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7 . 5 D O L B Y N R P R O C E S S O R C I R C U I T
Recording Mode Operation
1. Input signals are first applied to the MPX filter
where the FM broadcasting station's pilot signal
is employed, and where an 85kHz bias trap is
employed to prevent mis-operation of the Dolby
NR noise reduction system.
2. From the MPX filter, the signals are passed on
to the buffer amplifier whose output is divided
into 2 routes. The main signal is passed directly
to the adding amplifier, while the sub-signal is
passed via the side chain amplifier and clipper
before being recombined with the main signal
in the adding amplifier.
3. Besides being applied to the clipper, the side
chain amplifier output
is also passed via the
voltage
controlled
amplifier
and
integrating
amplifier back to the side chain amplifier input,
forming a variable filter circuit.
4. In addition to the above, the side chain amplifier
output is further applied to a high-pass filter and
rectifier where it is converted into a DC voltage
for control of the voltage controlled amplifier.
5. When the level of the signal passed through the
high-pass filter is low, the rectifier output DC
voltage will
be almost
"0",
resulting in a
minimum
tumover
frequency for the variable
filter.
The Ievel of the adding amplifier output
will thus be 10dB (above SkHz) higher than the
level of the main signal, thereby contraction the
dynamic range.
cT-Fs|5c,
6. The clipper produces a time lag in the signal
applied to the voltage-controlled amplifier, and
since it is not capable of responding to sudden
Ievel changes, no uncontrolled signals will be
a p p l i e d t o t h e a d d i n g a m p l i f i e r .
7. When the level of the signal passed through the
high-pass filter is high, the rectifier output DC
voltage will also be high, and the variable filter
turnover frequency will be increased. The sub-
signal level will therefore become almost "0", so
there will be no contraction of the dynamic
range.
Playback Mode Operation
Although each section of the Dolby NR Proces-
sor operates in the same way as during recording
mode, the sub-signal is derived from the output of
the
adding
amplifier.
And
since the
adding
amplifier is an inversion amplifier (where output
phase is opposite to input phase), the sub-signal
will be of opposite phase, thereby forming an NFB
l o o p .
In Dolby NR B noise reduction systems, con-
traction
and
expansion
occur
within
a fixed
frequency range determined by the variable filter
circuit.
And,
in
order
to
achieve perfectly
symmetrical
operation,
it is necessary to
fix
a
reference level for the operational point.
This is
the
so-called "Dolby
level",
below
which
no
contraction and expansion is performed.
A d d i n g A m p l i f i e r
M P X F i l t e r
B u f t e r A m o l i f i e r
S i d e c h a i n A m p l i f i e r
V o l t a g e . c o n t r o l l e d
A m p l i f i e r
R e c t i f i e r
H i g h p a s s F i l t e r
F i g . 7 - 4 D o l b y N R p r o c e s s o r c i r c u i t
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