Dbx Noise Reduction System - Akai GX-747 dbx Service Manual

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,~~ - -
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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)
+ 2 0 ~ - - - -
W/«fl////2
________ (dB)
0
-20
-40
-60
- 8 0 ' - - - - - . 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
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