Dolby Nr Circuit - Pioneer CT-F850 Service Manual

7.4 DOLBY NR CIRCUIT
The CT-F850 features a type B Dolby NR noise
reduction system where noise is reduced in the mid
to high frequency region only to eliminate much of
the inherent tape hiss. The S/N ratio is improved
by 10dB (max.) in the high frequency range (above
5kHz).
The Dolby NR circuit is incorporated in a
Dolby NR processor IC (PA4005) developed by
Pioneer. This IC employs a voltage controlled
variable gain circuit (VCA), rather than a variable
resistance element used in other more conventional
Dolby NR ICs. See Fig. 7-6 for an outline of
this Dolby NR processor.
Operation During Recording Mode
1. The input signal is applied to the buffer ampli-
fier via the MPX filter. This filter eliminates
the FM pilot signal, and also serves as a 85kHz
bias trap, thereby preventing any accidental
system mis-operation.
2. The buffer amplifier output is divided into 2
positions, the main signal being passed on
directly to the adding amplifier, while the sub-
signal is diverted through the side chain ampli-
fier and clipper before being rejoined with the
main signal in the adding amplifier.
3. Besides being passed on to the clipper, the side
chain amplifier output is also applied to the
voltage controlled variable gain circuit and
integrating amplifier to be fed back to the in-
put side again, thereby forming a variable filter
circuit.
4. Furthermore, the side chain output is also
applied to a high-pass filter amplifier and recti-
fier circuit. The rectified signal is then applied
to the voltage controlled variable gain circuit
as a control signal.
MPX filter Buffer amplifier
CT-FsS50
5. When the level of the signal passed through the
high-pass filter amplifier is low, the rectifier DC
voltage will drop to almost "'0''. The turnover
frequency of the variable filter will consequent-
ly be at a minimum. Under these conditions,
the adding amplifier output level will be 10dB
higher (for frequencies above 5kHz) than the
main signal input, and the dynamic range will
be contracted.
6. With a time delay being generated between the
signal applied to the voltage controlled variable
gain circuit and the main signal, the clipper will
not be capable of responding to sudden level
changes. No uncontrolled signal will thus be
applied to the adding amplifier.
7. When the level of the signal passed through the
high-pass filter amplifier is high, the rectifier
DC voltage will also be high, resulting in an
increase in the turnover frequency of the vari-
able filter. The sub-signal level will conse-
quently drop to almost ''0", and there will be
no contraction of the dynamic range.
Operation During Playback Mode
Although each block operates in the same way
as during recording, the adding amplfier becomes
an inversion amplifier (output phase becomes the
inverse of the input phase), resulting in the inver-
sion of the sub-signal phase, and the formation of
reducing type NFB loop.
In Dolby NR B noise reduction systems, con-
traction and expansion occur within the frequency
band determined by the variable filter circuit. But
in order to ensure complete symmetry in these
operations it is necessary to set a reference level for
the operational point. This is called the "Dolby
NR level", all signals above this level being con-
tracted and expanded accordingly.
Adding amplifier
Integrating
amplifier
_ controlled —
variable gain circuit
Rectifier High-pass filter amplifier
Clipper
Fig. 7-6 Dolby circuit
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