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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.
dBV
expresses
a
voltage
ratio.
OdBV
is
usually
referenced
to
1.0V
RMS.
Thus
0dBV=1V RMS, +6dBV=2V RMS.
+20dBV=10V RMS,
etc.
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 times
the
sound
pressure.
dBm
expresses
a
power
ratio.
OdBm
is
1
milliwatt
(.001
watts),
or
0.775V rms
delivered to a
600-ohm
load.
+3
dBm=2
milliwatts, or
1
.096V
into
600
ohms
(\/2
times
OdBm),
+
10dBm=10
milliwatts,
or
2.449V
into
600
ohms
(3.16
times
OdBm
),
etc.
dBV
and
dBm
differ
by
2.21
when
dealing
with
600-ohm
circuits.
However,
when
the
impedance
is
other
than
600 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
differ-
ence
between two
levels.
Thus,
the difference
between
lOdBV
and
15dBV,
the difference
between
OdBm
and
5dBm,
and
the difference
between
90dB SPL
and
95dB
SPL
are
all
differences of
5dB.
Decay Time
Decay
time
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
gain
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
or milliseconds)
during
which
the
compressing
or
expanding
amplifier returns to
90%
of the
normal
gain.
Very
fast
decay
times
can cause
"pumping"
or
"breathing"
effects,
whereas
ver\
slow decay
times
may
cause
moderate-level
program which
follows
high-level
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,
and
dbx
decoders
that retrieve
wide-dynamic
range
programs from
the
compressed programs on dbx-encoded
recordings.
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
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-emphasis
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-emphasis. For
example,
in
FM
tuners,
de-emphasis
is
used
to
compensate
for
special
equalization
(known
as
75-microsecond
pre-emphasis) 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
level
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
more
lifelike
than
programs
with
narrow dynamic
range.
Encoder
When
a
circuit
processes
an
original
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 encoder.
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
not
refer
to
frequency.
Th*
outline
it
ihe envelop*,
the
non
el it
within
th*
envelop*
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
take
many
milliseconds,
seconds
or
even minutes
to
rise
and
fall.
In
dbx
processing, the
envelope
is
what
"cues"
the
rms
level
detection
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
because
the
gain
would
change
much
too
rapidly for
audibly
pleasing
sound
reproduction.
EQ
(Equalization)
EQ
or equalization,
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
CCIR)
equalization,
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
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
than
the
tone
control
section of
an
amplifier.
Graphic
equalizers
have
many
controls,
each
affecting
one
octave, one-half octave, or one-third
octave
of
the
audio spectrum.
(An
octave
is
the
interval
between
a given
tone
and
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.)
Expander
An
expander
is
an
amplifier that increases
its
gain
as
the
level
of
the input
signal increases,
a characteristic that
"stretches" the
dynamic
range
of
the
program
(see
"expansion").
An
expander
may
operate over the
entire
range
of input
levels,
or
it
may
operate only
on
signals
above and/or below
a
given
level
(the
threshold
level).
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
wider
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. 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
input 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 Output
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
be
at
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.