JVC DD-9 A - Dolby C-Type Noise Reduction System

Dolby* C-Type Noise Reduction System:
Noise is reduced and MOL increased dramatically.
The Dolby C-type closely resembles the B-type in operation,
the system compatible with ANRS, yet offers even better
performance and more distinctive features. Namely:
® \|t provides about 20 dB above 1 kHz, and 15 dB even at
500 Hz of noise reduction.
® At 10 kHz (at OVU recording), it improves MOL by
4 dB maximum.
® It is immune to undesirable side effects, such as modu-
lation noise and breathing.
® Finally, it is tolerant of noise reduction encode/decode
errors. Level matching is not critical.
(* Dolby is a trademark of Dolby Laboratories, Inc.)
The basis of C-type noise reduction
Dolby C noise reduction solves the problem of achieving
a large amount of compression and expansion without
introducing undesirable side effects by the use of two pro-
cessing stages in series, each supplying 10 dB of compression
during recording and of expansion during playback. These
circuits operate at independent levels. One, identified as the
high-level stage in Figure B, is sensitive to signals at about
the same levels as Dolby B-type noise reduction, while the
other, the low-level stage, operates on signals of somewhat
lower level. Because the two stages operate in tandem with
each other, their effect is to multiply the signals (or add and
subtract in dB’s), so that a total of 20 dB of compression
and expansion, and thus of noise reduction, is accomplished.
Yet simultaneously, at no time is the signal subject to the
vagaries of a single compression or expansion action of
20 dB. In other words, the tandem two-level, two-stage con-
figuration provides a much more accurate control of the
signal than a single compander circuit would be able to
achieve.
Two conventional Dolby B IC circuits are used ina modified
way in C-type noise reduction to carry out the two-level,
two-stage scheme. Thus a desirable side benefit of C-type
noise reduction is that it can be executed from the begin-
ning with readily available and economical parts (the
development of a dedicated Dolby C integrated circuit in
the future would, however, simplify incorporating C-type
NR in products). Furthermore, one of the two stages can
be easily configured to provide the B-type characteristic at
the push of a switch, so cassette decks incorporating C-type
noise reduction can be economically equipped with Dolby
B NR as well for perfect compatibility with existing B-type
recordings.
a
eae
ee
et
:
MOL
Improved
by
3
Reference
Level
(160nwb/m)
——-+=
about
4dB
(10kHz)
SEP apo pe tots
Dolby*C ON
=P“ Noise Reduction OFF 4-
:
i
PSE SS ee eg ON Kei =
S/N Improved by about 20dB=--}—
_- (ikHz and above) =
Eieaehattwalis i
Output Level (dB)
BAS reenrears TARe FSI ARS
Noise Reduction OFF
Hi Tay eae
saith ae +)
ANRS/Dolby*B ON
i edt
t I se
|
SIN
Improved!”
{
{Dolby*c
ONT
E
“--H by about 15dB 5 -
20 50 100 200 500 1000 2000 5000 10000 20000
Frequency (Hz)
Dolby* C Improvement
Other developments
In addition to two-level processing, Dolby C-type noise
reduction incorporates a number of further innovations.
Two of these, shown in Figure B as anti-saturation and
spectral skewing networks, are carefully calculated fre-
quency response modifications introduced in the encoding
(record) process and reciprocally compensated for in the
decode (playback) process. Their purpose is two-fold: to
further guard against audible side effects, and to ensure
the practicality of the system in day-to-day consumer use.
The specific benefits of these innovations include the
reduction of encode-decode errors and a reduction of
upper-middle and high frequency tape saturation and its
side effects, such as high frequency losses and _ inter-
modulation distortion. Together with the two-level, two-
stage configuration, these new developments result in a
20 dB noise reduction system at least as free of side effects
as the 10 dB B-type system, and one which is just as practi-
cal in day-to-day use.
—~6-A—