Lexicon CP-3 Plus Supplementary Manual page 23

PLUS
CP-3 Theory
and Design
Pro Logic decoders remove dialog from
the left and right channels, while main-
taining stereo as much as possible.
Pro Logic requires phase accuracy. Com-
mon azimuth errors cause ghost dialog in
all channels unless the azimuth error is
corrected.
20
The manual balancing procedure does nothing to correct azimuth errors.
During the preparation of the master for a video tape or disc, misalignment
of the playback heads or skewing of the film produce small time differences
between the two channels. Azimuth is poorly controlled in both profes-
sional video recorders and optical film chains. In the final product, which
has been through many generations, it can easily be wrong by 50 microsec-
onds or more, and may vary as the tape or disk is played. At middle and high
frequencies it doesn't take much misalignment to generate large inter-
channel differences in phase, which are just what the decoder uses to do its
steering.
The Dolby encode/decode system deals with this problem by reducing the
treble in the surround, so the out-of-phase sibilants in the film do not
splatter annoyingly from the rear. This does not, however, reduce the
sibilants in the side speakers. Some non-Pro Logic decoders reduce these
side-channel sibilants by narrowing the spread of the front channels in the
presence of dialog; this compromise is unnecessary in the CP-3
PLUS
The CP-3 Decoder
The CP-3 PLUS decoder is unusual in a number of ways. First of all, it is entirely
digital. (Most surround decoders advertise that they are digital because
there is a digital delay line for the surround channel but the matrix and the
logic decoding are done in analog.)
Because the CP-3 is all digital, we can use some of the digital memory to
PLUS
delay all the output channels by 20 milliseconds — about the same as the
acoustic delay you get in the front row of a theater. (The surround channel
is delayed by an additional 15 to 30 milliseconds.) This delay allows plenty
of time for the CP-3 PLUS to determine the direction of sounds and adjust the
matrix before the sounds are sent to the amplifiers. This substantially
improves dialog and effects cancellation, as is immediately apparent from
the spread of ambient material or music, even in the presence of dialog. The
CP-3 PLUS can also sense and continuously correct both balance and azimuth
errors in the incoming material. All the time the film is playing, the CP-3
is checking balance and azimuth, keeping the dialog perfectly centered. The
result is superior steering. An added benefit is that the CP-3
front panel input balance control; the user need not bother with this
adjustment. You can check the quality of the balance and azimuth in a tape
or disc if you wish by turning the Auto Azimuth/Balance parameter off and
observing any changes in the location of dialog and effects.
Stereo Surround in the CP-3
As of this writing, the film industry is moving from a 4 channel audio
standard to a 5.1 channel standard. The new standard is similar to the old,
but has two surround channels instead of one, and an additonal low
bandwidth channel for a subwoofer. Once one has heard some good
examples of stereo surround channels the mono surround channel of Dolby
Surround is no longer satisfying.
As most of the information in the surrounds is ambient information and
PLUS
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