dbx 165 - Page 15

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^ithm of a
power rat^o or
20
times the toganthm of a voHage
or sound pressure ratio.)
If
the number
of
"dBV* are
referenced
to
a given
level, then the value of the
dB
number becomes specific.
dBv expresses a
voltage ratio. 0d8v is usually referenced
to
0.775V rms. Thus, 0dBv-0.775V. +6dBv- 1.56V
(twice OdBv),
+20dBv»7.75V (ten
times
OdBv),
etc.
dSV expresses a
voltage ratio arKi is similar
to
dBv,
but
OdSV is usually
referenced
to IV rms. Thus. OdBV is 2.2dB
higher than OdBv.
d6
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 centirr>eter (the
threshold of
human
hearing
at
1kHz).
As with dBV,
an increase of
6dB
SPL
is twice the sound pressure, and an increase of 20dB SPL
is an increase
of
10
times
the
sound pressure.
dBm expresses a power ratio. OdBm is 1 milliwatt (.001
watts), or 0.775V rms delivered to a BOO^hm load.
+3
dBm-2 milliwatts, or 1 .S5V into 600 ohms (twice
OdBm).
<^l0d6m-1
0
milliwatts, or
7.75V
into
600
ohms (ten
Times OdBm). etc. dBV and dBm are
numerically equal
when
dealing
with SOO^ohm
circuits. Howes«r, when the
impedance is other than
6(X)
ohms, the value of dBV
remains the same
if
the voltage is the same, whereas the
value of dBm decreases with increasing impedance.
dB alone, without any suffix, doesn't mean anything
unless it is associated with a reference. It may express the
difference between two levels. Thus, the
difference
between lOdBV and 1 6dBV, the difference between
OdBm and 5dBm, and the
difference between
BOdB
SPL
and 9Sd6
SPL are all differences of
5dB.
Decay
Time
Decay lime may mean
different
things, depending on the con*
text. A compressor's decay time is also known
as
its release time
or recovery time.
After
a compressor (or expander) changes its
gam
to
accommodate an incoming signal, and
the
signal is then
removed, the decay time is the amount of time required for the
circuitry
to
return
to
"normal."
More
precisely,
the decay time
is the interval (usually measured in microseconds) during which
the compressing or expanding amplifier returns
to 90%
of the
normal
gain. Very
fast
decay
times
can cause "pumping" or
"breathirtg" effects, whereas v«rv slow decay times may cause
moderate 'level program which follows high*levtl program or pro*
gram peaks to be too low in level.
Decoder
When a circuit restores an original program
from
a
specially
treated
version of that program, the circuit may be said
to
decode
the program. The equipment 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.
Typical decoders include: FM tuners
that
use multiplex
decoders
to
extract left and right stereo signals 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, end dbx decoders that retrieve
wide'dynamic
range programs from the compressed programs on dbx*encoded
recordings.
De>einphBsis
&
Pre-emphe$is
De*emphasis
end 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
specific
higher
frequencies,
the
encoding part
of
an
encoding
/decoding
system. De-emphasis is
an
attenuation
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-em^asis
attenuates high frequencies, thereby reducing tape modulation
noise and restoring
the original frequency response of the program
before it was dbx encoded. There are other types of pre-emphasis
and
de-emphasts. For example,
in
FM
tuners,
de-emphasis
is used
to
compensate for special equalization (known
as
75
-microsecond
pre*emphdsis) applied at the station's transmitter,
Dynamic Range
The 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 le>«l and
the maximum undistorted signal level. A program with wide
dynamic range has
a
large variation from the softest
to
the loudest
passages, and will tend to be nx)re lifelike than programs with
narrow dynamic range.
Encoder
When
a circuit processes an
origir>al
program to create a
specially treated version of that program, the circuit may be said
to encode the program. The equipment or circuit which performs
this
function
is
known
as
an enc^er. Encoded programs must
decoded only with complementary decoding circuitry. Typical
encoded programs include: FM multiplex broadcasts, matrix
quadraphonic
recordings, and dbx encoded recordings.
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
r>ot
refer
to
frequency.
In fact, any audio signal may be said to have an envelope. While
all audio frequencies rise and
fall
in instantaneous level from 40 to
40,000
times per second, an envelope may taka many milliseconds,
seconds or
even
minutes to rise
end fall. In dbx processing,
the
envelope
is what "cues" the rms level detectior> circuitry
to
com*
press and expand the signal; the peak or average level of individual
cycles of
a note
would be useless for level detection beceuse the
gam would change much too rapidly for
audibly pleasing
sound
reproduction.
EQ (Equalization)
EQ
or aquaMzation, is an intentional change in
the frequency
response of
a
circuit. EQ may be used for boosting (increasing) or
cutting (decreasing) the relative level of
a
portion of the audible
spectrum. Some EQ is used
for
achieving sound to suit
personal
listening tastes,
while other
types
of EQ are specifically designed
to
correct for non-linearities
in
the system;
these corrective EQ
"curves" include tape (NAB or COIR) equalization, and phono-
graph (RIAA) equalization. In
a
sense, the pre-emphasis and da-
emphasis used in dbx
processing
are
special forms of equalization.
There are two
common
types
of EQualization curves
(characteristics):
PEAKING
and SHELVING.
Shelving EQ is
used
in most Hi-Fi bass and treble tone controls. Peaking EQ is
used In Hi*Fi midrange tone controls, in graphic equalizers, and
many types
of professional
sound mixing
equipment.
EQ
is performed
by
an equalizer, which may be a specially built
piece
of
equipment, or it may be no more
then the
tone control
section
of
an
amplifier. Graphic equalizers have many controls,
each affactir>g one octave, one-helf octave, or one-third octave of
the
audio
spectrum. (An octave is the interval between
a
given tone
ar>d its repetition eight tones above or below on the musical
scale;
a
note which
is
an octave higher than another note is twice the
frequency of
the
first
note.)
Exper>der
An expander
is
an amplifier that increases its gain as the lavel of
tha input signal increases, e characteristic that "stretches" the
dynamic range
of
the program (see
"expansion").
An
expander may
operata ovar tha antire range of input levels, or it may operate only
on signals above and/or below a given level (the threshold
level).
Expansion
Exper>sion Isa process whereby the dynamic range of program
material is increased. In other words, the
difference between
the
lowest end highest audio levels is "stretched" into a wider dynamic
range.
Expemion 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-
par>der-type noise reduction systems, including dbx. Expansion is
achieved
with an expander, a special type
of amplifier
that increases
its gain
as
the level of the input signal increases. The amount of
expansion is expressed as a ratio of the input dynamic range to
the
output dynamic range; thus, an expander that takes a program
ir>put with 50dB of dynamic range and yields an output program
of
lOOdB dynamic range may be said to have a 1 ;2 compression
ratio.
Fundamental
A
musical note is usually comprised of
a
basic frequency,
plus one or more whole-number
multiples of that frequency.
The basic
frequency
is
known
as
the fundamental, and the
multiples are known as harmonics or overtones. A pure tone
would consist
of only the fundamental.
Ground
Compensated
Ou^ut
This is a
sophisticated 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 the
following device should
beat the same level (potential).
However, where grounding Is not
"right" (where
so-called "ground loops" exist), this circuit
calculates
the ground error and
adds
a correction signal to the high
side of the
output, thereby cancelling much
of
the
hum, buzz and noise that
might
otherwise have been introduced by ground loops.