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Using The Reverb Program Lexicon, Inc.
affordable large-screen TVs, Dolby Pro Logic, which
added logic steering to the basic matrix decoder, and
Home THX cinema, which imposed common standards
on home surround equipment. Pure audio recordings
with matrix surround encoding began to appear,
although when reproduced with Pro-Logic or THX
matrix decoders these recordings could be
disappointing.
In the early 1990s Dolby Digital, DTS, and MPEG
surround arrived. These systems provided discrete
digital signals for each channel and a separate Low-
Frequency Effects (LFE) that provided very low
frequency sound effects. Squeezing 5.1 discrete digital
channels into a recording involved some data reduction,
but the reduction schemes used psychoacoustical
principles to make the resulting artifacts less audible.
Dolby Digital soundtracks were the first to make the
transition from movie theaters to the home, via laser
disc. With the advent of DVD, whose popularity rapidly
eclipsed laser discs, Dolby Digital and DTS tracks
became more widely available for home theater.
The discrete digital technologies require specialized
hardware for playback, and the original source material
cannot be recorded on home tape recorders, VCRs, or
broadcast over conventional broadcast equipment.
More importantly more and more consumers were
installing surround sound systems in their homes, and
there was a great need for a technology that would
allow standard two channel recordings to be
reproduced in a surround system. Not with Dolby Pro
Logic, which narrowed the front image and provides a
monaural surround signal, but with a system that
offered a wide front image over a large listening area,
and the listener envelopment present in the original
recording location.
To solve both problems Lexicon developed LOGIC7
matrix technology, which provides a method for
releasing surround recordings on standard stereo
compatible CDs. More importantly, LOGIC7 and
LexiconLogic provide a method of playing the millions of
standard two channel music recordings with all the
advantages of discrete surround.
LOGIC7 also allows Dolby surround films to be played
with a wide frontal image and full rear envelopment.
LOGIC7 encoded recordings can be broadcast or
played on any currently available reproduction
equipment, and yet they provide full five channel
surround on a device with a LOGIC7 decoder, and
decent four channel surround when played with Dolby
Pro Logic. LOGIC7 uses matrix technology to
compress the spatial aspects of a recording so it can be
delivered on a two channel format. There is no data
reduction of the sound waveforms themselves, so the
sound quality can be very high.
Competing with LOGIC7 and the current data-reduced
discrete technologies are two audio disc formats based
on DVD technology, DVD-Audio and SACD. These
promise to deliver discrete 5.1-channel surround
without data reduction. These encoding technologies
promise higher fidelity than CD, thanks to 96-kHz/24-bit
recording—a signal format for which the 960L is already
prepared. The eight-channel design and modular
construction of the 960L make it ready for future
surround advances, whatever they may be.
Algorithms
Random Hall
Random Hall is a hall effect with gradual build-up, well
suited to complex sounds like orchestral music. Its
reverberators change over time in controlled random
ways to avoid the buildup of tinny, grainy, metallic, or
other colorations.
The early reflections are user adjustable in amplitude
and delay. Some skill is needed to set useful reflection
patterns. Since the reflections are adjustable, they are
not randomized. Once set, the reflection pattern is
fixed. The pattern can be expanded or contracted in
time using the "Delay Master" control, and the overall
level of the pattern can be set with the "Early Level"
control.
The most important user parameters for Random Hall
are the level controls – the "Early Level" and "Reverb
Level". Early Level is a master control for all the early
reflections. Lowering the control to zero eliminates the
reflections, and their associated sense of distance from
the sound source. The "Reverb Level" control is a
master control for the late reverberation. With this
control you can set the exact amount of reverberance
and envelopment.
The apparent size of the space supplied by the late
reverberation is set with the Shape, Spread, and Size
controls. Of these controls the Shape and Spread are
the most natural. As Shape is raised from zero to about
30% the onset of the late reverberation goes from
abrupt to gradual. The effect on the apparent size of the
space is quite dramatic. Spread has little or no effect
until Shape is at about a quarter of its range, at which
point Spread affects the length of both the buildup and
sustain. At this point, the sustain will be approximately
the time value indicated by the Spread display, in
milliseconds. At still higher settings of Shape, a
secondary sustain appears in the envelope at a lower
Sound in Space: A Short History of Stereo and Surround, Continued
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