GLOSSARY
Asperity Noise
This is a swishing type
of background noise that occurs with tape
recordings in the
presence of strong low frequency
signals, especially
when there are no high
frequency signals
to
mask the hiss. Asperity
noise is caused by minute
imperfections in the surface of the tape,
including variations in the magnetic particle
size in the tape’s oxide
coating. The
imperfections increase or decrease the strength
of
the
magnetic field passing the play head in a random
manner, resulting
in audible noise. Asperity noise may be
present even when no
program is recorded. When a program is recorded, asperity noise
becomes superimposed
on
the
signal, creating modulated asperity
noise,
or "modulation noise." Using high-quality tape
with
a
calendered surface
helps
reduce
asperity and modulation noise
(calendered tape is pressed smooth
by
high-pressure
rollers).
Attack Time
Attack time may mean different things,
depending on the
context. In music, the time it takes
for
a
note
to
reach its full
volume is the attack time of the note. Percussive instruments
have
short attack
times (reach maximum volume quickly) and wind
instruments have long attack times (reach
maximum volume more
gradually).
When a compressor (or expander) changes the
level of
an
incom-
ing
signal, the circuitry actually requires a finite amount of time to
complete that change. This
time
is
known
as
the attack time. More
precisely, the attack time is the interval (usually
measured in milli-
seconds or microseconds) during
which the compressing or expand-
ing amplifier changes its gain from the initial value to 63%
of
the
final value.
Aux Input (Aux Level)
Aux inputs, an abbreviation for
auxiliary inputs, are low
sensitivity jacks
provided on most hi-fi and semi-professional
equipment. Aux inputs (also known as "aux
level" or "line level"
inputs) have "flat" frequency response and are
intended
to be used
with preamplified signals.
Aux-level (line-level) signals are medium-
level, higher than microphone levels, but not enough power to
drive a speaker. The advantage to these
levels
is that they
are less
susceptible
to hum
and noise than are microphone levels. Typical
items which might be connected to aux inputs are tape
machine
"play" outputs, tuner outputs,
and dbx "play"
outputs.
Mic-level
or phono-level signals are considerably lower in level than aux inputs
(approx. -60 to -40dBV), so they will not produce adequate
volume
when connected
to
an aux input. Moreover, phono cartridge outputs
require RIAA equalization which
is
not
provided
by aux
inputs.
Bandwidth
Bandwidth refers to the "space"
between
two
specific
frequencies
which are upper and lower limits;
alternately,
band-
width refers to the absolute value
of
the
range of frequencies
between those
limits.
Thus, a
filter which passes frequencies
from
1
,000Hz
to
10,000Hz may be said to have a
bandwidth of 1 kHz-
10kHz,
or it may
be said
to
have a 9kHz bandwidth
(10kHz
minus
1kHz equals 9kHz).
Bandwidth is not
necessarily the same as frequency response.
Bandwidth may be measured at low levels, and
frequency response
at higher
levels. Moreover, bandwidth may refer only to certain
portions of the circuitry
within
a piece
of equipment, whereas
frequency
response may refer
to
the overall
performance of the
equipment. Thus, while the overall input-to-output
frequency
response of dbx type II equipment is 20Hz to 20kHz, the band-
width
of
the RMS
detection circuitry within that equipment is
30Hz
to
10kHz.
Bass
The low audio
frequency range below approximately 500Hz.
For the purpose
of discussion
or
analysis, the
bass
range may be
further divided into
upper
bass (250 to
500Hz), mid bass
(100-200Hz),
low bass (50-100Hz), and
ultra-low bass (20-50Hz).
Bass Boost
An
accentuation of the lower audio
frequencies
(bass
frequen-
cies).
whereby they are made louder
than other frequencies.
Biamplif ied
Descriptive of a sound system which utilizes
a
low level cross-
over
network
to divide
the full-spectrum
audio signal
into
low and
high frequency ranges. These ranges are then fed
to
separate
power amplifiers, which in turn feed low frequency speakers
(woofers) and high frequency speakers (tweeters).
Bias
Bias, as the
term
is
used in tape recording, is a very high fre-
quency signal (usually over 100kHz) that is mixed
with the
program being recorded in order
to
achieve linear magnetization
of
the tape. If only the
audio program were applied
to
the recording
head,
a
very distorted recording would result because lower-energy
portions of the
program
would not be
able
to
overcome the initial
magnetization threshold of the tape (known as hysteresis).
The frequency of the bias signal is not critical,
so
long as the
record and erase bias are synchronized. However, the bias
energy level has
a
direct effect
on the recorded level, background
noise,
and the distortion. It is
sometimes necessary to reset the bias
level for
optimum
performance
with different
types
of
recording
tape, and professional
tape machines are equipped with continu-
ously variable
bias controls; many consumer
tape machines are now
equipped with bias selector
switches.
Clipping
Clipping
is a very distorted sound.
It
occurs
when the output
capabilities of an amplifier are
exceeded, and the amp can no longer
produce
any more voltage, regardless of how
much additional gain
or how much more input signal
is present. Clipping is relatively
easy
to see on an oscilliscope, and
it
is
sometimes audible as an increase
in
harmonic distortion.
In severe cases of clipping
(hard clipping),sine-
waves begin to resemble square
waves, and the sound quality is very
poor. Often, the maximum
output level of
an
amplifier
is defined
as that level where clipping
begins to occur. There is
a
phenomenon
known
as
input clipping,
and this may occur
where the input signal
is so high in level that it
exceeds the level-handling ability of
the
transformer
and/or
of
the input amplifier. Clipping
also occurs
when tape is saturated
by excessive record levels.
So-called "soft clipping"
is usually the result of transformer
saturation, and it
may be somewhat less objectionable
than the
"hard
clipping" that occurs when
output voltage limits are reached.
Aside from degrading
the sound quality, clipping
can damage loud-
speakers.
Output clipping may
be avoided by reducing the level of
the input signal, reducing
the gain of the amplifier, or
using a
larger amplifier.
Input clipping may
be avoided by reducing the
level
of
the incoming signal,
and then increasing the gain of
the
amplifier.
Clipping Level
This
is the signal level at which
clipping just begins
to occur.
Clipping
level is not always
easy to
define.
It may
be a matter of
visually judging the waveform
on an oscilliscope
as the level is
increased;
alternately, clipping level
may be defined
as
the level
at
which harmonic distortion
reaches
a
given value.
Tape clipping, or
saturation,
is
defined
as the
3%
harmonic
distortion level.
Compression
Compression
is a process whereby
the dynamic range of program
material
is reduced. In other
words, the difference
between the
lowest and highest
audio levels is "squeezed"
into a smaller dynamic
range.
A compressed signal
has higher average level, and
therefore
may have more
apparent loudness
than an uncompressed signal,
even though the peaks
are no higher in level.
Compression is
achieved with
a compressor, a special type of amplifier
that
decreases its gain
as the
level of
the input signal increases. The
amount of
compression is expressed
as a
ratio of
the input dynamic
range to the
output dynamic range; thus,
a
compressor
that takes
a
program input
with lOOdB of
dynamic range and yields an
output
program of
50dB dynamic range may be said
to
have
a
2:1
com-
pression
ratio.
Compressor
A compressor
is an amplifier that decreases its gain
as the level
of
the input signal increases
to reduce the dynamic range of
the
program
(see "compression"). A compressor
may operate over the
entire range of input levels,
or it may operate only
on signals above
and/or below
a given level (the threshold
level).
Crossover Frequency
In loudspeaker
systems and multi-amplifier audio systems, the
transition frequency (actually
a
frequency range)
between bass and
midrange or midrange and treble
speakers or amplifiers.
Crossover Network
A circuit which divides the audio spectrum
into two or more
frequency bands for distribution
to
different
speakers (high level
crossover)
or
different
amplifiers which then feed different
speakers (low level crossover).
High level crossovers are
usually built into the speaker cabinet,
and are passive (they require no power supply).
Low level cross-
overs are
used
in biamplified
or triamplified sound
systems.
They
are usually self-contained, and come before the
power amplifiers.
Low level crossovers
may be passive or active; active low
level
crossovers are known
as
"electronic
crossovers."
Damping Factor
The ratio of loudspeaker
impedance to the amplifier's
output
source impedance. Damping describes the
amplifier's ability to
prevent unwanted,
residual speaker movement. The higher
the
numerical value, the
better the damping.
DB (Decibel) also, dBv
dBV dB SPL dBm dB
One dB is the smallest change in loudness the average
human ear
can detect.
OdB
SPL is the threshold of
human hearing whereas the
threshold of
pain is between 120 and 1
30dB
SPL. The term
dB
is
an
abbreviation for decibel, or
1 /1 0
of
a
Bel.
The
decibel
is a ratio, not
an absolute
number, and
is
used to express the difference between
two power, voltage or sound pressure levels.
(dB
is
10 times the
logarithm of
a
power ratio
or 20 times the logarithm of
a
voltage
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