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).
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