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dbx 161 Руководство по эксплуатации
111
or sound
pressure
ratio.) if the number of "dB's " are referenced
to a given level, then the value of the dB number becomes spe-
cific.
or
"breathing"
effects, whereas very slow decay times may cause
moderate-level program which follows high- level program or pro-
gram peaks to be too low in level.
dBV
expresses a vo ltage ratio.
OdBV
is usually referenced to
1.0V RMS.
Thus
OdBV
=
1 V RMS, +6dBV
=
2V RMS,
+20dBV
=
10V RMS, etc.
Decoder
When a circu it restores an original program from a specially
treated version of that program, the
ci
rcuit may be said to decode
the program
.
The equ
i
pment or circuit which performs this
function
is
known
as a decoder. Decoders must be used only with
programs which have been encoded by complementary
encoding
circuitry.
T
ypical decoders
i
nclude: FM tuners that use multiplex
decoders to
ex
tract
left
and right stereo signa ls
from
left -plus -right
and left -minus -right signals, matrix quadraphonic
decoders
that
extract four channels of program from the stereo program on
encoded recordings, and dbx
decoders
t hat
retrieve
wide-dynamic
range programs
from the compressed programs on dbx -encoded
recordings.
dB
SPL
expresses a Sound
Pressure Level ratio.
dB SPL
is
a
measure of acoustic pressure (loudness), not acoustic power,
which would be measured in acoustic watts
.
OdB SPL is
equal to
0.0002
dynes/square centimeter
(the threshold of
human
hearing
at
1
kHz). As with dBV, an
increase
of 6dB
SPL is twice the sound pressure, and an increase of 20dB
SPL
is an increase of 10 t
im
es the sound pressure.
dBm
expresses a power ratio. OdBm is
1
m
i
lliwatt
(.00
1
watts), or
0.775V
rms de
l
ivered to a 600 -ohm load
.
+3
dBm
=
2
milliwatts,
or
1
.096V into 600
ohms
(V2
times OdBm),
+
10dBm=10
milliwatts,
or 2.449V into 600 ohms (3 .16 times
OdBm
) , etc. dBV and
dBm
differ by
2.21
when deal ing with
600-ohm circuits.
However, when the impedance is other
than 600 ohms
,
the
val
ue of dBV remains the same if the
voltage is the same, whereas the value of dBm decreases with
i
ncreas ing impedance.
De-emphasis
&
Pre-emphasis
De-emphasis and pre -emphasis are related processes
that
are
usually
done
to avoid audio noise
in
some storage or transmission
medium.
Pre-emphasis is a boost at specif ic higher frequenc ies, the
encoding part of an encoding/decoding
system.
De-emphasis
is an
attenuat ion at the same frequencies, a reciprocal decoding that
counteracts
the pre-emphasis. In dbx noise
reduction,
de -emphasis
is performed
by
the decoder (the play circuitry).
The
de-emphas is
attenuates high
frequencies,
thereby
reducing
tape modu lation
noise and
restoring
the original frequency response of the program
before it was dbx encoded. There are other types of pre-emphasis
and de -emphasis. For examp le, in FM tuners,
de-emphas
is is used
to compensate fo
r
special equa
li
zation (known as 75 -microsecond
pre-emphasis) applied at the stat ion's transm itter.
dB
alone, without
any suffix, doesn't mean anything un less
it
is
associated w ith a reference. It may express the differ
-
ence between two levels. Thus, the difference
between
1 OdBV
and
15dBV,
the difference
between
OdBm
and
5dBm, and the difference
between
90dB SPL and
95dB
SP
Lare
al I
differences of 5dB.
Decay
T
ime
Dynamic Range
Decay time may mean different
things, depending on the con-
text.
A
compressor's decay t
i
me is also known as its re lease time
or recovery time. After a compressor (or expander) changes its
gain to accommodate an incoming signal, and the signal is then
removed, the decay time is the amount of t ime required for the
circuitry
to return to "normal."
More precise ly, the decay time
T
he dynamic range of a program is the range of signal levels
from the lowest to the highest level. In equipment,
the dynamic
range
is the "space," in dB, between the residual noise
level
and
the maximum
undistorted
signal level.
A
program with wide
dynam
i
c range has a large variation from the softest to the loudest
passages, and will tend to be
more
life
li
ke than programs with
narrow dynamic range.
Encod
er
is
the interva
l
(usua
l
ly measured in microseconds or
milliseconds)
during which the compressing or expanding amplifier
returns to
90
%
of the norma
l
gain.
Very
fast decay times can cause "pumping"
When a circu
i
t processes an orig ina
l
program
to create a
specially treated version of that program, the circu
i
t may be said
to encode the program.
T
he equipment or circuit which performs
this funct ion is known as an encoder. Encoded programs
must
decoded only with complementary
decoding circu itry.
Typical
encoded programs
include: FM
multip lex broadcasts, matrix
quadraphonic
recordings, and dbx encoded record
ings.
Envelope
In music, the envelope of a note
describes the change in average signal
level
from initial attack
,
to peak level,
to decay time, to sustain, to release
time. In other words, the envelope
describes the level of the note as a
function of time. Envelope does not
Thtouth
ntt ,stheenn lOPt,
refer
tQ
frequency.
1he
s1
9nllo1 n
w1t
h1n th• en velop
e,
In fact, any audio signal may be said to have an envelope.While
al
l
audio frequencies rise and fall
in instantaneous
level
from
40
to
40,000 times per second, an envelope may take many mi
l
liseconds,
seconds or even minutes to rise and fall.
In
dbx
processing,
the
envelope is what
"cues"
the rms level detection c ircuitry
to com
-
press and expand the signal; the peak or average
level
of ind ividual
cycles of a note would be useless for level detection because
the
gain wou ld change
much
too rapidly for audibly
pleasi
ng sound
reproduct
i
on.
EQ
{Equalization)
EQ or equa
li
zation, is
an
intentional
change in the frequency
response of a circuit.
EQ
may be used for boosting
(in
creasing) or
cutting (decreas ing) t he
relative
leve
l
of a port ion of the aud ible
spectrum. Some EQ
is
used for ach ieving sound to suit persona
l
listening tastes, whi le other types of EQ are specifica
lly
designed
to correct for non
-Ii
nearities
in the system; these correct ive EQ
"curves"
include
tape
(NAB
or
CC
IR) eq ualizat ion, and phono-
graph (RIAA)
equalization.
In a sense, the
pre-emphasis
and de-
emphasis used in dbx processing are special forms of equalization.
There are two common types of
EOualization
curves
(characteristics):
PEAKING
and
SHELVING.
Shelving
EQ is
used in
most
Hi -Fi
bass and treb le tone contro ls.
Peaking
EQ is
used in Hi-Fi midrange
tone
controls, in graphic equalizers, and
many
types of professiona
l
sound mixing equ
ip
ment.
EQ is performed
by
an
equalizer, which may be a special
ly
built
piece of equipme
n
t, or it may be no more than the tone contro
l
section of an amp
li
fier.
Graphic
equalizers
have
many contro ls,
each affecting o ne octave, one
-half
octave, or one-third octave of
the audio spectrum. (An octave is the interva
l
between a given tone
and its repet it ion eight tones above or be low on t he musical scale;
a note which
is
an octave
higher
t han another note is
twice the
frequency of the first note.)
Expander
An expander is an ampl
i
fier that increases its gain as the level of
the
input
signal
increases,
a characteristic
t hat "stretches"
the
dynamic range of the program (see "expansion").
An expander may
operate over the entire range of
i
nput
levels,
or
it
may operate only
on signals above and/or below a given level (the thresho ld leve
l
).
Expansion
Expansion is a process whereby the dynamic
range
of
program
material is increased. In other words, the
difference
between the
lowest and
highest
audio levels is "stretched"
into a w ider
dynamic
range. Expansion is sometimes used to restore dynamic range that
has been lost through compression
or limiting
done in the original
recording or broadcast; expansion
is
an
integral
part of com
-
pander-type noise
reduction
systems, including dbx.
Ex
pansion is
achieved with an expander, a
special
type of amplifier
that
increases
its
ga in as the leve
l
of the input signal
increases. The
amount of
expans ion is expressed as a ratio of the input dynam ic range
to
the output dynamic range; thus, an expander that takes a program
input with 50d
B
of dynamic range and yields an output
program
of
1 OOdB dynamic
range may be said to
have
a 1 :2 compress
io
n
ratio.
Fund
amental
A musical note
is
usual
ly comprised of a basic frequency,
plus
one or
more
whole-
num
ber
mu
ltip les of that frequency.
The basic
frequency
is known
as the fundamenta
l
, and
the
mult
ipl
es are
known
as
harmonics
or overtones. A
pure
tone
would cons ist of o
nly
the fundamental.
Ground Compens
ated
Output
This
is
a sophist icated output circuit that senses the potential
difference
between the ground of
the dbx unit
and the shield
ground of unbalanced inputs to which the
dbx
unit is connected.
Ideally, the dbx unit and the input of t he following
device should
be at
the
same level (potential).
However, where grounding is not
"right"
(where
so-call
ed
"ground
loops"
exi
st),
this
circuit calculates
the ground error and adds a correct ion signal to the high side of the
output,
thereby cancel ling
much
of the hum, buzz and
noise
that
might otherw ise have been introduced
by
ground
loops.
IV