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By LEO SIMPSON
Dolby Surround
Sound: How It Works
You’ve been to the movies. You’ve seen the
pictures. You’ve heard the sound. And now
you want it all at home, in your living room.
And while you may not be able to afford the
latest widescreen TV or LCD video projection
system, you can at least have the big sound of
the movies. Read on, to find out how Dolby
Surround Sound works.
Many people, if they think about
it at all, may assume that the latest
Dolby Surround Sound used to such
great effect in today’s action movies
is somehow related to the “failed”
surround sound systems of the early
1970s. These were variously referred
to as surround, four channel and
quadraphonic. There where three
competing systems, all of which were
incompatible with each other and all
required two additional speakers in
the rear corners of the listening room,
to reproduce extra channels from
specially encoded stereo LP records.
Older readers will remember the
names of those systems as “Sansui
QS”, CBS “SQ” and JVC’s CD-4.
Fig.1: this shows the arrangement of speakers
used in a typical Dolby stereo cinema system.
The film sound track provides two stereo
channels & these are fed to the Dolby processor
to provide four main channels, Left, Centre,
Right & Surround, together with an optional
Subwoofer channel for extra bass.
LEFT
SUB
“SQ” and “QS” sound systems were
“matrixed” systems in which the rear
channel signals effec
tively rode on
top of the normal stereo signals and
were recovered with relatively simple
phase-splitting circuitry. The resulting
rear channels did not have much separation from each other or from the
front channels (about 7dB) but they
did give an illusion of extra space or
“ambience” to the sound quality.
The CD-4 4-channel system, by contrast, was much more elegant and did
provide quite good separation between
front and rear channels (up to 20dB)
but it was also much more expensive,
push
ing the existing LP recording
technology to the absolute limits.
Briefly, a CD-4 encoded record has
the regular stereo signals plus a high
frequency carrier at 30kHz. This was
phase and frequency modulated with
rear difference signals (up to 45kHz)
CENTRE
SCREEN
POWER
AMPLIFIERS
OPTIONAL
SUBWOOFER
RIGHT
Lt
Rt
DOLBY STEREO
CINEMA
PROCESSOR
CENTRE
LEFT
SURROUND
SURROUND
4 Silicon Chip
RIGHT
DOLBY
A-TYPE NR
OR
DOLBY SR
Lt
Rt
BASS
EXTENSION
4-CHANNEL DECODER
OPTICAL
PREAMP
ADAPTIVE
MATRIX
7kHz
LOW-PASS
FILTER
AUDIO
DELAY
MODIFIED DOLBY
B-TYPE NR
DECODER
Fig.2: the block diagram of the Dolby stereo cinema processor. The key point
to note is that the surround channel is fed via an audio delay of typically 20
milliseconds. This is partly to provide ambience or reverberation & partly
to allow your ears get the cue for direction from the front speakers.
which were subsequently recovered
from the outputs of the RIAA preamplifier.
It required a phono cartridge with
a special elliptical (Shibata) stylus,
very low compliance and extra special
tracking capability. If you played a
CD-4 disc with a conventional stereo
cartridge, the high frequency carrier
would quickly be ploughed out and
the disc would no longer provide four
channel sound.
After a few years, consumers were
so confused with the claims and counterclaims for the different systems that
they avoided the issue entirely and
four channel sound just died.
Dolby Surround Sound
Dolby Surround Sound is quite
different from all three of the above
ill-fated systems and is continually
growing in its acceptance by movie-goers and consumers in the home.
So let’s see how Dolby works.
Dolby stereo surround sound based
on optical sound tracks was introduced
to movie theatres in the mid-seventies
but its arrangement of the channels is
quite different to that used in previous
home surround sound systems. Fig.1
shows the arrangement of channels
and speakers used in a typical Dolby
stereo cinema system.
In essence, the film sound track
provides two stereo channels, Lt (left
total) and Rt (right total) and these are
fed to the Dolby processor or decoder
to provide four main channels, Left,
Centre, Right and Surround, together
with an optional Subwoofer channel
for extra bass.
Notice that all the Surround speakers are fed with the same signal – there
is no left rear and right rear, just surround and this signal happens to be
delayed, by 20 milliseconds or more,
with respect to the front speakers.
Why three front speakers?
The trouble with having just two
stereo speakers in a cinema is that the
screen is too wide. For people at the
sides of the theatre, the nearest speaker
predominates. This sounds and looks
silly when the source of the sound is
clearly at the centre or on the other
side of the screen.
This is where the centre channel
comes into its own and provides much
better sound localisation. The centre
channel should not be thought of as
merely a “fill-in” speaker but as quite
separate from that provided by the
two stereo channels. Indeed, when
the centre channel is used in a typical
movie, it is used only for dialogue and
the stereo speakers are silent while it
is in use.
Once you become aware of it, you
SUBWOOFER
3rd OCTAVE
EQUALISER
RIGHT
3rd OCTAVE
EQUALISER
CENTRE
3rd OCTAVE
EQUALISER
LEFT
PARAMETRIC
EQUALISER
SURROUND
will often notice the profound switch
between stereo sound over to the
centre channel and then back out
again. In effect, Dolby provides three
separate front channels: they are used
separately for dialogue while the left
and right are used for stereo music
accompaniment.
The surround speakers are used
during action scenes, with the delay
providing a great deal of ambience,
reverberation, echo or whatever.
While you may think that having all
the surround speakers driven from the
one source would make rear sounds
somewhat vaguely located, your ears
get their cues from the front speakers
so that the apparent localisation is
quite strong.
Dolby decoding
Fig.2 shows the block diagram of
the Dolby stereo cinema processor
which is featured in the setup of Fig.1.
This shows input preamplifiers for the
projector’s optical pickups, followed
by Dolby A noise reduction, and
then the two stereo signals are fed to
the 4-channel decoder. This is partly
based on the 4-channel matrix systems
of the early 1970s and the key to it is
the box labelled “Adaptive Matrix”.
The outputs of this matrix become the
LEFT
CENTRE
Lt
-3dB
SURROUND
-3dB
BPF
DOLBY NR
ENCODER
+90ø
-90ø
RIGHT
Rt
Fig.3: the basic Dolby surround encoding setup. The encoder accepts four
separate input signals – left, centre, right & surround (L, C, R & S) – & creates
two final outputs, left-total & right-total (Lt & Rt).
October 1994 5
DIGITAL
DECODER
A
systems both are four channel systems, their method of
operation is quite different.
MONO
Dolby encoding
While you might expect
the Dolby signal encoding
process to be complicated, it
LEFT
is actually simpler in princiDIGITAL
2 CHANNEL
DECODER
ple than the decoding. Fig.3
STEREO
B
shows the basic encoding setRIGHT
up. The encoder accepts four
separate input signals, left,
DOLBY SURROUND DIGITAL
PROGRAM MATERIAL
centre, right and surround
(L, C, R & S), and creates two
LEFT
4 CHANNEL
ANALOG
DIGITAL
final outputs: left-total and
CENTRE
ANALOG
PRO LOGIC
DECODER
DOLBY
RIGHT
right-total (Lt & Rt).
DECODER
C
SURROUND
SURROUND
The L and R inputs go
straight to the Lt and Rt outputs without modification.
LEFT
The C input is divided equalCENTRE
ly to Lt and Rt but with a 3dB
DOLBY
5 OR 5.1 CHANNEL DOLBY
RIGHT
SURROUND
SURROUND DIGITAL
level reduction (to maintain
DIGITAL
LEFT SURROUND
(SUBWOOFER OPTIONAL)
a constant acoustic power in
DECODER
RIGHT SURROUND
the mix).
SUBWOOFER
The S input is also dividFig.4: this diagram shows the various decoding possibilities from Digital Dolby
ed equally between Lt and
Surround which is encoded on to optical tracks between the film sprocket holes.
Rt but it goes through three
Note that it provides left & right surround channels but this is not possible via Dolby
more processing steps: (1)
encoded video tapes.
bandwidth limiting from
100Hz to 7kHz, (2) encoding
four channels – left, centre, right and
After the delay line, the surround with modified Dolby B noise reducsurround – but the surround signal signal is filtered to remove any noise tion, and (3) ±90° phase shifts which
goes through a little more process- above 7kHz and then passed through are applied to produce a 180° phase
ing before it is fed to the surround a modified Dolby B noise reduction difference between the signal compoamplifiers.
decoder so that the net result is that nents added to Lt and Rt.
First, it goes through an audio delay when no surround signal is supposed
A number of points should be made
line which is there for two reasons. to be present, the rear speakers are about the overall encoding process.
First, it provides the echo or reverber- quiet.
First, there is no loss of separation
ation which results in the “big” sound
Note that all channel signals are between the original left and right
of cinemas. Second, by delaying the subjected to substantial equalisation signals. Second, there is also no theo
surround sound, your ears get the cue before being fed to their amplifiers retical loss of separation between the
for direction from the front speakers and loudspeakers. It should be clear centre and surround sign
als. This
and this provides the localisation re- by now that while Dolby Surround follows because the surround signal
ferred to above.
and the ill-fated quadraphonic sound is recovered by taking the difference
INPUTS
Lt
Rt
LEFT
INPUT
BALANCE
CONTROL
RIGHT
SURROUND
MASTER
VOLUME
CONTROL
L-R
L/R
BALANCE
SURROUND
TRIM
SURROUND
ANTIALIAS
FILTER
AUDIO
TIME
DELAY
7kHz
LOW PASS
FILTER
MODIFIED
DOLBY BTYPE NOISE
REDUCTION
UNIT
Fig.5: this is the block diagram of a “passive” Dolby surround sound decoder which
does not incorporate the directional enhancement feature of Pro-Logic decoders.
6 Silicon Chip
OUTPUTS
LEFT
RIGHT
SURROUND
Lt
VCA
LEFT
VCA
RIGHT
L+R
VCA
CENTRE
L-R
VCA
SURROUND
PASSIVE
DECODER
Rt
Fig.6: directional
enhancement could
be provided if each
decoded output had its
own voltage controlled
amplifier (VCA).
However, this does not
work well as dialogue
in the centre channel
could cause the music
in the stereo tracks
to be pumped up &
down.
CONTROL
CIRCUIT
between the Lt and Rt signals and any
identical centre channel components
will be cancelled in the surround
output. Similarly, since the centre
channel (when decoded) is derived
from the sum of Lt and Rt, the equal
and opposite surround sound components will be cancelled out.
This concept of precise cancellation to maintain separation between
the centre and surround channels
presupposes that the two main transmission channels have virtually identical gain and phase characteristics.
If they don’t, separation between the
centre and surround channel signals
will be poor.
Digital Dolby
In 1992, a new 35mm format called
SR.D was introduced by Dolby Laboratories. Between the film sprocket
holes on SR.D prints is a 6-channel
digital sound track. The older Dolby
stereo Surround information is also
present so that cinemas without the
digital sound equipment can still
show them.
Dolby Surround at home
All the movies which have been
subsequently released on videotape
or broadcast from TV stations have
the original Dolby surround encoding information still present in their
stereo sound tracks. In 1982, Dolby
introduced a surround decoder for
the home. When driving the required
number of amplifiers and loud
speakers, this can provide a convincing reproduction of theatre sound in
the home.
Dolby Pro-Logic
The most recent development in
home surround sound equip
ment
is the Dolby Pro-Logic decoder, first
introduced in 1987 and now more or
less standard in deluxe home theatre systems. This gives a substantial
enhancement to sound localisation
compared with the so-called “passive”
decoder depicted in Fig.5.
Before we describe the Pro-Logic
system we should discuss some other
means of directional enhancement
which have been tried. Directional
enhancement refers to any technique
that attempts to improve the separation between channels by modifying
the outputs of the matrix decoder. The
first of these is “gain riding” whereby
each decoded output has its own
voltage controlled amplifier (VCA), as
shown in Fig.6.
Consider the case where the dialogue is present in the centre channel
(so that Lt = Rt). This means that
the centre channel will have the dialogue but so will the left and right
hand speakers. To enhance the centre
channel, the decoder could increase
the gain of the centre amplifier and
reduce that for the stereo channels.
The same procedure could be used to
isolate the left channel when only left
LEFT
Lt
INPUTS
The new digital system provides
five full range channels for left, centre and right speakers, plus separate
left surround and right surround
speaker arrays in a configuration
known as stereo surround. A sixth,
bass only channel for subwoofers
gives rise to the de
scription “5.1
channels”. Fig.4 shows the various
decoding possibilities from Digital
Dolby Surround.
Dolby’s digital sound has been a
major factor in the success of recent
movies, particularly “Jurassic Park”.
Fig.5 shows the block diagram of a
Dolby surround sound decoder and
this can be compared with the Dolby
stereo cinema processor shown in
Fig.2. Note that the centre channel is
lacking.
At present, the six channel digital
sound encoding is not available via
the video tape format but it may
become available on future digital
video discs.
DOLBY
PROLOGIC
ADAPTIVE
MATRIX
INPUT
BALANCE
CONTROL
Rt
MASTER
VOLUME
CONTROL
RIGHT
CENTRE
CENTRE
TRIM
SURROUND
NOISE
SEQUENCER
ANTIALIAS
FILTER
SURROUND
AUDIO
TIME
DELAY
L/R
BALANCE
7kHz
LOW PASS
FILTER
MODIFIED
DOLBY BTYPE NOISE
REDUCTION
UNIT
SURROUND
TRIM
OUTPUTS
LEFT
RIGHT
CENTRE
SURROUND
Fig.7: the block diagram of a Dolby Pro-Logic decoder. If you compare this with the passive
decoder depicted in Fig.5, you will see that the main difference is in the Pro-Logic Adaptive
Matrix & the addition of the centre channel.
October 1994 7
INPUTS
Lt
Rt
FULL-WAVE
RECTIFIER
BANDPASS
FILTERS
FULL-WAVE
RECTIFIER
LEFT/RIGHT DOMINANCE SENSE
LOGDIFFERENCE
AMPLIFIER
DUAL
TIME
CONSTANT
E
POLARITY
SPLITTER
ER
THRESHOLD
SWITCHES
L+R
L-R
FULL-WAVE
RECTIFIER
FULL-WAVE
RECTIFIER
LOGDIFFERENCE
AMPLIFIER
DUAL
TIME
CONSTANT
Below: this photo shows our soon to
be published Dolby Pro-Logic Sur
round Decoder which is presently
being assessed by Dolby Laboratories
in California, USA.
8 Silicon Chip
EL
E
ER
E CL
EC
E SL
ES
OUTPUTS
LEFT
COMBINING
NETWORKS
RIGHT
CENTRE
SURROUND
E
POLARITY
SPLITTER
FRONT/REAR DOMINANCE SENSE
Fig.8: this diagram shows the Adaptive Matrix used in the Dolby Pro-Logic
decoder. The four signals are fed to full wave rectifiers & then to logarithmic
detection circuits to determine the dominant signal for subsequent directional
enhancement.
channel signal was present, by turning
down the gain of the other channels.
Unfortunately, this system of gain
riding with VCAs does not work with
real film sound tracks. In particular,
stereo music is usually present along
with dialogue. If dialogue is the domi
nant factor and is used to vary the gain
of relevant channels, the volume of
music will inevitably be pumped up
and down.
E
To solve this problem, Dolby has
come up with the concept of “signal
dominance” – the sound that is most
dominant in the sound mix at any instant in time. This can then be used to
vary the gains of the relevant channels
and thus give the desired directional
cues.
Dolby also suggest that if a decoder
is to detect and use signal dominance
to set the channel gains, it needs two
additional characteristics to work effectively. First, it must be fast enough
to provide enhancement on an instantaneous basis when the signal peaks
are prominent enough to be heard as
separate events. Second, it must sense
when the relative signal dominance
ES
8VCAs
falls below a threshold where it is
no longer necessary to provide any
substantial directional enhancement.
For these reasons, the Pro-Logic
decoder has sensing circuits which
ignore the absolute signal levels but
respond to the logarithmic difference
in levels between signals and thereby
determine direction of signal dominance. Thus it can provide the correct
degree of directional enhancement or
gain adjustment of the four channels.
Fig.7 shows the block diagram of
a Pro-Logic decoder. If you compare
this with the so-called passive decoder
depicted in Fig.5 you will see that the
main difference is in the Pro-Logic
Adaptive Matrix and the addition of
the centre channel.
The detail of the Adaptive matrix
is shown in Fig.8 and it has two sections: Left/Right dominance sense and
The construction of our proposed Dolby Pro-Logic Surround Decoder will be
quite straightforward, with most of the parts mounted on a single large PC
board. It is based on a Dolby Pro-Logic chip set from Mitsubishi.
Front/Rear dominance sense.
In practice, the decoded channels
from a Dolby Pro-Logic decoder can
have a separation of up to 30dB which
is more than adequate to provide
strong directional information. The
proof is in the listening, of course, and
any action movie gets a major benefit
from the Dolby sound track.
Loudspeakers & amplifiers
For a typical home Dolby surround
setup you need quite a lot of equipment, apart from either a hifi stereo
VCR (if you wish to play back Dolby
encoded tapes) or a stereo TV set. If
you have a conventional stereo VCR
(ie, without hifi sound) you’ll be wasting your money since tape hiss will be
a problem.
You will need a stereo amplifier
and a pair of stereo speakers for the
front channels and a stereo amplifier
for the rear speakers. It is normal
practice to have a stereo amplifier to
drive the rear speakers so that you can
easily set the balance between them.
For the centre channel, you have two
approaches. The first is to use the
“phantom channel” option available
on most Dolby decoders. This divides
the centre channel sound between the
front stereo speakers and can give a
good result in most home living rooms.
The second approach is to use a
centre channel amplifier and centre
channel speaker which sits on top
of or below the TV screen. Centre
channel speakers must have full
magnetic screening otherwise they
will seriously degrade the picture
quality from your TV, to the point
where it is completely unwatchable.
To emphasise this point, if you place a
conventional unshielded loudspeaker
within 30cm of your TV’s screen, you
will magnetise the shadow mask inside and thoroughly “screw up” the
colour purity. This will cause horrible
colour blotches all over the screen and
the set will then require de-guassing
to restore its picture quality.
The big problem about using a
centre channel speaker, even if it is
shielded, is that it should have the
same sound quality as the main ste-
reo speakers, as far as the treble and
midrange is concerned. If it is a lesser
quality speaker, the transition from
centre to stereo speakers will be very
noticeable – the centre channel will
“squawk”.
Most Dolby decoders have integral
amplifiers for the rear and centre
channel speakers and of course, you
can purchase a full-on stereo receiver
with Dolby Pro-Logic decoding builtin. This will have five power amplifiers
so that external power amplifiers will
not be necessary.
Rear speakers
Since the frequency bandwidth
of the surround signal is limited to
between 100Hz and 7kHz, the requirements for the rear surround speakers
are not demanding and virtually any
small pair of speakers will do the job.
Do-it-yourself Pro-Logic
Dolby Pro-Logic decoders are licensed by Dolby Laboratories who
maintain strict control over technical
standards. As this article is being written, a fully licensed design by SILICON
CHIP staff is currently under review
by Dolby Laboratories. We hope to
SC
publish the design very soon.
October 1994 9
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