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
megnetic field passing the play head in a rar>dom 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 e compressor (or expender) 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 90%
of the
final
value,
Aux Input (Aux
Level)
Aux inputs, an abbreviation for auxiliary inputs, are low
sensitivity jacks provided on most hr*fl and semi-professional
equipment. Aux inputs (also known as "aux level" or "lir>e level"
inputs) have "flat"
frequency
response and are intended to be used
with preemplified 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 then are microphone levels. Typical
items which might be connected to aux inputs are tape machine
"pley"
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 rwt 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
,OOOHz
to
10,0O0Hz may be said to have a bandwidth of 1
kHz-
10kHz, or
it
may
be
said
to
have
a
9kHz bandwidth (10kHz minus
1 kHz equals 9kHz).
Bandwidth is not necessarily the same as
frequency response.
Bandwidth may be measured at low levels, and frequency response
at
higher levels. I^^reover, bandwidth rr>ay 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
30Hzto 10kHz.
Bass
The low audio frequency range below approximately 5(X)Hz.
For the purpose
of
discussion or analysis,
the
bass range may be
further divided irtto upper
bass (250 to
500Hz), mid bass
(100-200Hz), low bass (
50-1
OOHz), and ultra-low bass (20-50HZ).
Bass Boost
An accentuation of the lower eudio frequencies (bass frequen-
cies). whereby they are made louder then other
frequencies.
BiampI if 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, v^ich
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 recordmg
tape, ar)d
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 admtiondl gam
or how much more input signal is present. Clipping is relatively easy
to
see on an oscilliscope, and it is sometimes audtble
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 i$ 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-handNng ability of
the
transformer and/or of the »nput 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, dipping can damage loud*
speakers.
Output
clipping
may be
avoided
by
reducing
the level
of
the
input
Signal,
reducing the gam of the
amplifier,
or
using
a
larger
amplifier. Input clipping may be avoided by reducing the
level of
the incoming
signal, and then mcreasmg the gam of the
amplifier.
Clipping
Level
This is The signal level at which clipping |u$t begins
to
occur.
Clipping level is
not
always
easy to
define. It may
be a matter
of
visually ludgmg the waveform on an oscilliscope as the
level
is
increased; alternately, clipping level may be defined
as
the level
at
which harmonic
distortion
reaches
a given
value.
Tape
clipping,
or
saturation, is defined as the
3%
harmonic distortion level.
Compression
Compression i$
a
process whereby the dynamic range of program
material
r$
reduced.
In
other words, (he 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
m
level.
Compression is
achieved
with a compressor, a
special
type
of amplifier
that
decreases its gam
as
the level of the input signal increases. The
amount of compression is expressed as a ratio of ihe input dynamic
range
to
the
output
dynamic range; thus,
a
compressor
that takes
a
program
input with 1 0OdB of dynamic range and yields an
output
program of
^dB
dynamic range
may be
said
to
have
a 2 f com*
pression ratio.
Compressor
A compressor is an amplifier that
decreases
its
gam 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 mput 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
auoio systems, the
transition frequency (actually
a
frequency range) between
bass
and
midsartge 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
tnamplified 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,
(he
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 hun^an hearing whereas the
threshold of pam 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
I
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