Akai GX-F44R

Vil. DOLBY C-TYPE
TAPE NOISE LEVEL
NOISE REDUCTION
TAPE NOISE LEVEL
~=— AUDITORY SENSE CURVE
AUDITORY SENSE CURVE
Fig. 8
High-Level Stage
Variable
filter
Side
chain
Control
circuit
Spectral
skewing
Input
1. OUTLINE
Doiby C-type is to improve the noise reducing effect
of the usual Dolby B-type. The B-type has the NR
effect of 10 dB at 5 kHz or more but C-type has the
NR effect of 20 dB at 1 kHz or more. Fig. 8 shows
the audible curve of man at low level and the noise
level of the cassette tape.
In the bands of about 200 Hz or lower and of 10 kHz
or higher the hiss noise of the tape is not heard. On
the contrary, in the range of 200 Hz to 10 kHz the
noise is apt to be heard. For this reason, the hiss noise
of the tape is reduced by getting the encode charac-
teristic closer to the audible curve as shown in Fig. 9.
Since the difference at this time between the noise
level and the audible curve is about 20 dB, C-type sets
the noise reduction amount at 20 dB. For the high
band (more than 10 kHz) the NR amount is reduced
to improve the characteristic.
Fig. 9
Low-Level Stage
Variable
filter
Control
circuit
Anti~saturation
network
Output
Fig. 10 Block Diagram of Encoder
2. CIRCUIT COMPOSITION
Fig. 10 shows a simple block diagram at the time of
encoding.
In Dolby system, the NR amount per one processor
is specified at 10 dB maximum. This is because as the
compression amount increases, the possibility of side
effect (dynamic error, etc.) increases. Therefore, the
C-type uses two processors with the NR amount of
10 dB each, one as high level stage and another as
low level stage. By providing these two processors
with different operational regions, the overlapping of
dynamic operation is prevented, one operating the
linear operation, and therefore, as a whole, it is the
compression operation of one 10 dB processor, but
20 dB effect is obtained.
Each processor uses the sliding band system and dual
pass system as in the case of B-type. The input signal
is separated into the Main Pass and Side Chain, and
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