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
$»r>e-
wave input at
tOOHz may produce
200Hz, 300H2,400Hz, 500Hz,
600Hz and even 700Hz energy,
plus lOOHz.at its output (these
being the 2nd. 3rd, 4th,
6th, 6th and 7th order
harmonics).
Usually, only the first
few harmonics are significant, and
even-order
harmortics (i.e.
2nd and 4th) are less
obiectionabie than odd*order
harmonics (i.e. 3rd and Sth); higher
harmonics may be
negligible >n
comparison
to
the fundamental
(100Hz)
output.
Therefore, rather than
specifying the level ot each harmonic com-
ponent. this
distortion is usually expressed as T.H.O. 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 m
dB.
between
the nominal operating signal level and the maximum
signal
level. The input headroom
of
a circuit that is meant to accept
nominal -lOdB levels, but can accept up to
*i‘18d6 without
overdrive or excessive distortion, is 28dB
(from -10
to
equals
28dB).
Similarly, the
output headroom
of
a
circuit that is meant
to
supply nominal +4d6m drive levels,
but that
can produce
+24d0m 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 20d6
above nommal operating
Signal levies.
I.M. (Intermodulation Distortion)
Intermoduiation 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
impiuse response
would tend
to
have
good
transient response.
Level Match
The dbx noise reduction system is unhke
competitive systems
m
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
bemg
changed m
level,
This
level
is
known
as
the level match pomt (transition point). Some dbx equipment
provides for
user adjustment of the level match point, for monitor-
ing purposes only. Although (his is no( 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 m
level
as you
switch from monitonng "live"
program
to
monitoring dbx-proces$ed program.
Limiter
A
limiter
is a type
of compressor, one with
a
10 1 Or greater com-
pression ratio. A hmiter with
a
high compression ratio
(
120.
1)
can
be set
so
that no
amount of increase
m 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 ILine Input)
Line level refers
to a
preamplified audio signal, m contrast
to
mic
level,
which describes a
iower-ievel
audio
signal.
The
actual
signal levels vary. Generally, mic level is riommally -50dSm (with
typical dynamic range of *64dBm to '^lOdBm). Lme
level
signals
vary, depending on the audio system.
Hi-Fi
line
levels are nominally
T
5d6 V,
whereas professional line levels are nominally '*4dBm or
8dBm (with
typical
dynamics
ranging from -BOdBm
to
+24d6m).
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 nomtrtal impedance of
a
mic level input.
Modulation Noise
Modulation noise is
a
swishing type of background hiss that
occurs with tape
recordirvgs m
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 r>oi$e has typically been "masked," hidden
by the
domirtant 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 signal?,
but
in fact
it is
minimized
by
dbx’s pre-emphasis and
de-emphasis.
Octave
In music Of audio, an interval between
two
frequencies havmg
a
ratio of
2:1.
Overshoot
When a compressor or expander changes its
gain
in
response to
a
fast increase or decrease
m level, the maximum gam change should
be directly proportional
to the actual
signal
level. However, m some
compressors the level detection and gam changing circuitry develop
a kind of "inertia," over-reacting
to changes in level, increasing or
decreasing the gam more than the fixed ratio
asked
for.
This over-
reaction
IS known as overshoot, and it can cause audibly non-lmear
compression (distortion),
dbx
circuits have minimal
overshoot,
so
they provide highly linear compression and expansion.
Peak
Level
An audio signal continuously vanes m level (strength,
or
maximum voltage) over any penod of time,
but at
any
mstant. 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).
Pnase 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
ol
the same program.
In other words,
phase
shift increases
or
decreases
(he 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,
E xcessive
phase shift can give
a
tunnel-hke quatitv
to
the sound. Phase shift
also
can
degrade the
performance of
compander type noise
reduction systems which depend or>
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 drived 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
(6,000
ohms or higher) loads.
Also
known
as a
mam 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
Tima
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 m signal level. The shorter the nse 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
mathematicallv 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
fer complex
audio
Signals, RMS value is more difficult
to
calculate. RMS level is
Similar
to
average level, although
not
identical (Average
level (s a
slower measurement).
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