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dbx
Noise
Reduction
System
The dbx Noise
Reduction System serves
to
greatly
reduce the
noise generated during
recording
and
playback across
the
whole audible
frequency spectrum (reduction
by
more
than
about 30 dB) and
also
to
improve
the
linearity
at
high
input
levels.
As
a
result, both
recording
and
playback
can
be
performed without
impairing
the wide
dynamic range
of
the
original
sound
There are
two
kinds
of
dbx Noise
Reduction
System: type
I
and
type II.
Type
I
is
used for
open-reel
decks
and
industrial applications while Type
II,
which
is
featured
in
this
deck, is used for
cassette applications.
Features
1
Reduced
noise over the whole
audible frequency
range
(more
than
about
30
dB reduction).
2
The noise
is
compressed
at a
high
recording
level
for
recording
to
enable
recording
with
minimal
distortion
and
a
wide
dynamic
range.
3.
The
linear logarithmic compression and
expansion
do
not
make
the
sound
quality
undergo change
with level
mis-
matching.
Principle of
basic
operation
The dbx system
works
to
expand
the dynamic
range by com-
pressing
(encoding) the signals
and
then expanding (decoding
them
As shown
in
the figure, the input signal level is
halved
during
recording onto
the tape.
During playback
the
halved
level
is
doubled
to restore the original signal. The figure shows
that
high signals are greatly
expanded
(from
"+10d8"
to
"
+20"
dB) while
low
signals
are
given
a
low
expansion
(
"
-40
dB"to"-80d8"
)
This results in a great
improvement in the
dynamic range
and
simultaneously
in
a
great reduction
in
tape
h
iss.
lnput
Encoding
Decoding
Output
ttt4d44fi/u^r,1l{{
Tape hiss
Expansion in
Dynamic
Range
The figure below
shows
the
dynamic range characteristics
First the overall
frequency response
is
indicated
at a
signal
level
where the
distortion
is
made 3%
al
l
kHz. When the
"dbx
in"
position
is
selected,there
is
an
increase of
about"l3dB"at
the 3%
point
over
the "out"
posrtion,
indicating that
the
distortion with
high input levels
is
greatly
improved.
ln
addition,
the
noise distribution is indicated
at
the
"dbx
in"
"dbx out"
positions when the noise level is
recorded without
any
signals
being supplied and that noise
on
the tape
is
played
back.
With recording
and playback
at
the
"dbx
in"
position,
it is
clear
that the
noise level is
greatly
reduced.
Because
of the
great
reduction
in the
noise
level
and
the great
expansion
in
the
signal
level, the dynamic range
at
the
"dbx
in"
position
is
vastly
improved,
and
a
dynamic
range which
is
vastly superior
to
that
ol
a2-lrack
38
cm/sec. open-reel
tape
is
yielded.
Notes:
oThere
is
no
compatibility between
the dbx
Noise
Reduction Systems and other similar
systems
Tapes
recorded
with the dbx
system should therefore
be
played back with the dbx system.
oRecord ordinary discs (not "dbx
encoded
discs")
with
the
noise reduction selector switch
at
any
position
except
"disc".
The
sound
quality will
change
if
this
switch
is set to
the
"disc"
position for ordinary
record
play
+2AdB
0dB
\
r
ape
saru
level
alton
+zadB
0dB
rt0dB
0dB
2AdB
-z
'l0dB
2AdB
40d
B
30d
a
40d
a
40d
B
60d
B
Tape
hiss
60d
B
80d
B
80dB
Encoding/decoding
process
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