dbx 161 Руководство по эксплуатации - Страница 12
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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.
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
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
signal
level.
However,
in
some
compressors
the
level
detection
and
gain
changing
circuitry
develop
a
kind of
"inertia," over-reacting to
changes
in level,
increasing 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
overshoot,
so
they provide highly
linear
compression and expansion.
Peak
Level
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
another
way
to
describe
phase
shift.
Some
circuitry,
such
as
record
electronics
and
heads,
will
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
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
effects
can be very
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
main
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
player), 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
edge of
a
square-wave
input.
RMS
Level
RMS
level
(Root
Mean
Square)
is
a
measurement
obtained by
mathematically
squaring
all
the
instantaneous
voltages
along
the
waveform,
adding
the
squared
values together,
and
taking the
square
root of that
number.
For simple
sine
waves,
the
RMS
value
is
approximately 0.707
times the
peak
value,
but
for
complex
audio
signals,
RMS
value
is
more
difficult
to calculate.
RMS
level
is
similar to
average
level,
although not
identical
(Average
level
is
a
slower
measurement).