3. dbx NOISE REDUCTION
SYSTEM
The dynamic range
of
actual sounds
of
orchestra,
rock
band,
etc. far exceeds 100 dB.
On
the
other
hand, the
dynamic
range
of
recording systems such
as
disk and tape
has
so far been limited to 50-70
dB
due
to
the
saturation
level and noise characteristics.
The
dbx
noise
reduction
system
is
one
of
the attempts
to improve
on
the
above drawback.
TAPE
LINE
IN
PUT
o----<XJ--
.__ _
____.
MIC
INPUT,~~
--
11
In this system, the dynamic range in decibels
of
the
input
signal
is
halved during recording, and doubled
during playback after recording
to
return
it
to
original range. The system makes
it
possible to
record and playback signals whose dynamic range
exceeds 100
dB
through a tape deck whose dynamic
range is the normal 50-60 dB. (Refer
to
Fig. 6-6)
DECODER
-----o
LINE OUTPUT
(dB)
Saturation
level
(dB)
+20~----
W/«fl////2
________
(dB)
0
-20
-40
-60
-80'-----.T
Source
Signal
+10
ENCODER
The dbx system largely consists
of
the encoder and
decoder. The basic operation
of
the encoder/decoder
is
as
follows:
Encoder operation
As
shown in Fig. 6-7, the
input
signal range in
decibels is halved. For example,
-60
dB
input signal
is
changed
to
-30
dB, and +20
dB
signal
is
reduced
to+
10 dB. After the encoder, the signal
is
sent to the
recording amplifier, and recorded on the tape. That
is, the 80
dB
dynamic range
of
the original signal
is
reduced
to
40 dB. In this case, when the user tries
to record the original signal with the tape deck
having 60
dB
dynamic range, the 20
dB
difference
from the 80 dB signal range should have been
sacrificed
by
the tape noise
or
saturation distortion,
but
thanks
to
the operation
of
the dbx encoder,
the dynamic range
of
40
dB
thus obtained covers the
80
dB
input
range sufficiently.
Decoder operation
The encoded playback
output
signal
of
the tape deck
which has passed through the playback amplifier
from the
PB
head
is
sent to the decoder. The
dynamic range
in
decibels is doubled according
to
the decode characteristic shown in Fig.
6-
7. For
Tope
-----~
+20
--
---
--
0
--
------
-20
-40
-60
...._
___
..J-80
DECODER
Output
Signal
Fig. 6-6 Encode and Decode
example, a
-30
dB
signal
is
changed
to
-60
dB,
and
the noise level
is
also lowered
by
30 dB, which
gives
the so-called noise reduction effect. Similarly, a + 10
dB
signal
is
increased to +20 dB,
so
that
the saturation
characteristic on the tape has apparently been im-
proved
by
10
dB. The saturation ceiling
level
is
increased, hence the dynamic range
of
the
tape
deck
is effectively increased, and the original sound
can
be
reproduced without its wide dynamic range being
affected.
(dB)
a
.J
~-10
~-20
!:i
-30
~
-40
::,
0
-so
-60-+Ec----+-------I
-10
-80
-80
-70
-60-50·40
-30
-zo
-10
0
+10
+20
(dBJ)
INPUT LEVEL
Fig. 6-7 Input/Outpl,lt
-----------------
SERVICE MANUAL GX-747dbx
----------------
11
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