dbx 161 Manuel d'instruction - Page 13
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VI
I
Sub Harmonic
A
sub-mul
t
ip le
of
the fundamental
frequency
.
For
ex
ample,
a wave the frequency of
which
is half
the
fundamenta
l
frequency
of another
wave
is
cal
led the second
sub
harmonic of that
wave.
Sub Woofer
A
louds peaker
made specifically
to
reproduce the lowest of
audio
frequencies,
usu
al
ly
between 20Hz and
1
OOHz.
Synthesizer
An ELECTRONIC
MUSIC SYNTHESIZER
is
an aud io
processor
th
at has a
bu
i
lt
-in
sound
generator (oscillator),
and
that alters
the enve lope
of the
sound
wit
h
voltage
contro
l
led
circuitry.
Synthesizers
ca
n
produce
familiar sounds
and serve as
musical
in
strume nts, or they
can create many
uniq ue sounds
and
effects
of
their own.
A SUB HARMONIC
SYN
T
HESIZER
is a
device
whi ch
is not
used
to
create
m
usic,
but
to
enhance an ex isting audio
program.
In the case of
t
he
dbx Mode
l 100
, the
unit
creates a new
signa
l
that corresponds to
the
volume of
t he i
nput
signal
,
b ut
is
at
1
/2 t
he
freq uenc
y
of
the
inpu
t
sign
al.
Tape Saturation
T here 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 h igher
levels.
This
phenomenon
is called
tape
saturation
because
the
magnet
ic
oxide
particles of the tape are literally
saturated
with
energy and can not accept any more magnetization.
T.H.D.
(T
otal
Harmonic D
istor
t
ion) (See "Harmonic
Distortion")
Threshold
Threshold
is
the
level at whic
h
a compressor or l
i
miter ceases to
have
linear gain, and begins
to perform
its
gain -changing function
(i.e., w here the output level
no
lo nger
rises
and fal ls
i
n direct
proportio
n
to the
i
nput level).
In
most systems, the
threshold
is a
point above wh ich
the
leve
l
cha
nges, al
t houg
h
there
are compressors
t hat
raise signal
levels be low a thresho
ld
point.
Some
compander-
type noise reduction systems, suc
h
as
Do
lby
~
* have upper
and
lower threshold
between
which
the
gai
n
c hanges; these systems
req
ui
re carefu
l
level ca
l
ibration
for
proper encode/decode
perfor
-
mance. dbx noise reduct ion systems
have
no threshold at which
compression or
ex
pansion factors
change,
so level
cal
ibration is
not critica
l
.
*
'Dolby'
is
a
trademark of Dolby
®
Laboratories,
Inc.
Triamplified
Similar
to biamp
l
ified.
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 o ne for
h
igh f requencies
.
T
he amplifiers are connected direct
ly
to the woofers, midrange
drivers and tweeters w
i
thout a
passive,
h igh-
level
crossover network
.
Tuner
A
unit which rece ives rad io broadcas
ts
and conve rts t
hem
into
audio frequency signa ls. Ma
y
be part of a receiver
.
VCA
(Voltage
Controlled
Amplifi er)
T
raditiona
ll
y, amplifiers
have been designed to increase signal
levels (to provide gain)
.
I
f an amp
lif
ier were
required to decrease
the
level
(to attenuate),
it could
become
unstable
,
and m ight even
oscillate
.
The gain (amount of amp
l
ification)
i
n
t hese traditiona
l
amp I
i
fiers wou Id be adjusted by one of three
methods
( 1) attenuat-
ing the audio signa
l
fed to the input of the amplif ier, (2) attenuating
the audio output
of the amplifier,
or (3) changing
the
negative feed-
back (feed
i
ng more or less signa
l
from the output
back
to the
input,
but in reversed polar
i
ty).
The VCA is a specia
l
type of amplifier
that can
be
used to
increase or decrease levels over a wide dynamic
range. Instead
of
usi
ng signal attenuation
or negative feedback, the gain (or loss) is
adjusted by means of an external
de
contro
l
voltage. dbx has a
unique,
pat ented
VCA
design that has extreme ly
low
noise and
very wide dynamic range; the
dbx VCA
is the heart of dbx no ise
reduction equipment.
Woofer
A
loudspeaker wh
ich rep
roduces o
n
ly low
frequencies.
I
NPUT
Tracking Accuracy
--OUT
PUT-~
2
1
Comp, cn1ori
120: 1
Co mp1eu1o
n
(hm
111n
g )
-Th
1e1hold
---
---
- -
-
~~
----
Tracking
refers to the abi lity of one
ci
rcuit to
"follow"
t he
c hanges of anot
her
circu
i
t. When two
volume controls are adjusted
in
ex actly
the
same way, the corresponding
" sam
eness" of the
output
levels can be
ex
pressed as t he 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.
T
he corre-
sponding
"sameness"
of the origina
l
signal and the encoded/
decoded signal ca
n
be
exp ressed as t he track
in
g accuracy of the
noise
reduction system. (dbx
systems are non-critica
l
for the
operator, and are
built
to close tolerances, so t hat
tracking
accuracy is exce
ll
ent, 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
fu
ll
dynamic
range, there must
be
some level which passes
through the
unit
without
being raised or lowered (where gain is
unity).
This un
i
ty gain level is the transition
leve
l
or tra nsition point.
The trans
i
t ion
point
is a "window"
1
dB wide, in a dbx encoder
(compressor),
al I
signals above t he
transition
point are decreased in
level,
and all signa ls be low the
point
are increased in level. Con
-
versely,
in
a dbx decoder (expander), all signals above the
transition
point are increased
in level,
and al
l
signals below the
po
i
nt are decreased in leve l.
The
transition
level is similar to a
"threshold,"
except it does not refer to a point at which
compress ion or expansion
factors
change.
VI I l