dbx 165 Manuel d'instruction - Page 16
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Ha rmonic
D
istortion
Harmonic distortion
consists
of
signal components
appearing
at the output of an amplifier
or other circuit
that were
not present
in the
i
nput
signal, and
that
are
whole-number
mult
i
ples
(harmonics)
of the input signal.
For
examp le, an amplifier
given a pure sine-
wave
input
at
1
OOHz
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
f
ica
nt,
and even-order
harmonics
(i.e.
2nd
and
4th)
are le$S
object
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
t
otal 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.
Har
mo nics
Overtones which
are integral
multiples
of the
fundament
al.
H
eadroom
Headroom
refers
to the "space,"
usually expressed
in
dB,
between the
nomina
l
operating signal
leve
l
and the maximum
signal
level.
The
in
put headroom
of a circuit
that
is
meant to accept
nominal
-1 OdB leve ls,
but can
accept up
to +18d8 without
overdrive or excessive
distort
ion,
is
28d B
(from
-1 0
to
+
18
equa ls
28dBl. Similarly,
the output
headroom
of a circuit
that
is
meant
to
supply nominal
+4dBm dr
ive leve
ls, 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
le
vies.
I.M
.
(I
ntermo
d
ula
t
ion
D ist
o rtion)
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
i
le
some types of harmonic
distort
ion
are
musical
,
and
not
particularly
objectionable,
most I.M.
distortion
is
unp
leasan
t
to the
ear.
Impulse Response
Related to
the
rise
1,me
of
a c
ircui
t, the
impu lse response
,s a
measurement of the abil
i
ty
o
f
a circuit
10 respond to
sharp sounds,
suc
h
as
percussion
i nstrumen
ts
or
plucked
strings
.
A
circuit
with
good
impluse response would tend to
have
good
tra
nsient response.
Level Match
The
d
bx
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
l
inearly,
with respect
to
decibels, over the full dynamic
range of
the program.
By
necessity,
there is an arbitrary
signal
le vel
which passes through the
encoder
and decoder without
be
ing
changed
i
n
level.
This leve
l 1s
known as
the
level match
point
(transition
point). Some dbx
equipme
nt
prov ides for
user
adjustment
of the
l
evel
matc
h
po
i
nt,
fo
r
monitor-
in
g 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
t
o
the nominal
(average) signal
le
ve!,
there
w
il
l be no
increase or
decrease ,n
level as you
sw itch from
mon,toring
"live"
program
10 mon1ton!'lg dbx-processed
program.
L imiter
A lim
i
ter
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
i
ll
be able
to ra,~e the
outp
ut
level beyond
a
preset
val ue.
Th
e
difference
between
li
mit
i
ng and
compression
is that compression
gently
"shrinks"
dynamic
range,
whereas limit
in
g
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 Leve
l
( Line Input)
Line
leve
l
refers 10 a
preamp lif,ed
audio s,gnal,
in
contrast to
mic
leve
l
, wh,ch desc
ri
bes
a
lower
-leve
l 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
+
1
Od
Bm).
L
i
ne level
signals
vary, depending on the
audio
system.
H1-F1 line
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
+2
4d8m
).
Line ,nputs are
simply inputs that
have
sensitivi
ties
,n ten
ded
for
li
ne 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
leve
l
,nput.
II I
M
odulation
N
oise
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
leve
l
of the
rec
o rded
si
gnal; 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
-f
requency
signals, but
i
n
fact it is
minim
iz
ed by db
x
's
pre-emphasis and de-emphas
is.
Octa ve
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
gai
n
change
should
be directly
proportional
10 the
actual signal
leve
l.
However,
in
some
compressors the
level
detect
i
on and gain changing
ci
rcuitry
develop
a kind
of "inert
ia,"
over-reacting
to changes
in leve
l,
increas
i
ng or
decreasi:1g the gain more than
th
e
fixed ratio asked
for.
This over-
reaction is
known
as overshoot,
and
11
can cause aud ibly non-linear
compression
(d
isto
rt
ion
).
dbx circu
i
ts
have
minimal
overshoot,
so
they provide high
I
y I in ear compression
and
exp
ansion.
Peak Level
An audio signal continuously
var ies in
leve
l
(strength
.
or
maximum
voltage) over any period of 1,me, but
at any
1
n~1an1, the
level
may be
highe
r
or lower than
the
average
.
Th
e maximum
instantaneous
value
reached by a signal
is its
pea
k 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 t
i
me 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
shi
ft
relative 10 the other,
the
effec
t
s
can be very noticeable,
and not
ver'/ desirable.
Excessive
phase
shift
can give
a t
un
nel-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
leve
l
detection
circu
i
try.
Power A mplifier
A
unit that takes
a
medium-level
signal (e.g ., from
a
p
re-
amp
l
ifier) and amplifies
it
so
i
t
can drive a loudspeaker
.
Power
amp
l
ifiers 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
l
ifier, the powe
r am
pl
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
te
d
ampl
ifier or of a
receiver
.
Pre-Emp hasis
(
See
"de-emp hasis")
Receiver
A
single unit that combines tuner, preamp
and
power
amplifier
sect
ions.
Re lease Time or Release Rate
(See
"decay time" and "attack
time")
R
ise Time {Attack
Time)
Th
is is the
ability
of
a circuit
to follow
(or
"track")
a
sudden
increase
i
n signal
level.
The shorter the
rise
ume, the better
the
frequ
ency response.
Rise
1
ime
,s usually
spec
i
f ied
as
the
interval
(in
microseconds)
required to
respond
to the
leading edge
of
a
square-vvave
in
put.
RM
S Level
RMS
IP.ve
l
(Root
Mean Square)
is
a
measurement
obtained by
ma
th em atical
ly
squaring all
the
instantaneous
voltages along the
waveform,
adding
th
e
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
i
lar 10
average level,
although
not identical
(Average
level
is a
slovver measurement).