Sub Harmonic
A
sub-multiple of the fundamental frequency. For example,
a
wave the frequency of which is half the fundamental frequency
of another wave is called the second sub harmonic
of
that wave.
Sub Woofer
A
loudspeaker made specifically
to
reproduce the lowest of
audio frequencies, usually between 20Hz and 100Hz.
Synthesizer
An ELECTRONIC MUSIC SYNTHESIZER is an audio
processor that has
a
built-in sound generator (oscillator), and
that alters the envelope
of the sound with voltage controlled
circuitry. Synthesizers can produce familiar sounds and serve as
musical instruments, or they can create many unique sounds
and effects of their own.
A SUB HARMONIC SYNTHESIZER is a device which is not
used to
create music, but
to
enhance an existing audio program.
In the case of the dbx Model 100, the unit creates a new signal
that corresponds
to
the volume of the input signal,
but
is
at
1/2 the frequency
of
the input signal.
Tape Saturation
There is a maximum amount
of energy
that can be recorded
on
any given type of magnetic tape. When a recorder "tries”
to
record
more energy, the
signals
become distorted, but are
not
recorded at
any higher levels. This phenomenon is called tape saturation
because the magnetic oxide particles of the tape are literally
saturated with energy and cannot accept any more magnetization.
T.H.D. (Total Harmonic Distortion) (See "Harmonic Distortion”)
Threshold
Threshold is the level at which a compressor or limiter ceases to
have linear
gain,
and
begins
to
perform its gain-changing function
(i.e., where the output level no longer rises and
falls
in direct
proportion to the input
level).
In most systems,
the threshold
is a
point above which the level changes, although there are compressors
that raise signal levels below a threshold point. Some compander-
type
noise
reduction systems,
such as Dolby,®’ have upper and
lower threshold between which the gain changes; these systems
require
careful level calibration for proper encode/decode perfor-
mance. dbx noise reduction systems have no threshold at which
compression or expansion factors change, so level calibration is
not critical.
’'Dolby' is a
trademark of
Dolby®
Laboratories, Inc.
Tracking Accuracy
Tracking refers to the ability
of
one circuit to
"follow” the
changes
of another circuit. When
two
volume controls are adjusted
in exactly the same way, the corresponding "sameness”
of
the
output levels can be expressed as the
tracking accuracy of the
controls.
The level detection circuitry in a dbx encoder senses
the signal
level, changes the
gain, and creates an encoded signal. The corre-
sponding "sameness” of the original signal and the
encoded/
decoded signal can be expressed as the
tracking accuracy of the
noise
reduction system,
(dbx systems are non-critical for the
operator, and are built
to
close tolerances,
so
that tracking
accuracy is excellent, even
if
the
encoder and decoder are in
different pieces of dbx equipment.)
Transition Level (See Level Match)
When
a
circuit has uniform
compression
or
expansion through-
out its
full
dynamic
range, there must be some level which passes
through the unit
without being raised or lowered
(where gain is
unity). This unity gain
level is the transition
level
or transition point.
The
transition point is
a
"window” IdB wide,
in
a
dbx encoder
(compressor), all signals above the
transition point are decreased in
level,
and
all signals below the point are increased in
level.
Con-
versely. in a dbx decoder (expander),
all signals above the
transition point are increased in level, and all signals
below
the
point are decreased in level. The transition
level
is
similar
to a
"threshold,” except it does
not refer
to a
point at which
compression
or expansion factors change.
VIII
Triamplified
Similar
to
biamplified.
A sound system where a passive cross-
over network creates three frequency ranges, and feeds three power
amplifiers: one
for
bass, one
for
mid. and one
for
high frequencies.
The amplifiers are connected directly
to
the woofers, midrange
drivers and
tweeters
without a passive, high-level crossover network.
Tuner
A unit which receives radio broadcasts and converts them
into audio
frequency signals. May
be
part of
a
receiver.
VCA
(Voltage Controlled Amplifier)
Traditionally, amplifiers
have been designed to
increase
signal
levels (to provide gain).
If
an amplifier were required to decrease
the level
(to
attenuate),
it
could
become
unstable,
and
might
even
oscillate. The gain (amount of amplification) in these traditional
amplifiers would be adjusted
by
one of three methods (1
)
attenuat-
ing the audio signal fed
to the
input of
the
amplifier,
(2) attenuating
the audio output of the amplifier, or
(3)
changing the negative feed-
back (feeding more or less signal from
the output back to the input,
but in reversed polarity).
The
VCA
is
a
special
type
of amplifier
that can be used to
increase
or decrease levels over a wide dynamic range. Instead of
using signal attenuation or negative feedback,
the gain (or
loss)
is
adjusted by means
of
an external dc control voltage, dbx has
a
unique, patented
VCA
design that
has
extremely low
noise and
very wide
dynamic range; the dbx VCA is the heart of dbx noise
reduction equipment.
Woofer
A loudspeaker which reproduces only low frequencies.
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