Lexicon CP-1 Theory Of Operation page 30

Multi-Channel Music & Cinema Systems
Theory and Design
tion of the original. When played back through a Lexicon processor running
our Logic 7 effect, surround information is maintained in stereo, and 5-2-5
or 5-2-7 encoding/decoding is possible, and the match to the original
soundtrack can be very close indeed.
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 in a way similar to 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 side, and then from left side to both left and right rear
loudspeakers. The steering between the side and rear speakers is accom-
plished through a shelf filter rather than with level. In the Logic 7 algorithm,
there is a 3dB decrease in the treble content of the side speaker when sound
is steered moderately to the rear, but the low frequency content of the side
and the rear speakers remains equal. As sound pans further to the rear the
shelf filter in the side is lowered, so the treble is further reduced. When the
sound is fully to the rear, there is a 6dB per octave low pass filter in the side
speakers, with the 3dB point at about 400Hz.
The result is that the low frequencies are never eliminated or reduced in the
side speakers. This is important because it is the side speakers which are
most capable of delivering fully spacious and strong bass to the listening
area. 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.
Many listening rooms place the favorite listening position on a sofa along
the side or rear wall of the room. It is not generally possible to place speakers
behind this piece of furniture — particularly because they would be likely
to be directly behind the heads of the listeners. However, it is well known
that sounds which come from far behind the listener (at least 150 degrees
from the front) are perceived as more psychologically exciting than sounds
which come merely from the sides.
The 5 Speaker Enhance effect uses Head Related Transfer Functions (HRTF)
to simulate seven channel performance in five speaker systems. It is
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