Dolby 212 b schematic

DOLBY B, C, AND S NOISE

REDUCTION SYSTEMS:

Making Cassettes Sound Better

In this age of digital audio it should come as little surprise to anyone that cassette tapes do not have the
same basic fidelity as compact discs (CD). The dynamic range available, between the noise floor and the
highest signal levels that can be recorded without distortion, is a fundamental limitation of magnetic audio
tape itself. Even if you make cassette recordings from noise-free sources like CD, when the music stops
between songs, tape hiss intrudes. What we call hiss is the noise created by the magnetic particles on the
tape, and it can obscure the quality of the music and become annoying. Dolby noise reduction has made
it possible to protect the music from tape noise, and helped make cassette the most popular audio
product ever devised. Here's how it works.

TABLE OF CONTENTS

Common To All Three Systems
Dolby B-Type Noise Reduction
Dolby C-Type Noise Reduction

Dolby S-Type Noise Reduction

Dolby Noise Reduction Systems Technology Comparison Chart

DOLBY B, C, AND S NOISE

REDUCTION SYSTEMS:

Making Cassettes Sound Better

COMMON TO ALL THREE SYSTEMS

Dolby noise reduction is a two-step process:

Step 1.

When music is being recorded, it is encoded just before reaching the tape. The purpose of encoding is to
raise the level of soft, high-frequency passages so they become louder than the tape's noise. During the
trip through the Dolby encoder, loud passages (that hide tape hiss) are not altered. Soft, high-frequency
passages (that tape hiss affects) are made louder than normal as they are recorded on the tape.

Step 2.

When playing back the tape, the sound is decoded by a precise mirror-image process of the encoding in
Step 1. The loud sounds are left unaltered, while the soft, high-frequency sounds are lowered back down

to their original levels. (You may have noticed that Dolby B tapes sound brighter when played without any
Noise Reduction decoding. Now you know why! You are hearing the encoded sound, not the original.

NOISE REDUCTION TAKES PLACE DURING DECODING. Tape Hiss is added to the recording during
the recording process. In step 1 we learned that the Dolby encoder boosted (made louder) the soft, high­frequency passages before the signal reached the tape and before tape hiss was mixed in.

During Step 2, the Dolby decoder doesn't "know," as it scans the signal coming off the tape, that tape
noise has been added to the music–it just goes about the business of reducing the encoded sounds to
their original levels, with the noise automatically getting the same treatment. The result? Completely
restored musical balance but with less hiss in the reproduced sound (see figure 1).

Figure 1. How Dolby noise reduction works.

Such encode-decode systems are generally called "companders." They compress the range between
loud and soft when recording and expand the range back again on playback, and reduce noise in the
process. While Dolby B-type, C-type, and S-type noise reduction systems all operate as companders,
there are many differences in the amount of noise reduction, the methods used to achieve it, and the level
of technology used in each.

DOLBY B, C, AND S NOISE

REDUCTION SYSTEMS:

Making Cassettes Sound Better

DOLBY B-TYPE NOISE REDUCTION

Dolby B-type noise reduction was developed in 1968 to address the hiss of slow-speed consumer tape­recording formats, such as 3-3/4 inches per second open-reel, compact cassette, and, later, the stereo
linear tracks on VHS video cassette tape.

Magnetic tape can only hold so much signal. If you put too much signal onto the tape, it will overload or
"saturate." The louder the signal that is being recorded, the closer the tape becomes to being "saturated."
If you were recording loud (high level) signals and at the same time you boosted them significantly for the
purpose of noise reduction, the tape would over-load and the recording would become distorted and
harsh sounding. Making matters worse for today's high-tech music styles, the bass and treble extremes of
the audible spectrum of magnetic tape have even less capacity than the midrange.

"If it isn't broke, don't fix it"

It is very important that the use of noise reduction not take away from the ability to record loud sounds by
unnecessarily boosting them. Loud sounds already hide noise on their own, so any increase would only
mean the overall recording level would have to be turned down to prevent distortion. All Dolby Noise
Reduction systems apply the principle of "least treatment" in their design. This means that the loud
signals are not processed, so there is no chance of causing tape overload when Noise Reduction is being
used.

Least treatment essentially means that if there is no benefit to be gained by changing the audio signal,
then don't change it. This is the reason that the compression-expansion process is only used on softer
(low level) signals (see Figure 2). For example, suppose you want to record a solo, loud rap on a bass
drum. The main part of the drum's sound is at low frequencies and may already approach the tape's
overload point. Even so, the drum is not able to hide the hiss noise in the treble range. The Dolby B-type
noise reduction encoder does not provide a boost to the drum signal, so the tape will not over-load
(saturate). However, it does provide a full boost over the rest of the frequency spectrum. During playback
through a Dolby B-type decoder, there is effective noise reduction and no tape over-loading. Dolby B-type
Noise Reduction does this by using a SINGLE, SLIDING COMPRESSION-EXPANSION BAND of
frequencies. A sliding band is basically a filter that can shift its frequency breakpoint up and down. In B­type noise reduction, the frequency where the noise reduction action just begins can change from about
300 Hz all the way out to 20,000 Hz.

Figure 2. Record processor characteristics.

When the audio signal is very low or contains no treble frequencies, the band slides to the lowest
frequency point (all the way to the left in Figure 3), giving a maximum of 10 dB noise reduction above
4,000 Hz. As the band slides to higher frequencies, less and less of the spectrum is covered, so there is
less effect. The band will not slide up, however, until the sounds being recorded are loud enough to hide
the noise on their own. By moving the sliding band quickly enough to follow the music being played, the
full effect of the noise reduction is maintained. In Figure 3, the sliding band slides up (in frequency) out of
the way of the loud bass drum signal (the "dominant frequency"), and keeps the noise reduction working
at higher frequencies, where tape hiss would otherwise be audible.

Figure 3. Effects of dominat signals within the NR band also vary with the system. To effect a given
decrease in gain (2 dB shown) at the dominant frequency, the NR band of a sliding-band system slides
up, reducing NR at frequencies below the signal.

In all Dolby Noise Reduction systems, the key to the operation is in the positioning of the sliding bands. In
order for the final decoded sound to be faithfully restored, the bands in the decoder must track the
positions of the bands in the encoder as closely as possible. The recorded audio signal tells the decoder
how to operate. If, for some reason, the level or the frequency response of the encoded signal is changed
before it reaches the decoder, mistracking of the sliding bands will occur. How audible this becomes has
to do with several factors, including the nature of the music, the listening conditions, and the sensitivity of
the listener. By limiting its overall range to 10 dB, B-type Noise Reduction is not very susceptible to

audible mistracking. Still, it is advisable to make sure that the tape formulations used with any recorder
are compatible with its design, and that the proper settings are used for bias and record calibrations.

TECHNICALLY SPEAKING

In Figure 4 you can see that the compression expansion process is only operating at middle-level signals
(not real loud, or real soft passages). At high signal levels (loud passages), there is no dynamic action
and the system acts as a unity-gain amplifier. At low signal levels (soft passages) the system acts only as
a fixed-gain amplifier. By restricting the system's compression-expansion action to only the middle-level
signals, distortion caused by overshoots is minimized. See Figure 5 for a Block Diagram of Dolby B-type
Noise Reduction.

Figure 4. Dolby B-type bilinear compression and expansion.

Figure 5. B-type noise reduction system-block diagram.

Dolby B-type noise reduction is now the standard in tens of millions of cassette decks, mini-component
systems, boom boxes, personal portables, and car stereo components; it provides 10 dB of noise
reduction above about 4,000 Hz.

DOLBY B, C, AND S NOISE

REDUCTION SYSTEMS:

Making Cassettes Sound Better

DOLBY C-TYPE NOISE REDUCTION

Introduced to the public in 1980, Dolby C-type provides 20 dB of noise reduction above about 1,000 Hz,
fully doubling the amount given by B-type. Additional features were incorporated to deal specifically with
the difficulties of recording on slow-speed consumer tape formats.

Increased Noise-Reduction Bandwidth

Cassette tape gives a noise spectrum, without Noise Reduction, which appears to be concentrated in the
high frequencies. That's why we refer to hiss rather than roar or rumble. Noise is less noticeable when no
one part of the audible spectrum is apparently dominant. At the other extreme, noise with a distinct pitch,
due to dominant energy in a narrow region of the spectrum, is especially irritating. Listening to tape hiss
with B-type noise reduction reveals that the noise is not only lower in level, but that it sounds more evenly
balanced across the frequency spectrum, because the concentrated noise at high frequencies has been
reduced. However, as you continue to reduce high frequency noise, as in Dolby C-type, obviously the
middle and low-frequency components of the tape noise become relatively more significant. If you were to
reduce the high frequency noise by twice as much in the region of the spectrum where B-type noise
reduction operates, middle frequencies would dominate in the resultant noise, and you might well
describe it as a roar rather than a hiss. As you apply more and more high-frequency noise reduction to
cassette tape, you need to extend the region of that noise reduction lower and lower in frequency so that
no one area of the spectrum becomes (apparently) dominant.

Referring to Figure 6, you see that the Dolby B-type system begins to take effect in the 300-Hz region and
increases its action until a maximum of 10 dB of noise reduction is achieved in the 4,000 Hz and above
region. What you hear is an overall reduction of noise. The remaining noise is not noticeable because it
appears to be spread evenly over the frequency range.