Lexicon CP-1 Theory Of Operation page 13

Multi-Channel Music & Cinema Systems
Theory and Design
Panorama
First-order correction travels to the left
ear, where it will be heard unless canceled
by an additional correction. When these
higher-order corrections are supplied,
accurate cancellation is possible.
10
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, Lexicon processors use
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 processor.
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.
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 fre-
quency.
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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