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logarithm
of a
power ratio
or
20 t
i
mes the logar
i
thm of a voltage
or sound pressure
ratio.)
If the
number of "dB's"
are
referenced to
a
given level,
then the
value of the dB number becomes
specific.
dBv
expresses a
voltage
ratio. OdBv is
usually
referenced
to
0. 775V rms.
Thus,
OdBv=O. 775V, +6dBv=1.55V
(twice OdBv), +20dBv=7.75V
(ten times
OdBv),
etc.
dBV
expresses a voltage ratio and
is
similar
to
dBv, but
OdBV is
usually
referenced to 1 V rms. Thus, OdBV is 2.2dB
higher than OdBv.
dB SPL
expresses a Sound
Pressure Level
ratio. dB SPL is a
measure of acoustic
pressure
(loudness), not acoustic povver,
which
would be measured in acoustic watts.
OdB
SP Lis
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
1.55V into 600
ohms (twice OdBm),
+10dBm =10 milliwatts,
or 7.75V
into
600 ohms (ten
times
OdBm),
etc. dBV and dBm are numerically
equal
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
difference between two levels.
Thus,
the difference
between 1 OdBV and 15d BV. 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) 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
very 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.
D&-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 thr 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 or iginal program to create a
specially treated version of that program, the circuit
may
be
said
II
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
,.,,. ....... '""'
,m,lope,
refer
tQ
frequency.
theJl.t,,.i i1w
ilhin
~en11tlope
,
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" therms 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 1 OOdB 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.