40
Glossary
panding amplifier returns to 90% of the normal
gain. Very fast decay times can cause "pump-
ing" or "breathing" effects, whereas very slow
decay times may cause moderate-level pro-
gram that 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 pro-
gram, the circuit may be said to decode the pro-
gram. The equipment or circuit which performs
this function is known as a decoder. Decoders
must only be used with programs which have
been encoded by complimentary encoding cir-
cuitry. 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 de-
coders that extract four channels of program
from the stereo program on encoded record-
ings, and dbx decoders that retrieve wide-dy-
namic range programs from the compressed
programs on dbx-encoded recordings.
De-emphasis and Pre-emphasis
De-emphasis and pre-emphasis are related pro-
cesses that are usually done to avoid audio
noise in some storage or transmission medium.
Pre-emphasis is a boost at specific higher fre-
quencies, the encoding part of an encoding/ de-
coding system. De-emphasis is an attenuation
Glossary
Encoder
When a circuit processes an original program
to create a specially treated version of that pro-
gram; the circu it may be said to encode the pro-
gram. The equipment or circuit which performs
this function is known as an encode r. Encoded
programs must be decoded only with comple-
mentary decoding circuitry. Typical encoded
programs include: FM multiple broadcasts, ma-
trix quadraphonic recordings, and dbx encoded
recordings.
Envelope
In music, the envelope of a note describes the
change in average signa l level from initial at-
tack, to peak level, to decay time, to sustain, to
release time. In other words, the envelope de-
scribes the level of the note as a function of
time. Envelope does not refer to frequency.
I
I
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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
at the same frequencies, a reciprocal decoding
that counteracts the pre-emphasis. In dbx noise
reduction, de-emphasis is performed by the de-
coder (the play circuitry). The de-emphasis at-
tenuates high frequencies, thereby reducing
tape modulation noise and restoring the origi-
nal 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 transmit-
ter.
Dynamic Range
The ratio between the loud est (greatest ampli-
tude) signals and the softest (least amplitude)
signals that either are contained in a certain
piece of music or can be reproduced accurately
by a certain piece of equipment. Dynamic
range is normally expressed in decibels (dB)
where each ratio of ten in power corresponds
to a dynamic range of lOdB (e.g. a ratio of 100
in power corresponds to a dynamic range of
20dB, a ratio of 1000 in power corresponds to a
dynamic range of 30dB). A program with wide
dynamic range has a large variation from the
softest to the loudest passages, and will tend to
be more life-like than programs with narrow
dynamic range.
dbx 160XT
milliseconds, seconds, or even minutes to rise
and fall. In dbx processing, the envelope is
what "cues" therms level detection circuitry to
compress and expand the signa l; the peak or av-
erage 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 (de-
creasing) the relative level of a portion of the
audible spectrum. Some EQ is used for achiev-
ing sound to suit personal listening tastes,
while other types of EQ are specifically de-
signed to correct for non-linearities in the sys-
tem; these corrective EQ "curves" include tape
(NAB or CCIR) equalization, and phonograph
(RIAA) equalization. In a sense, the pre-empha-
sis and de-emphasis used in dbx processing are
special forms of equalization.
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 con-
trols, 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 41
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