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Harmonic Distortion
Harmonic distortion
consists of
signal components appearing
at t he output of an amplifier
or other circuit that were
not present
in
the input signal, and t hat are whole-number
mu
lti
ples (harmonics)
of
the
input signa
l.
For examp le, an
am
plif ier given a pure sine-
wave input at
100Hz
may produce 200Hz
,
300Hz,
400Hz,
500Hz,
600Hz
and even
700Hz
energy, p lus
1
OOHz,
at its output
(these
being
the 2nd, 3rd
,
4th, 5th, 6th
and 7th order harmonics!.
Usually,
only the
first
few harmonics are sign
i
ficant, and even-order
harmonics
(i.e. 2nd and
4th)
are less ob jectionable
than odd-order
harmonics
(i.e. 3rd and
5th);
higher
har
monics may be
negligible in
compar ison to the
fundamental
(100Hz)
output.
T
herefore, rather than specifying the leve
l
of each harmonic com•
ponent,
this distortion
is
usually
expressed as
T
.H.D. or
Total
Harmonic
Distortion
.
While T.H.D.
is t he total power of all
harmonics generated by the circuitry,
expressed as a percentage
of the tota
l
output power,
the "mixture"
of differe
n
t harmonics
may
vary
in different
equipment
with the same
T
.H.D. rating.
Harmonics
Overtones
w hic
h
are i ntegral
mu
ltip les of the fundamental.
Head
room
Headroom
refers
to
the
"space," usually
expressed
in
dB,
between the
nominal
operat
i
ng signa
l
level
and
the
maximum
signal
level. The inp
ut
headroom of a circu it
that
is meant to accept
nominal
-10
dB
levels, but can accept up to +18dB
without
overdrive or excessive
distortion,
is
28dB
(from -10 to +18 equals
28dB). Similarly,
t he output
headroom of a circuit that is meant
to supply nomina
l
+4dB
m
drive
levels, b ut that can
produce
+24dB
m
before
clipping
is 20dB.
A circuit that lacks adeq uate
headroom is more like
ly
to distort by clipp ing transient
peaks,
since
these
peaks
can
be
10 to 20dB above nomina
l
operating
signa
l
levles.
1.M.
(Intermodulation
Distortion)
Intermodulation
distortion
consists of
signal
components
appear
in
g at the output
of an amplifier
or ot her ci rcu it 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
inp ut
frequencies. 1.M. distortion,
like
har
monic
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
objectionab le,
most I.M.
distortion
is
unp
l
easant
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 i nstruments or plucked strings. A circuit with
good
impulse
response would tend to have good transient response.
Leve
l
Match
The
dbx
no ise reduction
system is unlike competitive
systems
in t hat there
is
no one t hresho
ld
at w
hi
ch compression or expansion
begins. Instead,
compression occurs linear ly, 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
wit hout being changed in
level.
This level is known as
the
leve
l
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 sett
in
g 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
lim
iter is a
type
of compressor, one with a 10:1 or greater com-
pression ratio. A
Ii miter
with a high compression ratio ( 120: 1) can
be set so
that
no amount
of
increase in
the
input signa
l
will be able
to raise the output level beyond a
preset
va lue.
The
difference
between
limiting
and compression
is
that compression gently
"shrinks"
dynamic range, whereas limiting
is
a way to place a fixed
"
ceil
in
g" on max imum level, without
changing the dynamic range
of program be
lo
w that "ceiling,"
or thresho ld.
Line Level
( Line
I
nput)
Li
ne
level
refe rs to a preamp
l
ified audio signal,
in
contrast to
mic
l
evel, 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 +1 OdBm).
Line level
signals
vary, depending on t he audio
system.
Hi-Fi line levels are
nominally
-15dBV, whereas
professional
line levels are nomina
lly
+4dBm or
+8dBm (with typical dy nam
ic
s
ranging
from
-50
dBm to +24dBm).
L
ine inputs are simply
inputs
that
have
sensitivities
intended
for
l
in
e level (preamp
li
fied) signals
.
Often,
the nominal impedance of a
line level
input
wil
l
be
different
t han the nominal impedance of a
mic level input.
Modul
ation
Noi
se
Modulation
noise is a swishing type of background
h
iss that
occurs with tape
recordings
in the presence of strong
low
frequency
signals.
The
noise
depends
on the level of t he recorded signa
l
; the
higher the
recorded
signal level, t he
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.
reduction systems which depend on peak o
r
average level
detection circuitry.
Octave
In mus ic or audio, an
interval
between two frequencies
having
a ratio of 2:
1.
Overshoot
When a compressor or expander changes
its
gain
in
response to
a fast increase or decrease
in level,
the maximum gain change should
be directly
proportional
to the actual signa
l
level. However, in some
compressors the level detection and
gai
n changing circuitry
develop
a kind of "inertia,"
over-reacting
to changes in
level,
increas ing or
decreasing the gain more than the fixed ratio asked for
.
This
over-
reaction
is
known as overshoot, and it can cause audibly non-linear
compression (distortion).
dbx circuits have minimal overs hoot, so
they provide highly linear compression and expans ion.
Peak
Lev
el
An audio signal continuously
varies in level (strength, or
maximum voltage) over any
period
of time,
but
at any instant, the
level may be higher or lower than the average.
The
maximum
instantaneous value reached
by
a signal is its peak level (see
RMS level).
Phase Shift
"Time shift"
is anot her way to describe phase
sh
ift.
Some
circuitry,
such as record electronics and
heads,
wi ll delay some
frequencies of an audio program with
respect
to other
portions
of
the same program.
In
other words, phase shift increases or
decreases
the delay time as the frequency incre<1ses. On an abso lute basis,
phase shift cannot be heard, but when two signals are compared to
one another, one having a phase shift re lative to the other,
the
effects can be very noticeab le, and not very desirable. Excessive
phase shift can give a tunnel-like
qual
i
ty to the sound.
Phase
shift
also can degrade the performance of compander type noise
Power Amplifier
A unit
that takes a med ium-level signa
l
(e.g.,
from a
pre•
amplif ier) and amp
l
ifies it so it can dr ive a louds peaker. Power
amplifiers can operate into
very
low impedance
loads (4-16
o
hm
s),
whereas preamplifiers
operate on
ly i
nto low
impedance
(600
ohms) or high impedance (5 ,000 ohms or
higher)
loads
.
Also
know
n
as a
main amplifier,
t he power amp lifier may be bu
il
t into
an
i
ntegrated amp
lifi
er or a rece iver.
Preamplifier
A
device whic
h
takes
a
small
signa
l
(e.g., from
a
microphone,
record player), or a
medium-level
signal (e.g.,
from
a tuner or tape
recorder), and amp
l
ifies it or
routes
it
so
it can drive a power
ampl if
ie
r.
Most
preamplifiers
in
corporate
tone
and
volume
co
n-
tro ls.
A
preamp
may
be a
separate
component,
or
part
of an
int
egrated amp
li
fier or of a
receiver.
Pre-Emphasis
(See
"de-emphasis")
Receiver
A
si ngle unit that combines
tuner, preamp
and
power
amplifier
sections.
R
elease
Time
or
Release Rate
(See
"decay
time"
and
"attack
time")
Rise Time (Att ack Time)
Thi
s is the ability of a circuit to
follow
(or "track")
a sudden
increase in signal
level.
The shorter the rise
ti me,
t he better the
frequency
response. Rise time is usually specified as the interval
(in microseconds)
required to respond
to
t he leading edge of a
square-wave input.
RMS Level
RMS
leve
l
(Root
Mean Square)
is a
measurement obtained by
mathematically
squari
ng al
l
t he
instantaneous voltages
along the
waveform, adding the squared values together, and taking t he
square root of t hat
number.
For simp le sine waves, the
RMS
value
is
approximately
0.707
times the peak value, but for comple
x
audio
signals, RMS value is
more
difficu
l
t to calculate.
RMS
level is
simi lar to average leve
l
, although
not
ident
ical
(Average
level
is a
slower
measurement).
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