dbx 160 Manuale di istruzioni - Pagina 10
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GLOSSARY
Asperity
No
ise
This
is
a swishing type of background noise that occurs with tape
recordings
in
the presence of strong low frequency signals,
especia
lly
when there are no high frequency signals to mask the hiss. Asperity
noise is caused by minute
imp
erfections in
the surface of
t
he
tape,
includ
ing
variations in the magnetic particle size in the tape's oxide
coating.
The
imperfections
inc rease 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
record
ed.
When a program
is recorded,
asperity noise
becomes superimposed on the signal, creating
modu
lated 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
h igh-pressure
rollers).
Attack Tim
e
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
inst
ruments 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, t he circuitry
actually
requires
a finite amount of
time
to
complete
that
change
. T
his 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
amp
li
fier changes
its
gain from the initial value to 63% of the
fi
nal
value.
Aux
Input
(Aux
Lev el)
Aux
inputs,
an abbreviation
for
au
xi
liary inputs, are low
sensitivity
jacks provided on most hi-ti 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
preamp lified
signals. Aux-level (line-level) signals are medium-
level,
higher than
micropho
ne
levels,
but not
enough pow
er
to
drive a speaker.
The
advantage to these
levels is
that
t hey are
less
susceptible tci hum and noise than
are
microphone
levels. Typical
items
which might be connected to aux
inp
uts
are tape machine
"play"
outputs, tuner
outputs, and
dbx "play"
outputs.
Mic-level
or phono-level signals are considerably
lower
i
n
level than aux
inp
uts
(approx. -60 to
-40dBV),
so
they will not produce
adequate
volume
when connected to an aux
input.
Moreover, phono cartridge outputs
require R IAA
equal ization
which
is
not
provided by
aux inputs.
Bandwidth
Bandwidth
refers to
the
"space" between two
specif ic
frequencies which are
upp
er
and lower limits; alternately,
band-
width refers to
the absolute
value of the range of frequenc
ies
between those
limits.
Thus, a
filter
which passes frequencies
from
1,000Hz to 1
O,OOOHz
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 9kHzl.
Bandwidth
is
not necessarily the same as frequency
respo nse.
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
pi
ece
of
equipment,
whereas
frequency response may
refer to
the overall performance of the
equipment.
Thus, while the overall
i
np
ut-to-output
frequency
respo
nse of
dbx type
11
eq uipment is
20Hz
to
20kHz,
t he
band-
width of the RMS detection circuitry
within
that
equipment
is
30Hz to
1 Ok
Hz.
Bass
The low audio frequency range be low approximate ly 500Hz.
For
the purpose of discussion or analysis, the bass
ra
nge
may be
further div ided
i
nto
upper
bass (250 to 500Hz),
mid
bass
(
100-200Hz),
low
bass (50-1 OOHz), and ultra- low bass (20-50Hz).
Bass Boost
An accentuat ion of the
lower
aud io frequencies ( bass frequen-
cies), whereby they are made lo uder than other frequencies.
Biamplified
Descriptive of a sound system which uti
li
zes
a
low
level cross-
over network
to
divide the
fu
l
I-spect
rum
audio signa
l
in
to
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 1 OOkHz) that
is
mixed with the
program being recorded
in
order to achieve
Ii
near 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 leve
l
has
a
direct
effect
on the recorded
leve
l,
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 eq ui
pp
ed
with continu-
ously variable
bia
s
controls; many
consumer tape
machines
are
now
equipped
with bias
sel
ector
switches
.
achieved with
a compressor, a
special type of
ampl
ifier
that
decreases
it
s
gain as the level of the input signal
inc
reases.
The
amount of compression
is expressed as
a
ratio of t
he
input
dynamic
range
to the
output dynamic range; thus, a compressor
that
takes
Clipping
Clipping is
a
very distorted
so
und.
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. Clipp
in
g
is
relatively
easy
to
see on
an
oscilliscope, and
it
is
sometimes
audib le 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
leve
l
of an amplifier
is
defined
as that
level where clipping begins to occur.
T
here
is
a phenomenon
known
as
input
clipping
,
and
this
m
ay
occur where
the
input
signal
is
so high
in
level
that
it
exceeds the level-handling abi
l
ity of the
transformer and/or of the
input
am
p
lifier.
Clipping
also
occurs
when tape
is saturated
by
excessive
record levels.
So-called "soft clipping"
is
usually
t
he
resu
lt
of transformer
saturation,
and
i
t
may be
somewhat
less objectionable
than the
"hard
clipping"
that occu rs
when output
voltage
l
im
its
are
reac
hed.
Aside from degrading the sound quality,
clipping can damage
lo
ud
•
speakers.
Output clipping may be avoided by reducing the
level
of
the input
signal, reduci
ng
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
amp
li
fier.
Clipping
Level
T
his is
the signa
l
level
at which
c lipping
just begins to occur.
Clipping level
is
not
always
easy to
define. It may be
a
matter
of
visually
judging
the
waveform
on an oscil liscope
as
the level is
increased;
alternately,
cl
ipping
level
may be defined
as the
level
at
which harmonic distortion
reaches
a given
value. Tape
clippi
ng,
or
saturat
ion
, is
defined as the 3% harmonic distortion
leve
l
.
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 sma
l
ler
dynamic
range.
A
compressed signal
has higher average
level,
and therefore
may have
more
apparent
loudness than an
uncompressed
signal,
even tho
ugh
the peaks are no higher
in
level. Compression
is
a program
input
with
1
OOdB of dynamic range and yields an output
program of 50d B dynamic range may be said
to
have a 2: 1 com-
pression
ratio
.
Compressor
A compressor
is
an
amp
li
fie
r
that decreases
its gain as
the level
of the input signal
inc
reases
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
leve
l
(the threshold
leve
l
).
Crossover Frequency
In
lo
udspeaker
systems and mult i-amp
li
fier
audio
systems, the
transition
frequency (actua
ll
y a frequency
range)
between
bass
and
midrange or
midrange
and
treb
le
speakers or amp
l
ifiers.
Crossover
Network
A circuit which divides the
audio
spectrum into two or more
frequency
bands for
distr ibution to different
speakers (
high
level
crossover) or different amp
li
fiers which then feed different
speakers (low
level
crossover).
High
leve
l
crossovers are usually built into the speaker cabinet,
and are passive (they requ
ire
no
power supply)
.
Low
level cross•
overs are used
in
biamplified
or triamp
li
fied sound systems
.
They
are usually self-contained,
and come
before the
power
amplifie rs.
Low
level crossovers
may
be passive
or act
ive;
active
low level
crossovers
are
known as
"electronic
crossovers."
Dam
p
ing
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
i
n
loudness
the average human ear
can detect. OdB SPL is the
thresho
ld
of human hearing whereas the
threshold of pain
is between
120 and 130dB SPL. The term dB
is
an
abbreviation
for decibel,
or
1
/
1
O of
a
Bel. The decibel
is
a ratio, not
an
absolute
numb
er, and
is used to
express
the difference between
two
power,
vo
ltag
e
or sound pressure
levels.
(dB
is
1 O times the
logarithm of
a power ratio or 20 times the
logarithm
of a voltage
11