Lexicon CP-3 Plus Supplementary Manual page 24

CP-3 PLUS Digital Surround Processor
music, stereo significantly increases listening enjoyment. As listeners be-
come 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. Arti-
facts 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.
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.
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, repro-
ducing 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 on-
screen 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 rear, and then from left rear 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 ambi-
ence is obvious. On films with agressive sound effects the combination of
surround steering and localization can convince listeners they are listening
to a true discrete surround mix.
PLUS
CP-3 Theory
and Design
Auto Azimuth and Auto Balance fea-
tures in the CP-3 PLUS allow accurate de-
coding of the new 5.1 channel film stan-
dard.
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