dbx 165A Manuale di istruzioni - Pagina 14
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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
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 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
ro
llers).
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 lon'g attack times
(reach
maximum volume
more
gradually).
When a compressor (or expander) changes the
level
of
an incom-
ing
signal, the circuitry
actually
requi
res
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
fina
l
val
ue.
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
know
n
as "aux
leve
l"
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
leve ls,
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.
Typ
ical
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 R IAA
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 ,OOOHz to
1
O,OOOHz may be said to have
a
bandwidth
of 1 kHz
-
1 OkHz, 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
hig her 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
equip
ment
.
Thus, while the overall
input-to-output
frequency
respo
nse
of dbx
type
II equipment
is 20Hz to 20kHz, the band-
width of the RMS detection
circuitry
within
that equipment
is
30Hz to 1 OkHz.
Bass
Th
e low audio frequency range below approximately
500Hz.
For the purpose
of discuss ion
or
ana
lysis,
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
B
oost
An accentuation
of the
lower audio frequencies
(bass
frequen-
c ies), whereby they are made
louder
than other
frequenc
ies.
Biampl
if
ied
Descriptive of a sound system which utilizes
a
low level cross-
over network to divide
the ful l-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 1 OOkHz)
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-ene
rgy
portions of
the
program would not be
able to
overcome
the initia
l
magnetization
threshold
of the tape
(known as
hysteresis).
The frequency
of the
bias signal
is
not
critica
l,
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
record
i
ng
tape, and
professional tape mach
ines
are equipped
w
it
h
continu-
ously variable
bias
controls; many consumer
tape
machines are
now
equipped
w
ith
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 adciit ional gain
or
how
much more
input
signal
is
present. Cl
ipp
ing
is
relatively easy
to see on an oscil
I iscope,
and
it is
sometimes audible as an increase in
harmon
ic
distortion.
In
severe cases of clipping (hard clipping),sine-
·
waves
begin to
resemble
square
waves.
and
the
sound
qua
lity
is
very
poor.
Often, the
maximum output
leve
l
of an arnplifier
is
de
fined
3S
that
level
where
clipping
begi
ns
to occur.
There is
a phenomenon
known
as
inp
ut
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
l
im
i
ts
are reached.
Aside from degrading the sound quality, clipping can
damage loud
·
speakers. Output
clipping may
be
avoided by
reducing the
level
of
the
i
nput
signal, reducing the gain of the amplifier,
or using a
lar
ger
amplifier.
Input
clipping may be avoided
by
reducing
the
level
of the
incoming
signal, and then increasing the
gain
of
th
e
amp
I
ifier.
Clipping
Level
Th
is
is
the signal
le
vel
at
wh
ich
clipping
just
begins
to
occur.
Clippi
ng
level
is
not always easy
to
define. It may be a matter
of
visually judging the
waveform
on an oscilliscope as the
leve
l is
increased;
alternatP.ly, clipping
level
may be defined as the
level
at
which
harmonic distortion
reac
hes a given value. Tape
clipping, or
saturation,
is
defined as
the 3
%
harmonic distortion
level.
Compression
Compress ion
is
a process whereby
the dynamic range of
program
material
is
reduced.
In other words, the differenc
e
between the
lo
west
and
highest audio
levels is
"squeezed"
into
a smaller dynamic
range.
A compressed
signal
has h igher
average
level,
and
therefore
may
have more apparent loudness
than an uncompressed
signa
l,
even
though
the
peaks
are no
high er
in
level
.
Compressio
n is
achieved with a compressor, a special type of amplifier
that
decreases
i
ts
gain as the
level
of
the
input signal increases.
The
amount of compression
is
expressed as a
ratio o
f
the
input dynam
ic
range to the output
dy
nam
ic
range;
thus,
a compressor
that
takes
a program
input
with 100dB of
dynam
ic
range and yields an output
program
of
50dB dynamic
range
may be said
10
have a 2
:
1
com
-
pression
ratio.
Compressor
A compressor is an amplifier
that decrease
s
its ga,n
as
the
level
of the
input signal
increases to
reduce
the dynamic
range
of the
program
(see
"compression").
A
compressor may
opera
t
e
over the
entire range of
i
np
ut levels,
or
it may
operate only on signals above
and
/or
below a given
leve
l
(the threshold
leve
l)
.
Crossover
Frequency
In
loudspeaker systems
and
multi-amp
lifi
er
aui:iio
systems,
the
transition
frequency
(actually a
frequency
range) between bass and
m
idcange
or midrange and treb le speakers or amp
li
fiers.
Crossover
Network
A
circ
uit wh
ic
h
divides the audio
spectrum
into
two
or
more
frequency bands for distribution
to
different
speakers (high level
crossover) or different amp
li
fiers which then feed different
speakers (low
level
crossover).
High
level
crossovers are
usually
built
in
to
the speaker cabinet,
and are
passive
(they require no
po1111er
supp
ly
). Low level cross-
overs are
used
in
biamplified
or triamplified
sound systems. They
are usually
self-contained,
and come before the power amp
l
ifiers
Low
level
crossovers may be
passive
or
act ive; ac
tive
lovv level
crossovers are kno\{\/n as "electronic
crossovers."
Damping
Factor
The
ratio
of loudspeaker
impedanc
e
to the
am
plifier
's
output
source
impedance.
Dampi ng describes the amplifier's
ability
10
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
threshold
of
human
hearing
whereas
the
threshold of
pai
n is
between 120 and
130dB
SPL. The
term dB
is an
abbr~viation
for decibe
l
,
or 1/10 of a Bel.
The
decibel is a
rat
io
,
no
t
an absolute number, and
is
used
10
express the
d
ifference
betwee
n
two power, voltage or sound pressure
levels.
(dB
is
10 times
th
e