Teac 450 - The Dolby Noise Reduction System Explained

The Dolby noise reduction system
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Principle and Outline of the Dolby System
Dolby System comes in two types, A Type and B Type. The
A Type is designed for studio use and comprises a complex
and enormous circuitry structure which divides the record/
playback frequency band (20Hz — 20 kHz) into four bands.
The B Type, designed for popular use, features simpler
circuitry and is easy to handle.
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The Dolby System mentioned in this manual refers to B-Type
Dolby System unless otherwise indicated.
Inter-deck Compatibility of Dolby-encoded Tapes
Cassette tapes recorded by a Dolby-system recorder (Dolby-
encoded cassette tapes) give the best results when reproduc 
ed by a similar Dolby-system recorder or player. Needless to
say, no Dolby effects will be demonstrated when such tapes
are played back by an ordinary cassette tape deck, but the
treble range may be heard somewhat emphasized when very
soft signals are reproduced. This slight emphasis of the treble
range is almost unnoticeable unless compared with the
original source program or with the sound reproduction
made through the Dolby circuit.
For playback of Dolby-encoded tapes with an ordinary tape
deck, it is advisable to somewhat de-emphasize the treble
range.
Effects of the Dolby System
In the frequency range of 1 kHz. or higher where the tape
hiss and transistor noises are more conspicuous, the B Type
Dolby system incorporated in the 450 can reduce such
noises by 5 dB at 1 kHz and 10 dB at 10 kHz (in terms of
S/N ratio) by compressing and expanding the signals of 1
kHz or higher. (Refer to the diagram above.)
Thanks to the Dolby system incorporated, your 450 gives
dynamic, clear-tone recording and playback practically free
of the tape hiss and transistor noises formerly considered
unavoidable in a cassette system.
herent in the tape and amplifier. When playing back this
recording, the reproduction level is lowered to the original
input level or “ expanded. ” Through this unique manipla-
tion, the tape noise is reduced below normal levels to in 
audibility.
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Tape hiss and transistor noise from the cassette deck itself
are problems inherent in any cassette tape recording. The
magnetic coating of a cassette tape is so thin that its
magnetic saturation point is very low. This makes it difficult
to improve the S/N ratio through high-level recordings as
often practiced with an open-reel tape.
Numerous ways have been contrived and tested over the
years to reduce tape hiss and transistor noises. Nevertheless,
most of the noise reduction systems marketed today employ
a high-frequency filter for reducing these noises. Use of a
filter results in a somewhat changed tone quality, especially
in the treble range, the frequency range considered most
important in music recording and playback.
The newly perfected Dolby system is an epoch-making noise
reduction system in that the noise level itself is lowered to
improve the S/N ratio. The best of the noise reduction
systems available today, it enables you to make recording and
playback featuring extra-low noise and wider dynamic range.
Your 450 is a Dolby-system cassette tape deck and there 
fore assures you of the revolutionarily noise-free recording
and playback which only the Dolby system can provide.
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The principle of the Dolby System can be described as
follows:
When the input level is lower than a certain reference level in
Record, the recording level is raised higher than the input
level to “ compress ” the input signal, thereby enlarging the
difference between the input level and the noise level in-
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S/N RATIO IMPROVEMENT ACHIEVED BY DOLBY SYSTEM
1. WITHOUT THE DOLBY SYSTEM
2. WITH THE DOLBY SYSTEM
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