48
Glossary
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 en-
ergy, the signals become distorted , but are not
recorded at any higher levels. This phenome-
non is called tape saturation because the mag-
netic oxide particles of the tape are literally
saturated with energy and cannot accept any
more magnetization.
THO (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 di-
rect proportion to the input level). In most sys-
tems, the threshold is a point above which the
level changes, although there are compressors
that raise signal levels below a threshold point.
Some companger-type noise reduction systems,
such as Dolby , have upper and lower thresh-
old between which the gain changes; these sys-
tems require careful level calibration for proper
encode/ decode performance. dbx noise reduc-
tion systems have no threshold at which com-
pression or expansion factors change, so level
calibration is not critical.
Glossary
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 encod er
senses the signal level, changes the gain, and
creates an encoded signal. The corresponding
"same ness" of the original signal and the en-
coded/ decoded signal can be exp ressed as the
tracking accuracy of the noise reduction sys-
tem. (dbx systems are non-critical for the opera-
ta-, and are built to close tolerances, so that
tracking accuracy is excellent, even if the en-
coder and decoder are in different pieces of dbx
equipment.)
Transition Level (see Level Match)
When a circuit has uniform compression or
compression throughout its full dynamic range,
there must be some level which passes through
the unit without being raised or lowered
(where gain is unity). The unity gain level is the
transition level or transition point. The transi-
tion point is a "window" ldB wide. In a dbx en-
coder (compressor), all signals above the
transition point are lowered in level, and all sig-
nals below the point are raised in level. Con-
versely, in a dbx decoder (expander), all signals
INPUT
,ou TPUT7
2 1
CompreSlion
120·1
Compredon
Umirlng)
Tl·ueJliOkJ,, - - - - - - - - ,-----. - • -
Threshold and Compression Ratio Settings
(How they define compression and limiting):
When the level of an audio signal is at or below
threshold, the input and output levels will be
unchanged . Above threshold, compression oc-
curs at a level defined by the COMPRESSION
ratio setting. If the ratio is 2:1, then when an
above-threshold input goes up lCkiB, the out-
put will go up SdB. When the input goes down
1CkiB, the output will go down SdB (unless it
goes below threshold). If a signal with a dy-
namic range of 60dB is put through a 2:1 com-
p~ssor, the dynamic range of the output signal
will be 3CkiB. If the ratio is infinity :1, the output
level is constant regardless of the input level,
once the input signal exceeds the selected
threshold . A compression ratio of above 10:1 is
commonly called limiting.
dbx 160XT
above the transition point are increased in
level, and all signals below the point are de-
creased in level. The transition level is similar
to a "threshold," except it does not refer to a
point at which compression or expansion fac-
tors change.
uiamplified
Similar to biamplified. A sound system where a
crossover network creates three frequency
ranges, and feeds three power amplifiers: one
f<?r bass, one f~r. mid, and one for high frequen-
cies. The amplifiers are connected directly to
the woofers, midrange drivers and tweeters
without a passive, high-level crossover net-
work.
Tweeter
A loudspeaker which reproduces only high fre-
quencies, usually from 8kHz up to 20kHz.
VCA (Voltage-Controlled Amplifier)
:radition~lly, amplifiers have been designed to
increase signal levels (to provide gain). If an
amplifier were required to decrease the level (to
attenuat~), it could t>e:come unstable, and might
e_ven oscillate. Th~
9
a1n (amount of amplifica-
tion) of these trad1honal amplifiers would be
adjusted by one of three methods: (1) attenuat-
ing the audio signal fed to the input of the am-
plifie~,_(2) attenuating the audio output of the
amphf1er, or (3) changing the negative feedback
(feeding more or less signal from the output
49
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