
decreases
its
gain
as
the level
of
the
input
signal
increases
to
reduce
the
dynamic
range
of
the
program
(see
"compression").
A
compressor
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).
Crossover
Frequency
In
loudspeaker
systems
and
multi-amplifier
audio
systems,
the
transition
frequency
(actually
a
frequency
range)
between
bass
and
midrange
or
midrange
and
treble
speakers
or
amplifiers.
Crossover
Network
A
circuit
which
divides
the
audio
spectrum
into
two or
more
frequency
bands
for
distribution
to
different
speakers
(high
level
crossover)
or
different
amplifiers
which
then
feed
different
speakers
(low
level
crossover).
High
level
crossovers
are
usually
built
into
the
speaker
cabinet,
and
are
passive
(they
require
no
power
supply).
Low
level
cross-
overs
are
used in
biamplified
or
triamplified
sound
systems.
They
are
usually
self-contained,
and
come
before
the
power
amplifiers.
Low
level
crossovers
may
be
passive
or
active;
active
low
level
crossovers
are
known
as
"electronic
crossovers."
Damping
Factor
The
ratio of
loudspeaker
impedance
to the
amplifier's
output
source
impedance.
Damping
describes
the
amplifier's
ability
to
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
abbreviation
for
decibel,
or
1
/1
0
of
a Bel.
The
decibel
is
a
ratio,
not
an
absolute
number,
and
is
used
to
express
the
difference
between
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
Page 23
or
sound pressure ratio.)
if the
number
of
"dB's
"
are referenced
to a
given level,
then
the
value of
the
dB number becomes spe-
cific.
dBV
expresses a
voltage ratio.
OdBV
is
usually
referenced
to
1.0V RMS.
Thus
0dBV=1V
RMS,
+6dBV=2V RMS,
+20dBV=10V
RMS,
etc.
dB
SPL expresses
a
Sound
Pressure Level ratio. dB
SPL is a
measure
of
acoustic
pressure (loudness),
not
acoustic
power,
which
would
be
measured
in acoustic watts.
OdB SPL is
equal to
0.0002
dynes/square
centimeter (the threshold
of
human hearing at
1
kHz).
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
expresses a
power ratio.
OdBm
is 1
milliwatt
(.001
watts),
or
0.775V rms
delivered
to a 600-ohm
load.
+3
dBm=2
milliwatts, or 1
.096V
into
600
ohms
{\J2
times
OdBm),
+10dBm=10
milliwatts,
or
2.449V into
600
ohms (3.16
times
OdBm
),
etc.
dBV and
dBm
differ
by 2.21
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
differ-
ence
between two
levels.
Thus,
the difference between
10dBV
and
15dBV, 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 or milliseconds)
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 ver\
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
as a
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.
De-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 the
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
Page 24
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
refer
to frequency.
the
5
iqnai
i
5
within
the
envelope.
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"
the
rms
level
detection
circuitry
to com-
press
and
expand
the
signal;
the
peak
or
average level
of
individual
cycles
of
a
note
would
be
useless
for
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
1
OOdB
dynamic 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
senses the
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.
Page 25
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
multiples
(harmonics)
of
the
input
signal. For example,
an amplifier
given
a pure
sine-
wave input
at 100Hz
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
harmonics are
significant,
and
even-order
harmonics
(i.e. 2nd and 4th)
are less
objectionable
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
harmonic
com-
ponent,
this
distortion
is
usually
expressed
as T.H.D.
or
Total
Harmonic
Distortion.
While
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
with the
same
T.H.D. rating.
Harmonics
Overtones
which are integral multiples
of
the
fundamental.
Headroom
Headroom
refers to the
"space,"
usually
expressed
in
dB,
between
the
nominal operating
signal level
and the
maximum
signal
level.
The
input
headroom of
a
circuit
that
is
meant
to
accept
nominal
-10dB
levels,
but
can
accept
up
to +18dB
without
overdrive
or excessive distortion,
is
28dB
(from
-10
to
+18
equals
28dB).
Similarly,
the output
headroom
of a
circuit
that is
meant
to supply
nominal
+4dBm
drive
levels,
but
that
can produce
+24dBm
before
clipping
is
20dB.
A
circuit
that lacks
adequate
headroom
is
more likely
to distort
by
clipping
transient
peaks,
since
these
peaks
can be
10 to 20dB above
nominal
operating
signal levies.
I.M.
(Intermodulation
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
harmonically
related
to
the
input,
and
that are
the result of
interaction
between
two or
more input
frequencies.
I.M.
distortion,
like
harmonic
distortion,
is
usually
rated
as
a
percentage
of
the
total
output
power of
the
device.
While
some
types
of
harmonic
distortion
are
musical,
and
not
particularly
objectionable,
most
I.M. distortion
is
unpleasant
to the ear.
Impulse
Response
Related
to
the
rise
time of
a circuit,
the impulse
response is
a
measurement
of the
ability
of
a circuit
to
respond
to sharp
sounds,
such
as
percussion
instruments
or plucked
strings.
A
circuit with
good impulse
response
would
tend
to
have
good transient
response.
Level
Match
The
dbx noise reduction
system
is unlike competitive
systems
in
that there
is no one
threshold
at which
compression
or expansion
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
is known
as
the
level
match point (transition
point).
Some dbx
equipment
provides for
user
adjustment of the level
match point, for
monitor-
ing
purposes only.
Although
this
is
not necessary for
proper encode/
decode
performance,
by
setting
the
level
match
point
to
be
approxi-
mately
equal
to the
nominal
(average) signal level,
there
will be
no
increase
or decrease in level
as you switch from monitoring "live"
program
to monitoring
dbx-processed
program.
Limiter
A
limiter
is
a
type of compressor, one with
a
10:1
or greater
com-
pression
ratio.
A
limiter with
a high
compression
ratio
(120:1)
can
be
set
so
that no amount of
increase in
the input
signal will
be
able
to
raise
the
output level
beyond
a
preset value.
The difference
between limiting
and
compression
is that
compression
gently
"shrinks" dynamic range, whereas limiting
is a
way
to place
a
fixed
"ceiling"
on maximum level,
without changing the
dynamic range
of
program below
that "ceiling," or threshold.
Line
Level (Line Input)
Line level
refers
to a
preamplified
audio signal, in
contrast
to
mic level,
which
describes
a
lower-level
audio signal. The
actual
signal levels
vary.
Generally,
mic
level
is nominally
-50dBm
(with
typical
dynamic range of
-64dBm to
+10dBm).
Line
level
signals
vary,
depending
on the audio
system. Hi-Fi
line levels
are nominally
-1
5dBV, whereas
professional line
levels are nominally
+4dBm or
+8dBm (with typical
dynamics ranging from
-50dBm to +24dBm).
Line inputs are
simply
inputs that have
sensitivities intended for
line
level (preamplified)
signals. Often,
the
nominal
impedance
of a
line level
input
will be
different than
the nominal impedance
of a
mic level input.
Page 26
Modulation
Noise
Modulation
noise
is
a
swishing
type
of
background
hiss
that
occurs
with
tape
recordings
in
the
presence
of
strong
low
frequency
signals.
The
noise
depends
on the
level
of
the
recorded
signal
the
higher
the
recorded
signal
level,
the
higher
the
modulation
noise
Modulation
noise
has
typically
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
modulation
noise
could
become
audible.
This
would
happen
primarily
with
strong,
low-frequency
signals,
but
in
fact
it
is
minimized
by
dbx's
pre-emphasis
and
de-emphasis.
Octave
In
music
or
audio,
an
interval
between
two
frequencies
having
a
ratio
ot
Z
\
1
.
Overshoot
When
a
compressor
or
expander
changes
its
gain
in
response
to
Hp HVrirt.'
6856
°
r
^
ln
,6Vel
'
thG
maximu
™
gain
change
should
be
directly
proportional
to
the
actual
signal
level.
However
in
some
compressors
the
level
detection
and
gain
changing
circuitry
develop
Ho
'
. lu
6rt,a
'
over
-
reactin
9
to
changes
in
level,
increasing
or
decreasing
the
gam
more
than
the
fixed
ratio
asked
for.
This
over-
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as
.
oy
*
x
rs
*\
oot
'
and
it
can
cause
audibly
non-linear
compression
(distortion),
dbx
circuits
have
minimal
overshoot
so
they
provide
highly
linear
compression
and
expansion.
Peak
Level
An
audio
signal
continuously
varies
in
level
(strength
or
SS2SVTW
J
°^
anV
Peri
°
d
°
f
W
'
but
at
an
*
i"^nt,
the
level
may
be
higher
or
lower
than
the
average.
The
maximum
RMS
n
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hed
bV
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S
itS
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,evel
(see
Phase
Shift
"Time
shift"
is
another
way
to
describe
phase
shift
Some
circuitry,
such
as
record
electronics
and
heads,
will
delay
some
frequences
of
an
audio
program
with
respect
to
other
portions
of
th! H
8
JE
e
Pmgram
'
u
,n
°
ther
words
<
Phase
shift
increases
or
decreases
the
delay
time
as
the
frequency
increases.
On
an
absolute
basis
Phase
shift
cannot
be
heard,
but
when
two
signals
are
compared
to
one
another
one
having
a
phase
shift
relative
to
the
other
the
fi*J5ft!
V6rV
noticeable
<
and
not
very
desirable.
Excessive
Phase
shift
can
give
a
tunnel-like
quality
to
the
sound.
Phase
shift
also
can
degrade
the
performance
of
compander
type
noise
reduction
systems
which
depend
on
peak
or
average
level
detection
circuitry.
Power
Amplifier
A
unit
that
takes
a
medium-level
signal
(e.g.,
from
a
pre-
amplifier)
and
amplifies
it
so
it
can
drive
a
loudspeaker
Power
amplifiers
can
operate
into
very
low
impedance
loads
(4-16
ohms)
whereas
preamplifiers
operate
only
into
low
impedance
(600
ohms)
or
high
impedance
(5,000
ohms
or
higher)
loads
Also
known
as a
mam
amplifier,
the
power
amplifier
may
be
built
into
an
integrated
amplifier
or
a
receiver.
Preamplifier
A
device
which
takes
a
small
signal
(e.g.,
from
a
microphone
record
p
aver),
or
a
medium-level
signal
(e.g.,
from
a tuner
or
tape
recorder),
and
amplifies
it
or
routes
it
so
it
can
drive
a
power
amplifier.
Most
preamplifiers
incorporate
tone
and
volume
con-
trols.
A
preamp
may
be
a
separate
component,
or
part of
an
integrated
amplifier
or
of
a
receiver.
Pre-Emphasis
(See
"de-emphasis")
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)
This
is
the
ability
of
a
circuit
to
follow
(or
"track")
a
sudden
increase
in
signal
level.
The
shorter
the
rise
time,
the
better
the
frequency
response.
Rise
time
is
usually
specified
as
the
interval
(in
microseconds)
required
to
respond
to
the
leading
edqe
of
a
square-wave
input.
RMS
Level
RMS
level
(Root
Mean
Square)
is
a
measurement
obtained
by
mathematically
squaring
all
the
instantaneous
voltages
alonq
the
waveform,
adding
the
squared
values
together,
and
taking
the
square
root
of
that
number.
For
simple
sine
waves,
the
RMS
value
,S a
T°oftTo
tel
y
°' 707
times
the
peak
va,ue
<
but
for
complex
audio
signals,
HMS
value
is
more
difficult
to
calculate
RMS
level
is
similar
to
average
level,
although
not
identical
(Average
level
is
a
slower
measurement).
Page 27
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 100Hz.
Synthesizer
An
ELECTRONIC
MUSIC
SYNTHESIZER is an
audio
processor
that
has
a
built-in
sound generator
(oscillator),
and
that
alters
the
envelope
of
the
sound with voltage
controlled
circuitry. Synthesizers
can
produce
familiar sounds
and serve
as
musical
instruments,
or they can create many
unique
sounds
and effects
of
their own.
A
SUB
HARMONIC
SYNTHESIZER is
a device
which is not
used to create music,
but to
enhance an existing
audio program.
In
the case of the dbx
Model
1
00,
the
unit
creates
a
new signal
that corresponds
to
the
volume
of
the input signal,
but is at
1
12
the
frequency of the
input
signal.
Tape Saturation
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 oxide particles of the tape are
literally
saturated
with
energy
and
cannot accept
any more magnetization.
T.H.D.
(Total Harmonic
Distortion)
(See
"Harmonic
Distortion")
Threshold
Threshold
is
the
level
at
which a
compressor or limiter
ceases to
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
as Dolby®* have upper
and
lower threshold between which the gain changes; these
systems
require careful 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.
*'Dolby' is a
trademark
of
Dolby® Laboratories,
Inc.
Tracking Accuracy
Tracking refers
to
the
ability
of one
circuit to
"follow" the
changes
of another
circuit. When
two
volume controls are adjusted
in exactly
the same way,
the corresponding
"sameness" of the
output
levels
can
be expressed as
the
tracking accuracy
of the
controls.
The
level detection circuitry in
a dbx
encoder senses the
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
accuracy 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 dbx
equipment.)
Transition Level
(See
Level Match)
When
a
circuit has
uniform compression
or
expansion through-
out its
full
dynamic
range, there must be
some level which passes
through the unit without
being raised
or lowered
(where
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
similar
to
a
"threshold,"
except it
does
not
refer
to a point at
which
compression or expansion
factors change.
Page 28
Triamplified
Similar
to
biamplif
ied.
A
sound
system
where
a
passive
cross-
over
network
creates
three
frequency
ranges,
and
feeds
three
power
amplifiers:
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
frequency
signals.
May
be
part
of
a
receiver.
VCA
(Voltage
Controlled
Amplifier)
Traditionally,
amplifiers
have
been
designed
to
increase
signal
levels
(to
provide
gain).
If
an
amplifier
were
required
to
decrease
the
level
(to
attenuate),
it
could
become
unstable,
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
less
signal
from
the
output
back
to
the
input
but
in
reversed
polarity).
The
VCA
is
a
special
type
of
amplifier
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
of
an
external
dc
control
voltage,
dbx
has
a
un.que,
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.
Page 29
.
1
NOTES
Page 30
dbx
71 Chapel
Street
Newton,
MA
02194
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.
1079.2M-600104
Printed in
U.S.A.
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