The history of
multi-channel sound reproduction goes back seventy years. Today's
surround sound has its origin in film and theater reproduction. In
this area there had been many successful experiments prior to
multi-channel reproduction entered the home market. Quadraphony was
the first consumer based system that had a commercial significance
although on small scale. During the last decade surround sound
systems have dominated the multi-channel surround sound markets.
Today the most significant company in this field is Dolby
Laboratories. It has developed de facto standards for both cinema
and home systems.
and early adopted reproduction systems contained one speaker. In the
early years of recorded or transmitted sound, the level of
technology did not allow more channels. Today it is still considered
to be the most common audio reproduction system. It is also dominant
in TV because of its simplicity and low cost. Monophonic
reproduction has many advantages and in some cases it is the only
technique that can be used. The main disadvantage is the lack of
directivity information and itís inability to re-create a
realistic, acoustical, three dimensional image of an original
directivity information in reproduction can be achieved by adding
one extra speaker channel in front of the listener. This system is
known as stereo. The improvement when compared to monophonic
reproduction was significant. In most cases, it is relatively easy
to add one extra channel to the existing system but further
additions are not always possible. An LP-record is an example of a
media, where only one extra sound track can be added easily. Today,
most the recorded music is produced as stereophonic. As well, most
FM radio stations are transmitting stereophonic sound. Stereophonic
reproduction by and large works relatively well. However,
stereophonic systems require higher quality equipment than
Figure 1 Speaker positioning and listening area in
a two channels stereophonic system. Satisfactory stereophonic
sounfield is possible to achieve in a relatively small area on the
axis between the speakers.
best listening area in a two channel stereo environment is limited
to quite a narrow area in the axis between speakers (figure 1).
The other disadvantage is that information on the room acoustics of
the original performance is very difficult to reproduce via two
has been long known that two channels can not provide realistic
directionality. Multi-channel reproduction has its origin early in
this century and since those days it has been studied intensively.
Today we have technically and economically functioning multi-channel
experimental starting point of multi-channel audio reproduction was
Bell Laboratories in the late twenties. There it was demonstrated
that three channels give a better depth effect than two channels.
From those first experiments it took almost forty years to produce
economical, quality sound equipment for the consumer market. During
those years experimenters and researchers discovered the theoretical
basis of multi-channel reproduction.
the four decades prior to 1960, several multi-channel sound
reproduction systems were developed and demonstrated. At the same
time, the quality of sound reproduction was increasing. When the
experiments of multi-channel sound started, the only consumer format
was monophonic. At that time AM radio transmissions and 78 rpm shell
lack records were used. There was no media that could offer more
than one channel. For this reason those early experiments were
conduced in public formats like theaters and conference rooms.
Although the experiments showed the possibilities of multi-channel
reproduction and listeners were mostly satisfied, the time was not
yet ready for a consumer product.
new inventions were still needed to decrease the over all costs of a
multi-channel system. In the late forties Williamsson introduced an
amplifier using a high degree of feedback. As a consequence the
distortion of the amplifier was very low. The next step towards
multi-channel reproduction was taken in the fifties. Speaker
technology had developed greatly. High quality closed and vented
boxes were introduced onto the market. At that time HiFi as a hobby,
started. An invention to economically produce multi-channel sound
was still needed. The answer came in the sixties in the form of
four-track magnetic tape recorders and playback machines. Now the
infrastructure was in place for a practical multi-channel sound
systems for consumers based on prerecorded four channel magnetic
phonograph records and radio broadcasting could not carry
multi-channel sound, consumer multi-channel sound systems remained
the hobby of small amateur groups. At that time there was no great
commercial interest in multi-channel sound reproduction. The outlook
changed in the late sixties when Peter Scheiber described and
demonstrated a technique for recording and reproducing four-channel
sound through a two-channel medium. This was the starting point of
intensive research on 4-2-4 matrixing systems. Once the basic
technique became widely approved, it took only a short time to
develop commercial systems for phonograph records and FM
broadcasting. Besides matrixed systems, discrete channel systems
were studied and phonograph records were developed. Matrixed systems
were called SQ, stereo-quadraphony (compatible), and discrete
channel systems were known as CD.
Figure 2 Schematic diagram of matrixed 4-2-4
multi-channel audio system. Four input channels have been encoded to
two channels for recording or broadcasting. For reproduction encoded
two channels have been decoded back to four channels.
typical or conventional quadraphony system as it was called
contained an acoustical arrangement in which a stereophonic system
was fitted with additional speakers located
behind a listener. Also sc. unconventional four channel systems were
introduced. In one system speakers were placed at the sides, front
and rear of the listener. These two quadraphony systems are
presented in figure 3.
Figure 3. On the left side is presented the
conventional speaker arrangement in quadraphony. A newer system is
on the right. Normally multichannel audio was studied as an
unlimited array of speakers surrounded the listener. The speakers
were spaced from each other by an angle.
Besides four channels systems, three
channel solutions were also studied. Two of these systems had
commercial significance. The first of them was ambiophony in which
one speaker was added to the normal two channel stereo system behind
the listener. Another view of three channels reproduction offered
was the Finnish basic system called Orthoperspecta. In both of these
two systems, the rear channel was supplied with the difference
between the normal stereo left and right channels. The aim was to
produce some type of surround signal.
Unlike ambiophony, Orthopespecta used a mono front channel and two
rear speakers supplied by the same signal. These two, three channels
systems are presented in figure 4.
Figure 4 Two three speakers approaches to
multichannel reproduction. The ambiophony is on the left and
Orthoperspecta on the right. Orthoperspecta was in production also
by the mid seventies four-channel records were available for
consumer use, commercial success was not achieved. Quadraphony
failed for many reasons. Different record companies and stereo
equipment manufacturers backed different incompatible
encoding-decoding systems. Producers and recording engineers did not
have clear vision of how best to use these extra channels and most
consumers couldn't hear any advantage with the system. The price of
these early systems was also very high, discouraging most people
from buy them. People never associated quadraphony as the surround
sound used in films. Home multi-channel sound was nearly forgotten
for fifteen years. Only three channel systems had some significance,
because they used a two channel sound source.
was situation specifically in Europe. However during in that time
the theoretical basis for multi-channel reproduction was
established. Consumers eventually became reacquainted with
multi-channel sound. The film-industry was interested in the new
multi-channel sound reproduction.
This was because theaters audiences were shrinking under the
competition from television. High quality multi-channel sound gave
theater owners a competitive advantage TV could not.
only four channels medium at the end of sixties was prerecorded
magnetic tapes. The system was expensive and the supply of program
material was low. Discrete four channel sound could not be
transmitted via radio or recorded onto records. The turning point in
the development of quadraphony was the introduction of matrixed
systems [PESC70]. Instead of four independent channels only two had
to be stored or transmitted. All existing stereophonic systems were
able to convey quadraphony. This was the starting point for the
consumer format of multi-channel audio.
principle of matrixed quadraphony is to encode four input channels
into two either for recording or broadcasting. Coded channels are
decoded back to four ones for reproduction (figure 2). The
requirements for an ideal matrixed system are [PECH70]
** Ability to produce correct 360 degree localization in
** No degradation of signal quality (noise, frequency,
** Four channel compatibility using standard components and
construction wherever possible
** Stereo compatibility: the ability to reproduce the four
channel program on all standard two channel
with all sounds in the four channel program
with correct stereo localization
** Mono compatibility: Monaural playback possible on all
standard medium without losing or altering
relative level of any sound in the four channel program
** Adaptability to standard practices for software
** Full playing time within a given format compared with the
equivalent of stereo
** Usable with all major recording media and broadcast
requirements were difficult and impossible to fulfill completely.
Matrixed four channel encoding can be described as following
chx are input channels,
chxt are recorded
channels, xx and yx
are coefficients of matrixed channels. The rank of coefficient
matrix is two. There are only two orthogonal vectors and for the
coefficient matrix there is no inverse one. For complete decoding
four orthogonal vectors are needed.
incomplete decoding is possible. Degradation of crosstalk between
channels in reproduction can not be avoided in this case. Channel
separation decreases remarkably and will be only a few decibels.
Decoding is the reversal of encoding. It is described with the
are decoded channels,
are coefficients of
THE FILM SOUND
first sound reproduction method used in films was synchronized
At the same time an optical soundtrack was developed that stayed in
use for two decades. In the late forties new inventions
improved significantly the sound quality of phonographic records.
Micro groove records took the first step and high quality tape for
recorders was also introduced. As a consequence the frequency
response on the new records was 30 Hz...15 kHz within +/- 2 dB.This
compared favorably to the 6 kHz high frequency limit and 45 dB
signal to noise ratio of optical soundtracks. The improvements were
film industry adopted the new magnetic recording system within a
short period because of its obvious benefits. In production it
shortened the time from recording to final soundtrack. In the early
fifties two new film formats,
using magnetic sound stripes were introduced. A new stereophonic
sound was promoted with a new wide screen film format. The film
stereo started out with a minimum of four channels. To play these
films an extra playback head, like those in tape recorders, was
needed in the projectors.
In a normal set-up at least one of the channels contained rear
information; known at that time as the effect channel. The rear
channel was used only occasionally during dramatic effects because
of the high noise level.
5 Typical film sound production organization in the mid eighties
market for film-stereo shrank in the late sixties and early
seventies for many reasons [DOLA94]. The magnetic method was
expensive and the film industry was in a turmoil because of
competition from TV. The magnetic stripe was also sensitive to
disturbance. In some cases the content of the sound track could be
wiped out of because of the poor quality of the reproduction
equipment. In these conditions it was difficult to maintain
sufficient quality sound and the popularity of multi-channel
mixers, however, continued experimenting, almost as hobby. Six-track
70 mm film offered consistent signal-to-noise ratios for all
channels. The effect channel could be used for continuous low level
ambient sounds. This gave more realism overall and the effect
channel got a new name, the surround channel. The method was called
surround sound. In the rear of the theaters a pattern of speakers
was installed to produce a diffuse sound field.
production two different strategies have been used. A typical
production organization for film sound is presented in figure 4.5.
The production sound group works simultaneously with other
production groups recording the sound and picture. In many cases
this leads to an unsatisfactory result. Outside disturbances like
Figure 6 35 mm Dolby Stereo playback containing two
optical matrixed soundtracks identified as Lt and Rt. [DOLA94].
of airplanes and filming equipment cause background noise that is
very difficult to avoid. Also with the microphone technique in use,
there were problems, especially during dialogue recording. These and
other reasons have led to the practice of recording the soundtrack
afterward. Sound staff record only a reference track that can be
used to help the actors to remember the correct nuances during the
post production phase. Similarly in dancing scenes only a few
musicians are used with the final full orchestra being added during
post production method is very widely used today. Dialogue, music
and effects are recorded afterward and the film is mated with the
soundtrack. This has many advantages. There are no unwanted sounds
in recording studios. In many cases prerecorded effect libraries are
used. In these conditions a very accurate sound field can be created
and the mixer can create the most effective sound possible. The post
production method is used also for dubbing, when the original
dialogue is changed to a different language. At this time the
soundtrack in many old films have been rerecorded, too. Perhaps the
most famous post recording company at this time is Lucas film and
it's Skywalker group, LucasArts.
the mid seventies, Dolby Laboratories introduced a new sound
technology for 35 mm film based on optical soundtracks instead of
magnetic ones. In the system one extra sound stripe was fitted into
the same space as the previous track to ensure compatibility with
mono reproduction. To achieve acceptable signal to noise ratio, a
compression technique is used for noise reduction. This solution
also decreased the costs of the sound tracks.
channels are not sufficient to produce good stereo sound in a
cinema. In normal two-channel stereo, the maximum distance between
speakers is 4.5 meters without discontinuity in the stereo image.
Locations off the center of a wide movie screen it is also
impossible to achieve satisfactory localization. To fill the hole
and to make the narrow stereo operating area wide enough an extra
channel is needed between the left and right speakers. The task was
to fit four channels into two soundtracks on the film. The solution
was the 4-2-4 matrix techniques first used for quadraphonic home
stereo. The system contains left, center, right and surround
channels and it is known as Dolby Stereo (figure 6). The block
diagram of Dolby 4-2-4 cinema decoder is presented in figure 7.
The optional subwoofer output is formed from decoded channels.
Figure 7 Dolby Stereo Cinema Processor [DOLA94]
digital sound format was introduced in 1992 by Dolby. The new sound
track was added between perforated holes on the side of film. The
old analog stripe remains unchanged and contains normal Dolby
surround information. The new digital track provides stereo surround
sound and an extra limited band effect channel compared to previous
analog system. Dolby stereo digital is presented in figure 4.8. On
the left of the picture the sound track arrangement previously
described and on the right side, the speaker placement. Dolby stereo
digital uses a new bit compression technology that is known as AC-3.
Figure 8 Dolby Stereo Digital multichannel film
sound reproduction system [DOLA94]
Figure 9 Fantasia was the first film shown publicly
with stereo sound utilizing three optical tracks on separate 35 mm
film played in sync with picture. [DOLA94]
surround and stereo digital, SR*D, are methods to code and encode
multi-channel sound. There are no special requirements for the sound
equipment or the quality of sound in the theaters. To overcome this
situation Lucasfilm started to develop requirements and standards
for theater sound reproduction. The result of this work was THX. THX
describes accurately the minimum requirements for theater acoustics
and equipment to reproduce the film soundtrack as the mixer
intended. Besides sound, THX specifies the minimum requirements for
picture quality. Theaters that fulfill these standards can use the
THX logo as a quality label. To maintain a high level of performance
Lucasfilm monitors these theaters strictly so that the public can be
assured of a satisfactory cinema experience.
MULTI-CHANNEL SOUND IN TV
the early eighties, the time was ripe for home surround sound.
Stereo video cassette recorders and Laser disks were introduced,
offering high quality sound channels. TV manufacturers launched the
first models with stereo amplifiers and stereo video broadcasting
was ready for testing. Unlike sound only reproduction, television
never used traditional quadraphony. Instead TV had adopted the sound
system that was in use in theaters.
Surround was introduced in late 1982. The original soundtrack stays
unchanged when the film is transferred onto stereo videocassette,
Laser Disk or transmitted on stereo TV. A few years later, Dolby
introduced Pro Logic, improving remarkably the channel separation.
It also made it possible to decode the center channel. Market
acceptance of Dolby Surround has been very good.
timeline for multichannel sound reproduction is presented in figure
9 for both consumer and cinema format.
surround is a variation of quadraphony systems. Four channels are
used with the rear speakers used to create ambiance for the front
speakers. Three front channels, left center and right are used for
larger listening areas. The system contains two normally enclosed or
two cardioid or one gradient radiating rear or side speakers
supplied by one mono channel. A normal system with two side speakers
is presented in figure 4.10.
surround belongs to the family of 4-2-4 matrixed quadraphony. Four
input channels have been converted to two channels. The conversion
operation is [PESC76]
where L = left input channel
C = center input channel
R = right input channel
S = surround input channel
and Rt signals can be
recorded on any normal stereo media such as a CD or HiFi stereo
video tape. Also all stereo transmission methods can be used. In
reproduction the opposite operation is used to produce four output
channels. Decoding for playback is
Figure 10 Speaker arrangement in Dolby surround
system. Rear left and right speakers reproduce same signal in phase.
Center channel is also known as dialogue channel
Figure 11 Channel separation map of Dolby surround
sound system. Crosstalk from left and right channels to center and
surround channels and wise-versa is only 3 dB [RODR92].
equation (3) signals Lt
and Rt are
Lt and Rt
into equation (4.4) the decoded four channel signals are
Channel separation between
C' and L' and R' is only 3 dB. The situation is the same between the
decoded surround and left and right channels. Crosstalk between the
left and right is good, as well as between the center and surround
channels. The separation map is presented in figure 4.11.
12 Block dia-gram of Dolby B and Modified Dolby B
13 Block diagram of Dolby surround encoder
of low channel separation a set of psychoacoustical methods have
been used to improve perceived directivity. A seven kHz low-pass
filter decreases the leakage that tend to increase in magnitude
towards the high frequency. Secondly, there is a delay in the
surround channel. If two similar signals arrive at different times
the sound is localized in the direction of the first received signal
[HHA72]. Also, modified Dolby B-type noise reduction is used, partly
to reduce noise in the surround channel but also to decrease the
front channel leakage. Image dominance over sound helps
significantly improve sound localization on the screen while
decreasing psychoacoustic crosstalk from the surround channel.
front to back separation, has been perceived at better than 3 dB.
Together with a phantom center image, Dolby surround can offer
satisfactory spatial effects. In some cases the center speaker have
also been used to improve dialogue localization at the expense of a
lower stereo image. The block diagram of the Dolby surround encoder
is presented in figure 13 and the decoder in figure 14. Low-pass
filters and modified Dolby B noise reduction are used both in the
encoder and in the decoder.
14 Dolby surround sound decoder block diagram. The system is also
known as passive decoder [RODR92].
the 4-2-4 matrixed quadraphony system, since four directional input
signals are encoded to two channels, some directional information
has been lost. In reproduction
original directional sound field is impossible to create in the
listening room. To decrease crosstalk between channels many methods
have been proposed.
approach is to feed that part of a frequency range that contains
speech to the front speakers and the balance of the frequencies to
the remaining four speakers. In another system, the sound level of
each four loudspeaker is adjusted without changing the relative
contribution of the two channels. This reduces the crosstalk. The
system is known as gain riding. Another possibility is to produce
the four output signals from the two ones according to a special
mathematical algorithm varying the relative contributions of the two
channels. The objective is to reduce the effect of crosstalk. The
system is known as the variable matrix approach.
signals derived from encoded two channels is widely used. Two
signals have been proposed to control separation for rectangle
quadraphony. Both front and rear channels have their own control
signal. The front-rear level is controlled. Systems containing up to
ten control signals have been introduced.
enhancement has also been developed for Dolby surround sound. Two
alternative solutions can be used to improve channel separation. An
enhancing circuit can be connected in cascade with the decoder or
constructed as a part of the decoding process. Dolby has solved the
enhancement problem with an adaptive matrix [MTAD89] [MTAD90]
[MTD91] that is included in the decoder. The system is known as
Dolby surround Pro Logic. An extra block, Pro Logic adaptive matrix
has been added to the surround decoder. To help the level alignment
of the user a noise sequencer is also used. The directional
enhancement system is presented in figure 16 and the block diagram
of the adaptive matrix is in figure 4.17. Encoders stay unchanged
and channel separation is dependent only on the decoder [MTAD89]
15 Active Dolby surround decoder block diagram [RODR92]
16 Block diagram of Dolby Pro Logic adaptive matrix
crosstalk between some channels is only 3 dB, there is still enough
directivity information to re-create almost the original sound field
by using a more complex algorithm than a simple add and subtract
function. Channel separations in the left-right and in the
center-surround axes are over 40 dB. First dominance vectors have
been derived from band-pass filtered Lt and Rt.
Left, right and center surround dominance vectors DLR and DCS are respectively
a is a constant. Four directional control signals EL, ER, EC and ES have been derived from
low-pass filtered dominance vectors
the next step, four multiplicands FL,
FR, FC and FS are formed
a is referred to the base of logarithm in equation (7) and b is a
constant (that has the best match to the referred directional
sensation). A vector V is defined as
matrixes GX have been
used by Dolby Pro Logic. These matrixes have been derived from
signals where only one is reproducing the matrixed sound. The size
of matrix GX is five by two. Now the output signals L', R', C' and S' of
adaptive matrix can be written
adaptive matrix described above will detect the direction of one
individual sound very accurate. For more complex signals directivity
is lower. If one source is
Figure 17 Separation map of Dolby Pro Logic
adaptive matrix surround sound system [RODR92]
it is easily detected and reproduced in the correct direction. The
balance of the sources forms an ambiance signal. Channel separation
is increased to about 30 dB compared to 3dB without the adaptive
matrix. The separation map is presented in figure 17.