4-1-2 PRE-EMPHASIS CIRCUITS
(Refer to Figs. 6-11 and 6-12)
r---------
-7
I RI I
I0K
C6
I00P
I
I
Rl2
33K I
,------,
I
______
__.
I
I
C3
R5 I I C7
I00P
I
I 0,01 IOK I I
--------
I
I I
IC
I }JPCl252H2 I I
FROM R6 V
CA
H.P.F I
C4
I
33K
I I
I I
1------~+--
➔
TO
REC
AMP
I I I
L
_J
'---+------,
►
TO
8,
P.F
- - - -
LPre
Emphosis Circuit
___
_.,,,..I
I
(dB)
10
5
0
.,
-5
>
~
-10
I I
L
_________
J
Fig.
6-11
Pre-emphasis Circuit
I
Pre
-emphasis
\
De-emphasis
-15
'----------~--~
10
100
IK
I0K
I00K
(Hz)
Freq
Fig. 6-12 Pre-emphasis and De-emphasis
Characteristics
The
primary role
of
this circuit is
to
play the same
role
as
the equalizer used with the record
or
other
recording media
by
pre-emphasis
of
the high frequency
band
to
reduce high band noise, and
to
reduce the
breathing phenomenon (Note 1 ).
During playback, the high frequency band
is
de-
emphasized
to
return the signal
to
the original state.
In Fig. 6-11, the time constant
of
the Low band
is
determined in the first stage, and the time constant
of
the high band and the emphasis value
is
determined
in
the last stage.
C3
and
C4
provide
AC
coupling
to
prevent the
DC
offset
of
the previous stage from
being modulated
by
the change in the
VCA
gain.
The first stage also serves to convert the
input
voltage
to
a current source, while the last stage converts the
current
to
a voltage output.
C7
is an oscillation
preventing capacitor. (Refer to Fig. 6-11)
Note
I.
It is generally well-known that noise
(hiss
noise)
added
by
the tape recorder
or
tape
deck
has
a
high
content
of
high frequency
band
com-
ponents.
In the dbx system, compander gain changes
according
to
the level
of
the
intermediate
and
low frequency band components which · are the
main components in the input,
but
hiss
noise
also varies according
to
the gain
change.
As
a
result, hiss noise changes
as
the level
of
the
main signal component changes, and
"bre8thing"
noise
is
heard. This
is
called
the
breathing
phenomenon.
Of
course intermediate and low
frequency
band
noise changes similarly,
but
since
in
term
.ediate/
low band signal components are stron. g, this
noise is masked
by
these strong signals,
a.11d
it
is
not
sensed
by
the ears.
Hiss
noise consisting
of
high frequency band components is
not
oiasked
by the signal components and is
therefore
easily
detected by ear.
----------------SERVICE
MANUAL GX-747dbx
----------------
13
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