DC-1 Theory and Design
DC-1
Theory
and
Design
Lexicon
Acknowledgements
The DC-1 is manufactured under license from Dolby Laboratories Licensing Corporation. "Dolby", "AC-3", "Pro Logic", and the double-D symbol are trademarks
of Dolby Laboratories Licensing Corporation.
The DC-1 is manufactured under license from Lucasfilm Ltd. U.S. patent numbers 5,043,970; 5,189,703; and 5,222,059. Foreign patents pending. Lucasfilm
and THX are trademarks of Lucasfilm Ltd.
"Auto-Azimuth" and the A-Z logo
AUTO AZIMUTH
are trademarks of Lexicon, Inc.
Copyright ©1996 Lexicon. Inc. All Rights Reserved. Lexicon Patent: U.S. no. 4, 862, 502; other patents pending on the DC-1.
Lexicon, Inc.• 3 Oak Park • Bedford MA • 01730-1441 USA•Tel: 781-280-0300 • Fax: 781- 280-0490
Lexicon Part #070-11278
Printed in the United States of America
DC-1
Theory
and
Design
DC-1 Digital Controller
DC-1Theory
and Design
Introduction
to the DC-1
All of the operating modes of the Lexicon DC-1 Digital Controller have a
common goal: to draw you, the listener, more deeply into a musical performance or a film. For music the DC-1 uses true stereo digital processing to
recreate the original recording space or to create a new one of your choosing.
For films it incorporates Dolby Digital surround processing, the format that
utilizes Dolby Laboratories’ AC-3 technology. For two channel surround
encoded films it includes Lexicon’s 5 and 7 channel implementations of the
original Dolby Pro Logic surround decoder. These improvements yield a
greatly increased sense of surround and envelopment, as well as spectacular
left/right surround effects.
A great deal of effort went into designing an instrument which would be
flexible enough to satisfy the most critical listeners and yet be simple to
operate. Essentially, the DC-1 is a signal processing computer that can be
custom-programmed for any specific system. The increase in impact of a
musical performance or film when heard with the DC-1 is incredible.
To recreate the experience of being at a live musical performance the DC-1
draws on recent studies of concert-hall acoustics, and applies this research
to home listening rooms. Our auditory sense is quite adept at interpreting
clues about our physical environment. Even with your eyes closed, it is
possible to get a good mental picture of the room or hall you are in by
listening to the ambience, or reflected sound energy, in the room. We are not
aware of our auditory sense in everyday life because it confirms what our
eyes identify as the environment. When we listen to recorded music,
however, there are no visual clues and we rely completely on our sense of
hearing. The introduction of two-speaker stereo systems over thirty years
ago brought dramatic improvement to high fidelity music reproduction.
With a carefully-designed system, and good recording, it became possible to
produce a good sonic picture of the original event. Unfortunately, our
listening rooms do not approximate the acoustics of a good concert hall, an
intimate jazz club, or a magnificent cathedral - our ears tell us where we
really are. The Lexicon DC-1 is designed to overcome this fundamental
limitation to two-speaker reproduction and bring us closer to the ultimate
goal of transporting ourselves to the original musical event.
The DC-1 utilizes four basic algorithms: 5 channel or 7 channel Surround,
Ambience, Reverberation, and Panorama. These algorithms create effects
specifically designed to accurately reproduce any program material in your
home. The object is to increase the sideways-moving sound in a room, thus
increasing Spatial Impression, or SI. The DC-1 increases SI by either extracting it from the original recording, with the Panorama or Surround effects,
or by generating a new acoustic environment with Ambience or Reverberation
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Lexicon
The DC-1 surround effects include a novel matrix technology which is
capable of extracting 5 or 7 channels from a two channel encoded original
in such a way that the left and right channels maintain full separation during
music. Although matrix decoders have been previously made with more
than 4 channels, they all reduce the separation in the rear channels when
decorrelated material such as music is being decoded. The matrix effects in
the DC-1 maintain full separation and envelopment during the music
sections of films, and for music CD recordings.
The Ambience (Nightclub and Concert Hall) and Reverberation (Church
and Cathedral) algorithms transform the listening room into a new acoustic
space, letting you choose an environment which matches your music or
your mood. Unlike most ambience processors, the DC-1 provides full stereo
processing, preserving the critical SI information in the recording and
expanding upon it. The Ambience algorithm generates the side and rear
reflection patterns of idealized rooms and concert halls. The larger spaces
add the true depth and realism of a concert hall to classical and popular
music, while the smaller spaces are ideal for jazz and rock. The Reverberation algorithm is similar, but places more emphasis on rich, dense reverberant decay than on early reflections. It is especially good for simulating large,
highly reverberant spaces such as churches, stadiums, and cathedrals.
When a listener is in the correct spot the Panorama effect provides an almost
ideal re-creation of the original recording space. It works by using digital
signal processing to cancel the crosstalk between the listener’s ears, effectively spreading the sound from the two front loudspeakers in a wide arc in
front of the listener. With the optional addition of rear speakers, Panorama
can be startling in its realism.
The requirements for processing sound for home theater are somewhat
different than those for music and the DC-1 programs available using the
new Surround algorithm meet all of these requirements. Lexicon invented
the technology that permits the most accurate reproduction of film sound
in the same system that is used for music listening, and the software-based
DC-1 is optimized for each of these unique tasks. The Music Surround effect
is specifically designed to optimally play conventional stereo music
through any system which includes side or side-located rear speakers.
Additionally, the DC-1 is able to perform automatic analysis and error
correction to compensate for problems in the source material.
The new 5 channel and 7 channel surround decoding allows the stereo
music in soundtracks to be reproduced with a full stereo spread, unencumbered by the relatively narrow separation of the front speakers. Left-tosurround and right-to-surround pans are also enhanced. Instead of sound
jumping from a front channel to both surrounds, the left and right side
speakers can function independently to facilitate smooth and fluid pans.
Sounds directed to hard left and hard right (the main left and right speakers)
will not appear in the side surrounds.
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DC-1 Digital Controller
DC-1Theory
and Design
This technique overcomes the limitation of the monaural surround channel
inherent in these formats, while remaining true to the front hemisphere
directional cues. The audio imaging tracks the picture image in a way which
fulfills the intent of the sound mixer. The drama of this effect is most
noticeable with strong stereo music soundtracks, and on films with strongly
panned effects.
For film viewing, the DC-1 also provides effects for expanding monaural
film sound tracks (Mono Logic), general TV viewing (TV Matrix) and, of
course, direct two-channel stereo playback (Two Channel).
The Dolby Digital version of the DC-1 also incorporates Dolby Digital
processing, a film sound format based on Dolby AC-3 technology which has
been used in cinemas since 1992. Dolby Digital processing features five
completely independent channels, each offering full range fidelity, and a
bass-only effects channel, referred to as “5.1 channels”. Six of the DC-1’s
eight discrete 20-bit A/D converters are dedicated to processing Dolby
Digital encoded signals. Standard AC-3 decoding is available in the Dolby
Digital effect. However Lexicon has gone beyond this decoding with our
proprietary extension of the 5.1 standard to 7 channels, using Logic 7 matrix
technology. This is included in the 5.1 Logic 7 effect, along with some of the
LucasFilm Home THX enhancements. All the LucasFilm enhancements to
the AC-3 standard are available in the 5.1 THX effect, but without the
Lexicon 7 channel technology. The DC-1 also includes the 5.1 Two Channel
effect, which mixes the 5.1 discrete channels of an AC-3 original to two
channels, in such a way that the sound will play back with 5 or 7 channels
of surround when decoded with the DC-1.
The THX and Dolby Digital versions of the DC-1 incorporate Lucasfilm
Home THX Cinema processing. This program compensates for the tonal
and spatial characteristics of film soundtracks mixed for the cinematic
environment. Home THX provides reproduction of film soundtracks
recorded in Dolby Digital or Dolby Pro Logic. In the DC-1, the two channel
input THX Cinema effect includes a parameter which allows it to utilize the
Lexicon 5 channel surround technology, although 7 channel operation is
inhibited if dipole speakers are selected. All matrix decoding algorithms in
the DC-1 (including the original Dolby Pro-Logic effect) utilize a patented,
completely digital surround decoder, the only one with automatic correction of inter-channel phase and channel-balance errors (the most common
audio problems in currently available video releases of films). The integration of Lexicon’s proprietary digital signal processing with these new
technologies results in a home theater experience that is unmatched in the
industry.
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DC-1Theory
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Lexicon
Lateral Sound
Concert Hall Acoustics
For decades the study of concert-hall acoustics relied on certain basic
measurements to characterize halls, principally the time it takes a sound to
drop in level (decay) by 60 decibels — referred to as RT60, or “reverberation
time”. Approximately the time it takes a hand clap to subside to inaudibility, reverberation time is measured as a function of frequency, usually in
bands one octave wide, over the range of audible frequencies. Although this
measurement is of fundamentalimportance in predicting the rate at which
sound decays, it does not predict how loud or how audible the sound decay
will be, and it does not predict the intelligibilty of speech or the clarity of
music. Other measures are clearly needed to determine hall quality and
suitability for a particular function or type of music.
The search for accurate and reliable measures of hall quality has been long
and contentious. Measures of intelligibility or clarity were the first to be
developed, and many have been proposed. The most common measures,
(Clarity or C80, Centertime, Deutlichkeit, etc.) have been shown to be highly
correlated in actual halls. They thus are closely related, and have common
virtues and faults. None of them appear to accurately predict intelligibility,
although they are still useful.
An accurate measure for the quality of reverberance, and for the properties
of spaciousness and envelopment, has proved much more difficult. A. H.
Marshall and Michael Barron noticed in laboratory experiments that reflections arriving from the side were important to the perception of spaciousness. Although this perception clearly applies equally to both early and late
arriving sound, Barron concluded that the early arriving reflections were
the most important. This conclusion (which in hindsight is misleading at
best) has dominated academic thought on hall acoustics for many years.
Fortunately, the best working acousticians gave lip service to the idea, and
continued to build halls with a combination of tradition and blind luck.
What was really needed was a method to study halls in the laboratory, but
under realistic conditions. One step toward this process was provided by
Manfred Schroeder. In an effort to answer the question of why some halls
sound so much better than others, Schroeder devised a method for comparing them without transporting his subjects from hall to hall. Using a dummy
head with microphone diaphragms in place of ear drums, Schroeder made
binaural recordings in many halls. These recordings, played back through
earphones, gave excellent reproduction of spatial qualities. Unfortunately,
the stereo image tended to appear entirely inside the head, spoiling the
accuracy of such recordings for some listeners.
Halls with similar RT 60 sound very different. The best have large amounts of
sideways moving reflected sound.
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DC-1 Digital Controller
To overcome this problem, Schroeder played his recordings through a pair
of loudspeakers in an anechoic chamber, using a special electro-acoustic
technique (developed by Atal, Schroeder, Damaske and Mellert) to eliminate crosstalk between the listener’s ears. Normally each speaker is heard
by both ears but Schroeder’s system canceled the sound reaching the right
ear from the left speaker and vice-versa. Provided that the listener held his
head in exactly the right spot, the sound had all the excellent localization
properties of earphones but was properly located outside the listener’s
head.
This technique, generically known as interaural crosstalk cancellation, was
found to work well even without the anechoic chamber, and music presented this way can be quite enjoyable. Lexicon incorporated this technique
in many products as “Panorama”. Dwane Cooper coined the name
“transaural reproduction” for similar effects. This allowed the first direct
comparisons of specific halls. From these studies Schroeder concluded that
the best halls were all characterized by having large differences in the sound
between the two ears in the dummy head. Very simply, the best halls gave
the most stereo. But there is more to it than that. We have continued
laboratory work, using both Panorama and headphones and have utilized
our skill in reproducing different types of reverberation to study the effects
of reflected energy at all time delays and angles. Our results have shown
that differences between the ears are created by sound arriving from the
sides, but the optimum angle is a function of frequency. Below 700Hz the
optimum angle is completely from the side (or lateral). As the frequency
rises the optimum angle becomes a cone centered on a line drawn between
the ears. The interior angle of the cone rises from 0° at 700Hz to about 30°
at 1000Hz, to about 60° at 2000Hz.
DC-1Theory
and Design
Sound from the side is vital to listener
comfort and involvement . . . It must
really be from the side!
In an anechoic chamber, this sideways
sound is missing . . . and music sounds
unpleasant.
This angular dependence has some very interesting consequences for the
perception of spaciousness, envelopment, and reverberance. Most home hifi systems have the front loudspeakers separated by at least ±30°. Such a
separation is effective for frequencies above 1000Hz, and the front speakers
alone create substantial spaciousness, at least at the frequencies which
convey the most speech (and music) information. In the best seats in concert
halls the orchestra is at least this wide. The high frequency direct sound is
inherently spacious, even without the aid of reflected energy. Yet it is clear
that halls increase the spaciousness of the sound. With careful listening both
in actual halls and in the laboratory, it becomes apparent that the increased
spaciousness is perceived primarily at low frequencies and in the gaps
between notes. Hall spaciousness can occasionally also be heard when a
note is held, particularly during a brass chord with a little vibrato, but this
perception is relatively rare. Spaciousness and reverberance are never
heard at the attacks of notes. In the best halls these are always clear and dry.
In an ordinary room, the room supplies
these directions, and the sound is
cramped, but tolerable. The overall impression, however, may be muddy due to
unwanted frontal reflections.
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DC-1Theory
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Lexicon
The reason is simple — music is highly effective at masking its own reflected
energy. Most of the time, note follows note with very little space between.
It is only in the gaps between notes that our ears have time to perceive the
background sound which the hall creates. When that sound has a strong
lateral component, a sense of spaciousness is created.
The confusing frontal reflections can be
absorbed, leaving the essential lateral
ones. This is better, but not ideal.
The time delay of the lateral reflected energy turns out to be very important.
The ear is relatively insensitive to reflected energy in the first 150ms or so
after a note ends, and musical masking increases this dead time to over
300ms. The greater the amount of reflected energy that comes to our ears
after this time, the greater the sense of spaciousness and reverberance. Thus,
both the loudness of the reverberation (the total energy relative to the direct
sound) and the reverberation time are important to our perception. In
natural halls the loudness and the reverberation time are linked by the hall
volume. In a very large hall the reverberation level tends to be low, but the
lack of level can be compensated by an increase in reverberation time. This
effect can be clearly heard in an organ concert in a great cathedral. The
organ, although often quite distant, is completely clear, yet it is bathed in a
marvelous quiet reverberance. Similarly, small rooms with very little
absorption can be loud and muddy, but seldom seem spacious or reverberant.
This understanding of the importance of late arriving lateral energy is quite
new, and its effect on hall design has yet to be seen. The best halls have high
late arriving lateral energy for obvious architectural reasons, such as a long
narrow shape with high coffered ceilings. Fan-shaped halls have better
sight lines, are more adaptible to multiple uses, and hold a greater number
of seats for their total volume than shoe-box halls. However, their greater
number of seats increases the total absorption of the audience, and decreases the strength of the later reverberance. These differences can sometimes be overcome. A notable example is the Boston Symphony’s fanshaped Tanglewood music shed. Although it used to be thought that
diffraction from the edges of the overhead reflectors provided the needed
spaciousness, it is more likely that the high internal volume of the hall and
the low absorption at low frequencies combine to make the low frequency
reverberance unusually audible for such a hall. Middle and upper
frequences are dry unless one is distant from the orchestra.
With the DC-1, confusing short reflections can be absorbed; the DC-1 will
supply the essential lateral sound —
which can simulate a much larger space.
The more absorbent the playback room,
the better it will sound, and the closer it
will sound to a real hall, or larger environment.
6
In a small playback room, a spacious and enveloping sound can be achieved
only if several conditions are met. First, significant energy should reach the
listeners from the side. Second, this energy must be different on the left and
right, i.e. it should be stereo, with excellent separation. Third, at least some
of this energy should have a time delay of at least 100ms. These conditions
can only be completely satisfied by placing loudspeakers at the sides of the
listeners, and then driving those loudspeakers with a stereo signal which
contains either ambient information from the original venue, or ambient
information synthesized in the processor. Achieving all of these conditions
is the guiding principle behind the DC-1.
DC-1 Digital Controller
DC-1Theory
and Design
Lateral Sound in your Listening Environment
In an ordinary listening room, conventional stereo set-ups (with loudspeakers separated by 60° or less) provide a feeling of spaciousness only at upper
frequencies. It is the room reflections excited by the speakers which provide
low frequency lateral energy. A standard speaker arrangement in the
artificial environment of an anechoic chamber is exceedingly detailed and
precise, but unpleasant. Some lateral reflections are needed to make the
sound musical.
Ordinary two-speaker stereo works as well as it does because sidewaysmoving reflections can be excited at low frequencies by two loudspeakers
if they are placed asymmetrically in the room, or if they are driven with outof-phase low-frequency information. (3,4) (Out-of-phase bass is intentionally provided in the best stereo recordings.) Another reason is that most
listening rooms have reflective surfaces to the sides of the listener. A
popular listening room treatment puts absorptive material at the front of the
room, leaving the walls by the listener reflective. This improves the clarity
by removing the front reflections, while retaining those from the side. This
also explains the appeal of loudspeakers that produce lots of sidewaysreflected energy.
Unfortunately, in most two-speaker set-ups the mid- and high-frequency
lateral sound is not optimal unless the speakers are unusually widely
placed. The listener can hear a little of the original hall, stretched between
the stereo loudspeakers, but never really becomes a part of it. What is worse,
the lateral sound that exists in most playback rooms has so little delay that
the ear can not separate it from the direct sound. The reflections generate
some room sound but they also cause coloration and muddiness. Small
rooms usually sound better if these reflections are broken up (with wall
hangings, furniture or bookcases) or absorbed (with curtains or soundabsorbent panels). When this is done the room becomes quieter and clearer,
but not in any way like the original hall.
The Lexicon DC-1 resolves this deficiency by supplying appropriate signals
to loudspeakers at the sides of the listener. The DC-1 gives the listener a
choice: the sideways energy cues of the original recording can be extracted
by the DC-1, or a different hall sound can be generated and supplied from
the correct directions.
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DC-1Theory
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Lexicon
Ambience Extraction
The Panorama and Surround effects in the DC-1 extract the original acoustic
cues from the recording and present them to the listener from the correct
directions. We call this ambience extraction. The Panorama effect can do this
even with only two loudspeakers. Panorama uses crosstalk elimination to
fool the ear into thinking there is a continuous band of loudspeakers
extending all the way to the sides of the listener. Panorama also supplies a
delayed and filtered L-R signal to rear speakers. On a good recording this
successfully recreates the original recording space, although the ideal
listening area may be small.
The Music Surround and Music Logic effects also provide ambience extraction. They divide the front energy among three loudspeakers and supply an
ambience signal to the surround loudspeakers. Because a center speaker is
provided, the left and right speakers can be placed far enough to the sides
of the listener to directly excite significant spaciousness, and the resulting
image is wide but seamless. This setup can sound similar to Panorama, and
it works over a large listening area.
Pro Logic, THX Cinema, TV Matrix, Logic 7, and Music Logic also provide
steering. Steering works by enhancing the directionality of the loudest
sound in a mix; it steers the sound out of loudspeakers where it is not needed
and into the ones closest to its direction in the image. In popular music the
loudest sound is usually the vocals, which will be preferentially steered
toward the center loudspeaker. Some steering is frequently beneficial to
music recordings played with a surround speaker arrangement, and it is
essential for films. The latest of the surround programs, Music Logic and
Music Surround, apply the steering in a special way. In these programs
whenever there is no particular loudest sound, the left and right surround
loudspeakers mimic the left and right front loudspeakers, but with added
delay and frequency contouring. This provides an ambience signal with
maximum left and right difference and maximum spaciousness. When
there is a strong signal to the left or right this signal is cleanly removed from
the rear speakers, and reproduced only in the front. The result is a convincing five or seven channel reproduction of a two channel original recording.
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DC-1 Digital Controller
DC-1Theory
and Design
Ambience Generation
The ambience and reverb effects in the DC-1 — Nightclub, Concert Hall,
Church and Cathedral — synthesize the side and rear sound of several
different acoustic spaces. Rather than extracting the acoustic cues of the
original hall from the recording, they generate a new environment, effectively enlarging and improving the acoustics of your listening room. In
these effects the original stereo channels are presented basically unaltered
to the main loudspeakers, and new signals are generated for the side and
rear loudspeakers.
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DC-1Theory
and Design
Panorama
Lexicon
Loudspeakers placed on either side of the listening position are the most
effective and foolproof way to produce added Spatial Impression. Since it
is not always possible to have side loudspeakers, the DC-1 uses crosstalk
elimination to simulate them when they can’t physically be there. In
Panorama the front speakers are driven entirely by the front digital outputs
of the DC-1.
Versions of the Atal/Schroeder/Damaske/Mellert technique mentioned
earlier have appeared in several consumer signal processors under various
trade names, as well as in a line of loudspeakers that achieved a similar effect
acoustically. These have all been what we call “first-order” devices. To see
what this means, imagine there is a sound coming from the left channel only.
This sound will travel to the left ear of the listener, then diffract around the
listener’s head and be heard by the right ear. If we take the left-channel
sound, delay it just the right amount, invert it in phase and feed it to the right
speaker, it will arrive at the right ear just in time to cancel the crosstalk from
the left speaker.
First-order correction travels to left ear,
where it will be heard unless canceled by
an additional correction. When these
higher-order corrections are supplied,
accurate cancellation is possible.
The main problem with a first-order device is that the subtracting signal is
also heard by the opposite ear. In our example, the canceling signal from the
right loudspeaker will diffract around the head to the left ear, interfering
with the left-speaker sound and producing a “comb filter” which colors the
sound in an obvious and unpleasant way. Furthermore, the listener’s head
is not well represented by a simple delay line. Both the delay and the
amplitude of the opposite-ear sound vary in complicated ways with frequency.
Lexicon’s implementation, called Panorama , was designed using measured
data on sound diffraction around the head to shape the frequency spectrum
of the canceling signal. This signal is then itself canceled by a second signal,
and so on, so that both the crosstalk and the signal that is canceling it are
eliminated.
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DC-1 Digital Controller
DC-1Theory
and Design
Imagine a click in the left speaker . . .
Sound from speaker L travels to the left
ear and also to the right ear, a time ∆t
later.
If we supply a negative delayed signal to
the right speaker, this crosstalk can be
canceled.
This works extremely well when the room is well damped and the listener’s
head is correctly positioned. The first order devices described earlier
required the listener to sit on the center line between the loudspeakers, and
to arrange the angle between the speakers to correspond to the modeled
delay. Although the DC-1 provides adjustments to compensate for offcenter listening and for varying speaker angles, maintaining a consistent
listening position is still important and becomes more so with increasing
frequency. With wide speaker angles, a movement of as little as 1 inch can
make a perceptible difference. Fortunately, the effect is usually fairly good
everywhere within a zone about one foot wide.
To achieve the fullest Panorama effect, your main loudspeakers should
have good imaging. The smaller speakers that tend to be used with video
systems may have an inherent advantage here but the most important
requirement is that the two speakers have identical frequency response and
symmetrical dispersion. It is not necessary, or desirable, to turn your
listening room into an anechoic chamber but moving the speakers away
from the walls can be helpful, as can adding absorption (as provided by
carpets, curtains and/or sound-absorbent panels) to reduce the reflectivity
of the floor, walls and ceiling.
In a well-damped room with loudspeakers mounted on stands away from
the walls, the Panorama effect can be very exciting, giving the closest
possible approximation to the actual hall used for the recording. With true
binaural recordings (made with a modern dummy head with accurate
external ears and proper equalization) the playback can be uncannily
realistic. And, unlike previous versions of this technique, the Panorama
mode adds virtually no coloration to the original signal.
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Lexicon
Panorama is used in two ways in the DC-1. First, it is designed to reproduce
as closely as possible the sound actually recorded by the engineer. If the
recording has good natural ambience, Panorama will spread that ambience
around the listener, giving a true impression of the original hall.
The Low Frequency Width control provides an important adjustment to the
bass in Panorama. This control is a simple implementation of a Spatial
Equalizer (a function which Alan Blumlein referred to as a “shuffler”). One
of the ways ordinary stereo excites SI is through the out-of-phase low
frequency energy in the recording. The Low Frequency Width control
allows the amount of out-of-phase bass in a recording to be adjusted. Even
when the Effect level of the Panorama control is all the way down, the Low
Frequency Width control is active, allowing the user to experiment with this
property of sound.
Panorama can be used with music, films,
or from within the Reverb and Ambience
programs to simulate side speakers if the
listener is inside the effective area between loudspeakers.
Recording engineers have only recently become aware of Spatial Equalization and many older recordings are greatly improved by increasing the low
frequency width a little. When the rest of the Panorama mode is not used (by
turning down the Effect control) just turning LF Width up a bit can make
ordinary recordings quite spacious. The user should exercise caution,
however, since some recordings (such as those on Telarc) use microphone
techniques which already contain sufficient out-of-phase low frequency
energy.
The crosstalk cancellation in Panorama increases the low frequency width
as well as the high frequency width of a recording. Recordings in which the
engineer deliberately added large amounts of low frequency width will
sound too wide and phasey when played with either Panorama Normal or
Panorama Wide. Negative values of the Low Frequency Width parameter
can bring the low frequencies back in line with the higher frequencies and
make the playback with Panorama more effective. The Binaural setting of
Panorama greatly increases low frequency width and should only be used
with true binaural recordings, which have very little out-of-phase low
frequency energy. A few compatible binaural recordings are becoming
available, in which the low frequency width has been increased to match the
requirements of loudspeaker playback. These recordings may sound best
when played with the Normal setting.
Panorama is capable of simulating side loudspeakers effectively, but cannot
mimic sound sources to the rear of the listener. So we have added a delayed
Left minus Right signal which can be sent to the side and rear loudspeakers.
The delay is adjustable, as is the treble rolloff.
For a listener in the ideal position, Panorama, with surround (side or rear)
speakers, gives a nearly ideal re-creation of the original recording area.
When the front speakers are close together, the Panorama Effect is less precise
but more dramatic, and it works over a
larger area.
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DC-1 Digital Controller
DC-1Theory
and Design
Panorama is also used as an element in the ambience and reverb simulation
effects, where it can synthesize side loudspeakers which are not present in
the installation. In these effects, the stereo inputs to the DC-1 are fed directly
to the front loudspeakers, with the digital outputs of the DC-1 mixed in
according to the setting of the Effect Level control. The Ambience or Reverb
side outputs are sent through Panorama before being mixed into the front
loudspeakers, so the added sound spreads beyond them and does not
interfere with the original material. When side speakers are present, the
Panorama Effect parameter is automatically turned down and no mixing
occurs into the front speakers.
Speaker alignment is important. A 6"
difference in the distance to the rear wall
can greatly change the effective area,
unless compensated for by the LISTENER POS parameter.
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DC-1Theory
and Design
Ambience
Lexicon
While Panorama recreates the space that already exists in the recording, the
Ambience effects actually generate the side and rear reflection patterns of
ideal halls. The reflections were determined by computer ray-tracing using
architectural data, augmented by Lexicon’s decades of experience with
digital concert hall simulation.
The ambience simulation is done in stereo. Instead of feeding a monaural
signal derived from the combined left and right channel inputs of the
processor, the DC-1 has two input points corresponding to instruments
placed on the left or right side of the stage. From these, the loudness and
delay of the reflections for the side and rear loudspeakers are calculated. By
using full stereo for the inputs to the simulation programs, the spread of the
soundstage is automatically preserved in the simulation process. This
obviates the need for adjustment on every different recording, a chore
required for decent results on some other systems.
Early research with quadraphonics involved extensive experimentation
with speaker placement, and confirmed that additional speakers beside the
listener sounded better than the conventional approach of putting pairs of
speakers in front and behind. Our research into speaker placement with
ambience confirmed the previous results of others: The side speakers
should be directly to the side of the main listening position, plus or minus
about 20°. The spatial impression is greatly reduced outside of this critical
angle. The primary reason for this result is the critical importance of the
lateral energy at frequencies below 700Hz, which is usually insufficient in
playback rooms. Thus, speakers which can reproduce frequencies down to
100Hz or below are recommended for the sides. If tiny speakers with
subwoofers are used for the sides, it is best to use a separate subwoofer for
each side, and place the subwoofers on opposite sides of the room.
The best way to generate spatial impression (SI) is with appropriate signals from
loudspeakers at the side. Reverb, Ambience and the Surround programs can all
be used to generate these signals.
14
The sides are the most important additional speakers, much more so than
the rears. Keep in mind that, although you can use the left and right main
speakers to simulate a phantom center speaker, you cannot produce spaciousness with one speaker in the front and one at the rear . (You can
perform this experiment for yourself with the DC-1 using the Ambience
effect: Try the side speakers both in their usual positions and in the front
corners. If your listening room has enough absorption to damp its side-wall
reflections, the side location will provide much more spatial impression,
and will sound substantially better.) The DC-1 mixes some reverberant
sound into the front speakers, to create an overall impression.
DC-1 Digital Controller
DC-1Theory
and Design
The effectiveness of the DC-1’s ambience simulation is heavily dependent
on the source material and the playback room. If the playback room is large
and reverberant, its reflections may dominate those generated by the
DC-1. Carpet, drapes and furniture can all be used to break up or absorb
undesirable reflections, making it easier to hear the processor’s output. The
balance between the side, rear and front speakers is also very important. If
the channels are set up properly, no single speaker will be audible by itself.
Although the ambience effect in Nightclub and Concert Hall can provide
some reverberation (adjustable with the Liveness parameter), for long
decay times it is better to use the reverb effects: Cathedral and Church. The
Concert Hall ambience is not intended to be used on material which is more
appropriate to a smaller ambience, such as a small hall or a club. Highly
percussive material is almost always better in Nightclub, which is quite
successful in livening and expanding popular music.
Placement of side loudspeakers is critical! Placement B sounds much better
than A, especially when the room is well
damped.
15
DC-1 Theory
and Design
Lexicon
Reverb
While the Ambience effect simulates the early reflections of real halls, the
Reverb effect, used in Church and Cathedral, is more concerned with what
happens to the sound after the first hundred milliseconds or so. The first
reflections are not intended to simulate any particular hall and no real shape
will be audible.
The Reverb effects produce a rapidly increasing echo density that smooths
out impulsive sounds. The decay in these effects is unusually smooth and
natural and can create the effect of a church or a very reverberant hall. The
early sideways reflections are weaker than they are in the Ambience effects.
For both reverb and ambience simulation, the stereo input is fed directly to
the front loudspeakers. Some of the reverb is also added to the front
channels to produce a convincing sense of surround.
Direction is critical to maintaining clarity in the Reverb as well the Ambience effects. The recording engineer has probably put as much reverberation in the recording as the music can withstand. Adding more through
speakers located in front of the listener is generally not a good idea, since
these effects combine with the sound from the front speakers, making the
music muddy.
Delay and reverb in the rear can occasionally be helpful but the ear is not
particularly good at distinguishing between front and rear sounds and, as
with the ambience effects, it is at the sides that reverb information is most
needed.
Reverb is very good for simulating a large
reverberant space.
16
DC-1 Digital Controller
The requirements for processing sound for film viewing are quite different
than those for music listening. Music demands recreation of the original
performance venue, or the evocation of an appropriate setting, as well as the
creation of a believable soundstage. When visual images are introduced, not
only is strong audio imaging necessary to reinforce the illusion that dialog
originates at the screen image, but a diffuse soundfield must be created
which envelops the viewer without distracting attention from the screen.
DC-1 Theory
and Design
Surround
In addition, films are designed to provide an enveloping experience in
large, reverberant auditoriums where background noise is a significant
consideration, where the screen dominates the field of vision, and where the
sound must be as uniform as possible over a large seating area — in short,
quite a different environment from your living room.
When all of these factors are taken into consideration, it becomes obvious
that no single speaker system or single method of processing is optimal, or
even adequate, for all types of music as well as for audio/video material.
The DC-1 is able to optimize the listening experience of any material
precisely because it provides such a wide range of processing options. This
flexibility is readily apparent in the variety of Surround effects available to
the user, each optimized for maximizing listener involvement in different
types of material played on a system whose speakers are laid out primarily
for films.
TV MATRIX provides surround effects to enhance television viewing of
monaural, stereo and stereo-synthesized programs. This program has the
most flexibility in adjustment of any of the Surround effects.
MUSIC LOGIC and MUSIC SURROUND use a unique ambience extraction
method which can provide spectacular results with music, as well as
providing seven channels.
PARTY (Full Range) allows unprocessed music to be played over all the
speakers for background music, or for maximum acoustical output of the
system.
MONO LOGIC expands the music and effects on monaural films into the
surround channels while leaving dialog in the front center.
PRO LOGIC provides the same decoding used in Dolby Stereo theater
systems, using as many as eight speakers for front, center, side, rear and
subwoofer channels. All side and rear channels are driven in parallel, or
monaural.
17
DC-1 Theory
and Design
Lexicon
THX CINEMA combines Dolby Pro Logic decoding and the spectral enhancements of the LucasArts Home THX Cinema system. In this effect, the
side and rear speakers are driven in parallel as in Pro Logic, but there is a
decorrelation effect between the left and right side channels. A Stereo
Surround parameter in the DC-1 provides further enhancement, making
the effect similar to Logic 7, but with no differentiation between the sides
and rears.
LOGIC 7 takes decoding further with the introduction of true seven channel
decoding using a stereo surround process. Steered signals are panned to the
front, side and rear speakers so that surround information follows the visual
movement on-screen. Ambient surround information, such as stereo music
in the soundtrack, is reproduced at full bandwidth and in stereo in the rear
channels. Sound effects which pan to the left or right of the listener are
reproduced with full realism.
DOLBY DIGITAL decodes six discrete channels of digital information from
Dolby Digital soundtracks. Improvements to film sound reproduction
include improved dynamic range, superior channel separation, a dedicated
subwoofer channel, and discrete sound effects.
THX 5.1 provides Home THX Cinema enhancements for film soundtracks
recorded in the Dolby Digital format.
5.1 LOGIC 7 optimizes Dolby Digital sound when played in systems with
both side and rear speakers. The timing and equalization of left surround
and right surround information is different for side and rear channels to
provide a more theatrical presentation of Dolby Digital film soundtracks.
5.1 MUSIC enhances playback of Dolby Digital music recordings or Dolby
Digital film soundtracks with strong musical content.
5.1 TWO CHANNEL mixes Dolby Digital 5.1 information for two-channel
playback. These soundtracks can be recorded onto two-channel formats for
later playback through a surround effect such as Logic 7, or they can be
played back through left and right front speakers.
TV Viewing
Television sound playback presents a challenge simply because of the wide
variety of recording and processing techniques used in program production and transmission. Programs and commercials are recorded in mono,
stereo, and surround sound, then subjected to compressors, erratic stereo
synthesizers, sleepy production interns, and other electronic tortures. Often
all of this can be experienced on one channel within five minutes.
The TV Matrix effect makes use of directional steering to enhance dialog in
the center channel and remove it from the left and right channels, while
maintaining as much stereo separation as possible. In addition, it contains
several interesting variable parameters. In the preset version of TV Matrix
18
DC-1 Digital Controller
DC-1 Theory
and Design
these parameters are set for pleasing results with a wide variety of programming, but they can be changed to produce a versatile mode for film sound
— and some music as well.
The Front Steering parameter, which controls the steering among the front
three channels, is preset at Full (the appropriate setting for Dolby Surround
encoded films). Lower settings cause centrally located sounds to be diminished in the side speakers and vice-versa.
The rear noise reduction parameter (Dolby B NR) is preset to ON, meaning
that the high-frequency sound in the rear channels is being decoded and
frequency limited by the Dolby B circuit. This control may be turned off for
non surround-encoded material, or when full-range surrounds are desired.
The Side Assign parameter can be used to connect the side speakers in
parallel with the front speakers. The resulting wide frontal image can be
effective on a variety of films and music, but for most material, the standard
setting is recommended.
The Re-Equalizer applies the THX main equalization to all the channels.
Stereo surround is also available in this mode.
The Music Surround effect in the DC-1 is designed to optimally play
conventional stereo music through any system which includes side or sidelocated rear speakers.
Enjoying Music
in Surround
Conventional stereo generally relies on the acoustics of the playback room
to regenerate the important side, or lateral, energy which is present in nearly
all music performance spaces. Speakers which are arranged for film viewing, however, often have the front loudspeakers too close together for
optimal reproduction of the spaciousness of the original soundfield. With
the DC-1, additional speakers located near the sides of the listeners can add
to the playback spaciousness. In the ambience and reverb effects (Nightclub, Concert Hall, Church and Cathedral), the DC-1 calculates and generates the ambient sound from a larger room and presents this sound through
the additional speakers. This works well, but the ambience is generated by
the processor — not contained in the recording. In the surround effects,
surround channel information is extracted from signals encoded on the
recording by the sound mixer — the DC-1 does not add anything to the
sound.
On films, this surround information is intended to be reproduced in
monaural from an array of speakers all around the rear of the listener.
Unless there is enough energy accidentally encoded into the surround
channel to give significant sideways energy, surround speakers won’t
contribute very much to the listening experience of ordinary stereo music.
In addition, with all speakers except the fronts reproducing the same
monaural signal, no directional effects are possible. There may be some
19
DC-1 Theory
and Design
Lexicon
sense of the music coming from all around you, but the violins and cellos are
equally loud from both sides.
Music Surround solves this problem in a novel way. The side speakers
reproduce the left and right front loudspeaker signals with two additions:
delay and inverse steering.
The delay serves a simple purpose. Sounds are kept from being localized to
the sides by an approximately 10ms delay inserted between the front and
side speakers. The center speaker is steered, and can be delayed relative to
the fronts. The center delay allows the center speaker to appear to be at
exactly the same distance from the listener as the front speakers, even if it
is placed (as usual) on a line between the front speakers.
Inverse steering acts to remove a strongly steered signal from certain
directions. As an example, assume you have a strong signal in the left
channel of a film. With normal steering, the processor enhances the level of
that signal in the left speaker, and actively removes it from the other
speakers. Inverse steering actively removes this signal from the left side
speaker, while keeping its level strong in the left front speaker. This signal
removal is done cleverly, so the level of any unsteered signal which might
be present at the same time (such as music in a film, or the rest of the
orchestra in a music recording) is not significantly reduced. The sonic result
is a much wider soundstage and a very spacious sound. When music is
played, any loud instrument or sound effect is reproduced from the front
speakers, not distractingly located off to the side.
Rear speakers in Music Logic and Music Surround, as in the stereo surround
film effects, are driven by a similar signal to the side speakers, but with some
additional delay and steering. Thus, in a 7 channel setup when a sound
effect is intended to come from the rear, it is reproduced primarily from the
rear speakers with deliberate additional help from the side speakers. (In
THX Cinema and Pro Logic, the rear speakers and the side speakers are
driven in parallel. In the Pro Logic effect, all four side and rear speakers are
driven in mono, and in the THX Cinema effect, there is a decorrelation
between the left and the right, but the signal being reproduced is the same
mono signal used in Pro Logic.)
In the Music Surround and Music Logic effects, the front left and right
outputs are attenuated by the volume and balance controls, but are otherwise unaltered by the processor. This absence of main front steering is ideal
for playing stereo music, where the original stereo signals are reproduced
from the front speakers with absolutely no alterations. The Music Surround
effect really shines on this material — the center speaker adds a little
stability to the front image without being at all obvious, while the side and
rear speakers add a tremendous amount of ambience. For film, the lack of
steering of the main speakers is noticeable on dialog, and the Logic 7 effect
gives better results.
20
DC-1 Digital Controller
DC-1 Theory
and Design
The Party effect is provided to allow music to be played over your entire
system. All speaker outputs in this mode are simply fed a stereo signal. The
left front and left side speakers are driven by the left input signal. The right
front and right side speakers are driven by the right input signal. The center
and rear channels are both driven by the sum of the left and right inputs. A
High Pass parameter allows you to remove bass from side and rear speakers
which might not be able to handle it. Center Level and Subwoofer Level
controls are also provided. This effect is primarily useful for large audiences, or background music when entertaining.
The goal of the DC-1 film surround effects is to maximize viewer involvement and to faithfully reproduce the director’s intentions for the soundtrack
in your own listening environment. Before explaining the way each of the
film surround effects (Mono Logic, Pro Logic, THX Cinema, TV Matrix, and
Logic 7) accomplish this goal, it is important to understand something about
the way film soundtracks are made and presented in the theater.
Film Surround
In the early 1940’s, large movie studios owned their own theaters and took
responsibility for their own quality standards. During this period, movie
theaters had the best sound reproduction heard anywhere — each major
studio had a master sound engineer to ensure that the sound systems in that
studio’s theaters performed properly. A decade later, the studios were
forced to sell off their theater holdings in an anti-trust action, and quality
became the responsibility of independent theater owners. Since each theater could choose films from any studio, it was no longer practical, or
feasible, for the studios to monitor the quality of each theater. At the same
time, the impact of television caused a decline in theater attendance which
left little money for individual theater owners to reinvest in their facilities.
As a result, sound technology in theaters froze. Despite the advances being
made in recording and in home music systems during this period, film
sound remained essentially unchanged through the 50’s and 60’s. In fact, by
the end of the 1960’s the average teenager had a music system at home
which was considerably superior to theater systems.
This situation began to change in the 1970’s with the introduction of Dolby
Stereo. This technological breakthrough, which allowed four channels of
sound to be recorded onto the two available optical soundtracks of a 35mm
movie print, yielded spectacular results — and created a demand for
improved film sound tracks, and for better-sounding theaters. Theaters that
upgraded their sound systems were rewarded with larger audiences. Over
the next decade, Dolby Stereo became an established standard for film
sound recording — but theater sound systems, although improved, varied
in their ultimate accuracy.
21
DC-1 Theory
and Design
Lexicon
In the 1980’s a new movie theater sound system was created under the name
THX. This system addressed, for the first time, the design of a theater
speaker system which could faithfully reproduce the film director’s intentions. The THX Sound System complemented and enhanced the playback
of Dolby Stereo, and encompassed standards for power amplifiers, speakers, and the acoustics of theaters themselves, to assure the best possible
reproduction of movie soundtracks. By 1990, this system was available in
over 500 theaters across the country. The recent availability of film on video
tape and laser disc has led to a demand for home audio/video systems
which equal the best theater sound. The Home THX Cinema system was
designed to address this need, and the DC-1 incorporates it into the THX
Cinema effect.
Research at Lexicon has taken the possiblities of THX decoding to a further
level with the introduction of true 7 channel decoding using a stereo
surround process. This option is available via the Surround Effect parameter in THX Cinema, and is further refined in the Logic 7 effect. The variety
of DC-1 surround film effects allows you to have the highest possible sound
quality for video material — whether you select classic monaural movies or
the latest releases with all of their special effects — simply by changing
effects.
Dolby Digital began appearing in theaters in 1992. Unlike analog Dolby
surround, with its mono, band-limited surround channel, Dolby Digital has
two completely independent surround channels. As a result, film sound
engineers can implement true stereo surround effects, providing theater
audiences with an expanded sense of depth, localization and overall realism. In 1995, films with Dolby Digital soundtracks were first released on
laser video discs, allowing home theaters equipped with Dolby Digital
playback systems to reproduce the same multi-channel digital sound heard
in cinemas.
Playback of Dolby 5.1 soundtracks in the home also benefits from the Home
THX system. The acoustics, equalization curves and speaker arrays used in
mixing remain the same, whether mixing a four-channel analog Dolby
soundtrack, or a 5.1 digital soundtrack. In the DC-1, the THX 5.1 effect
provides THX Home Cinema enhancements for playback of Dolby Digital
source material. The 5.1 Logic 7 effect incorporates Lexicon enhancements
in addition to some of the THX parameters to reproduce Dolby Digital
soundtracks with a greater sense of spatial realism, creating a more cinematic presentation.
Other delivery formats featuring Dolby Digital are in development. Dolby
Digital has been chosen as the digital audio format for DVD, a new digital
playback medium for film and music, and it has been selected by the Grand
Alliance to provide digital surround for the HDTV system being developed
in the U.S.
22
DC-1 Digital Controller
The problem of mono-to-stereo conversion is an old one. One time-honored
solution is to break the incoming signal into frequency bands, sending some
to one channel and the rest to the other. When the filters are complementary
(when the sum of the two output channels equals the original input channel)
this solution can give stereo spread without ruining the tonal balance. When
the filters are non-complementary, they can produce an unpleasant fake
stereo effect.
DC-1 Theory
and Design
Monaural Films
Some effort has been made to design filter pairs for film sound which leave
voice frequencies unchanged while spreading out the music. More recent
designs have gone in another direction, using digital or analog delay lines
to produce a comb filter effect. So far, these attempts have not been very
successful.
The principal element of film sound is dialog and the principal rule in
reproducing it is to assure that it appears exclusively in the center channel.
Broadcasters, who have an interest in converting mixtures of dialog and
music to synthesized stereo, have built circuits designed to turn off the
stereo synthesizer when voice appears. Unfortunately, the switch from
mono to stereo is often abrupt and the chances of dropping into mono by
mistake during music are high. One basic problem with films, especially
modern ones, is that music or background effects which should be spread
out into the side speakers frequently appear at a low level beneath the
dialog.
The DC-1 Mono Logic effect electronically identifies certain properties of
film speech and removes it from the stereo synthesis. This allows music and
effects in the dialog to be spread out while leaving the dialog centered. The
remaining music and effects are directed to the input of a room simulation
mode that creates a space the size of a large room or small theater. The room
simulator has outputs for left, right, side and rear surround speakers.
The monaural input sound from the film is unchanged in the center speaker,
so that all the dialog and music that the director expected to come from the
screen still does — with no modification or reverb. Partly because of the
acoustical character of the room synthesizer, the result is often so successful
that switching from Mono Logic to Pro Logic or THX may make a surprisingly small difference.
The most critical adjustment in Mono Logic is the Effect Level. Ideally the
film’s music and effects should appear to come from the front but with the
added sense of a large space surrounding you. The side and rear speakers
should not be individually audible.
During the early days of film stereo,
dialog was sometimes mixed (by
"panning" the monaural dialog
track) to come from the same part of
the screen as the image of the actor.
Subjective reactions to this technique were varied, and technical
problems with some magnetic sound
tracks helped to discourage the practice, so modern movies are seldom
mixed with panned dialog. In a home
system with a good Pro Logic decoder, however, the effect can work
quite well; recent releases with
panned dialog include "Yellow Submarine" and "Superman I." In most
films though, all dialog comes from
the center channel.
High frequency attenuation is provided via the variable Treble Rolloff
parameter and the Academy Filter On/Off parameter, which recreates the
proper tonal balance of older, monaural films which were recorded with a
much narrower and brighter frequency response than current films.
23
DC-1 Theory
and Design
Stereo Films
Lexicon
A Dolby Stereo film sound track has four basic components: left and right
channels, a center front channel and a surround channel. The first three are
fed to speakers arrayed behind the movie screen, while the surround sound
goes to speakers on the side and rear walls of the theater. The four channels
are recorded on separate magnetic tracks and are combined by the Dolby
Stereo matrix encoder into two stereo channels called Lt and Rt (left total
and right total) during the final mixing process. The original left and right
channels go directly onto the left and right channels of the Dolby Stereo mix.
The center channel is fed equally to both channels, in phase, and the
surround track is fed equally to both channels, but 180 degrees out of phase.
(It’s actually a little more complex than that, but the the end result is
essentially the same for this discussion).
The center channel contains the most energy, including the dialog; music is
normally mixed so that it appears to come from the front, with reverberation
or ambience coming from the surrounds. The surround channel ambient
information is a powerful tool for the film sound mixer. It is this information
that helps us believe that the scene in the film is real. For special effects,
music can be encoded to come from all around the listener or even from
behind. In any case, with music and ambient effects there is almost always
a substantial spread across the front of the loudspeaker array.
Films originally have four channels: one
for dialog and three for music and effects.
To make a Dolby Stereo film, these are
combined to two.
Sound effects can come from any direction around the listener and it is the
job of the decoder to duplicate as closely as possible the film mixer’s
placements.
When the movie is shown the two Dolby Stereo tracks must be decoded and
separated into the original four. The basic Dolby Surround decoder does
this in a rather rudimentary way: it supplies a signal to the center channel
which is just the sum of the two input channels. This signal contains the
dialog. However, the left and right signals still contain dialog too, so the
dialog is spread out among the three front speakers. Similarly, the Dolby
Surround decoder takes all out-of-phase signals and sends them to the
surround speakers, while leaving the original out-of-phase components in
the left and right front speakers.
The basic Dolby Surround decoder has high channel separation between
left and right decoded audio, and between center and surround. The
separation between left or right and center, or between left or right and
surround, however, is only a few dB. The simple Dolby Surround decoder
does pretty well with music (although sometimes the center channel is too
loud) but, because any sound will be reproduced in at least three loudspeakers, effects are smeared and often unconvincing .
With conventional surround any sound
comes from at least three directions.
24
A Pro Logic decoder, like the professional Dolby Stereo cinema processor,
both enhances the dialog in the center and removes it from the left and right,
while maintaining as much stereo separation as possible. This is a form of
directional steering. Properly done, steering prevents the dialog from
appearing in the other channels and enhances its plausibility.
DC-1 Digital Controller
The requirements for film sound are quite different from those for the
playback of music. The most important track in most films is the dialog
(assuming you aren’t watching 10,000 Years BC or Quest For Fire...). When
the two stereo channels are played back through two speakers with no
decoder, dialog will appear to come more or less from the center, but only
for those listeners on the center line of the main stereo pair.
DC-1 Theory
and Design
The Importance of Dialog
The situation is similar with music and sound effects. For example, if the
sound was intended to be in the left, the decoder will remove it from the
center and surround channels. If it was intended to be halfway between left
and center, the Pro Logic decoder presents it equally to the left and center
speakers and removes it from the right and surround channels.
The Pro Logic decoder can give good stereo spread and precise control over
front-to-back perspective. But the real strength of Pro Logic decoding
emerges when music and dialog occur at the same time. When dialog is
present, the center channel information must be removed from the left and
right channels without reducing the spread or loudness of the music.
Pro Logic decoders sense both the direction of the loudest sound and the
difference in level between it and any ambient information. They then use
this information to direct the steering. The accuracy with which this is done
is even more important in a home decoder than in a professional model,
because the small size of the playback room makes decoding errors more
audible than they are in a theater. The level detection must be very fast, and
the matrix must adapt very quickly or there will be a time lag between the
audibility of a sound and its correct steering. Since phase relationships
determine how the sound is steered, Pro Logic decoding puts unusual
demands on the accuracy of the phase and balance of the input channels.
Other Pro-Logic decoders have a front panel control for adjusting input
balance and for best results a user should carefully adjust this for each mode.
But what if the channel balance varies during playback?
Simple logic decoders turn down the left
and right speakers during dialog. This
seriously affects music and effects.
The manual balancing procedure does nothing to correct azimuth errors.
During the preparation of the master for a video tape or disc, misalignment
of the playback heads or skewing of the film produce small time differences
between the two channels. Azimuth is poorly controlled in both professional video recorders and optical film chains. In the final product, which
has been through many generations, it can easily be wrong by 50 microseconds or more, and may vary as the tape or disk is played. At middle and high
frequencies it doesn’t take much misalignment to generate large interchannel differences in phase, which are just what the decoder uses to do its
steering.
The Dolby encode/decode system deals with this problem by reducing the
treble in the surround, so the out-of-phase sibilants in the film do not
splatter annoyingly from the rear. This does not, however, reduce the
sibilants in the side speakers. Some non-Pro Logic decoders reduce these
side-channel sibilants by narrowing the spread of the front channels in the
presence of dialog; this compromise is unnecessary in the DC-1.
Pro Logic decoders remove dialog from
the left and right channels, while maintaining stereo as much as possible.
25
DC-1 Theory
and Design
Pro Logic requires phase accuracy. Common azimuth errors cause ghost dialog in
all channels unless the azimuth error is
corrected.
Lexicon
The DC-1 decoder is unusual in a number of ways. First of all, it is entirely
digital. (Most surround decoders advertise that they are digital because
there is a digital delay line for the surround channel but the matrix and the
logic decoding are done in analog.) Because the DC-1 is all digital, we can
use some of the digital memory to delay all the output channels by 10
milliseconds — about the same as the acoustic delay you get in the front row
of a theater. (The surround channels are delayed by an additional 15 to 30
milliseconds.) This delay allows plenty of time for the DC-1 to determine the
direction of sounds and adjust the matrix before the sounds are sent to the
amplifiers. This substantially improves dialog and effects cancellation, as is
immediately apparent from the spread of ambient material or music, even
in the presence of dialog. The DC-1 can also sense and continuously correct
both balance and azimuth errors in the incoming material. All the time the
film is playing, the DC-1 is checking balance and azimuth, keeping the
dialog perfectly centered. The result is superior steering. An added benefit
is that the DC-1 needs no front panel input balance control; the user need
not bother with this adjustment. You can check the quality of the balance
and azimuth in a tape or disc if you wish by turning the Auto Azimuth/
Balance parameter off and observing any changes in the location of dialog
and effects.
As of this writing, the film industry is moving from a 4 channel audio
standard to a 5.1 channel standard. The new standard is similar to the old,
but has two surround channels instead of one, and an additonal low
bandwidth channel for a subwoofer. Once one has heard some good
examples of stereo surround channels the mono surround channel of Dolby
Surround is no longer satisfying. As most of the information in the surrounds is ambient information and music, stereo significantly increases
listening enjoyment. As listeners become accustomed to placing the surround speakers out to the sides instead of behind them, stereo surrounds
become even more important.
The decorrelation circuit required in previous THX decoders overcomes
some of the limitations of the mono surround channel. While it does not give
directionality to the signal, it at least increases the sense of spaciousness and
reduces the tendency of the surround speakers toward localization. Artifacts from the decorrelation itself are usually mild. However, when an event
is specifically steered to the surrounds the decorrelation can diffuse the
signal too much. For example, if a jet flies from the front to the rear, the
sound should not sound sharp and well localized in the front, and then
disappear into a diffused mush in the rear.
Auto Azimuth and Auto Balance features in the DC-1 allow accurate decoding of matrix surround.
26
Lexicon has developed a new technique for decoding a stereo surround
signal from a standard matrix encoding. This yields a 4-2-5 encoding/
decoding process. When the source film was originally a 5.1 channel film the
resulting 4-2-5 decoding is much closer to the original if the 5.1 Two Channel
effect is used to create a 5.1 channel original. 5-2-5 or 5-2-7 encoding/
decoding is possible, and the match to the original film can be very close
indeed.
DC-1 Theory
and Design
DC-1 Digital Controller
The decoding works by combining very high quality standard Pro Logic
decoding with the technology of the Music Logic program. The front
channels (left, center, and right) are decoded with standard Pro Logic, but
with the addition of an adjustable delay for the center channel. The two rear
channels operate in a dual mode. When the film surround content is
primarily music or ambience, the rear channels maintain full stereo, reproducing the left and right front channels with the addition of frequency
contouring and delay. (In the TV mode the contouring is adjustable —
allowing full bandwidth surround if desired.) When the front hemisphere
contains steered information, such as dialog or sound effects, these signals
are cleanly removed from the stereo surrounds. Thus, dialog and all onscreen sound effects remain in the front where they belong. The new
decoding depends critically on our ability to remove unwanted sound from
the stereo surrounds, and this would not be possible without the accuracy
afforded by our Auto Balance and Auto-Azimuth circuits.
As sound effects move toward the rear, the processor decodes differently.
Effects which move from left to rear pan smoothly from the left front
loudspeaker to the left side, and then from left side to both left and right rear
loudspeakers. Effects which pan from right to rear behave similarly. Once
full rear is reached, the decoder outputs are identical to a standard Pro Logic
decoder, so full compatibility is maintained. The action of the decoder has
been extensively tested both at Lexicon and at Lucasfilm to ensure the intent
of the film director is accurately reproduced.
Although films have not yet been deliberately mixed to take advantage of
the capabilities of the new decoding, the improvement on standard films
can be dramatic. The improvement in spaciousness with music and ambience is obvious. On films with aggressive sound effects the combination of
surround steering and localization can convince listeners they are listening
to a true discrete surround mix.
Based on Dolby AC-3, an audio encoding technology developed by Dolby
Laboratories, Dolby Digital delivers six discrete channels of digital sound.
Five channels: Left, Center, Right, Left Surround and Right Surround are
full-range (3Hz to 20,000Hz). The sixth channel is called the Low Frequency
Effects (LFE) channel. It can contain additional bass to emphasize the
impact of scenes such as explosions or crashes. Because the LFE channel has
a limited frequency response, it is often referred to as the ".1" channel. When
added to the five full-range channels, the Dolby Digital surround system is,
therefore, commonly referred to as having "5.1" channels.
Dolby Digital
As all channels are discrete digital tracks, channel separation and dynamic
range are greatly improved over analog surround sound. One advantage is
a dramatic improvement in localization of dialog. In addition, full-range
surround effects can be independently directed to left surround or right
surround speakers for a heightened sense of realism. Another benefit is
increased dynamic range, which allows subtle audio cues to be more easily
heard while giving loud effects, such as explosions, life-like impact.
27
DC-1 Theory
and Design
Lexicon
The LFE channel delivers deep bass information for special effects and
music soundtracks, but is not always used. Recordings made without the
LFE channel are referred to as "5.0" soundtracks.
THX Enhancements
THX with Lexicon Stereo Surround 5channel decoding combines full Pro Logic
decoding with stereo surrounds.
Home THX Cinema is designed to present film viewers with the same audio
impressions perceived by the film director on the "dubbing stage" — the
studio in which the final mix of the soundtrack is established. These stages
feature a large screen for viewing the picture and a massive audio console
for controlling every aspect of the soundtrack. THX attempts to precisely
reproduce the experience intended by the director in a home environment.
In general, this requires not only considerations of the characteristics of a
small room (as opposed to a large dubbing stage), but many specific system
performance requirements. A complete THX system for accurate translation of dynamic motion picture soundtracks requires a specific array of
speakers which are designed to produce the soundtrack’s full frequency
response and dynamics without distortion, amplifiers which meet exacting
specifications for distortion, noise, stability and dynamic power, and a
Dolby Pro Logic controller with electronic enhancement.
As a complete discussion of the theory of THX design is beyond the scope
of this booklet, we refer you to LucasArts for detailed information on all
aspects of the Home THX Cinema system. (LucasArts Entertainment Company, THX Division, P.O. Box 2009, San Rafael, California 94912 Telephone:
415-662-1900) Here, we will limit our discussion to the advantages of the
DC-1 as a THX controller.
The main requirements of any THX controller are:
Dolby Pro-Logic Surround decoding
Subwoofer Crossover
Re-equalization
Surround Decorrelation
Surround Timbre-Matching
Dolby Pro-Logic decoding is not only necessary to decode soundtracks
which are encoded in a matrixed surround sound format such as Dolby
Stereo or Ultra*Stereo, but enables all motion pictures (including the more
than 5000 made in matrix stereo) to be reproduced as they were heard in the
film studio dubbing stage — no matter what delivery system is used. The
DC-1 is a completely digital, Dolby Pro Logic decoder with patented
automatic correction of inter-channel phase and channel-balance errors.
With this technology, home theater speakers not only reproduce the appropriate audio information, but correct errors which occur in the multiple rerecording processes of most video software releases.
28
DC-1 Digital Controller
DC-1 Theory
and Design
The Subwoofer Crossover feature enables the subwoofer speaker to reproduce only frequencies in the low bass range, leaving the front, center, sides,
and rears to reproduce frequencies above 80 Hz. Dolby Digital adds
another challenge to bass reproduction in home theaters, as bass frequencies can be mixed into all five channels, as well as the LFE channel. Adapted
for Dolby Digital, this multichannel crossover is designed to blend low
frequency information from the front and surround channels with the LFE
channel. This facilitates optimum placement for imaging and smoothness,
protects smaller speakers, and uses amplifiers more efficiently.
Re-Equalization is provided as a control to compensate for the fact that the
soundtracks in films sound "bright" when listened to in a home environment. This results from a combination of the way we perceive sound in large
halls vs. small rooms, and the theater equalization which has become
standardized throughout the movie industry. This problem can be more
pronounced with Dolby Digital soundtracks due to the addition of three
full-range channels in the mix (Center, Left Surround and Right Surround).
The re-equalization control restores the sound to its natural balance, and
reduces listening fatigue dramatically by reducing excessive high frequencies.
As the surround channel in a Dolby stereo film is monaural, the signal is
neither spacious nor enveloping. In a theater, the quantity and placement of
surround speakers compensates for this; in the home, arrays of 12-18
speakers are generally impractical. To diffuse the perceived sound, the
DC-1 (in the decorrelated mode) provides a decorrelation algorithm which,
after the monaural signal is split in two, alters the phase/time relationships
between channels slightly. This eliminates the mono effect from the rear
channel and creates an enveloping soundfield. When the "Stereo Surround"
parameter is selected, the rear channels become stereophonic, and the
decorrelation is unnecessary. This setting is recommended.
In Logic 7, stereo surround 7 channel
decoding features full-range stereo effects in side and rear speakers.
Although Dolby Digital has discrete surround channels, not all of the
surround information on these channels is stereophonic. Dolby Digital rear
channel information is often mono. Raindrops, for instance, are typically
recorded in mono. When played back through the array of surround
speakers in a theater, there is no need for left channel and right channel
raindrops, as the speaker system presents this sound as an enveloping
environmental effect. The Home THX Adaptive Decorrelation circuitry is
active when surround channel information is predominanatly mono. When
stereo effects are present, decorrelation is not used. The result is consistently
spacious surround sound and dramatic stereo effects.
Timbre Matching compensates for the difference in characteristics of
sounds coming from different locations. In real life, timbre differences help
us differentiate sounds which originate from the sides and rear from those
originating from the front. Some people, however, find that these natural
changes in timbre are undesirable in a film. Timbre-Matching provides
equalization to the surround channels, reducing the perceived change in
character of sounds which are panned from the screen into the surrounds.
29
DC-1 Theory
and Design
Lexicon 5.1 Enhancements
Lexicon
5.1 Logic 7 combines the re-equalization, subwoofer crossover, and adaptive de-correlation features of 5.1 THX Cinema with Lexicon's Logic 7
matrix technology. 5.1 Logic 7 differentiates between the side and rear
speakers with a combination of delay and directional steering. For
soundtracks with music or effects such as wind noise or applause, this
creates a more spacious and enveloping soundfield and has a large effective
listening area. For sound effects that pan to the side and rear of the listener,
the result moves more convincingly through the sides and into the rear
speakers. 5.1 Logic 7 produces the most exciting, and at the same time the
most natural reproduction of 5.1 channel encoded material.
5.1 Logic 7 includes a parameter for surround level which has a default
setting of +3dB. As of this writing, extensive listening to available 5.1 laser
discs has convinced us that they sound better balanced (and closer to their
2-channel versions) with a surround boost of 3dB.
5.1 Two Channel provides a means for mixing the 5.1 channels of an AC-3
film into two stereo channels while preserving all the original directional
effects when the mix is decoded through Logic 7 or TV Matrix. 5.1 Two
Channel includes a parameter for surround level. Like the surround level
parameter in Logic 7, the optimum setting in our experience is +3dB. The
default setting is 0dB. There is also a parameter for reducing the level of the
LFE channel.
30
DC-1 Digital Controller
To some degree, speaker placement will depend on whether your priorities
lean more toward music listening or film viewing. Fortunately, the goals are
the same for both listening situations: stable localization (imaging) in the
front, and diffuse, enveloping surround sound from the sides and rear.
The placement of speakers will be somewhat restricted in your home by the
need to have the sound closely associated with both the screen and your
seating area. Within this limitation, however, there are many possible
variations, for example in the height, angle and distance from the walls.
These choices, again, will depend on the type and number of speakers you
are using, as well as the physical characteristics of your room. Some of the
general effects caused by room reflections are explained in the section “The
Listening Room.”
Additional information relevant to your particular system should be available from your speaker manuals. THX speakers in particular are designed
for specific placement and the manufacturer’s recommendations should be
followed for best results. With that in mind, here are some general guidelines:
DC-1 Theory
and Design
Speaker Placement
CAUTION: Speaker magnets
can distort the TV image. If
you see any picture discoloration in the area closest to
the speaker, the speaker is
too close to the screen.
If only two speakers are used, the user is free to experiment. The standard
±30° speaker positon works quite well in many cases for both music and
film. For a single listener you may want to try placing the speakers relatively
close to the sides of the screen. With this configuration and a single listener,
the Panorama effect should be used for both music and films. Be sure to
adjust the speaker angle parameter in the Setup menu to correspond to the
actual angle between the speakers.
With multiple speakers, the main front speakers can again be ±30° if you are
comfortable with occasional sound effects which extend beyond the screen.
Spreading the speakers further apart provides a wider, more exciting
soundstage, and most people readily accept an auditory sound field which
is wider than the visual field. If you wish to put the main speakers close to
the screen, the processing in the DC-1 will make up for the lost spaciousness
in both music and film. The front speakers should be placed at equal heights
and at equal distances from the front wall. Try angling the speakers in
toward the listening position (toe in) and keep them away from the side
walls.
A center speaker is so important for realistic dialog placement that, if it is not
possible to have a center speaker, it is frequently better to plug the Center
output of the DC-1 into the audio input of your video monitor and use its
built-in speaker (if it has one) than it is to run the dialog through the main
loudspeakers. (The DC-1 provides a control for this setup which splits bass
out of the center channel and directs it to the left and right speakers.)
The Center speaker should be located directly beneath or above the screen
(shielded.) Turning a speaker which is designed to be used vertically on its
side will significantly alter the soundwave patterns it is intended to produce.
31
DC-1 Theory
and Design
Lexicon
Right, Left and Center speakers should be positioned at equal heights and
at equal distances from the front wall. Depending on their height, they
should be tilted to aim vertically at the listening area — they should not
necessarily be angled in from the side walls toward this area. In most setups,
although the left and right speakers are about the same distance from your
listening position, the center speaker is often closer to you, causing sound
from the center speaker to reach you earlier than sound from the left and
right. Equalizing the path length (the distance from the speaker to your ear)
of the three front speakers in your system will improve the image. A Center
Delay parameter (along with other “Listener Position” controls) is provided
in the Setup menu of the Base and THX versions of the DC-1. The Dolby
Digital version provides Time Alignment, which electronically compensates for differences in the distance of all speakers from the listener position.
Optimum side speaker placement depends on your room and listening
position, the type of speakers used, whether you are primarily interested in
film sound or music, and aesthetics (which we’ll leave to you). If you have
THX-type surround speakers, follow the manufacturer’s recommendations
for placement. If you are sitting within six to seven feet of the main speakers,
place the side speakers directly to the sides of the listening area. If you
normally sit further away from the main speakers, the sides should be
somewhat forward of the listening position, preferably angled back towards it. A bit of experimentation will prove very helpful. The goal is to
blend the side speakers’ energy with the main speakers; you should not be
distinctly aware of output of the side speakers.
Side speaker height should be near the level of the ear. Placement near the
ceiling/wall boundary may help disperse the sound, and will reinforce the
bass response of the speaker, but very high placement can reduce the
spaciousness that the true stereo processing of the DC-1 provides. If
pedestal or wall mounting proves impractical, ceiling-mounted speakers
will suffice. Place them far apart and equidistant from the listening area.
Rear speaker placement is somewhat less critical. Here the main goal for
both film and music is to produce a diffuse and enveloping soundfield. The
height of rear speakers will depend on both room size and furnishing. In
general, placement of the rear speaker(s) above the listening area can be
very effective. Note, however, that placing these speakers too high in a
narrow room, or at the ceiling, may reduce the spaciousness of the sound.
For maximum envelopment the rear speakers should be widely spaced —
preferably near the side walls.
Subwoofer placement is less critical, but in some positions in your room
may cause standing wave paterns and/or a rattling of furnishings. Refer to
“The Listening Room” section, and specifically to your subwoofer manual
for recommendations. The Subwoofer Output is a monaural signal created
by summing the left and right inputs, then filtering out frequencies above
80 Hz at a rate of 24dB per octave. For the tragically technical, this is a
Linkwitz-Riley LPF -6dB at 80Hz.
32
DC-1 Digital Controller
If you are using THX speakers, be aware that they have significant design
differences from standard (non-THX) speakers. Extensive information is
available from the manufacturer on their design and on maximizing their
effectiveness in a room. Although not contradictory to those recommendations, the following guidelines are intended as a generalized reference for
those using standard speakers. Most manufacturers assume that THX systems will not include separate side and rear speakers. In our experience,
however, adding a pair of widely spread rear speakers to a five channel THX
system makes a very significant improvement to the sound. Such a system
allows the full benefits of Logic 7 and 5.1 Logic 7 to be heard.
DC-1 Theory
and Design
The Listening Room
The physical characteristics and furnishings in your listening room will
affect the way sound is reflected and dispersed through the room. A great
deal of research has been done in the field of room acoustics and a superb
environment can be constructed with this research in mind. In this discussion, however, we will limit ourselves by assuming that your room is
already built, and that your goal is to get the best sound possible from your
system in that room. Even within this limited scope, there are many factors
which may affect the quality of sound. Fortunately, where there are problems, there are also some relatively simple solutions.
All rooms have acoustical characteristics determined by their dimensions
and structural materials. The hard surfaces of the room reflect sounds back
and forth in patterns which are likely to interfere with the sounds generated
by your system. The nature of these reflections, and their effects, is dealt
exhaustively in the wealth of material available on Room Acoustics — here
we will simplify by stating that the first general goal is to dampen or diffuse
these echoes — to create an acoustically neutral room which doesn’t
interfere with the environment your system creates. The most obvious way
to do this is to add absorbent material to those surfaces from which the most
offensive reflections arise, and to break up undesireable reflective patterns
with uneven surfaces. Fortunately, furnishing the room with carpets,
drapery and furniture goes a long way toward accomplishing this. (Absorbent materials can be commercially-available acoustic panels, fiberglass,
dense foam, drapes, or upholstered furniture. Diffusive materials can be
commercially-available panels, or irregular furniture, such as bookcases.)
Now, what we really want to do is “tune” the room to maximize the clarity
of dialog within a localized front soundstage, and to create a diffuse, nonlocalized soundfield for effects. This can often be done by adjusting speaker
placement, by altering room furnishings or by some combination of both,
but to do this effectively, we must look briefly at the kinds of interference
you may encounter. Following are some basic problems and suggested
solutions.
33
DC-1 Theory
and Design
Room reflections
Lexicon
Room reflections cause blurring of the image of a localized front soundstage. Sounds which are ideally perceived as coming from the screen, reach
the listener from other directions, spoiling the illusion. These unwanted
reflections can be eliminated by placing absorbent or diffusive material at
the point of reflection. To maximize the localization of the soundstage, an
attempt should be made to deaden the area immediately behind and
adjacent to the front speakers.
Speaker alignment is important. If front speakers are placed at unequal
distances from a wall, the tonal balance between the two speakers will
change. This is particularly important in placement of speakers in the front
of the room where sounds should pan realistically. Care should be taken to
place speakers the same distance from the front wall. An attempt should
also be made to achieve approximate symmetry in the distance between
each of these speakers and the side walls. In addition to being placed
consistently with regard to room surfaces, the front speakers should be
placed well away from side walls.
Room reflections interfere with the image
of a localized front soundstage.
These unwanted reflections can be either
absorbed …
…or diffused.
34
DC-1 Digital Controller
DC-1 Theory
and Design
Room echoes cause interference which results in an unnaturally “bright”
sound. These echoes can also be absorbed or diffused. Use heavy insulating
drapes over large expanses of glass.
Room echoes
Bass frequencies have long wave lengths — these may be as long as one of
the dimensions of your room. This can cause over-emphasis of some
frequencies and cancelling out of others. This effect is most noticeable in
rooms which have two or more dimensions which are equal (for example,
length and width). This type of room will exhibit irregular bass response in
different parts of the room. Keep in mind that this effect is worst in nearly
empty rooms. Large pieces of furniture will break up the reflections
between parallel surfaces quite nicely. Moving speakers further in from the
walls can also make a noticeable improvement.
Irregular bass response
Note that speakers placed against a solid wall can alter the bass and midbass response of your system — making it sound bass-heavy. Placing a
speaker in a corner (where 3 surfaces meet) will make any speaker not
designed for such placement sound muddy. Very thin walls will allow some
of this amplified bass energy to escape, reducing the effect. Moving the
speakers in from the walls also works nicely.
Structural resonance (pictures and windows rattling. etc.) can be caused
even by desireable bass frequencies. If the bass response of your system is
at the desired levels, these rattles can be eliminated by putting felt on picture
frames, or around window cracks.
Structural resonance
The listener should not be able to pinpoint the source of surround effects,
therefore output from the surround speakers should not be aimed directly
at the listening area. This is easily accomplished by raising the surround
speakers. Although surround speakers require some reflected sound to be
effective, these reflections should be diffusive — providing randomized
reflections in many directions. Bookcases and other irregular surfaces
provide this sort of diffusion, as do some commercially-available acoustic
panels.
Maximizing the effect of the
surround soundfield
35
DC-1 Theory
and Design
Lexicon
Creating a Custom Concert Hall Seat
The DC-1 allows you to create virtually any acoustic space you desire. It is
extremely simple to walk through the
parameters of a program, vary them to
your liking, name the space, and store it.
For this example, let's do a fairly radical
redesign — changing CATHEDRAL
into a reverberant hall. Begin by selecting CATHEDRAL, the most reverberant
DC-1 effect.
Think about a nice concert hall
where you would enjoy going out to see
your favorite performer. Consider a hall
of approximately 2,700-3,000 seats
with a balcony (and, of course, valet
parking). Get your performer's CD, cue
it up in the CD player, but don't start it yet
— we'll design the hall first.
First, let's decide how long the hall is,
as this will influence the reverb time and
the overall character. Using the MENU
buttons, enter the EFFECT ADJUST
menu, select HALL SIZE, and set it to 35
meters (meaning the simulated hall will
be 35 meters long).
How close to the stage do you want
to sit? Might as well go to the good
seats— how about row M in the center of
the orchestra section? Predelay approximates the amount of time between
the original sound of the performer and
the sound of the hall. The closer you are
to the stage, the longer it will take for the
sound to bounce off the back walls and
get back to you. Let's add a little extra.
(Some predelay is inherent in the mode
to keep the direct sound distinct.) Using
the MENU buttons, set PREDELAY to
64ms.
The Speech Detection feature is actually a special dialog recognition program which reduces the reverberation
on spoken voice. As we aren't using the
radio (and, therefore, have no announcers or commercials), let's turn Speech
Detection off by pressing MENU ▼.
Think about what the walls look like
in a good concert hall. Many irregularly
shaped objects like elaborate columns,
carved panels, draping and light fixtures
come to mind.
36
These objects absorb and diffuse
the sound. A good sounding space
never has a lot of high frequencies
bouncing back at you. Select
ROLLOFF to manipulate this part of the
hall. Change the value to 2.2kHz.
Next, select MID RT. The reverb
time of a space helps describe how
large it is. For example, a cathedral has
a much longer reverb time than a living
room. Let's modify the reverb time of our
factory preset to an appropriate value. A
hall of the size we are creating will have
a reverb time of approximately 2.00 seconds for the mid frequencies.
BASS RT describes how long the
low frequencies of the hall stay around
relative to the midrange. You have three
choices: 1.25 (warm), 1.0 (neutral), and
0.7 (dry). Typically, a hall of this size will
have a little longer bass reverb time.
(Hence, the preset value of 1.25.) This
one is your choice — use MENU ▲ and
▼ to change the value.
Lastly, hold the Back BALANCE button down to move the balance all the
way to the back. Now it's time to listen to
your creation. Start your CD. It should
sound like you are standing in the lobby
of the hall with the doors closed. Slowly
move the balance to the front of the hall
one step at a time by repeatedly pressing Front BALANCE. (Don't hold it
down.) You are now walking down the
aisle to take your seat. When the sound
feels right, stop pressing the balance
key and "take your seat." If you want to
make the effect even more dramatic,
increase EFFECT LEVEL (in the EFFECT ADJUST menu).
To rename your new hall, use MENU
▼ to select CUSTOM and scroll to EFFECT NAME. Use MENU ▲ and ▼ to
change each letter. Use SELECT to
move to another character. Once you've
named your hall, press DONE to exit the
menu. Your settings and new name are
automatically stored in place of the factory program CATHEDRAL. If you want
to restore the factory settings, go to the
CUSTOM menu for your new effect and
select RESTORE PARAMETERS.
Press SELECT to erase your settings
and restore the CATHEDRAL parameter settings. Select RESTORE EFFECT NAME and press SELECT to
change your effect's name back to CATHEDRAL.
It's fun, and pretty easy, to create
almost any acoustic space you want.
For the megalomaniacs among you try
creating your own private Taj Mahal —
in the CATHEDRAL effect, increase
MID RT to 17.04 and turn EFFECT
LEVEL all the way up. Play a CD for a
couple seconds, then hit pause and
close your eyes ... Just remember, you
can't hurt anything by experimenting,
and the factory presets can't be permanently modified, so don't worry about
losing them forever.
Enjoy!
DC-1 Digital Controller
1. Schroeder, M.R., Gottlob, D. and Siebrasse, K.F.,”Comparative Study of
European Concert Halls: Correlation of Subjective Preference with Geometric and Acoustic Parameters”, J. Acoust. Soc. Am., vol. 56, pp. 1195-1204
(1974).
DC-1 Theory
and Design
References
2. Barron, M., and A.H. Marshall, “Spatial Impression Due to Early Lateral
Reflections in Concert Halls: The Derivation of a Physical Measure”, J.
Sound Vibration, vol. 77, pp. 211, 232 (1981).
3. Griesinger, D., “Spaciousness and Localization in Listening Rooms and
Their Effects on the Recording Technique”, J. of the Audio Eng. Soc., vol. 34
no. 4, pp. 255-268 (1986).
4. Griesinger, D., “New Perspectives on Coincident and Semi Coincident
Microphone Arrays”, J. of the Audio Eng. Soc., 82nd Convention, London(1987) Preprint # 2464 (H-4).
5. Damaske and Mellert, “Ein Verfahren zur richtungstreuen
Schallabbildung des oberen Halbraumes Hber zwei Lautsprecher”,
Acustica, vol. 22, pp. 153-162 (1969/70)
6. Bishnu S. Atal and Manfred R. Schroeder, “Apparent Sound Source
Translator” - U.S. Patent Disclosure, Patent No. 3,236,949, Feb. 22, 1966.
7. Borish, J., “An Auditorium Simulator for Domestic Use”, J. of the Audio
Eng. Soc., 33 (5) p. 330 (1985).
8. Blumlein, A.D., British Patent 394,325, 14 June, 1933, reprinted in J. of the
Audio Eng. Soc., Vol. 6, pp. 91-98, 130 (April, 1958).
9. Griesinger, D., “Theory and Design of a Digital Audio Signal Processor
for Home Use”, . of the Audio Eng. Soc., 37 pp. 40-50.
10. Marimoto, and Pˆsselt, C., “Contribution of Reverberation to Auditory
Spaciousness in Concert Halls”, J. Acoustical Soc. Japan (E)10, pp. 87-92, 2
(1984).
11. Bradley, J.S., “Contemporary Approaches to Evaluating Auditorium
Acoustics”, Proc. of the AES 8th International Conference, 1990, pp. 59-69.
12. Griesinger, D., “Multichannel matrix surround decoders for two eared
listeners”, Preprint from the Los Angeles Conference of the AES, Nov. 1996.
13. Griesinger, D., “Spaciousness and envelopment in musical acoustics”,
Preprint from the Los Angeles Conference of the AES, Nov. 1996
37
DC-1 Theory
and Design
38
Lexicon
Lexicon, Inc.
3 Oak Park,
Bedford MA 01730-1441 USA
Telephone: 781-280-0300
Fax: 781-280-0490
Lexicon Part #070-11278
Printed in the United States of America
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