Lexicon CP-3 Plus Supplementary Manual page 9

PLUS
CP-3 Theory
and Design
The Panorama
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.
6
Loudspeakers placed on either side of the listening position are the most
effective and foolproof way to produce added Spatial Impression. Since it
Mode
is not always possible to have side loudspeakers, the CP-3
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 CP-3 PLUS
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
PLUS uses crosstalk